The Spine in Three Dimensions

The spine is a stack of small bones that somehow manages to be strong enough to carry the upper body, flexible enough to let a dancer bend in half, and protective enough to shelter the spinal cord through a lifetime of movement. Nearly every topic in this book, from manipulation and posture to pain and examination, rests on knowing this structure in three dimensions. This chapter builds that mental model one layer at a time: the regions and curves, the bones, the joints and ligaments that tie them together, the muscles that move and steady them, and the neural tissue they protect.

Section 1Regions, curves and the motion segment

Five regions, thirty three bones

The vertebral column (spinal column, backbone) runs from the base of the skull to the tip of the tailbone. In a typical adult it is about 70 cm long in men and about 60 cm in women, and roughly a quarter of that length is made up of intervertebral discs rather than bone. It is divided into five regions, and the commonly taught counts are worth memorizing exactly:

• Cervical: 7 vertebrae (C1 to C7), forming the neck.
• Thoracic: 12 vertebrae (T1 to T12), each carrying a pair of ribs.
• Lumbar: 5 vertebrae (L1 to L5), the large weight bearing bones of the low back.
• Sacral: 5 vertebrae that fuse in adulthood into one sacrum.
• Coccygeal: usually 4 small segments (the range is 3 to 5) that fuse partly or completely into the coccyx.

That gives 33 vertebrae at birth and, because the sacral and coccygeal segments fuse, about 24 separate movable vertebrae above the sacrum in the adult (7 + 12 + 5). Natural variation is common. Some people have a sixth lumbar segment, some have a lowest lumbar vertebra partly fused to the sacrum (sacralization), and some have a first sacral segment that behaves like a lumbar vertebra (lumbarization). These are called transitional segments, and they matter in practice because they can change how vertebral levels are counted on an image. A practitioner who assumes every spine has exactly five lumbar vertebrae can mislabel a level.

Curves: primary and secondary

Seen from the front or back, a healthy spine is straight. Seen from the side, it forms a gentle S shape made of four sagittal curves. Two curves are concave forward, called kyphoses (singular kyphosis): the thoracic and the sacral. Two curves are concave backward, called lordoses (singular lordosis): the cervical and the lumbar. The terms describe the shape of a normal curve; in everyday clinical language, "kyphosis" or "lordosis" is also used loosely for an exaggerated curve, so read the context carefully.

The kyphotic curves are called primary curves because they are present before birth. The fetus is curled into a C shape, and the whole column is one long kyphosis. The lordotic curves are called secondary curves (or compensatory curves) because they develop after birth in response to function. The cervical lordosis begins to form in the first months as the infant lifts the head against gravity, and it becomes well established when the child can sit with the head upright. The lumbar lordosis forms later, as the child begins to crawl, stand and walk, roughly around the end of the first year, and it continues to mature through childhood. A useful way to remember the order is to follow the milestones from the top down: head control first, then standing.

Secondary curves are produced mostly by wedging of the discs rather than of the bones. The front of a cervical or lumbar disc is taller than the back. In the thoracic region the reverse is partly true of the vertebral bodies themselves, which are slightly taller behind than in front, so the thoracic kyphosis is more "built into" the bone. This is one reason the thoracic curve changes less with posture than the lumbar curve.

Why curves help

A curved column sounds weaker than a straight one, but the alternating curves serve several purposes. They allow the column to flex slightly under load and spring back, spreading force through discs, ligaments and muscles instead of concentrating it on bone. They keep the head and trunk balanced over the pelvis so that the line of gravity passes close to the joints that carry it, which reduces the muscle work needed to stand. They also enlarge the thoracic and pelvic cavities.

Normal curve size varies widely between healthy people. Commonly cited standing values are roughly 20 to 45 degrees of thoracic kyphosis and roughly 40 to 60 degrees of lumbar lordosis, but ranges differ by measurement method, age and sex. A curve outside a reference range is a finding to interpret, not a diagnosis on its own; many people with large or small curves have no pain, and many people with pain have average curves. Abnormal curvature in the frontal plane, a sideways curve with rotation, is scoliosis, discussed in later chapters on posture and examination.

The motion segment

Because the spine is made of many repeating units, it is useful to study the smallest unit that shows the column's behavior. This unit is the motion segment, also called the functional spinal unit (FSU). It consists of two adjacent vertebrae plus all the soft tissue between them: the intervertebral disc, the two facet joints, and the ligaments that connect the two bones. The muscles that cross the segment are usually discussed separately, but they act on it constantly. Physicians and researchers describe motion as occurring "at L4 to L5," meaning in the motion segment formed by L4, the L4 to L5 disc, and L5.

At each typical motion segment the two vertebrae meet at three joints. In front is the interbody joint, the vertebral bodies joined by the disc, a cartilaginous joint (specifically a symphysis). Behind are the paired zygapophyseal joints, usually called facet joints, which are synovial joints. Together they form the three joint complex. The idea behind the name is that these three joints share the load and share the motion, so a change in one affects the others. When a disc loses height, for example, the facets are pushed closer together and carry more load, which can contribute to facet joint wear over time. This is the logic behind the commonly described degenerative cascade, a sequence in which disc and facet changes influence each other over decades.

Anterior and posterior columns

A practical way to think about the motion segment is to split it into an anterior column and a posterior column. The anterior column (bodies and discs) is built for compression: it carries most of the axial load in upright standing and absorbs shock. The posterior column (arches, facets, spinous processes and their ligaments) is built for guidance and tension: the facets steer motion and resist shear and rotation, while the posterior ligaments resist being pulled apart in flexion. Surgeons and radiologists use a related three column model for spinal fractures (anterior, middle and posterior), in which the middle column is the back half of the body and disc plus the posterior longitudinal ligament. Injuries that damage two or more columns are generally considered more likely to be unstable.

Section 2Vertebrae by region

The typical vertebra

Almost every vertebra follows the same basic plan, and regional differences are variations on that plan. A typical vertebra has two main parts. In front is the vertebral body (centrum), a short cylinder of spongy (cancellous) bone wrapped in a thin shell of compact bone. The body is the weight bearing part. Behind is the vertebral arch (neural arch), a ring of bone that encloses the vertebral foramen. Stacked together, the vertebral foramina form the vertebral canal, which houses the spinal cord, its coverings and the nerve roots.

The arch has named parts that you should be able to point to:

• Pedicles: two short, thick pillars that connect the arch to the back of the body. The upper and lower edges of each pedicle have notches; the inferior notch of one vertebra and the superior notch of the one below make the roof and floor of the intervertebral foramen.
• Laminae: two flat plates that run from the pedicles backward and toward the midline, meeting to close the arch.
• Spinous process: a single process projecting backward from the junction of the laminae. It is the bump you feel down the middle of the back.
• Transverse processes: paired processes projecting laterally from the junction of pedicle and lamina.
• Articular processes: two superior and two inferior processes, each with a smooth cartilage covered facet. The inferior articular processes of one vertebra meet the superior articular processes of the one below to form the facet joints.
• Pars interarticularis: the narrow strip of lamina between the superior and inferior articular processes. A stress fracture here is spondylolysis, most common at L5.

The spinous and transverse processes act as levers for muscles and attachment points for ligaments. Longer levers give muscles more mechanical advantage.

Cervical vertebrae

The neck has the smallest and most mobile vertebrae. All seven cervical vertebrae share one feature found nowhere else: a hole in each transverse process called the transverse foramen (foramen transversarium). The vertebral artery usually enters the transverse foramen at C6 and ascends through C1 before turning medially over the posterior arch of the atlas and entering the skull through the foramen magnum. The C7 transverse foramen is small and usually carries only small veins. The typical cervical vertebrae (C3 to C6) also have:

• Small, wide, rectangular bodies with raised lateral lips on the upper surface called uncinate processes. These form the uncovertebral joints (joints of Luschka) with the body above. They guide motion and help keep the disc from bulging laterally, and bony spurs from them can narrow the intervertebral foramen in older adults.
• A large, triangular vertebral foramen to fit the cervical enlargement of the cord.
• Short, often bifid (split) spinous processes, typically C2 to C6.
• Transverse processes ending in anterior and posterior tubercles, separated by a groove for the spinal nerve. The anterior tubercle of C6 is especially large and is called the carotid tubercle.
• Articular facets set at roughly 45 degrees, facing upward and backward on the superior processes, which allows a generous range of flexion, extension, lateral flexion and rotation.

Three cervical vertebrae are atypical. The atlas (C1) has no body and no spinous process. It is a ring made of an anterior arch, a posterior arch and two lateral masses that carry kidney shaped superior articular facets for the occipital condyles. The atlanto-occipital joint allows the nodding "yes" motion, flexion and extension of the head. The axis (C2) has a tooth shaped projection, the dens (odontoid process), rising from its body. Developmentally, the dens is the body of C1 that fused to C2. The dens sits against the back of the anterior arch of the atlas and is held there by the strong transverse ligament of the atlas. The atlanto-axial joints allow the "no" motion: about half of all cervical rotation occurs here. The axis also has the first large, palpable spinous process below the skull. Finally, C7 is called the vertebra prominens because its long, non bifid spinous process is usually the most visible bump at the base of the neck (although T1 is sometimes as prominent, which is why palpation counts need confirmation).

Thoracic vertebrae

The defining feature of thoracic vertebrae is their relationship with the ribs. Each typical thoracic vertebra has costal facets (demifacets) on the side of the body where rib heads articulate, and transverse costal facets on the transverse processes where the rib tubercles articulate. A typical rib head (ribs 2 to 9) sits across the disc and touches two vertebrae: the body of its own number and the body above. Thus rib 6 articulates with T5 and T6 and with the transverse process of T6. Ribs 1, 10, 11 and 12 articulate with only one vertebral body, and ribs 11 and 12 have no joint with a transverse process.

Other thoracic features include heart shaped bodies of intermediate size, a small round vertebral foramen (the cord is narrowest here, and the canal leaves little spare room), and long spinous processes that slope steeply downward, especially in the middle thoracic region (about T5 to T8), where the tip of a spinous process can lie at the level of the body below. The thoracic facets are nearly vertical and lie close to the frontal (coronal) plane, facing backward on the superior processes. That orientation favors rotation and lateral flexion, but the rib cage stiffens the region overall.

A palpation guideline often taught for the thoracic spine, the rule of threes, estimates how far below its own transverse processes each spinous process tip lies; studies have questioned its precision, so treat it as a rough guide.

Lumbar vertebrae

The lumbar vertebrae carry the most body weight and have the largest bodies, which are wide and kidney shaped when viewed from above. They have no transverse foramina and no costal facets. The spinous processes are thick, square (hatchet shaped) and point almost straight backward. The transverse processes are long and slender. On the back of each superior articular process is a small bump called the mammillary process, and at the base of the transverse process is a smaller accessory process; both anchor the deep multifidus and intertransversarii muscles.

The lumbar facets are curved and oriented mostly in the sagittal plane in the upper lumbar spine, becoming more coronal (more turned to face front and back) at L4 to L5 and L5 to S1. The superior facets are concave and face medially and backward; the inferior facets are convex and face laterally and forward. This interlocking shape allows flexion and extension readily but limits axial rotation to only a few degrees per segment. The more coronal orientation at the bottom helps resist the forward shear that develops where the lumbar spine sits on the tilted top of the sacrum. L5 is the largest lumbar vertebra and is somewhat wedge shaped, taller in front than behind, contributing to the lumbosacral angle.

Sacrum and coccyx

The sacrum is a large triangular bone formed by fusion of five sacral vertebrae, usually completed by about the mid twenties. It sits wedged between the two ilia, forming the back of the pelvis. Key landmarks include:

• The sacral base (the top surface of S1), whose front edge projects forward as the sacral promontory.
• The ala (wings) on each side of the base.
• Four pairs of anterior sacral foramina and four pairs of posterior sacral foramina, through which the ventral and dorsal rami of S1 to S4 exit.
• The median sacral crest (fused spinous processes) on the back.
• The sacral hiatus, the lower opening of the sacral canal where the laminae of S5 (and sometimes S4) fail to meet, flanked by the sacral cornua. Clinicians use the hiatus for caudal epidural injections.
• The auricular surfaces (ear shaped) on each side, which form the sacroiliac joints with the ilia.

The sacral base is tilted forward. The angle between the top of S1 and a horizontal line, the sacral base angle (Ferguson's angle), is commonly taught as about 30 to 40 degrees in standing, although reported values vary. A steeper sacrum generally goes with a deeper lumbar lordosis. The coccyx, the tailbone, is a small triangle of usually four rudimentary segments. It anchors the gluteus maximus, parts of the levator ani and the anococcygeal ligament, and it bears some weight when a person sits leaning back. Pain here is called coccydynia.

Section 3Discs, facets and ligaments

The intervertebral disc

There are 23 intervertebral discs in the adult spine. The first lies between C2 and C3; there is no disc between the occiput and the atlas or between the atlas and the axis. The last true disc lies between L5 and S1, and a small rudimentary disc remnant may be present between the sacrum and coccyx. Disc height increases from the neck to the low back, and the discs together make up roughly a quarter of the height of the column above the sacrum. Because discs lose a little water under load during the day and regain it lying down overnight, people are measurably taller in the morning than in the evening, often by about 1 to 2 cm.

Each disc has three parts that work together as a pressurized system.

• Nucleus pulposus: the gel-like center. In a young adult it is about 70 to 90 percent water, held there by proteoglycans (especially aggrecan), large molecules whose negatively charged glycosaminoglycan chains attract water by osmosis. The nucleus also contains loose type II collagen. Because it is mostly water, it is nearly incompressible; when loaded it pushes outward in all directions, converting vertical compression into tension in the surrounding annulus. In the lumbar spine the nucleus sits slightly behind the center of the disc.
• Annulus fibrosus: the tough outer ring of about 15 to 25 concentric layers called lamellae. The outer annulus is rich in type I collagen (tensile strength); the inner annulus has more type II collagen and proteoglycan and blends into the nucleus. The collagen fibers in each lamella run at an angle of roughly 60 to 65 degrees from vertical, and the angle alternates direction in successive layers, like the plies of a radial tire. This crisscross arrangement lets the annulus resist twisting in both directions and contain the pressure of the nucleus. The posterior and posterolateral annulus is thinner than the anterior annulus, which is one reason disc herniations usually occur in a posterolateral direction.
• Vertebral endplates: thin layers of hyaline cartilage (about 1 mm thick) that cover the top and bottom of each disc and attach it to the vertebral bodies. The cartilage endplate rests on a perforated plate of bone, the bony endplate, with a raised rim called the ring apophysis. The endplate is the weakest part of the segment under pure compression, so a large compressive load tends to crack the endplate before it ruptures a healthy annulus. A herniation of nucleus through the endplate into the vertebral body is called a Schmorl's node.

Blood supply, nutrition and nerves of the disc

The adult disc is the largest structure in the body without its own blood supply. Small vessels reach only the outermost annulus. The cells of the nucleus and inner annulus are fed by diffusion: nutrients such as glucose and oxygen pass from capillaries in the vertebral bodies through the endplates, and waste such as lactic acid diffuses out. The trip can be several millimeters, which is long for diffusion. Movement and changes in load are thought to help by pumping fluid in and out, which is one reason activity is often described as good for disc health. When the endplate calcifies with age, diffusion slows, the disc cells struggle, proteoglycan content falls, and the nucleus loses water. These changes are part of normal aging and are very common on imaging in people without pain.

Innervation is also limited to the periphery. The outer third of the annulus receives sensory fibers from the sinuvertebral nerve (recurrent meningeal nerve), which re-enters the intervertebral foramen to supply the posterior annulus, posterior longitudinal ligament and dura, and from branches of the gray rami communicantes on the front and sides. In degenerated discs, nerve fibers can grow deeper into fissures of the annulus, a finding that has been linked with discogenic pain.

The facet joints

The zygapophyseal (facet) joints are true synovial joints, with articular cartilage, a synovial membrane, synovial fluid and a fibrous joint capsule. Many contain small inward folds called meniscoids (synovial folds or fat pads) that fill the gaps at the edges of the joint. The capsule is richly supplied with sensory receptors, including mechanoreceptors and nociceptors, so the facet can be a pain source and also a source of position information. Each facet joint is innervated by the medial branch of the dorsal ramus from the level at and the level above the joint. For example, the L4 to L5 facet joint receives medial branches from the L3 and L4 dorsal rami. This dual supply is why diagnostic medial branch blocks target two nerves for one joint.

Facet orientation is the single biggest reason each region moves differently. In the cervical spine the facets are angled about halfway between horizontal and vertical, so the neck can move in all directions. In the thoracic spine the facets lie in the frontal plane, which favors rotation and lateral flexion. In the lumbar spine they lie mostly in the sagittal plane and curve around each other, which favors flexion and extension and blocks rotation. Chapter 2 develops these motion patterns in detail.

The facets share load with the disc. In upright standing with a normal lordosis they are generally said to carry a modest fraction of the axial compressive load (figures from about 10 to 20 percent are commonly quoted), and this share rises in extension and when the disc loses height. Their larger roles are to resist shear, limit rotation and guide the path of motion.

Ligaments of the spine

Ligaments are bands of dense connective tissue connecting bone to bone. In the spine they limit motion at the end of range, protect the cord, and, because they contain sensory receptors, contribute to the body's sense of position. Two run along the bodies, and several connect the arches.

• The anterior longitudinal ligament (ALL) is a broad, strong band on the front of the bodies and discs, from the occiput and anterior tubercle of the atlas to the sacrum. It limits extension and is thick enough to resist forward bulging of the disc.
• The posterior longitudinal ligament (PLL) runs on the back of the bodies, inside the vertebral canal, from C2 (where it continues upward as the tectorial membrane) to the sacrum. It limits flexion. It is narrower than the ALL, and in the lumbar region it narrows further and is attached mainly to the discs in a pattern that leaves the posterolateral corners less covered. This is another reason herniations tend to go posterolateral rather than straight back.
• The ligamentum flavum (yellow ligament) connects the laminae of adjacent vertebrae. It is unusual for its high elastin content, which gives its yellow color and lets it stay taut without buckling into the canal when the spine extends. It limits flexion. With aging it may thicken and fold, narrowing the canal and contributing to spinal stenosis.
• The interspinous ligaments join adjacent spinous processes and limit flexion.
• The supraspinous ligament joins the tips of the spinous processes from C7 down to the sacrum. Above C7 it expands into the ligamentum nuchae, a midline sheet that anchors neck muscles.
• The intertransverse ligaments join transverse processes and limit lateral flexion to the opposite side.
• The facet capsules are ligaments too, and they limit flexion and rotation.

The upper cervical spine has special ligaments that protect the brainstem. The transverse ligament of the atlas holds the dens against the anterior arch; with vertical bands above and below it forms the cruciform ligament. The paired alar ligaments run from the sides of the dens to the occipital condyles and limit rotation of the head on the neck. The tectorial membrane covers these ligaments from behind. Rheumatoid arthritis and Down syndrome can weaken the transverse ligament, allowing atlantoaxial instability, a contraindication to forceful upper neck manipulation.

Section 4Muscles and fascia of the trunk

Three layers of back muscles

Back muscles are easiest to learn in layers, sorted by depth, function and nerve supply. The superficial and intermediate layers are called extrinsic back muscles: they lie on the back but developed with the limbs and ribs, and they are supplied by ventral rami (or, for the trapezius, the spinal accessory nerve). The deep layer contains the intrinsic or true back muscles, which move and stabilize the spine itself and are supplied by dorsal rami of the spinal nerves. That one distinction, ventral versus dorsal rami, explains a great deal of clinical anatomy.

• Superficial layer (moves the upper limb): trapezius, latissimus dorsi, levator scapulae, and rhomboid major and minor.
• Intermediate layer (thin respiratory muscles): serratus posterior superior and serratus posterior inferior, which attach to ribs.
• Deep layer (moves and stabilizes the spine), itself arranged from superficial to deep as the splenius group, the erector spinae, the transversospinalis group, and the small segmental muscles.

The intrinsic muscles

The splenius capitis and splenius cervicis wrap like a bandage (splenius means bandage) from the spinous processes of the lower neck and upper thorax upward and laterally to the skull and upper cervical transverse processes. Acting on one side they rotate and side bend the head toward the same side; acting together they extend it.

The erector spinae is the large muscle mass that forms the two ridges beside the midline of the low back. It arises from a broad common tendon attached to the sacrum, iliac crest and lumbar spinous processes, and it splits into three columns. From lateral to medial they are the iliocostalis, longissimus and spinalis, remembered by the phrase "I Love Spine." Working together on both sides, the erector spinae extend the spine and, importantly, control flexion by contracting eccentrically as we bend forward. Working on one side, they side bend the trunk toward the same side.

The transversospinalis group lies deep in the groove between the spinous and transverse processes. Its fibers run upward and medially from transverse processes to spinous processes. Three muscles make up the group, getting shorter and deeper as you go: semispinalis (spans about four to six segments, most developed in the neck and upper thorax), multifidus (spans about two to four segments, thickest in the lumbar region) and rotatores (span one or two segments, best developed in the thoracic region). On one side they rotate the trunk toward the opposite side. The lumbar multifidus deserves special attention because it has a large cross section close to the joints it crosses, which suits it to controlling small segmental motions. Imaging studies have reported reduced size and increased fat in the multifidus of many people with chronic low back pain, although whether this is a cause or a result of pain is still studied.

The deepest segmental muscles are the interspinales and intertransversarii. They are tiny and contain a very high density of muscle spindles, so they are thought to act mainly as position sensors rather than movers. In the upper neck, four pairs of suboccipital muscles (rectus capitis posterior major and minor, obliquus capitis superior and inferior) connect the occiput, atlas and axis. They fine tune head position, are supplied by the suboccipital nerve (dorsal ramus of C1), and bound the suboccipital triangle, through which the vertebral artery runs.

The abdominal wall and other trunk muscles

The front and sides of the trunk are closed by four abdominal muscles. The rectus abdominis runs vertically from the pubis to the lower ribs and xiphoid, divided by tendinous intersections, and is the main trunk flexor. The external oblique runs downward and forward ("hands in your pockets"), the internal oblique lies beneath at roughly right angles to it, and the transversus abdominis, the deepest, runs horizontally around the waist like a belt. Rotation pairs the external oblique of one side with the internal oblique of the other: turning the trunk to the right uses the left external oblique and right internal oblique. The transversus abdominis increases intra-abdominal pressure and tensions the thoracolumbar fascia. All four are supplied by the ventral rami of the lower thoracic nerves (about T7 to T12), with L1 contributing to the lower internal oblique and transversus.

Several other muscles complete the trunk "cylinder." The diaphragm forms its roof, and its crura attach to the upper lumbar bodies. The pelvic floor forms its base. The quadratus lumborum forms part of the back wall of the abdomen, and the psoas major runs along the sides of the lumbar bodies to the femur. In the neck, the longus colli and longus capitis (the deep neck flexors) lie on the front of the cervical bodies, while the scalenes and sternocleidomastoid move the neck and assist breathing.

The thoracolumbar fascia

The thoracolumbar fascia (lumbodorsal fascia) is a tough sheet of connective tissue that wraps the deep back muscles. In the lumbar region it has three layers. The posterior layer covers the back of the erector spinae and attaches to the spinous processes. The middle layer attaches to the tips of the lumbar transverse processes and separates the erector spinae from the quadratus lumborum. The anterior layer covers the front of the quadratus lumborum. The layers fuse at the lateral edge of the erector spinae to form the lateral raphe, which is where the transversus abdominis and internal oblique attach. Superficially, the latissimus dorsi takes origin from the posterior layer, and below, the fascia blends with the gluteus maximus fascia across the midline.

This arrangement makes the thoracolumbar fascia a link between the arms, trunk and legs. When the latissimus dorsi on one side and the gluteus maximus on the other contract together, as in walking or throwing, they tension the fascia diagonally across the low back. When the transversus abdominis contracts, it pulls on the fascia laterally and stiffens the lumbar segments.

Section 5The spinal canal, cord and nerve roots

The spinal cord and its coverings

The spinal cord begins at the foramen magnum as a continuation of the medulla and, in most adults, ends at about the level of the L1 to L2 disc as a cone-shaped tip, the conus medullaris. At birth it reaches lower, about L3, because during development the column grows faster than the cord. The cord is about 45 cm long in the adult and is not uniform: it widens at the cervical enlargement (cord segments about C4 to T1, supplying the upper limbs) and the lumbosacral enlargement (about L1 to S3 segments, supplying the lower limbs), where more neurons serve the limbs.

Three membranes, the meninges, surround the cord. The dura mater is the tough outer tube. Unlike the cranial dura, the spinal dura is separated from bone by the epidural space, which contains fat and a valveless internal vertebral venous plexus (part of the network often called Batson's plexus). Inside the dura is the delicate arachnoid mater, and beneath it the subarachnoid space filled with cerebrospinal fluid (CSF). The pia mater clings to the cord surface. Pia forms the denticulate ligaments, tooth-like lateral extensions that anchor the cord to the dura, and continues below the conus as the filum terminale, which anchors the cord to the coccyx. The dural sac itself ends at about S2.

Below the conus, the vertebral canal contains a bundle of lumbar, sacral and coccygeal nerve roots descending to their exits, called the cauda equina ("horse's tail"). Because there is no cord below about L2, a needle can be placed in the subarachnoid space at L3 to L4 or L4 to L5 to sample CSF (lumbar puncture) with little risk to the cord. A large central disc herniation or other mass in the lower canal can compress many roots at once, producing cauda equina syndrome: saddle area numbness, new bladder or bowel dysfunction, and leg weakness. It is a surgical emergency and one of the most important red flags in spine care.

Spinal nerves and how they are numbered

There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. Each forms from a dorsal (sensory) root, which carries the cell bodies of sensory neurons in the dorsal root ganglion, and a ventral (motor) root. The roots join in or just beyond the intervertebral foramen to form a short mixed spinal nerve, which divides almost at once into a dorsal ramus (to the deep back muscles, facet joints and skin of the back) and a larger ventral ramus (to the limbs and the front and sides of the trunk, forming the cervical, brachial and lumbosacral plexuses). The sinuvertebral nerve branches back into the canal.

Numbering trips up many students because the neck has 8 nerves but 7 vertebrae. The rule is:

• In the cervical spine, each nerve exits above the vertebra of the same number. C1 exits between the occiput and atlas, C6 exits between C5 and C6, and so on.
• C8 has no vertebra of its own and exits between C7 and T1.
• From T1 downward, each nerve exits below the vertebra of the same number: the L4 nerve exits through the L4 to L5 foramen.

Because the cord ends high, there is also a growing mismatch between cord segment level and vertebral level. In the lower thoracic region, the cord segments sit about two to three levels above the vertebrae of the same number, and all of the lumbar and sacral cord segments are packed into the region behind roughly T11 to L1. The lumbar and sacral roots must therefore descend a long way through the canal before they exit.

The intervertebral foramen and lateral recess

The intervertebral foramen (neural foramen) is the doorway through which each spinal nerve leaves the canal. Its boundaries are:

• Roof and floor: the pedicles of the vertebrae above and below.
• Front: the back of the vertebral bodies and the disc (and, in the neck, the uncovertebral joint).
• Back: the facet joint and its capsule, with the lateral edge of the ligamentum flavum.

Each of these boundaries can narrow it. A posterolateral disc bulge or herniation, an osteophyte from the uncovertebral or facet joint, a thickened ligamentum flavum, or loss of disc height that lowers the pedicle above can all reduce the space, a condition called foraminal stenosis. In the lumbar spine the nerve root occupies only the upper part of the foramen, beneath the pedicle, so loss of disc height pinches it from below and in front.

Just before a lumbar root enters its foramen, it travels in a gutter along the side of the canal called the lateral recess (subarticular zone), bounded in front by the body and disc, laterally by the pedicle, and behind by the superior articular process. Enlargement of that facet can trap the traversing root here. This geography explains a key clinical rule. At the L4 to L5 disc, the L4 root is already exiting high in its foramen, while the L5 root is traversing the canal on its way down to the L5 to S1 foramen. A typical posterolateral herniation at L4 to L5 therefore compresses the L5 root, the traversing one. A far lateral (foraminal) herniation at the same disc compresses the exiting L4 root. In the cervical spine the roots exit almost horizontally, so a C5 to C6 herniation usually affects the C6 root.

Blood supply of the cord

The cord is supplied by one anterior spinal artery, which runs in the front midline and feeds roughly the front two thirds of the cord, and two posterior spinal arteries, which feed the back third, including the dorsal columns. These longitudinal channels are reinforced along the way by segmental (radicular) arteries entering through the foramina. The largest is the great anterior radicular artery, the artery of Adamkiewicz, which usually enters on the left between about T9 and T12. The mid thoracic cord is a watershed zone with relatively little collateral flow. Veins drain into the valveless vertebral venous plexuses, which connect with pelvic and abdominal veins; this connection is one proposed route for the spread of prostate and other cancers to the vertebrae.

Words worth knowing

Spinal Biomechanics

Anatomy tells you what the spine is made of; biomechanics tells you what those parts do when you bend to tie a shoe, twist to look over your shoulder or lift a child from the floor. The same few principles, planes and axes, coupled motion, load sharing and stability, explain why some movements feel easy and others strain, why discs and facets fail in particular ways, and why advice about posture and lifting has changed over the years. This chapter turns the static spine of Chapter 1 into a moving, loaded structure you can reason about.

Section 1Planes, axes and coupled motion

The language of motion

To describe motion precisely, biomechanics uses three imaginary planes that pass through the body in anatomical position (standing upright, facing forward, arms at the sides, palms forward). Each plane has an axis perpendicular to it, and rotation in a plane happens around that axis, the way a door swings in a horizontal plane around the vertical axis of its hinges.

• The sagittal plane divides the body into left and right. Motion in it is flexion (bending forward) and extension (bending backward). It occurs around a frontal axis (also called the coronal or mediolateral axis) that runs side to side.
• The frontal plane (coronal plane) divides the body into front and back. Motion in it is lateral flexion (side bending) to the left or right, around an anteroposterior (sagittal) axis that runs front to back.
• The transverse plane (horizontal plane) divides the body into top and bottom. Motion in it is axial rotation to the left or right, around a vertical (longitudinal) axis.

Students often mix up the names of planes and axes because the sagittal plane rotates around a frontal axis and the frontal plane rotates around a sagittal axis. The trick is to picture the motion and ask what a skewer through the moving joint would look like: for nodding (flexion), the skewer runs from ear to ear, a side to side or frontal axis.

Six degrees of freedom

A vertebra moving on the one below can do more than rotate. It can also slide, or translate. Because there are three axes and the bone can rotate around each and translate along each, a motion segment has six degrees of freedom: three rotations (flexion and extension, lateral flexion, axial rotation) and three translations (forward and backward glide, side to side glide, and vertical compression or distraction). Research groups describe spinal motion in a three axis coordinate system with its origin in the vertebral body so that results from different laboratories can be compared. In real motion the rotations are much larger than the translations, which are usually a few millimeters at most. Translation becomes clinically important when it is excessive, as in the forward slip of one vertebra on another called spondylolisthesis.

Osteokinematics, arthrokinematics and the axis of rotation

Osteokinematics describes the motion of whole bones that you can see from the outside, such as "the lumbar spine flexes 50 degrees." Arthrokinematics describes what happens between the joint surfaces themselves: gliding, rolling and gapping. When the lumbar spine flexes, the inferior articular processes of each upper vertebra glide upward and forward on the superior processes of the vertebra below, and the facet joint surfaces partly separate. In extension the inferior processes glide downward and backward until their tips approach the lamina below, and the facet surfaces press together. Understanding these small glides helps explain why extension often provokes facet pain while sustained flexion stretches the posterior annulus and ligaments.

Because a vertebra both rotates and translates, the point around which it rotates is not fixed. At any instant there is a point that does not move, the instantaneous axis of rotation (IAR), and it migrates as the motion progresses. In the lumbar spine, the IAR for flexion and extension usually lies in the back half of the disc or near the upper endplate of the vertebra below. In degenerated or injured segments the path of the IAR becomes erratic and scattered, which is one way researchers have tried to detect abnormal motion. A related idea, the helical axis, combines rotation and translation into one screw motion and is used in three dimensional motion analysis.

Coupled motion

When the spine moves in one plane, it rarely moves only in that plane. Coupled motion is the consistent association of a primary motion around one axis with a secondary motion around another. The classic example is the pairing of lateral flexion with axial rotation. Coupling arises from the shape and orientation of the facets, the geometry of the curves, the discs and the ligaments, so it differs between regions.

• Lower cervical spine (C2 to C7): coupling is consistent. Lateral flexion is accompanied by axial rotation to the same side. When you side bend your neck to the right, the vertebral bodies also turn to the right, which means the spinous processes swing to the left (toward the convex side of the curve). The 45 degree facet slope makes this almost unavoidable: as one inferior facet slides down its sloped partner and the other slides up, the vertebra must turn.
• Upper cervical spine: the atlanto-occipital joint allows mostly flexion and extension, with small amounts of lateral flexion and rotation that tend to couple in opposite directions. At C1 to C2, rotation is coupled with a slight vertical translation, because the convex facets of the atlas ride down the convex facets of the axis as the head turns.
• Thoracic spine: the upper thoracic segments tend to follow the cervical pattern. In the middle and lower thoracic segments coupling is variable between studies and between people.
• Lumbar spine: coupling is small and inconsistent. Studies have found same side, opposite side and no coupling, and the pattern changes with posture, level and whether the movement starts from flexion, neutral or extension.

Fryette's laws and what research says

Early in the twentieth century, osteopathic physician Harrison Fryette proposed rules for coupled motion based on observations of models and patients. They are still taught in manual therapy programs, so it is worth knowing them accurately:

1. Law I (neutral): when the thoracic or lumbar spine is in neutral, side bending and rotation occur to opposite sides, and the pattern involves a group of vertebrae.
2. Law II (non-neutral): when the spine is substantially flexed or extended, side bending and rotation occur to the same side, and the pattern may involve a single segment.
3. Law III: introducing motion in one plane reduces the motion available in the other planes.

Law III is consistent with experimental findings and with everyday experience: you can rotate your trunk less when you are already bent far forward. Laws I and II have not held up well as universal rules. Systematic reviews of lumbar coupling studies, using cadavers, radiographs and three dimensional motion capture, have found that the direction of coupling varies between people, levels and starting postures, and no single pattern predicts it reliably. In the cervical spine below C2, by contrast, same side coupling is consistent in nearly all studies. A sound conclusion is that cervical coupling can be taught as a rule, while thoracic and lumbar coupling should be treated as individual and variable. This matters clinically because diagnostic or treatment approaches that depend on a fixed lumbar coupling pattern rest on an uncertain foundation.

Section 2Regional motion: cervical, thoracic and lumbar

What controls how much a region moves

Three features largely decide how much motion a region allows and in which directions. The first is facet orientation, introduced in Chapter 1: facets permit gliding along their surfaces and block motion that drives them into each other. The second is the disc height to body height ratio: a tall disc on a short body allows more tilt before the bones or ligaments stop the motion, the same way a thick rubber washer lets a bolt wobble more than a thin one. This ratio is about 2:5 in the cervical spine, 1:5 in the thoracic spine and 1:3 in the lumbar spine, which helps explain why the neck is the most mobile region and the thorax the least. The third is attached structures, above all the rib cage, which stiffens the thoracic spine, and to a lesser degree the large muscles and the pelvis.

The values below are representative segmental ranges from classic cadaver and imaging studies. They are useful for understanding patterns, but reported values vary considerably with age, method and individual, so treat them as approximate. Flexion and extension are given as a combined arc; lateral flexion and rotation are given to one side.

Cervical spine

The neck is built for mobility so that the eyes, ears and nose can be pointed wherever they are needed. The upper cervical complex (occiput, atlas and axis) and the lower cervical spine (C2 to C7) behave differently. The atlanto-occipital joints, deep cup-shaped joints for the occipital condyles, contribute a large share of nodding flexion and extension, while the atlanto-axial joints provide roughly half of all cervical rotation, about 40 degrees to each side. Rotation at C1 to C2 is limited mainly by the alar ligaments. Beyond about 40 degrees of head rotation, the lower cervical segments join in.

In the lower cervical spine, flexion and extension are greatest at C4 to C5 and C5 to C6, which is also where degenerative changes most often appear in middle-aged adults, possibly because these segments see the most motion. The uncovertebral joints guide flexion and extension and limit lateral translation. Commonly cited normal active ranges for the whole neck in young adults are roughly 45 to 60 degrees of flexion, 60 to 75 degrees of extension, about 45 degrees of lateral flexion and 70 to 80 degrees of rotation to each side. Range falls gradually with age, especially extension and rotation, so normal values must be judged against age.

Two terms describe head and upper neck positions in the sagittal plane. Protraction moves the head forward (lower cervical flexion with upper cervical extension, the chin poking forward) and retraction moves it back (lower cervical extension with upper cervical flexion, the "chin tuck"). These combined patterns show that the upper and lower cervical spines can move in opposite directions at the same time, a point that matters in posture assessment and exercise.

Thoracic spine

Although each thoracic segment allows only modest motion, there are twelve of them, so the region contributes meaningfully to trunk rotation, especially in the middle thoracic spine. The frontal plane facets permit rotation, but the rib cage stiffens the region: cadaver studies show that removing the ribs and sternum greatly increases thoracic motion. Flexion is limited by the posterior ligaments and by the ribs crowding together in front; extension is limited by contact of the long, overlapping spinous processes and laminae and by the anterior longitudinal ligament. Lateral flexion is limited by the ribs on the concave side.

The thoracolumbar junction (about T11 to L1) is a transition zone. The upper facets of T12 face like thoracic facets, while its lower facets face like lumbar ones, so rotation drops sharply from the thorax to the low back. A stiff region meeting a mobile one concentrates stress, which is one reason the thoracolumbar junction is the most common site of traumatic spinal fractures. The thoracic spine also moves with breathing: in inspiration the thoracic curve flattens slightly and in forced expiration it increases.

Lumbar spine

The lumbar spine is the region for flexion and extension. Segmental sagittal motion increases from L1 to L2 down to L5 to S1, so the two lowest segments together provide a large share of lumbar flexion and extension, and they are also the most common sites of disc herniation. Lateral flexion is moderate. Axial rotation is small, about 1 to 2 degrees to each side per segment, adding up to only about 10 degrees each way for the whole lumbar spine. The curved, sagittally oriented facets account for this: as a vertebra rotates, one inferior facet jams against its partner within a couple of degrees. Most of the trunk rotation people think of as "twisting the low back" actually happens at the hips and thoracic spine.

The lumbosacral junction combines large flexion and extension with a forward-tilted sacrum that creates anterior shear. The more coronal facets at L5 to S1, the strong iliolumbar ligaments connecting L5 to the ilium, and the disc and muscles resist this shear.

Lumbopelvic rhythm

Touching your toes is not just lumbar flexion. Forward bending combines lumbar flexion with anterior rotation of the pelvis at the hips, a coordinated pattern called lumbopelvic rhythm. In the classic description, the early part of the movement is dominated by lumbar flexion and the later part by hip flexion, and the order reverses on the way up: the hips and pelvis lead the return, and the lumbar spine extends last. In reality the two overlap throughout the movement. A typical adult might reach roughly 40 to 60 degrees of lumbar flexion and 60 to 80 degrees of hip flexion in full forward bending, though both vary widely.

Lumbopelvic rhythm explains several practical observations. Tight hamstrings limit pelvic rotation, so a person with short hamstrings must flex the lumbar spine more to reach the same point. A person with a painful or stiff low back often bends mainly at the hips and keeps the lumbar spine relatively straight. Fingertip-to-floor distance therefore measures combined hip and spine motion, not lumbar motion alone, which is why clinicians use other methods, such as the modified Schober test or inclinometers, to estimate lumbar flexion.

Section 3Loads on discs and facets

Basic load concepts

A load is a force or a moment applied to a structure. Five basic load types act on the spine: compression (pushing the vertebrae together along the long axis), tension (pulling them apart), shear (sliding one vertebra forward, backward or sideways on the next), bending (flexion, extension or lateral flexion, which compresses one side of the segment and stretches the other) and torsion (twisting about the long axis). Real activities combine several at once. Force is measured in newtons (N); a mass of 1 kg weighs about 10 N, so a 70 kg person weighs about 700 N. A moment (torque) is a force multiplied by its perpendicular distance from the axis, the moment arm, and is measured in newton meters (N·m).

When a tissue is loaded, it deforms. Stress is the load per unit area and strain is the deformation relative to the original length. Plotting load against deformation gives a load-deformation curve, and the slope of that curve is stiffness. Most spinal tissues are not linear springs: they are viscoelastic, meaning their response depends on how fast and how long they are loaded. Three viscoelastic behaviors matter in daily life:

• Creep: continued deformation under a constant load. A disc slowly loses height and a ligament slowly lengthens during prolonged sitting or standing.
• Stress relaxation: falling internal stress when a tissue is held at a constant length, as in a sustained stretch.
• Hysteresis: energy lost as heat during a loading and unloading cycle, seen as a loop on the load-deformation curve. It is part of how discs absorb shock.

Viscoelastic tissues are also stiffer when loaded quickly. A disc or ligament resists a sudden load more strongly than a slow one, but it may fail under a fast, high load in a way it would not under a slow one.

Compression and the disc

In upright postures, most of the compressive load on the spine passes through the vertebral bodies and discs. The body weight above the L3 to L4 level is roughly 60 percent of total body weight, but the compressive force on a lumbar disc in standing is usually larger than that, because the trunk muscles must also contract to keep the upper body balanced, and muscle forces compress the spine. During lifting, muscle forces dominate the total.

When a healthy disc is compressed, the incompressible nucleus pressurizes and pushes outward. The annulus fibers are pulled into tension around the circumference, much like the hoops of a barrel, and the annulus bulges slightly outward. The endplates bow toward the vertebral bodies. This system turns vertical compression into tension in the annulus, the tissue best suited to resist it. The weakest link under pure compression is the endplate, which typically cracks before a healthy annulus ruptures; reported failure loads for young adult lumbar segments are often in the range of about 5 to 10 kN, with much lower values in older people with low bone density. A degenerated disc, whose nucleus has lost water and pressure, transfers more load directly to the annulus and facets and distributes stress unevenly.

Intradiscal pressure in different positions

In the 1960s, orthopedic researcher Alf Nachemson measured pressure inside the third lumbar disc of volunteers with a needle transducer. His results, often shown as percentages of the pressure in relaxed standing, became some of the most reproduced figures in spine care: lying supine about 25 percent, side lying about 75 percent, standing 100 percent, sitting upright without support about 140 percent, sitting bent forward about 185 percent, and lifting with a flexed spine more than 200 percent. These values shaped decades of advice that sitting is "harder on the back" than standing.

In the late 1990s, Wilke and colleagues implanted a pressure sensor in the L4 to L5 disc of a healthy volunteer and recorded pressure through a full day. Many findings matched Nachemson: lying produced the lowest pressure (about 0.1 MPa), relaxed standing about 0.5 MPa, standing bent forward roughly doubled it, and lifting a 20 kg weight with a rounded back produced the highest values (about 2.3 MPa) compared with lifting the same weight close to the body with bent knees (about 1.7 MPa). The main difference was sitting: relaxed or slouched sitting produced pressures similar to or lower than standing, not higher. Pressure also rose overnight as the unloaded disc reabsorbed water. The newer data come from very few subjects, so neither set is the final word, but together they support a balanced message: forward bending and lifting with load far from the body raise disc pressure most, while ordinary sitting is not dramatically worse than standing.

Bending, torsion and shear

In flexion, the front of the disc is compressed and the back is stretched, the nucleus pressure rises, and the posterior annulus, posterior longitudinal ligament, ligamentum flavum, interspinous ligaments and facet capsules are tensed. The facet joints unload as their surfaces slide apart. Laboratory studies in which spinal segments were flexed and compressed thousands of times, as in repeated bending and lifting, produced progressive tears in the posterior annulus and eventually herniation of nucleus material, often after the inner annulus had already fissured. This is the mechanical basis for the idea that repetitive loaded flexion is a risk factor for disc injury, although living tissue also adapts and repairs, so laboratory cycle counts cannot be applied directly to people.

In extension, the posterior annulus is relaxed, the anterior longitudinal ligament and anterior annulus are stretched, and the facets carry more load, as their surfaces press together and the inferior articular tips approach the lamina below. Extension also narrows the intervertebral foramina and the central canal slightly, which is why people with spinal stenosis often feel worse standing upright or leaning back and better sitting or leaning forward.

In torsion, only about half the fibers of the annulus, those running in the direction of the twist, are stretched at a time, because the lamellae alternate. The lumbar facets protect the disc: as a lumbar vertebra rotates, its inferior facet on the side opposite the direction of rotation presses into the superior facet below, while the facet on the other side gaps. In right rotation of a lumbar segment, the left facet joint is compressed. This bony block stops lumbar rotation within a few degrees, before the annulus is strained to failure. Combined twisting and bending, however, increases annular stress and is a common component of disc injury.

Shear is especially important at the lower lumbar segments, where the lordosis and the forward-sloping sacrum create a tendency for the upper vertebra to slide forward. The facets (particularly their more coronal parts), the disc, the iliolumbar ligaments and the muscles resist it. If the pars interarticularis fractures, the facet block is lost and the vertebra may slip forward, producing isthmic spondylolisthesis, most often at L5 on S1.

How the disc and facets share load

The disc and facets form a load-sharing partnership. In upright standing with a normal lordosis, the disc carries most of the compression and the facets carry a smaller share, commonly cited in the range of about 10 to 20 percent. The facet share increases with extension, with prolonged standing (as the discs lose height through creep) and with disc degeneration, and in some studies of narrowed discs it rose substantially higher. The facets carry a large share of the resistance to shear and torsion at all times.

Section 4Stability and the neutral zone

Why the spine needs muscles to stand

A spine stripped of its muscles is surprisingly fragile. In classic experiments, an isolated cadaver thoracolumbar spine fixed at the base buckled under a vertical load of only about 20 N, and an isolated lumbar spine under roughly 90 N, far less than the weight of the upper body. In life the same spine carries loads of thousands of newtons. The difference is made by muscles and the nervous system that controls them. The ligamentous spine works like a flexible mast; muscles act like guy wires that stiffen it in every direction. Stability, in this mechanical sense, is the ability to return to a balanced position after a small disturbance rather than buckling or sliding away.

Panjabi's three subsystems

In 1992, biomechanist Manohar Panjabi proposed a model of spinal stability that remains the standard framework. It describes three interdependent subsystems:

• The passive subsystem: vertebrae, discs, facet joints, joint capsules and ligaments. It provides stiffness mainly near the end of range, when ligaments become taut and bones make contact. Its ligaments and capsules also contain mechanoreceptors that report position and load.
• The active subsystem: the muscles and tendons around the spine. They generate force and stiffness throughout the range of motion.
• The neural control subsystem: the nervous system, which receives information from receptors in the passive and active subsystems, determines what stability is needed, and sets muscle activation patterns, both in anticipation of a load and in response to it.

The subsystems can compensate for one another. A person with lax ligaments may stay well by relying on muscle control, while a person with good passive structures but poor control can still have trouble. Dysfunction that exceeds the compensating capacity of the other systems produces instability, symptoms or adaptations such as altered movement.

The neutral zone and elastic zone

If you apply a small moment to a motion segment in the laboratory and plot rotation against moment, the curve is not a straight line. Near the neutral position the segment moves a fair amount with very little resistance; further out it stiffens steeply. Panjabi named these regions:

• The neutral zone (NZ): the part of the range near neutral where motion meets minimal internal resistance. Here the segment is lax and stability depends heavily on muscles.
• The elastic zone (EZ): from the end of the neutral zone to the end of the physiologic range, where passive tissues resist increasingly.
• Together they make up the range of motion. Loading beyond it enters the plastic zone, where tissues are permanently deformed, and finally failure.

Laboratory studies found that the neutral zone increases with injury and disc degeneration, sometimes more than the total range of motion does, and that simulated muscle forces decrease it. Panjabi therefore suggested that an enlarged neutral zone, rather than an enlarged total range, may be a more sensitive marker of a segment that has lost stability, and that muscles help by keeping the segment within a safe, stiffer zone. These ideas are well supported in cadaver models; directly measuring the neutral zone in living patients remains difficult.

Muscles, co-contraction and motor control

Because the ligamentous spine is unstable on its own, stability in life comes from coordinated activity of many trunk muscles at once, a strategy called co-contraction. Modeling studies suggest that modest co-contraction, often only a small percentage of maximal effort, is enough for most daily tasks, and that higher levels are needed for heavy or unexpected loads. More co-contraction raises stability but also raises spinal compression and energy cost, so the nervous system balances the two.

Research in the 1990s reported that in people with low back pain, the transversus abdominis switched on slightly later than normal when they moved an arm quickly, a delay in its usual feedforward (anticipatory) activity. This finding inspired motor control exercise, which trains people to activate deep trunk muscles and then integrate that control into functional movement. Later studies showed that altered trunk muscle control in back pain is real but highly variable: some people brace and stiffen more rather than less. Systematic reviews of trials have found that motor control exercise helps people with chronic low back pain about as much as other forms of exercise, but not clearly more. The practical lesson is that many types of exercise help, and no single muscle needs to be isolated for most people.

Clinical instability

The word "instability" is used in two quite different ways. Clinical instability in the classic definition by White and Panjabi is the loss of the spine's ability, under physiologic loads, to maintain its normal relationships between vertebrae so that there is no initial or later damage to the cord or nerve roots, no major deformity and no incapacitating pain. Fractures, ligament ruptures, tumors, infections and some spondylolisthesis can produce this kind of instability, which often requires immobilization or surgery. For the cervical spine, commonly cited radiographic warning signs are more than about 3.5 mm of horizontal translation or more than about 11 degrees of angulation between adjacent segments on flexion and extension films.

In manual therapy and exercise settings, "instability" or "functional instability" is also used for segments thought to have poor control within their normal range, sometimes described as aberrant motion: catching, painful arcs or sudden shifts during movement. This concept is plausible and has guided treatment classification systems, but its signs are harder to measure reliably. It should not be confused with structural instability. Its opposite, a segment that moves too little (hypomobility), is a key idea behind joint manipulation and is discussed in later chapters.

Section 5Posture and load in daily life

The line of gravity in standing

Posture is the position of the body's parts relative to each other and to gravity at a given moment. Standing posture is often assessed with a plumb line, a vertical reference representing the line of gravity. In the traditional description of ideal standing alignment seen from the side, the line passes through or near the earlobe (external auditory meatus), the bodies of the cervical vertebrae, the shoulder joint, roughly through the lumbar vertebral bodies, slightly behind the hip joint (near the greater trochanter), slightly in front of the center of the knee and slightly in front of the lateral malleolus at the ankle.

Wherever the line of gravity falls in front of or behind a joint, it creates a moment that must be balanced. Because the line passes in front of the thoracic spine, gravity tends to flex it, and posterior ligaments and back extensors resist. Because it passes behind the hip, gravity tends to extend the hip, resisted largely by the strong iliofemoral ligament, so standing costs the hip muscles little. In front of the knee it tends to extend the knee, which locks it straight passively. In front of the ankle it tends to dorsiflex the ankle, so the calf muscles, especially soleus, stay lightly active and make constant small corrections. This is why quiet standing involves gentle postural sway and low but continuous muscle activity. The arrangement is efficient: in good alignment, much of the work is done by ligaments and bony stacking rather than effortful muscle contraction.

Moment arms: why distance matters more than weight

The most useful single idea for understanding spinal load is the moment arm. The back extensor muscles attach close to the spine, with an effective moment arm of only about 5 to 6 cm from the center of the disc. Any weight held in front of the body, including the weight of the head and trunk themselves, acts at a much longer moment arm. To balance the moment, the extensors must pull with a force many times larger than the weight, and that muscle force compresses the spine.

A simple worked example shows the scale. Suppose a person leans forward so that the 400 N of upper body weight acts 20 cm in front of the L5 to S1 disc, and holds a 100 N box (about 10 kg) 40 cm in front of it. The flexion moment is (400 N × 0.20 m) + (100 N × 0.40 m) = 80 + 40 = 120 N·m. If the extensors work at a 0.05 m moment arm, they must generate 120 / 0.05 = 2,400 N. Adding the roughly 500 N of weight above the disc gives a compressive load near 2,900 N, about six times the weight lifted plus body weight. If the box is held close, at 20 cm, its moment drops to 20 N·m, the extensor force falls to 2,000 N and the estimated compression falls by about 400 N. Bringing the load closer reduces spinal load more than almost any other change. This is a simplified two dimensional model that ignores abdominal pressure, co-contraction and ligament forces, but it captures the main relationship. For comparison, an occupational safety guideline from the 1980s set an action limit of about 3,400 N of compressive force at L5 to S1, with about 6,400 N considered a level that most workers could not tolerate.

Forward head posture and sitting

The same lever logic applies to the neck. An adult head weighs roughly 4.5 to 5.5 kg. When it is balanced over the cervical spine, the moment is small. When it drifts forward, as in looking down at a phone, the moment arm lengthens and the neck extensors must work harder. One widely cited computer model estimated that tilting the head forward 60 degrees raises the effective load on the neck to the equivalent of about 27 kg. That model is a useful illustration of the lever principle, but it was not a measurement in people, and studies have not shown a consistent link between how far a person's head sits forward and whether they have neck pain.

Sitting changes the lumbar curve. When a person sits, the pelvis tilts backward and the lumbar lordosis flattens, so slouched sitting places the lumbar spine in flexion, stretching the posterior annulus and ligaments over time through creep. A lumbar support and a reclined backrest of about 110 to 120 degrees reduce disc pressure and back muscle activity in laboratory studies. On the other hand, the in vivo pressure data above show that ordinary sitting is not dramatically more loading than standing, and population studies have not found that any particular sitting posture reliably causes back pain.

Creep, time and the "next posture"

Time matters as much as position. Holding the lumbar spine in full flexion for even 10 to 20 minutes causes creep in the posterior ligaments, and in laboratory studies the ligaments and reflex muscle protection took much longer than the loading period to recover. Near full forward flexion, the back extensors often fall electrically quiet as ligaments and fascia take the load, a pattern called the flexion relaxation phenomenon; many people with chronic back pain lose it and keep the muscles active. In the morning, discs are fully hydrated and stiffer in bending, so the posterior annulus is under more strain at a given flexion angle. In one randomized trial, people with chronic low back pain who avoided early morning lumbar flexion for six months reported less pain than a control group, a small example of biomechanics translating into simple advice.

These observations support a modern view summarized by the phrase "the best posture is the next posture." No single sitting or standing position has been shown to prevent back pain, and systematic reviews have found little or no association between posture measures and pain. Prolonged, unchanging positions are uncomfortable for most people, and variety appears more important than achieving an ideal shape. Similarly, reviews of workplace training in lifting technique have not shown that it prevents back pain, even though lifting with the load close and avoiding combined twisting and bending clearly reduce mechanical load. The message for students is to separate two questions: what increases load on tissues (well established) and what causes pain in people (much more complex, involving tissue tolerance, conditioning, sleep, stress and beliefs, as later chapters on pain explain).

Words worth knowing

Joints, Cavitation and Tissue Response

Put your hand on the back of your neck and turn your head: dozens of small joints are gliding, opening and closing beneath your fingers, each one wrapped in a capsule, bathed in fluid and wired with sensors. When a joint is stiff, injured or immobilized, the tissues around it change in predictable ways, and when a clinician moves it, sometimes there is a pop. This chapter explains what a joint really is, how examiners feel its motion, what the famous cracking sound is made of, and how injured tissue heals, scars and remodels.

Section 1Synovial joint structure

Three ways bones can meet

A joint, or articulation, is any place where two or more bones meet. Anatomists classify joints in two parallel ways. The structural classification asks what tissue binds the bones together: fibrous joints are held by dense connective tissue (the sutures of the skull, the interosseous membrane of the forearm, the distal tibiofibular syndesmosis); cartilaginous joints are held by cartilage (the hyaline synchondrosis of a growth plate, or the fibrocartilaginous symphyses such as the pubic symphysis and the intervertebral disc); and synovial joints have a fluid filled cavity between the bones, enclosed by a capsule. The functional classification asks how much the joint moves: synarthroses are essentially immovable, amphiarthroses are slightly movable and diarthroses are freely movable. Nearly all diarthroses are synovial.

The spine contains all three structural types. Each mobile segment, which spinal biomechanists call a functional spinal unit or motion segment (two adjacent vertebrae and the soft tissues between them), has a cartilaginous joint in front (the disc between the vertebral bodies) and a pair of synovial joints behind (the left and right zygapophyseal joints). The ligamentum flavum, interspinous ligaments and the fibrous posterior part of the sacroiliac joint add fibrous connections. Because the synovial joints are where the bones glide against each other, and where manual therapies apply most of their force, this chapter focuses on them.

The parts of a synovial joint

Every synovial joint shares the same basic plan, whether it is a knuckle, a knee or a facet joint in the lumbar spine.

• Articular cartilage. The ends of the bones are covered by hyaline cartilage, usually 1 to 4 mm thick in small joints and up to about 6 mm in the knee. It has no blood vessels, no lymphatics and no nerves. It is made of a few cells, the chondrocytes, scattered in a large extracellular matrix of type II collagen fibers and proteoglycans, chiefly aggrecan. Aggrecan carries many negatively charged glycosaminoglycan chains (chondroitin sulfate and keratan sulfate) that attract water, so cartilage is roughly 65 to 80 percent water by weight. The collagen network resists the swelling pressure of the proteoglycans, which gives cartilage its springy stiffness under compression.
• Joint capsule. A sleeve of connective tissue attaches around the margins of the articular surfaces. Its outer fibrous layer is dense, mostly type I collagen, and is often thickened into capsular ligaments. Its inner synovial membrane (synovium) lines every surface inside the joint that is not articular cartilage.
• Synovial membrane. The synovium is a thin, highly vascular layer with one to three cell layers on its surface. Type A synoviocytes are macrophage like cells that clear debris. Type B synoviocytes are fibroblast like cells that secrete hyaluronan (hyaluronic acid) and lubricin into the joint.
• Synovial fluid. The fluid is an ultrafiltrate of blood plasma enriched with hyaluronan and lubricin. A healthy joint contains only a small amount (a normal adult knee holds roughly 1 to 4 mL), clear and pale yellow, with fewer than about 200 white cells per microliter. Hyaluronan makes the fluid viscous and shear thinning: it is thick when the joint moves slowly and thinner when it moves fast. The fluid lubricates the surfaces and carries nutrients to chondrocytes, which have no blood supply of their own.
• Accessory structures. Many synovial joints contain fat pads, fibrocartilage menisci or discs (in the knee, the temporomandibular joint and the sternoclavicular joint), a labrum that deepens a socket (in the hip and shoulder), intracapsular or extracapsular ligaments, and nearby bursae, small synovial sacs that reduce friction where tendons glide over bone.

How a joint stays almost frictionless

Healthy cartilage sliding on cartilage in synovial fluid has a coefficient of friction around 0.001 to 0.01, lower than ice on ice. Three lubrication mechanisms work together. In boundary lubrication, lubricin and surface phospholipids bind to the cartilage surface, so that even when the surfaces press close together a slippery molecular layer separates them. In fluid film lubrication, a thin layer of synovial fluid is trapped and pressurized between moving surfaces. In weeping or hydrostatic lubrication, compression squeezes water out of the cartilage matrix into the gap, so the load is carried mostly by pressurized fluid rather than by the solid matrix. When the load is removed, the cartilage reabsorbs fluid like a sponge.

That sponge action is also how cartilage is fed. Because chondrocytes have no blood vessels, nutrients and waste move by diffusion and by the pumping effect of cyclic loading. A joint that is compressed and released many times a day, through walking and ordinary movement, keeps its cartilage nourished. A joint that is held still or held under constant load does not, a point we return to in Section 5.

Synovial joint shapes

Synovial joints are grouped by the shape of their surfaces, which sets their axes of motion.

The synovial joints of the spine

The zygapophyseal joints, usually called facet joints, form between the inferior articular process of the vertebra above and the superior articular process of the vertebra below. They are true synovial plane joints, with hyaline cartilage, a capsule, synovium and a small volume of fluid (about 1 to 1.5 mL in a lumbar facet joint). Their job is to guide and limit motion: they allow some movements and block others, and in extension they share part of the compressive load that would otherwise go through the disc.

The orientation of the facet surfaces changes from region to region, which largely explains why the neck rotates freely, the thorax rotates but resists flexion and extension, and the lumbar spine flexes and extends but resists rotation.

Other spinal synovial joints include the atlanto-occipital joints, the lateral and median atlantoaxial joints, the costovertebral and costotransverse joints where the ribs meet the thoracic vertebrae, and the anterior part of the sacroiliac joint, which is synovial in front and fibrous behind. The uncovertebral joints (joints of Luschka) between the uncinate processes of C3 to C7 are clefts that develop in the lateral disc during childhood; whether they count as true synovial joints is still debated.

Meniscoids and innervation

Spinal facet joints contain small inward folds of capsule and synovium called intra-articular inclusions or meniscoids. Three types are commonly described: thin connective tissue rims along the capsule edge, adipose tissue pads filling the gaps at the joint poles, and fibroadipose meniscoids, larger folds up to about 5 mm long that project between the articular surfaces. They likely help fill space, spread load and protect exposed cartilage at the margins of the joint as it opens and closes. One hypothesis, discussed in Chapter 4, proposes that a meniscoid can be pinched or trapped at the edge of the joint and contribute to acute locked back or wry neck.

Joint capsules are richly innervated. In the spine, each facet joint is supplied by the medial branches of the dorsal rami, typically from the level of the joint and the level above, which is why one painful facet can refer pain over two segments. Hilton's law captures a general rule: a nerve that supplies a joint also tends to supply the muscles that move it and the skin over it. The capsule and ligaments contain four classic types of receptor (after Wyke):

Section 2Joint play and end feel

Two kinds of joint motion

Ask someone to bend their elbow and you see the forearm swing through an arc. That visible movement of a bone through space is osteokinematic or physiological motion: flexion, extension, abduction, rotation and the other movements a person can make with their own muscles. Underneath it, the joint surfaces are moving relative to each other in small ways that cannot be seen and cannot be produced voluntarily. Those surface motions are arthrokinematics, and the small passive movements a clinician can produce by pushing or pulling one joint surface on the other are called accessory motion or joint play. The physician James Mennell, who popularized the term, argued that normal, pain free physiological movement depends on normal joint play, and that loss of joint play (which he called joint dysfunction) could cause pain and stiffness even when the bones looked normal on film.

Three arthrokinematic motions combine during any movement:

• Roll: new points on one surface meet new points on the other, like a tire rolling on a road.
• Glide (or slide): one point on a surface contacts new points on the other, like a skidding tire.
• Spin: one surface rotates around a fixed point on the other, like a top. The radial head spinning on the capitulum during forearm rotation is the classic example.

Pure rolling would roll a joint off its partner, and pure gliding would jam it. In real joints, roll and glide happen together in proportions set by the shapes of the surfaces.

The convex concave rule

Most joint surfaces are curved, with one convex partner and one concave partner. The convex concave rule (associated with Kaltenborn) predicts the direction of glide:

• When a convex surface moves on a fixed concave surface, it rolls one way and glides the opposite way. Example: as the convex humeral head elevates in the glenoid during abduction, it rolls up and glides down (inferiorly).
• When a concave surface moves on a fixed convex surface, roll and glide go in the same direction. Example: as the concave tibial plateau extends on the femoral condyles in an open chain knee extension, the tibia rolls and glides anteriorly.

Clinicians use the rule to choose the direction of a mobilization. If a shoulder will not abduct well, an inferior glide of the humeral head is the logical accessory motion to restore. The rule is a helpful starting point but a simplification: real joints are not perfect spheres, ligaments and muscles steer the surfaces, and imaging studies sometimes show glide directions that do not match the textbook prediction. The flat spinal facets do not follow it neatly at all; their arthrokinematics are better described as sliding of the inferior articular process up, down or apart relative to the one below.

Close packed and loose packed positions

Every synovial joint has one position in which the surfaces are most congruent, the capsule and ligaments are tightest and accessory motion is at a minimum. This is the close packed position. Examples are full extension of the knee, full extension of the elbow (humeroulnar), and extension with abduction and internal rotation of the hip. In close packing the joint is stable and behaves almost like a single solid unit, which is why it is a common position of injury when a large force strikes, and why it is a poor position for testing joint play.

The loose packed or resting position is where the capsule is most lax and joint play is greatest, typically somewhere in mid range (about 25 degrees of knee flexion, or about 55 degrees of abduction with slight horizontal adduction at the shoulder). Effusion pushes a joint toward this position because it gives the capsule the most volume, so a swollen knee is held slightly bent. Clinicians usually test accessory motion and begin gentle mobilization in the resting position.

Assessing joint play

To test joint play, the examiner stabilizes one bone, places the joint in its resting position and applies a gentle glide, distraction (separation) or compression to the other bone, noting both the quantity of movement and the quality of resistance. Kaltenborn proposed a simple 0 to 6 scale:

In the spine, chiropractors, osteopathic physicians and physical therapists assess segmental motion with static palpation (feeling for tenderness, position and tissue texture), motion palpation (feeling how a segment moves as the patient or examiner moves the spine), and posterior to anterior (PA) spring testing, in which the examiner presses on a spinous or transverse process and judges stiffness and pain. Research on these methods gives a clear, consistent message. Pain provocation (does pressing here reproduce the patient's pain?) has fair to good agreement between examiners. Judgments of motion and position alone have poor to fair inter-examiner reliability, with kappa values often below 0.4, and examiners frequently disagree even about which vertebral level they are touching. These findings do not make palpation useless, but they argue for combining it with history, pain response and regional movement testing rather than treating a single palpation finding as a diagnosis.

End feel

The orthopedic physician James Cyriax taught that the sensation the examiner feels at the end of passive range, the end feel, tells you what tissue is stopping the motion. A normal joint stops in a way that fits its anatomy; a different end feel, or a normal end feel at the wrong point in the range, suggests a problem.

The capsular pattern

Cyriax also observed that when the whole capsule of a joint is inflamed or contracted (as in arthritis or adhesive capsulitis), passive motion is lost in a characteristic proportion for that joint, called the capsular pattern. A loss of motion that does not follow the pattern, a noncapsular pattern, points to something more localized, such as a ligament adhesion, an internal derangement or an extra-articular lesion.

Section 3Cavitation: what the pop is

An old question with a modern answer

People have cracked their knuckles, necks and backs for as long as anyone has recorded, and for most of that history the sound was explained by guesses: bones snapping back into place, ligaments flicking over bone, or tendons snapping. The first careful laboratory work came in the late 1940s, when researchers pulled on finger joints while taking x-rays and found that a clear space appeared inside the joint at the moment of the crack. In 1971, Unsworth and colleagues repeated the experiment with better instruments and gave the explanation that dominated textbooks for decades: a sudden drop in pressure inside the joint causes gas dissolved in the synovial fluid to come out of solution, and the collapse of that bubble makes the sound. They also showed that once a joint has cracked, it cannot crack again for roughly 20 minutes, a refractory period during which the gas slowly redissolves.

The word for this process is cavitation: the formation of a vapor or gas filled cavity in a liquid when local pressure falls low enough. In joints the term is used for the gas event that produces the audible release, and many clinicians simply call the sound a "cavitation" or an "audible release."

The mechanics step by step

Synovial fluid normally sits at slightly below atmospheric pressure, and a healthy joint holds very little of it, spread as a thin film between two closely fitting surfaces sealed inside a capsule. Imagine pulling the two surfaces apart:

1. Initial separation. As force is applied (a pull on a finger, or a thrust on a spinal segment), the surfaces begin to separate. The capsule is a closed container, and the volume inside it increases, but no fluid can rush in from outside to fill the space.
2. Pressure falls. Because the volume increases while the amount of fluid stays the same, pressure inside the joint drops sharply. Measurements in finger joints have recorded strongly negative pressures just before the crack.
3. Gas comes out of solution. Synovial fluid contains dissolved gases, mostly carbon dioxide along with nitrogen and oxygen. When pressure drops below a threshold, a gas cavity suddenly forms, a process called tribonucleation when it occurs as two wetted surfaces are pulled apart.
4. Rapid separation and sound. The joint surfaces jump apart a little further, energy is released abruptly, and an audible crack is heard. On x-ray, a radiolucent (dark) gas shadow appears in the joint space.
5. Refractory period. The gas bubble persists for some time and slowly redissolves. Until it does, the joint cannot be cracked again by the same maneuver. Times of about 20 to 30 minutes are commonly cited.

Formation or collapse? The imaging evidence

Unsworth's model held that the sound came from the collapse of the bubble. That idea had a problem: the bubble can be seen on x-rays taken after the crack, so it clearly had not fully collapsed. In 2015, Kawchuk and colleagues filmed finger joints being pulled inside a magnetic resonance imaging scanner at high frame rates. The cavity appeared at the instant of the crack and remained afterward, supporting the view that the sound is linked to cavity formation (rapid separation and bubble inception) rather than to collapse. In 2018, a mathematical model by Chandran Suja and Barakat suggested that the sound could be produced by partial collapse of the bubble within milliseconds of its formation, leaving a smaller residual bubble behind.

The fairest summary today is this: the audible release is tied to the sudden creation of a gas cavity as the joint surfaces separate, and whether the acoustic energy comes from the formation itself, from rapid partial collapse, or from the surfaces snapping apart is still being studied. What is not in doubt is what the pop is not. It is not a bone being put back into place, and it is not a sign that something has broken.

Cavitation versus other joint sounds

Not every joint noise is cavitation. Clinicians distinguish several kinds:

Does knuckle cracking cause arthritis?

This is one of the most common questions a clinician hears. The best available evidence says no clear link. Cross sectional studies comparing habitual knuckle crackers with non crackers have not found more hand osteoarthritis in the crackers. A famous self experiment by a physician who cracked the knuckles of only one hand for about 60 years found no arthritis in either hand; it is a single person, so it proves little, but it fits the larger studies. The honest answer to a patient is that habitual knuckle cracking does not appear to cause arthritis, although a joint that cracks with pain, swelling or locking deserves examination.

Cavitation during spinal manipulation

A high velocity, low amplitude thrust to a spinal segment often produces one or more audible releases, and both patients and clinicians tend to value the sound as a sign that "something happened." Research tempers that belief in three ways.

• The pop is not always at the target. Studies that placed accelerometers on the skin over several vertebrae found that cavitations during thoracic and lumbar manipulation frequently came from joints one or more levels away from the intended segment, and that a single thrust often produced several cavitations. Spinal manipulation is less segment specific than many practitioners assume.
• Outcomes do not depend on the sound. Clinical trials in people with low back and neck pain have compared patients whose manipulation produced an audible release with those whose manipulation did not. Improvement in pain and function was similar in both groups. The sound appears to be a byproduct rather than the active ingredient.
• The pop does carry meaning for people. Patients often report feeling looser and relieved when they hear it, and this expectation can contribute to the overall response through the same mechanisms that shape any treatment experience (Chapter 4 discusses these nonspecific effects). Clinicians can respect that experience while explaining that the sound is not required.

Cavitation does produce a brief, measurable change. In the finger, joint separation and passive range of motion increase slightly after cracking and the joint feels less stiff for a time, consistent with the gas bubble occupying space and the capsule being slackened.

Section 4Tissue response to injury, healing phases

One process, three overlapping phases

Whether a sprained ligament, a strained muscle or a cut in the skin, vascularized connective tissue heals through the same broad sequence. The phases overlap, and their timing depends on the tissue, the size of the injury, the person's age and health, and how the area is loaded during recovery. The times below are typical teaching values for a moderate soft tissue injury, not fixed deadlines.

The inflammatory phase

Injury tears blood vessels and cells. Within seconds, damaged vessels constrict and platelets aggregate and trigger the clotting cascade, forming a fibrin clot that plugs the leak and serves as a scaffold for incoming cells. Platelets release growth factors such as platelet derived growth factor (PDGF) and transforming growth factor beta (TGF beta) that recruit and activate repair cells. After the brief vasoconstriction, nearby vessels dilate and become leaky under the influence of histamine, bradykinin and prostaglandins.

White cells arrive in a predictable order. Neutrophils dominate the first 24 to 48 hours, killing microbes and clearing debris. Macrophages, derived from blood monocytes, take over by about 48 to 72 hours. They engulf dead tissue and release cytokines and growth factors that switch the area toward repair; without macrophages, healing stalls. The classic signs of inflammation follow directly from these events: rubor (redness) and calor (heat) from increased blood flow, tumor (swelling) from leaky vessels, dolor (pain) from chemical sensitization of nociceptors and mechanical pressure, and functio laesa (loss of function) from pain and swelling together.

Inflammation is the first step of healing, not its enemy. Prostaglandins and other mediators sensitize nearby nociceptors (peripheral sensitization), so ordinary movement hurts and the area is protected in the early days. Problems arise when inflammation is excessive or does not resolve.

The proliferative phase

From about day 3, fibroblasts migrate into the clot along fibrin strands and begin producing new extracellular matrix: glycosaminoglycans, fibronectin and, above all, type III collagen, a thinner, more elastic collagen that can be laid down fast. New capillaries sprout into the area under the influence of vascular endothelial growth factor, and the combination of new vessels, fibroblasts and loose matrix forms granulation tissue, which looks red and granular in an open wound. Some fibroblasts become myofibroblasts, cells with contractile actin filaments that pull the wound edges together.

The new collagen is laid down somewhat randomly, and its cross links are weak. The repair is therefore fragile at first, but tensile strength rises steeply over these weeks. Gentle, progressive loading in this phase helps fibroblasts align new collagen along the direction of stress, which produces a more functional scar.

The remodeling phase

Over months, the scar is reorganized. Type III collagen is gradually replaced by thicker, stronger type I collagen; enzymes called matrix metalloproteinases break down disorganized fibers while fibroblasts lay down new ones; and covalent cross links between collagen molecules mature. Cell numbers and capillaries decline, so the scar becomes paler and less vascular. Fibers increasingly line up with the forces the tissue experiences, which is why mechanical loading during this phase matters so much. In skin, a scar reaches only about 20 percent of normal strength at 3 weeks and roughly 70 to 80 percent by about a year, and it rarely regains full strength. Ligaments follow a similar course: a healed ligament is often longer, weaker and less stiff than the original for a year or more.

Not all tissues heal alike

The main reason tissues differ in healing is blood supply. Repair cells, oxygen and nutrients all arrive by blood, so well vascularized tissues heal faster and more completely.

Notice that bone is the exception that regenerates rather than scarring: once remodeled, a healed fracture can be as strong as the original bone. Notice too that the disc is one of the poorest healers in the body, because its adult nucleus and inner annulus depend on diffusion through the vertebral endplates. A herniated disc often becomes less painful as the herniated fragment is broken down by macrophages, inflammation settles and the nervous system calms, not because the annulus has regrown to its youthful state.

Managing the phases

Advice for acute soft tissue injury has evolved. The old RICE formula (rest, ice, compression, elevation) emphasized rest. It was followed by PRICE (adding protection) and then POLICE (protection, optimal loading, ice, compression, elevation), which replaced rest with the idea of loading the tissue just enough. A more recent framework, PEACE and LOVE, recommends protection, elevation, avoiding anti-inflammatory modalities in the first days, compression and education for the acute stage, followed by load, optimism, vascularization (pain free aerobic activity) and exercise. It reflects concern that suppressing early inflammation could blunt repair signals, and evidence that progressive loading improves tissue quality. The evidence for ice and for routine anti-inflammatory drugs after soft tissue injury is limited and mixed; they can help with pain, but they are not essential to healing.

Section 5Immobilization, adhesion and remodeling

Connective tissue obeys its loads

Living tissues constantly adjust their structure to the forces placed on them. For bone this idea is known as Wolff's law: bone is laid down where it is stressed and resorbed where it is not. The parallel principle for soft tissue is sometimes called Davis's law: ligaments, tendons and fascia that are kept under appropriate tension lengthen and strengthen, while those left slack shorten and weaken. The cellular basis is mechanotransduction, the process by which cells convert mechanical signals into chemical ones. Fibroblasts, chondrocytes, osteocytes and muscle fibers sense stretch, compression and fluid flow through their cell membranes, integrins and cytoskeleton, and respond by changing which genes they express and how much matrix they build or break down.

This is why both too little and too much load are harmful. Moderate, varied, progressive loading keeps tissues healthy and guides healing. Excessive load damages tissue. Removing load entirely, as in casting, bracing, bed rest or guarding a painful area, sets off a cascade of changes that can outlast the original injury.

What immobilization does to each tissue

Much of what we know comes from animal studies in which one limb was immobilized for several weeks and compared with the free limb; human observations broadly agree.

The common theme is that tissues lose properties quickly when unloaded and regain them slowly when loading resumes. A two week immobilization may need two months or more of rehabilitation to reverse. That asymmetry is the strongest argument for keeping immobilization as short and as partial as safety allows.

How adhesions form

The word adhesion describes abnormal fibrous connections between tissue surfaces that should glide independently. In an immobilized joint, three processes work together. First, without movement, collagen fibers that are normally kept apart by water and proteoglycans come into close contact and form new cross links where they touch, binding fibers that should slide past each other. Second, fibrofatty tissue proliferates in the joint recesses and can bridge from synovium to cartilage. Third, if the joint was injured, the collagen of the repair scar is laid down without the orienting influence of motion, so it forms a disordered mesh that tethers neighboring structures. Motion during healing limits all three by keeping fibers sliding and aligning new collagen along the direction of movement.

In chiropractic theory, the concept of a fixation, a spinal segment that has lost some of its normal motion, has long been linked to the idea of intra-articular adhesions, along with other proposed mechanisms such as meniscoid entrapment and segmental muscle spasm. Laboratory work in rats, in which spinal segments were held still with small implanted devices, has shown that adhesions in the zygapophyseal joints become more numerous and larger the longer a segment is immobilized, supporting the idea that spinal synovial joints respond to disuse like other synovial joints. Whether the stiff segments found on examination in people with back pain contain such adhesions, and whether manipulation breaks them, has not been demonstrated in humans. It is reasonable to call adhesion one plausible, partly supported mechanism rather than an established explanation.

Creep, stress relaxation and why sustained stretch works

Ligaments, capsules and tendons are viscoelastic: their response depends on both how much and how fast they are loaded. Two viscoelastic behaviors matter clinically. Creep is continued elongation under a constant load: hold a gentle stretch and the tissue slowly lengthens. Stress relaxation is the gradual fall in tension when tissue is held at a constant length. Both explain why a stretch held for 30 seconds or longer gains more range than a brief bounce, and why sustained or repeated mobilization can restore motion to a stiff capsule.

Permanent lengthening of a contracted capsule requires more than temporary viscoelastic change. It requires remodeling: repeated, tolerable loading over weeks, so that fibroblasts break down old cross linked collagen and lay down new fibers at a longer resting length.

Remodeling: putting load back on purpose

Because tissues remodel according to load, rehabilitation is essentially a program of controlled mechanical signals, sometimes called mechanotherapy. Key principles include:

• Early, protected motion. In the 1970s and 1980s, the surgeon Robert Salter showed in animals that continuous passive motion after joint injury improved cartilage healing and reduced adhesions compared with immobilization. Modern postoperative care, from knee replacement to tendon repair, favors early, controlled movement whenever the repair can tolerate it.
• Progressive loading. Load is increased gradually across weeks, matched to the healing phase, so that the tissue is challenged but not overwhelmed. Tendon rehabilitation programs using slow heavy loading or eccentric exercise are good examples.
• Specificity. Tissues adapt to the specific type of load. Collagen aligns along the direction of tension, bone thickens where it is compressed and bent, and cartilage thrives on cyclic compression. Rehabilitation should load tissues in the directions they will need in daily life.
• Time. Remodeling continues for months. Feeling better does not mean the tissue has reached full strength, which is one reason reinjury is common when people return to full activity too quickly.

Words worth knowing

Manipulation and Mobilization Mechanics

Spinal manipulation is one of the oldest hands on treatments in the world and one of the most debated. Done well, it is a precisely controlled impulse lasting a fraction of a second; described badly, it becomes "putting bones back in place." This chapter replaces the myths with mechanics: how manipulation differs from mobilization, what forces and speeds are involved, what the barrier concept means, what the thrust actually does to joints and nerves, and how clinicians keep it safe.

Section 1Manipulation versus mobilization

Defining the terms

Manual therapists use many overlapping words, so it helps to start with clear definitions. Both manipulation and mobilization are passive techniques: the clinician moves the patient's joint while the patient relaxes. They differ mainly in speed, size of movement, where in the range the force is applied, and whether the patient can stop it.

• Spinal manipulation (often called spinal manipulative therapy, SMT, an adjustment in chiropractic, or a high velocity, low amplitude (HVLA) thrust) is a quick, short impulse delivered at or near the end of a joint's passive range. The movement is so fast that the patient cannot resist or stop it once it begins. It is often, but not always, accompanied by cavitation (Chapter 3).
• Mobilization is a slower, usually rhythmic or sustained passive movement within or up to the limit of the joint's range. It is low velocity, may be small or large in amplitude, and is applied in a way the patient can stop at any time.

In chiropractic, the word adjustment has historically referred to a specific thrust intended to correct a vertebral subluxation. Chiropractic schools of thought differ on how they define a subluxation: some use the term for a functional, biomechanical problem such as a restricted, painful segment, while others retain the founders' broader concept of a neurological interference affecting health. Chapter 9 explores this history. For this chapter, the mechanical definition is what matters: an adjustment is an HVLA thrust, and the research on manipulation applies to it.

The Maitland grades

The Australian physiotherapist Geoffrey Maitland developed a grading system that places each technique on the joint's available range. It is widely used in teaching and research because it makes "how hard" and "how far" describable.

Families of manual technique

Within chiropractic, osteopathy and physical therapy there are dozens of named techniques. Most fall into a few mechanical families, and knowing the family tells you more than the brand name does.

Two further distinctions are useful. A technique can be specific, aiming at one segment with a short lever contact on a vertebra, or nonspecific, using a long lever (for example the thigh or shoulder) that moves several segments at once. In practice, as Chapter 3 showed, even specific thrusts move and cavitate neighboring segments. A technique can also use direct force (moving toward and into the restriction) or indirect force (moving away from the restriction, toward ease), a distinction prominent in osteopathic manual medicine.

Section 2Force, speed and amplitude of a thrust

Anatomy of a thrust

When researchers place a pressure sensing mat or a force plate under a patient and record a manipulation, the force time curve has a consistent shape with three phases.

1. Preload (setup). The clinician positions the patient, takes up the slack in the soft tissues and applies a steady force toward the target. This brings the joint toward the end of its passive range. Preload is usually held for a short time and lets the clinician and patient sense whether the position is comfortable.
2. Thrust (impulse). A sudden, rapid increase in force to a peak, then a quick release. This is the "high velocity" part. In the spine, the thrust phase typically lasts on the order of 100 to 200 milliseconds.
3. Resolution. Force returns toward zero as the clinician releases the contact. Sometimes a brief drop just before the thrust, called a countermovement or dip, is visible when clinicians unload slightly before driving in.

Typical numbers

Force values vary widely among clinicians, patients, techniques and measuring methods, so treat the table below as approximate ranges reported across biomechanical studies rather than targets. Note that force is measured in newtons (N); 100 N is roughly the weight of a 10 kg (22 lb) mass.

The force measured at the hand or table is not the force that reaches the joint. Skin, fat and muscle absorb and spread part of it, and the remaining load is shared across several segments, the rib cage and the pelvis. Displacements at the target vertebra during a thrust are small, usually a few millimeters of translation and a few degrees of rotation. Studies using bone pins or high speed imaging have found that adjacent vertebrae move almost as much as the target, again showing that manipulation is less localized than it feels.

Velocity, rate and impulse

The defining feature of manipulation is not how hard it is, but how fast. Two physical quantities help describe this.

• Rate of force application (force divided by time to peak, newtons per second). A thrust that reaches 400 N in 100 ms has a rate of 4,000 N per second.
• Impulse (force multiplied by time, the area under the force time curve, in newton seconds). Impulse determines how much the body's momentum changes. A large force applied briefly can deliver the same impulse as a small force applied for longer.

Rate matters because tissues and sensors respond to it. Ligaments and capsules are viscoelastic (Chapter 3): a fast load meets a stiffer tissue, which transmits force with less time for creep. Muscles also behave differently: a thrust that is fast enough reaches its peak before the stretch reflex can generate a full protective contraction, which typically takes tens of milliseconds to begin and longer to build. This is one reason a relaxed patient and a quick thrust work better than a slow push into a guarding muscle. Animal experiments recording from sensory nerves in the back muscles have shown that muscle spindles fire more strongly when thrusts are shorter, with the greatest response at durations around 75 to 150 ms and a smaller response with very slow loads. In other words, the speed of the thrust shapes the burst of sensory input to the spinal cord, which is the starting point of the neurophysiological effects discussed in Section 4.

Amplitude and control

Amplitude is kept small for safety: the goal is to move the joint just beyond its passive limit, not to drive it toward the anatomical limit where tissues fail. Expert clinicians show remarkably consistent force time curves from thrust to thrust, while novices vary more in peak force and especially in preload and timing. Training with force sensing tables and feedback improves this consistency, a good example of how a motor skill can be measured and taught.

Clinicians also adjust parameters to the patient. Lower preloads and peak forces, shorter levers and less rotation are chosen for smaller, older or more fragile patients; mobilization or instrument based methods may replace HVLA thrusts altogether. Choosing the dose of force is part of clinical reasoning, much like choosing the dose of a medication.

Section 3The barrier concept and the paraphysiological space

Mapping a joint's range

To explain where a thrust acts, manual therapists divide a joint's motion into zones separated by barriers. A barrier here does not mean a wall; it means a point where the character of the resistance changes. The most influential version in chiropractic was published in 1976 by the Swiss chiropractor Raymond Sandoz, who drew a joint's range as a series of nested zones, and osteopathic manual medicine developed a closely related vocabulary. Combining the two gives the following sequence, moving outward from the neutral position:

1. Neutral position and the neutral zone. Near the middle of the range, the joint moves with very little resistance. In the spine, the neutral zone is the region of motion around neutral in which the passive tissues offer little restraint and the deep muscles do most of the controlling.
2. Active range and the physiological barrier. The patient can move the joint with their own muscles to the end of active range of motion. The point where active motion stops is the physiological barrier.
3. Passive range and the elastic barrier. An examiner can move the joint a little further than the patient can, taking up the elasticity of the capsule and ligaments. This extra region is sometimes called the end play zone. At its end is the elastic barrier, where the tissue resistance becomes firm. This is where the end feel of Chapter 3 is judged.
4. Paraphysiological space. Sandoz described a small additional zone beyond the elastic barrier that can be entered only by a sudden thrust, typically with cavitation. Entering it gives a slight extra separation of the joint surfaces without injuring the tissues.
5. Anatomical barrier (limit of anatomical integrity). Beyond the paraphysiological space lies the point where the capsule and ligaments are fully tensed and the bony anatomy stops further motion. Motion beyond this limit damages tissue: sprain, dislocation or fracture.

The restrictive barrier

The zones above describe a normal joint. In a stiff or painful joint, motion may stop earlier, at what osteopathic and chiropractic clinicians call a restrictive or pathological barrier. This barrier can be produced by several things: muscle guarding, a shortened capsule, adhesions, swelling, a trapped meniscoid, or simply pain and fear of movement. In osteopathic language, the clinician looks for the direction of ease (where the joint moves freely) and the direction of bind (where resistance is met early). Direct techniques engage the restrictive barrier and move through it; indirect techniques move toward ease and let tissues relax.

The aim of most manual techniques, in this model, is to restore the joint's motion so that the restrictive barrier moves back toward its normal position. A mobilization might gently and repeatedly push against the barrier; a muscle energy technique uses the patient's own contraction and post-contraction relaxation to let the clinician take up new slack; a thrust passes quickly through the end of the passive range.

Strengths and limits of the model

The barrier model is valuable because it gives clinicians a shared language for where force is applied and why the amplitude of a thrust must be so small. It also neatly explains the safety principle: the paraphysiological space is narrow, and the anatomical limit lies just beyond it, so a thrust must be brief and shallow.

But the model is a conceptual drawing, not a measured map. Some cautions:

• Barriers are not sharp lines. Tissue resistance rises smoothly along a curve (Figure 3.3 in Chapter 3). Where an examiner perceives the elastic barrier depends on their hands, the force they use and the patient's relaxation.
• Cavitation does not prove entry into a special space. Cavitation happens when sealed surfaces separate enough to drop pressure. It can occur at different points in the range depending on joint position and preload, and it can occur without clinically meaningful change.
• The extra motion is small and short lived. In finger joints, range increases by a few degrees after cracking and the effect fades as the gas redissolves. Similar measurements in the spine are difficult, and imaging after manipulation has not shown lasting changes in vertebral position.
• A restrictive barrier is often neurological. Under anesthesia, many joints that felt restricted move further, showing that muscle activity and protective responses contribute substantially to the end of range in a painful patient. This supports the idea that manual therapy works partly by changing nervous system output rather than only by stretching tissue.

Section 4Mechanical and neurophysiological effects

The question of mechanism

For much of the twentieth century, explanations of manipulation were mainly mechanical: something was out of place or stuck, and the thrust moved it back or freed it. Since the 1990s, researchers have increasingly emphasized that a brief mechanical impulse sets off a cascade of responses in the nervous system, and that these responses probably explain more of the clinical effect than any lasting change in bone position. A widely cited framework, proposed by Bialosky and colleagues in 2009, describes how a mechanical force triggers neurophysiological effects at three levels (peripheral, spinal and supraspinal), all influenced by the patient's expectations and the clinical context.

Mechanical effects

Joint gapping. Magnetic resonance imaging studies of the lumbar spine have shown that side posture positioning alone opens the zygapophyseal joints on the upper side, and that adding a manipulative thrust opens them further. Gapping separates cartilage surfaces and may briefly stretch the capsule. The extra gap measured after manipulation is small, on the order of a millimeter, and it is not known how long it lasts.

Release of an entrapped meniscoid. One hypothesis for sudden, painful locking of the neck or low back (an acute wry neck or "locked" back) is that a fibroadipose meniscoid at the edge of a facet joint is pinched or fails to re-enter the joint as it closes, buckling and pulling on the sensitive capsule. Gapping the joint, in theory, lets the meniscoid return. The anatomy makes this plausible, but it has not been demonstrated directly in patients.

Breaking adhesions. As Chapter 3 described, immobilized synovial joints, including spinal facet joints in animals, develop intra-articular adhesions. Manipulation might stretch or rupture them. This is reasonable in principle but unproven in human spinal pain.

Changes in segmental stiffness. Studies using instruments that measure posterior to anterior stiffness have found that, in some patients with low back pain, stiffness decreases right after manipulation, and that the patients whose stiffness drops most tend to report the greatest improvement. The change may reflect altered muscle activity as much as altered joint mechanics.

What is not supported. There is no good evidence that manipulation produces a lasting change in the position of a vertebra visible on imaging, and the displacements during a thrust are too small and too widely shared among segments to "put a bone back in place." Clinicians who explain manipulation to patients in those terms are using a metaphor, and it is better to choose words that do not imply fragility or misalignment, since such language can increase fear and dependence.

Neurophysiological effects

Sensory barrage. A thrust stretches muscles, tendons, capsules and ligaments very quickly, producing an intense, brief burst of firing in muscle spindles, Golgi tendon organs and joint mechanoreceptors (the type I to III receptors described in Chapter 3). This input arrives at the dorsal horn of the spinal cord over large, fast conducting fibers.

Spinal effects. Large fiber input can inhibit transmission of nociceptive signals in the dorsal horn, an idea that traces back to the gate control theory of pain (Chapter 6). Experiments have shown that manipulation can reduce temporal summation (the increasing pain felt with repeated identical stimuli, a marker of dorsal horn excitability). Electromyographic studies show brief reflex bursts in paraspinal muscles during the thrust, followed in some studies by reduced resting muscle activity. Recordings of the H-reflex, a measure of motoneuron excitability, show that manipulation transiently reduces motoneuron pool excitability, usually for less than a minute or so. These effects are real but short lived.

Hypoalgesia. Meta-analyses of experimental studies show that manipulation raises pressure pain thresholds, meaning more pressure is needed before a person reports pain, both near the treated area and at distant sites. The effect size is small on average and it fades over minutes to hours. Remote effects suggest that central mechanisms, not just local tissue changes, are involved.

Supraspinal and autonomic effects. Cervical mobilization and manipulation have been shown to produce brief signs of sympathetic nervous system excitation, such as increased skin conductance and changes in skin temperature and heart rate, together with hypoalgesia. This pattern resembles the activation of descending pain control systems from the periaqueductal gray in the midbrain. Studies of blood levels of endorphins, cortisol and other substances after manipulation have produced mixed results, and brain imaging studies are small and early.

Nonspecific effects. Every treatment comes with context: a clinician's explanation, touch, attention, confidence, and the patient's expectations and past experience. Trials show that patients who expect manipulation to help tend to improve more, and that sham manipulation (positioning without a thrust) often produces substantial improvement as well. These effects are not "fake"; they act through the same brain pain systems. Good clinicians harness them honestly, through clear explanation and a positive alliance, without making misleading claims.

What clinical trials show

Mechanisms matter less to a patient than outcomes, so it is important to know the clinical evidence in plain terms.

• Low back pain. Large systematic reviews have found that spinal manipulation produces small to moderate short term improvements in pain and function, similar in size to other recommended treatments such as exercise therapy and appropriate medication. Several major clinical practice guidelines include manipulation as one reasonable nonpharmacological option for acute and chronic low back pain, usually alongside advice to stay active and exercise. Benefits beyond a few months are less certain.
• Neck pain. Manipulation and mobilization, especially when combined with exercise, have evidence of short term benefit for nonspecific neck pain. Thoracic manipulation has also been studied for neck pain with some favorable results.
• Headache. There is limited evidence of benefit for cervicogenic headache (headache referred from the neck), and less for migraine and tension type headache.
• Conditions that are not musculoskeletal. For conditions such as asthma, high blood pressure, infant colic, otitis media and painful menstruation, well designed trials and reviews have not shown that manipulation is effective. Claims that spinal adjustments boost immunity or treat organ disease are not supported by evidence.

Section 5Safety, contraindications and adverse events

Thinking about risk honestly

Every effective treatment carries some risk, and patients deserve accurate information about it. For spinal manipulation, the picture has two very different parts: common, mild, short lived reactions that many patients experience, and rare, serious events whose frequency is hard to measure precisely. Clinicians need to know both, explain both, and screen carefully for the patients in whom the balance of benefit and harm shifts.

Researchers classify adverse events by severity. Mild or benign events are short in duration, mild in intensity and resolve without treatment. Moderate events are more intense or longer lasting and may need some care. Serious events cause hospitalization, lasting disability or death.

Common, benign reactions

Prospective studies that ask patients about symptoms after treatment find that roughly 30 to 60 percent report at least one mild reaction after a course of manipulation. The most common are:

• Local soreness or aching at the treated area
• Stiffness
• Headache (more often after cervical treatment)
• Tiredness, and less often radiating discomfort or mild dizziness

These reactions typically begin within 24 hours and resolve within 24 to 72 hours. They are similar to the soreness that follows starting a new exercise program. Notably, trials comparing manipulation with sham treatment or with exercise find that mild reactions are common in all groups, so not every reaction is caused by the thrust itself. Warning patients that brief soreness is expected and does not mean harm can reduce worry.

Serious adverse events

Serious events after spinal manipulation are rare, but they matter because they can be devastating. The main ones are:

• Cervical artery dissection and stroke. A dissection is a tear in the inner layer of the vertebral or internal carotid artery wall. Blood enters the wall, narrowing the vessel or forming a clot that can travel to the brain and cause a stroke. Dissections can follow major trauma, minor neck movements (turning to reverse a car, sneezing, yoga, a hair wash at a salon) or occur with no clear trigger; people with connective tissue weakness and recent infection appear more susceptible.
• Cauda equina syndrome. Compression of the bundle of nerve roots below the end of the spinal cord, usually from a large central lumbar disc herniation, causing saddle numbness, bladder or bowel dysfunction and leg weakness. Reports after lumbar manipulation are extremely rare.
• Worsening of a disc herniation or radiculopathy. Symptoms can increase after treatment, though it is often hard to tell whether the treatment caused the change or the condition simply progressed.
• Fracture. Rib or vertebral fractures can occur, especially in patients with osteoporosis, metastatic cancer or long term corticosteroid use.
• Spinal epidural hematoma and other bleeding complications, very rarely, particularly in patients on anticoagulants.

The cervical artery question

The link between neck manipulation and stroke has been studied and argued about for decades. Patients with an early vertebral or carotid dissection often have neck pain and headache before any neurological signs, and those are exactly the symptoms that bring people to a chiropractor or a primary care physician. Large population studies using health records found that people who had a vertebrobasilar stroke were more likely to have seen a chiropractor in the days before, but they were about equally likely to have seen a primary care physician, who does not manipulate the neck. This pattern suggests that much of the association reflects people seeking care for the early pain of a dissection already in progress. It does not prove that manipulation never contributes: a thrust could conceivably extend an existing small tear. Estimates of the risk of serious harm vary widely, from roughly one in tens of thousands to one in several million manipulations, because the events are rare and causation is hard to establish.

Because of this uncertainty, the international guidance for manual therapists emphasizes careful clinical reasoning about cervical arterial dysfunction rather than reliance on any single test. Older "premanipulative" positional tests, which held the neck in extension and rotation to see whether symptoms appeared, have poor sensitivity and are no longer recommended as a reliable screen on their own. Instead, clinicians look for risk factors and for symptoms of brainstem or cerebral ischemia. A useful memory aid lists the five Ds and three Ns, plus ataxia:

A patient with sudden, unusual neck pain or headache together with any of these neurological features needs urgent medical evaluation, not manipulation. Chapter 7 develops this examination logic and the other red flags in more depth.

Contraindications

A contraindication is a condition in which a treatment should not be used (absolute) or should be used only with modification and caution (relative). For HVLA manipulation, the commonly taught lists are:

An absolute contraindication to HVLA does not always rule out all manual care. A patient with osteoporosis might still benefit from gentle mobilization, soft tissue work and exercise, while a patient with suspected cauda equina syndrome needs emergency referral and no manual treatment at all. The key is that contraindications are specific to the technique, the region and the force.

Reducing risk in practice

• Screen thoroughly. A careful history (including recent trauma, medications such as anticoagulants and corticosteroids, cancer history and neurological symptoms) and a focused examination are the most important safety tools. Imaging is reserved for patients with red flags or specific indications, not used routinely.
• Obtain informed consent. Patients should hear, in plain language, what the treatment involves, what benefits are realistic, the common mild reactions, the rare serious risks and the alternatives, and they should know they can decline or stop.
• Choose the dose. Use the least force, rotation and amplitude that achieve the goal. In the neck, many clinicians limit end range rotation and favor lateral flexion setups or mobilization, especially in patients with any vascular risk factors.
• Reassess. Check the patient's response after each treatment, and stop or change course if symptoms worsen or new neurological signs appear.
• Communicate and refer. Work with physicians and other providers when the picture is unclear, and document carefully.

To put risk in context, other common treatments for back and neck pain carry their own hazards. Nonsteroidal anti-inflammatory drugs can cause gastrointestinal bleeding and kidney and cardiovascular problems, and opioids carry risks of dependence and overdose. These comparisons do not make manipulation risk free, and comparisons across different treatments are imprecise. They do show why guidelines weigh each option's benefits and harms for the individual patient.

Words worth knowing

Nerve Roots, Dermatomes and Reflexes

When a patient points to a stripe of numbness down the outside of the leg, or a reflex hammer meets a quiet ankle, the nervous system is telling you exactly which level of the spine to look at. Dermatomes, myotomes and reflexes link a symptom in the hand or foot to a single spinal segment. This chapter builds the map from the spinal nerve outward, then shows how to tell true root irritation from referred pain and from a nerve trapped farther down the limb.

Section 1Spinal nerve formation and the intervertebral foramen

From rootlets to rami

Each segment of the spinal cord gives off a series of small rootlets from its back and front surfaces. The rootlets on the back gather into the dorsal (posterior) root, and those on the front gather into the ventral (anterior) root. The two roots carry very different traffic. The dorsal root is purely sensory (afferent): it carries information from skin, muscle spindles, joints, ligaments and viscera into the cord. Their cell bodies sit outside the cord in a swelling on the dorsal root, the dorsal root ganglion (DRG). The ventral root is motor (efferent). It carries alpha motor axons to skeletal muscle, gamma motor axons to muscle spindles and, at T1 to L2 and S2 to S4, preganglionic autonomic fibers.

Just beyond the DRG, usually within the intervertebral foramen, the roots merge to form the spinal nerve. The spinal nerve is short, roughly a centimeter long, and it is mixed: from here outward every branch can carry both sensory and motor fibers. Almost as soon as it leaves the foramen the spinal nerve divides:

• The dorsal (posterior) ramus turns backward to supply the deep intrinsic back muscles (the erector spinae and transversospinalis groups), the skin of the back near the midline, and, through its medial branch, the zygapophyseal (facet) joints.
• The ventral (anterior) ramus is much larger. It supplies the muscles and skin of the front and sides of the trunk and all of the limbs. In the thoracic region the ventral rami stay separate as the intercostal and subcostal nerves. In the neck and the lumbosacral region they mix to form the cervical plexus (C1 to C4), brachial plexus (C5 to T1), lumbar plexus (L1 to L4) and sacral plexus (L4 to S4).
• The rami communicantes link the spinal nerve to the sympathetic chain. White rami (myelinated preganglionic fibers) exist only from T1 to L2; gray rami (unmyelinated postganglionic fibers) return to every spinal nerve so that sweat glands, blood vessels and arrector pili muscles everywhere receive sympathetic supply.
• The recurrent meningeal (sinuvertebral) nerve loops back through the foramen into the canal to supply the posterior longitudinal ligament, the outer third of the annulus fibrosus, the anterior dura and the vessels of the canal. It is a key reason the disc itself can hurt.

Thirty one pairs and the numbering rule

Humans have 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. The only mismatch with the vertebrae is in the neck, which has seven vertebrae but eight nerves. The first cervical nerve leaves between the skull and the atlas, so each cervical nerve from C1 to C7 exits above the vertebra that shares its number. The C8 nerve exits between C7 and T1, and from T1 downward every nerve exits below its numbered vertebra. This rule has a direct clinical payoff. A disc between C5 and C6 sits next to the foramen through which the C6 nerve leaves, so a posterolateral C5 to C6 herniation usually produces a C6 radiculopathy. In the lumbar spine the L4 nerve leaves below the L4 pedicle, through the L4 to L5 foramen.

A second mismatch develops during growth. The vertebral column lengthens faster than the cord, so in the adult the cord ends as the conus medullaris at about the L1 to L2 disc. Below that, lumbar and sacral roots must travel downward inside the dural sac before they reach their exit foramina. This bundle of descending roots is the cauda equina ("horse's tail"). A large central herniation in the lumbar spine compresses roots, not cord, so it produces a lower motor neuron picture with saddle numbness and bladder dysfunction (the cauda equina syndrome) rather than spasticity.

The intervertebral foramen

The intervertebral foramen (IVF) is the window between two adjacent vertebrae through which the spinal nerve leaves the canal. Its borders are:

• Roof: the pedicle of the vertebra above (its inferior vertebral notch).
• Floor: the pedicle of the vertebra below (its superior vertebral notch).
• Anterior wall: the back of the two vertebral bodies and the intervertebral disc between them; in the neck, the uncovertebral joint (joint of Luschka) also forms part of this wall.
• Posterior wall: the zygapophyseal (facet) joint, with its capsule and the lateral extension of the ligamentum flavum.

The contents are the dural sleeve holding the roots or spinal nerve, the dorsal root ganglion, the recurrent meningeal nerve, a spinal branch of a segmental artery, communicating veins and fat. In the lumbar spine the nerve occupies a relatively small part of the foramen, often quoted as about one third to one half of its area, and it lies in the upper part, tucked under the pedicle above. The disc lies at the level of the lower part of the foramen. This is why loss of disc height, which lets the upper pedicle drop toward the lower one, squeezes the exiting root.

In the neck the foramen is shaped more like a gutter that runs forward and outward at roughly 45 degrees, and the nerve fills a larger share of it. Uncovertebral spurs project into the front of the foramen and facet joint enlargement projects into the back, so cervical foraminal stenosis is common in middle age and is the most frequent cause of cervical radiculopathy, ahead of soft disc herniation.

What changes the size of the foramen

The foramen is a dynamic space. Flexion opens it, because the pedicles separate and the ligamentum flavum is stretched thin. Extension closes it, because the facets slide together, the ligamentum flavum buckles inward and the disc bulges slightly backward. Lateral flexion and rotation toward one side narrow the foramen on that side. This is why patients with foraminal stenosis often lean forward, find walking downhill harder than uphill, and get relief from sitting, and why the Spurling test (extension, side bending and rotation toward the painful side with gentle downward pressure) can reproduce cervical radicular pain.

Disc height loss, herniation, osteophytes, facet hypertrophy, ligamentum flavum thickening and spondylolisthesis all shrink the foramen over time. Compression of the root does not by itself cause pain. A healthy nerve that is compressed tends to go numb or weak. Pain arises when the root is also inflamed. Chemical mediators released from herniated nucleus pulposus (including phospholipase A2, tumor necrosis factor alpha and other cytokines), together with venous congestion and reduced blood flow inside the root, make the DRG and root hyperexcitable.

Section 2Dermatomes and myotomes

What a dermatome is, and is not

A dermatome is the area of skin supplied by the sensory fibers of a single spinal nerve root. A myotome is the group of muscles supplied mainly by the motor fibers of a single root. Both ideas come from embryology. Each embryonic somite carries its own nerve supply as it grows, which gives neat horizontal bands in the trunk. In the limbs the pattern is stretched and twisted, because the limb buds grow out from the body wall and then rotate: the upper limb rotates laterally and the lower limb rotates medially. That is why the dermatomes of the leg spiral around the shin and why the big toe, which is on the inner side of the foot, belongs to L5 while the little toe belongs to S1.

Dermatome maps are useful, but they are approximations. Three facts keep them honest. First, adjacent dermatomes overlap. Each root sends some fibers into the territory of its neighbors, so cutting a single root rarely produces a clean band of total numbness; usually there is reduced sensation in a smaller central zone, often called the autonomous zone. Second, the maps disagree. Classic maps were built by different methods and draw several borders differently; modern evidence based maps show the most consistent zones rather than sharp lines. Third, people vary. A given person may have a prefixed or postfixed brachial plexus that shifts everything up or down by about half a segment. For these reasons, examiners test key sensory points near the center of each dermatome rather than trying to trace borders.

Key sensory points

The standard examination tests one key point per dermatome for light touch and pinprick. A few landmarks make the trunk easy: the nipple line is about T4, the xiphoid process about T6, the umbilicus T10, and the inguinal ligament T12 to L1. The cape over the top of the shoulder is C4, which lies right next to T2 on the upper chest because the C5 to T1 territories have been pulled out into the arm.

Myotomes and how to test them

Most limb muscles receive fibers from two or three roots, so a single root lesion usually causes partial weakness rather than paralysis. Still, each movement has one root that contributes most, and testing a key movement for each root gives a quick and reasonably reliable screen. Strength is graded on the Medical Research Council scale: 0 no contraction; 1 a flicker of contraction without movement; 2 movement with gravity eliminated; 3 movement against gravity but not resistance; 4 movement against some resistance; 5 normal strength. Most root weakness falls within grade 4.

Always compare sides. For the large leg muscles manual testing is insensitive, so add functional tests: ten single leg heel raises for S1, walking on the heels for L4 and L5, and a single leg sit to stand or step up for L3 and L4.

Putting sensation and strength together

A root lesion is suggested when changes in sensation, strength and reflexes all point to the same level, on one side, and match the pattern of pain. A peripheral nerve lesion produces a different pattern: sensory loss with sharp, characteristic borders that cut across dermatomes, and weakness confined to muscles of one nerve even if those muscles come from different roots. For example, weakness of thumb abduction with numbness of the thumb, index and middle fingers but normal wrist extension and a normal brachioradialis reflex points to the median nerve at the wrist, not to C6.

Test both light touch (large fibers, dorsal columns) and pinprick (small fibers, spinothalamic tract). Patients often describe paresthesia (tingling or pins and needles, a positive symptom) before hypoesthesia (reduced feeling, a negative sign), and some develop dysesthesia (unpleasant, burning sensations) or allodynia (pain from a normally harmless touch).

Section 3Deep tendon and pathological reflexes

The stretch reflex arc

The deep tendon reflex, more accurately called the muscle stretch reflex, is the simplest reflex in the body and one of the most informative tests in the neurological examination. Tapping a tendon does not stimulate the tendon itself. It produces a quick, small stretch of the muscle, and that stretch is detected by muscle spindles.

Muscle spindles are small encapsulated sensory organs that lie in parallel with ordinary (extrafusal) muscle fibers. Their specialized intrafusal fibers are wrapped by sensory endings that give rise to large, fast type Ia afferent fibers that fire vigorously when the muscle is stretched quickly. The Ia fiber enters the cord through the dorsal root, its cell body in the dorsal root ganglion, and synapses directly on alpha motor neurons in the ventral horn that supply the same muscle. Because there is only one synapse between sensory input and motor output, the reflex is called monosynaptic. The alpha motor neuron fires, the muscle contracts and the limb jerks.

At the same time, branches of the Ia fiber excite a Ia inhibitory interneuron that suppresses motor neurons to the antagonist muscle. When the quadriceps is reflexly contracting, the hamstrings are told to relax. This is reciprocal inhibition. A separate receptor, the Golgi tendon organ, lies in series with the muscle at the musculotendinous junction and signals tension through type Ib fibers, which inhibit the same muscle (autogenic inhibition). Spindle sensitivity is set by gamma motor neurons, which contract the ends of the intrafusal fibers and keep the spindle taut. Descending pathways adjust this setting constantly, so reflexes change with anxiety, attention and injury to the brain or cord.

The reflexes you test and their levels

Each commonly tested reflex is carried by one or two root levels through one peripheral nerve. A reduced or absent reflex can therefore point to a root, provided the change is on one side and fits the sensory and motor findings.

Grading and reinforcement

Reflexes are graded on a five point scale: 0 absent; 1+ present but diminished (trace); 2+ normal; 3+ brisker than average, possibly with spread to neighboring muscles; 4+ very brisk, often with sustained clonus. The grade itself matters less than asymmetry and context. Many healthy people have 1+ or 3+ reflexes everywhere; an absent ankle reflex on one side only, in a patient with pain down the back of that leg, is meaningful. Ankle reflexes often fade in people over about 60, so a bilaterally absent ankle jerk in an older adult may be normal aging or a peripheral neuropathy rather than a root problem.

When a reflex seems absent, use reinforcement before calling it 0. The Jendrassik maneuver asks the patient to hook the fingers of both hands together and pull apart just as you tap a lower limb reflex; clenching the teeth or making a fist reinforces upper limb reflexes. A reflex that appears only with reinforcement is graded 1+.

Upper versus lower motor neuron signs

The upper motor neurons are the neurons of the motor cortex and brainstem whose axons descend in the corticospinal and other tracts. The lower motor neurons are the alpha motor neurons of the ventral horn and cranial nerve nuclei, along with their axons in roots and peripheral nerves. Injury at each level produces a recognizable cluster of signs, and telling them apart is one of the most important skills in spine examination, because a patient with neck pain and an upper motor neuron picture may have cord compression rather than a simple root problem.

Pathological reflexes

Some reflexes are normally absent in adults and appear only when descending control is damaged. They are called pathological reflexes, and their presence is a red flag for disease of the brain or spinal cord.

• Babinski sign (extensor plantar response). Stroke the lateral sole firmly from heel toward the little toe and across the ball of the foot. The normal adult response is flexion of the toes. Extension of the big toe, often with fanning of the other toes, indicates a corticospinal tract lesion. The response is normal in infants up to about one to two years, before the corticospinal tract has fully myelinated.
• Hoffmann sign. Hold the patient's middle finger and flick its nail downward. Flexion of the thumb and index finger is a positive sign. It suggests upper motor neuron involvement above the lower cervical segments, most often cervical myelopathy, although a few healthy people with generally brisk reflexes have a mild bilateral Hoffmann response.
• Clonus. A rapid, forceful dorsiflexion of the ankle that is then held produces rhythmic beats of plantar flexion. A few beats that die away can be normal; sustained clonus (more than about four or five beats, or continuing as long as the stretch is held) indicates an upper motor neuron lesion.

Among the superficial reflexes, the abdominal reflexes (T8 to T12) are lost with upper motor neuron lesions, and the anal wink and bulbocavernosus reflexes test S2 to S4, the segments that control the bladder, bowel and sexual function, and they are checked when cauda equina syndrome or conus damage is suspected.

Section 4Radiculopathy versus referred pain

Four words that are often confused

Leg and arm pain from the spine is described loosely even by clinicians, but each of these terms has a different mechanism, prognosis and meaning for referral.

• Radicular pain is pain that arises from irritation of a spinal nerve root or dorsal root ganglion, usually by a combination of mechanical compression and inflammation. It is felt in the limb, travels in a narrow band, and is often described as shooting, electric, lancinating or burning. In the lower limb it commonly passes below the knee and into the foot.
• Radiculopathy is a loss of function in a root: conduction is blocked, so the examiner finds numbness, weakness or a reduced reflex in that root's distribution. Radiculopathy is defined by objective neurological signs, not by pain. A patient can have radicular pain without radiculopathy, radiculopathy without much pain (painless foot drop), or both together.
• Somatic referred pain is pain felt in a region away from its source, where the source is a musculoskeletal structure of the spine such as the disc, a facet joint, the sacroiliac joint, ligaments or muscles. It is deep, dull, aching and hard to localize, it tends to stay in the buttock and thigh (usually above the knee), and it is not accompanied by neurological deficit.
• Visceral referred pain is pain felt in the body wall or limb whose source is an internal organ, such as the heart, gallbladder, kidney, pancreas or aorta.

"Sciatica" is a lay term that does not say whether leg pain is radicular or referred, so replace it with a precise term after the examination.

Why referral happens

Referred pain is explained by convergence. Sensory neurons from deep spinal structures, viscera and skin all synapse in the dorsal horn, and many second order neurons there receive input from more than one source. The brain has learned, over a lifetime, that activity in a given dorsal horn neuron usually comes from the skin and muscle it serves most often. When the same neuron is activated by input from a disc or an organ, the brain projects the sensation onto that familiar territory. This is called the convergence projection theory. Classic experiments injecting salt solution into the interspinous ligaments of volunteers produced aching, segmental limb pain with no root involved, and later studies showed that facets, discs and sacroiliac joints refer pain into the buttock, groin and thigh with large overlap.

Visceral referral follows the segment that supplied the organ's embryonic origin. Pain from the heart is referred to the chest, left arm and jaw (upper thoracic segments); pain from the diaphragm, through the phrenic nerve (C3 to C5), is referred to the tip of the shoulder; gallbladder disease refers to the right upper abdomen and below the right scapula; the kidneys and ureters refer to the flank and groin; the pancreas and a leaking abdominal aortic aneurysm can refer straight through to the mid back. Consider visceral causes when back pain is unchanged by movement or comes with systemic symptoms.

Which root, from which disc?

About 90 to 95 percent of symptomatic lumbar disc herniations occur at L4 to L5 or L5 to S1, the two levels that carry the most load and motion. Where the herniation sits around the circumference of the disc decides which root it meets. Clinicians describe positions from the midline outward: central, subarticular (also called paracentral or lateral recess), foraminal and extraforaminal (far lateral). The most common is posterolateral (subarticular), where the posterior longitudinal ligament is thinner.

• A subarticular herniation at L4 to L5 compresses the traversing L5 root. At L5 to S1 it compresses S1.
• A foraminal or far lateral herniation at L4 to L5 compresses the exiting L4 root, often its DRG, and tends to be very painful.
• A large central herniation can compress several roots of the cauda equina at once, on both sides, which is the setting for cauda equina syndrome.

Root syndromes worth memorizing

Real patients often show only part of each classic pattern. In the neck, C7 is the most commonly involved root, followed by C6. In the low back, L5 and S1 together account for the large majority of radiculopathies.

Provocation and neurodynamic tests

Nerve roots must slide and stretch as the limbs and spine move. An inflamed or tethered root becomes sensitive to tension, and several tests take advantage of this. The straight leg raise (SLR) lifts the straight leg of a supine patient; reproduction of the patient's familiar leg pain below the knee between about 30 and 70 degrees of hip flexion is a positive test for irritation of the L5 or S1 roots. Pain only in the back, or only a stretch in the hamstrings, is not a positive result. Adding ankle dorsiflexion or neck flexion and seeing the leg pain increase strengthens the finding. The SLR is sensitive (often quoted around 0.9 for disc herniation in surgical populations) but not specific, so a negative SLR helps rule out herniation while a positive one does not confirm it. The crossed SLR, in which raising the opposite leg reproduces the symptomatic leg pain, is the reverse: insensitive but quite specific, and it suggests a large, often central or axillary herniation.

The slump test builds tension in sequence (seated slump, neck flexion, knee extension, ankle dorsiflexion) and then releases neck flexion; if the leg symptoms ease when only the neck is lifted, the response is neural rather than muscular. The prone knee bend (femoral nerve tension test) tensions the L2 to L4 roots. In the neck, the Spurling test (extension, side bending and rotation toward the symptomatic side with gentle axial pressure) narrows the foramen and is specific but only moderately sensitive for cervical radiculopathy. The upper limb tension test is sensitive but not specific, and relief when the hand rests on top of the head (the shoulder abduction relief sign) supports a root source. Combining tests helps: when several of the Spurling test, upper limb tension test, relief with neck distraction and reduced rotation toward the painful side are positive together, the likelihood of cervical radiculopathy rises sharply.

Natural history, investigation and red flags

The outlook for radicular pain from a disc herniation is generally good. Most patients improve substantially within 6 to 12 weeks, and repeat imaging shows that many herniations shrink or disappear on their own, with the largest, extruded and sequestered fragments the most likely to regress because they are exposed to the immune system and resorbed. Guidelines therefore recommend a period of conservative care for most patients without serious deficits. Imaging with MRI is reserved for red flags, progressive deficits, or symptoms that persist beyond about six weeks and when the result would change management such as an injection or surgery. MRI findings must match the examination, because herniations are common in people without pain.

Electrodiagnostic testing (nerve conduction studies and needle electromyography) assesses function rather than structure. Because a root lesion is proximal to the dorsal root ganglion, the cell bodies and their peripheral axons remain intact, so sensory nerve action potentials in the numb area are typically normal in radiculopathy. In a plexus or peripheral nerve lesion, which damages axons beyond the ganglion, they are reduced. Needle EMG may show denervation in one myotome, including the paraspinals, after about two to three weeks.

Some nerve root presentations are emergencies. Cauda equina syndrome presents with some combination of saddle (perineal) numbness, new urinary retention or incontinence, fecal incontinence, sexual dysfunction and bilateral leg pain or weakness; it requires same day emergency imaging and surgical assessment, because outcome depends on how quickly the compression is relieved. Progressive or severe motor weakness (for example a new foot drop), signs of myelopathy, and radicular pain with fever, a history of cancer or unexplained weight loss also call for urgent referral.

Section 5Peripheral nerve entrapments

How a nerve is injured by pressure

Once a spinal nerve leaves the foramen and its fibers sort themselves into peripheral nerves, it still has to pass through tight places: fibro osseous tunnels, gaps between muscles, and points where it crosses a bony edge. At these sites chronic compression and repeated stretch can produce an entrapment neuropathy.

Nerves are sensitive to pressure because of their blood supply. Each nerve has its own small vessels (the vasa nervorum) running in the epineurium and perineurium. Experimental work shows that pressures of only about 20 to 30 mmHg slow venous outflow from the nerve, and that somewhat higher pressures impair axonal transport and then arterial inflow. Venous congestion causes intraneural edema, and with time the connective tissue thickens and the myelin thins. Early symptoms are intermittent and positional (tingling at night, or when the wrist or elbow is bent for a long time), reflecting reversible ischemia. Later symptoms are constant, with numbness and then weakness and wasting, reflecting demyelination and finally axon loss.

These stages map onto the classic grades of nerve injury: neurapraxia (myelin damage with a local conduction block and full recovery in days to weeks), axonotmesis (axons divided inside intact sheaths, with regrowth of about 1 mm per day) and neurotmesis (the whole nerve divided, needing surgical repair).

Carpal tunnel syndrome

Carpal tunnel syndrome, compression of the median nerve at the wrist, is the most common entrapment neuropathy, affecting several percent of adults, more often women. The carpal tunnel is formed by the concave arch of the carpal bones as its floor and walls and the transverse carpal ligament (flexor retinaculum) as its roof. The ligament attaches to the scaphoid tubercle and trapezium on the radial side and to the pisiform and hook of the hamate on the ulnar side. Through this unyielding tunnel pass the median nerve and nine tendons: four of flexor digitorum superficialis, four of flexor digitorum profundus and the flexor pollicis longus. Anything that adds volume inside the tunnel, such as tenosynovial thickening, pregnancy, hypothyroidism, rheumatoid arthritis or diabetes, raises the pressure on the nerve. Wrist flexion and extension both raise tunnel pressure, which is why symptoms are worse at night when people sleep with curled wrists and why prolonged gripping and vibration are risk factors.

Patients report numbness, tingling and burning in the thumb, index, middle and radial half of the ring finger, often waking them at night and relieved by shaking the hand (the "flick sign"). Two anatomical details separate carpal tunnel syndrome from a C6 or C7 radiculopathy. First, the palmar cutaneous branch of the median nerve leaves above the wrist and passes over the ligament, so sensation over the thenar eminence is normal in carpal tunnel syndrome but may be reduced in a root lesion. Second, carpal tunnel syndrome weakens only the median nerve muscles of the hand (abductor pollicis brevis, opponens pollicis and the lateral two lumbricals), causing thenar wasting in advanced cases, while reflexes, wrist extension and elbow strength stay normal.

Provocative tests include Phalen's test (holding the wrists fully flexed for about a minute), Tinel's sign (tapping over the nerve at the wrist produces tingling into the fingers) and the carpal compression test (direct thumb pressure over the tunnel for about 30 seconds). Each has only moderate accuracy; nerve conduction studies showing slowed median conduction across the wrist help when the diagnosis is uncertain. Treatment begins with a neutral night splint and activity changes; corticosteroid injection gives short to medium term relief; and surgical release is definitive for severe or persistent cases. Nerve gliding exercises and manual therapy are also used, and one randomized trial reported results similar to surgery at one year, but the evidence overall is limited and of mixed quality.

Other upper limb entrapments

The ulnar nerve is the second most commonly entrapped nerve, usually at the elbow in the cubital tunnel, where it runs behind the medial epicondyle under a fibrous band between the two heads of flexor carpi ulnaris. Elbow flexion both stretches the nerve and narrows the tunnel, so leaning on the elbow, sleeping with the arms bent and holding a phone to the ear provoke symptoms. Numbness affects the little finger, the ulnar half of the ring finger and the ulnar side of the back of the hand. Weak interossei and adductor pollicis produce a weak pinch with compensatory thumb flexion (Froment sign) and, in severe cases, clawing of the ring and little fingers. At the wrist, the ulnar nerve can be compressed in Guyon canal, between the pisiform and the hook of the hamate, as in long distance cyclists; this spares the dorsal hand sensation.

The radial nerve can be compressed in the spiral groove of the humerus (for example after sleeping with an arm over a chair back), producing wrist drop with sparing of the triceps. Its deep motor branch, the posterior interosseous nerve, can be trapped under the arcade of the supinator muscle, producing weak finger extension with no sensory loss.

Thoracic outlet syndrome describes compression of the brachial plexus, subclavian artery or subclavian vein as they pass through the interscalene triangle, the costoclavicular space or the space under pectoralis minor. Most cases are neurogenic, with symptoms often in the lower trunk (C8 and T1) distribution: medial arm and hand tingling, worse with overhead activity or carrying. Vascular forms are rarer but more serious. The classic provocative tests have high false positive rates in healthy people, so diagnosis requires a consistent overall picture.

Lower limb entrapments

Meralgia paresthetica is compression of the lateral femoral cutaneous nerve (L2 to L3) where it passes under or through the inguinal ligament near the anterior superior iliac spine. It is purely sensory: burning, tingling or numbness of the anterolateral thigh, with normal strength and reflexes. Tight belts, obesity, pregnancy and diabetes are typical contributors. Its sharply bordered patch of altered sensation and its normal motor exam separate it from an L2 or L3 radiculopathy.

The common fibular (peroneal) nerve winds around the neck of the fibula just under the skin, where it can be compressed by habitual leg crossing, a tight cast or brace, prolonged squatting or kneeling, or rapid weight loss that removes its fat padding. The result is foot drop (weak dorsiflexion and eversion) with numbness of the lateral leg and dorsum of the foot, which mimics L5 radiculopathy. In a fibular nerve palsy, foot inversion (tibialis posterior, tibial nerve) and hip abduction (gluteus medius, superior gluteal nerve) are normal, because those muscles are not supplied by the fibular nerve. In an L5 radiculopathy they are often weak, because they share the L5 root. Back pain and a positive straight leg raise also favor the root.

Other entrapments include tarsal tunnel syndrome (the tibial nerve beneath the flexor retinaculum behind the medial malleolus, causing burning in the sole), Morton neuroma (thickening of an interdigital nerve, most often between the third and fourth metatarsal heads, causing burning forefoot pain with tight shoes), and deep gluteal syndrome, a broader and more accurate term for sciatic nerve irritation in the buttock that includes the older idea of piriformis syndrome. It causes buttock pain that is worse with sitting; no single test is accurate.

Double crush and systemic risk

The debated double crush hypothesis proposes that compression at one point, such as the root, makes the distal nerve more vulnerable to a second compression, so examine the whole nerve when one lesion does not fit. Diabetes, hypothyroidism, kidney disease and vitamin B12 deficiency also make nerves more susceptible, so multiple entrapments should prompt a search for a cause.

Words worth knowing

Pain Neuroscience

Two people can have the same disc bulge on an MRI and live completely different lives: one barely notices it, the other cannot sit through a meal. Pain neuroscience explains why. Pain is not a direct readout of tissue damage but an output of the nervous system that weighs danger signals against everything else it knows, and understanding that process changes how we examine, treat and talk to people in pain.

Section 1Nociception versus pain

Two different things

The International Association for the Study of Pain defines pain as an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage. Three words in that definition do a lot of work. Pain is an experience, so it happens in a person, not in a tissue. It is both sensory and emotional, so fear, mood and meaning are part of it from the start. And it is linked to potential damage, so pain can warn before harm occurs and can persist after harm has healed. The accompanying notes stress that pain is always personal, that it is shaped by biological, psychological and social factors, that pain and nociception are different, and that a person's report of pain should be respected.

Nociception is the neural process of encoding noxious stimuli: the activity of nociceptors and the pathways that carry their signals. Nociception is a source of input to the brain. Pain is one possible output. Most of the time they travel together: you touch a hot pan, nociceptors fire, and you feel pain. But each can occur without the other:

• Nociception without pain. Soldiers with severe wounds and athletes injured mid competition sometimes report little pain until the threat has passed. Physicians in the Second World War described wounded soldiers asking for far less pain relief than civilians with comparable surgical wounds, a difference attributed to the meaning of the injury (a ticket home rather than a disaster).
• Pain without nociception. Phantom limb pain is felt in a limb that no longer exists. A widely cited case report describes a builder in severe pain after a nail went through his boot, whose pain eased once the boot was removed and the nail was found to have passed between his toes. The brain had concluded "danger" from what it saw.

Rare genetic conditions show the value of pain. People born with congenital insensitivity to pain, often caused by loss of function mutations in the sodium channel Nav1.7, cannot feel pain. They suffer repeated unnoticed injuries, burns, fractures and joint destruction. Pain is a protective system, and the goal of care is to restore its accuracy rather than to switch it off.

The four processes

It helps to break the path from stimulus to experience into four steps.

1. Transduction. A noxious stimulus (heat, intense pressure, chemicals) opens ion channels on the free nerve endings of nociceptors and creates a generator potential. Key transducers include TRPV1 (heat above about 43°C, acid and capsaicin, the active ingredient of chili peppers), TRPA1 (irritant chemicals), acid sensing ion channels and P2X3 receptors for ATP released from damaged cells. If the generator potential reaches threshold, voltage gated sodium channels (Nav1.7, Nav1.8 and Nav1.9 are concentrated in nociceptors) fire action potentials.
2. Transmission. Action potentials travel along the primary afferent to the dorsal horn of the spinal cord, where the nociceptor releases glutamate and neuropeptides such as substance P and calcitonin gene related peptide (CGRP) onto second order neurons, which carry the signal to the brainstem and thalamus and on to the cortex.
3. Modulation. At every relay, and especially in the dorsal horn, the signal is turned up or down by local interneurons and by pathways descending from the brain.
4. Perception. The brain integrates the incoming signal with context, memory, attention, emotion and expectation, and the conscious experience of pain may or may not emerge.

The fibers and pathways

Nociceptors are the peripheral endings of two kinds of small fiber. Aδ fibers are thinly myelinated and fast; they produce first pain, the sharp, well localized prick that makes you pull your hand away. C fibers are unmyelinated and slow; they produce second pain, the dull, burning, poorly localized ache that follows a second or so later and lingers. Many C fibers are polymodal, responding to heat, pressure and chemicals. Some C fibers are normally unresponsive and "wake up" only in inflamed tissue; these are called silent or sleeping nociceptors. Large Aβ fibers carry touch, pressure and vibration and do not normally signal pain, but they matter for the gate control theory and for allodynia.

In the dorsal horn, nociceptors synapse on two main types of projection neuron. Nociceptive specific neurons, concentrated in lamina I, respond only to noxious input. Wide dynamic range neurons, concentrated in lamina V, respond to a graded range of input from gentle touch to injury, and they receive convergent input from skin, muscle, joints and viscera, which is part of the basis of referred pain. Axons of the projection neurons cross the midline within a segment or two in the anterior white commissure and ascend in the anterolateral quadrant as the spinothalamic tract. A lateral division reaches the ventral posterolateral thalamus and then the primary and secondary somatosensory cortex, supporting the sensory discriminative features of pain (where, how strong, what kind). A medial division and related spinoreticular and spinoparabrachial pathways reach the brainstem, medial thalamus, amygdala and hypothalamus and from there the anterior cingulate and insular cortex, supporting the affective motivational features (how unpleasant, how urgent, what to do). Because the tract crosses early, a hemisection of the cord produces loss of pain and temperature on the opposite side below the lesion.

Brain imaging has not found a single pain center. Many regions activate together, including the somatosensory cortices, insula, anterior cingulate, prefrontal cortex, thalamus and amygdala, and the same regions are also active in other salient, threatening situations. This supports the view that pain is constructed by a distributed network (sometimes called the neuromatrix) that weighs evidence of danger.

Three mechanistic categories of pain

These categories are not exclusive. A person with chronic low back pain may have a nociceptive contribution from a facet joint, a neuropathic contribution from a root and a nociplastic contribution from a sensitized nervous system, in proportions that change over time.

Section 2Peripheral and central sensitization

A system that learns

The nociceptive system is not a fixed set of wires. After injury its sensitivity rises, so that stimuli that were harmless now hurt and stimuli that hurt now hurt more. This sensitization is normally helpful: an inflamed ankle that hurts when you lean on it encourages you to protect it while it heals. Sensitization usually fades as tissue recovers. When it persists or spreads, it becomes a major driver of chronic pain. Two terms describe what patients feel:

• Allodynia is pain from a stimulus that does not normally provoke pain, such as light touch of clothing on sunburned skin.
• Hyperalgesia is increased pain from a stimulus that is normally painful, such as a pinprick that feels unusually sharp. Primary hyperalgesia occurs in the injured tissue itself; secondary hyperalgesia occurs in the surrounding uninjured tissue and reflects changes in the spinal cord.

Peripheral sensitization

When tissue is injured, damaged cells, platelets, mast cells and immune cells release a mixture of chemicals often called the inflammatory soup: bradykinin, prostaglandins (especially PGE2), histamine, serotonin, ATP, hydrogen ions, nerve growth factor (NGF) and cytokines such as tumor necrosis factor alpha and interleukins 1β and 6. Some of these activate nociceptors directly (bradykinin, ATP, acid). Others sensitize them. Prostaglandins and NGF trigger intracellular signaling (protein kinases A and C) that phosphorylates TRPV1 and sodium channels, lowering their thresholds and increasing their firing. NGF also travels back to the cell body in the dorsal root ganglion and increases the production of channels and neuropeptides. Silent nociceptors are recruited. The net result is primary hyperalgesia and tenderness at the site of injury.

Activated nociceptors also act in reverse. Impulses spread along branches of the same nerve ending and release substance P and CGRP into the tissue, causing vasodilation, leaky vessels, swelling and mast cell degranulation. This neurogenic inflammation produces the red flare around a scratch and helps explain why nerve activity itself can sustain inflammation. Nonsteroidal anti-inflammatory drugs work largely by blocking cyclooxygenase and the prostaglandins it makes; ice, compression and relative rest reduce the inflammatory drive; and topical capsaicin, which overstimulates and then defunctionalizes TRPV1 bearing endings, shows how the peripheral terminal can be targeted directly.

Central sensitization

Central sensitization is an increase in the responsiveness of nociceptive neurons in the central nervous system to their normal or subthreshold input. It was first described in the dorsal horn in the early 1980s and is now recognized at several levels of the neuraxis. Several mechanisms contribute:

• Wind-up and NMDA receptors. Glutamate released by nociceptors acts on AMPA receptors for fast transmission. The NMDA receptor is normally blocked by a magnesium ion. Repeated C fiber input, together with substance P and CGRP, depolarizes the dorsal horn neuron enough to expel the magnesium. Calcium then flows in and activates kinases that make the neuron more excitable. In wind-up, repeated identical C fiber stimuli at a rate of about one per second or faster produce a progressively larger response; the perceptual counterpart is temporal summation, in which repeated identical pinpricks feel increasingly painful.
• Long term potentiation. Intense input can strengthen dorsal horn synapses for hours to days, a form of pain memory that resembles learning in the hippocampus.
• Receptive field expansion. Sensitized neurons respond to input from a larger area, so pain and tenderness spread beyond the injured tissue (secondary hyperalgesia).
• Disinhibition. Inhibitory interneurons that release GABA and glycine become less effective after nerve injury, so the brakes on transmission weaken. Aβ touch fibers can then access pain pathways, producing allodynia.
• Glial activation. Microglia and astrocytes in the spinal cord respond to injury by releasing cytokines and brain derived neurotrophic factor, which amplify neuronal excitability. Glia are now seen as active partners in persistent pain.
• Descending facilitation. Brainstem pathways can turn the dorsal horn up as well as down (see Section 3). Stress, poor sleep and threat can tip the balance toward facilitation.

Recognizing a sensitized nervous system

No single test proves central sensitization in a patient, and the term is sometimes used too loosely as a label for any pain that is hard to explain. Clinicians look for a pattern: pain that is disproportionate to the nature and extent of injury; pain that is widespread or spreading; hypersensitivity to many stimuli including pressure, cold, light, sound and smell; pain that is unpredictable or poorly related to mechanical loading; and associated fatigue, poor sleep, low mood and difficulty concentrating. Research tools include pressure pain thresholds measured at sites far from the painful area, temporal summation protocols and tests of conditioned pain modulation. Recognizing this pattern does not mean the pain is "not real" or "all in the head." It means the nervous system has become more protective, and treatment should target that system as well as any local tissue.

Section 3The gate control theory and descending modulation

The gate control theory

Before the 1960s, pain was often taught as a simple specificity model: pain receptors connect to a pain pathway that connects to a pain center, like a bell rope, so the amount of pain should match the amount of damage. Clinical experience did not fit. Pain could be blocked by distraction and intensified by fear, and rubbing a bumped elbow helped. In 1965 the psychologist Ronald Melzack and the neuroscientist Patrick Wall proposed the gate control theory, which placed a modulating mechanism in the dorsal horn of the spinal cord.

The theory has four elements. Large diameter fibers (Aβ, carrying touch and vibration) and small diameter fibers (Aδ and C, carrying nociceptive signals) both excite transmission (T) cells that project to the brain. Both fiber types also connect to inhibitory interneurons in the substantia gelatinosa (SG, lamina II). Large fiber input excites these interneurons, which inhibit transmission and "close the gate." Small fiber input inhibits the interneurons, which "opens the gate." The fourth element is central control: pathways descending from the brain can open or close the gate according to attention, emotion and past experience. Pain depends on the balance of these inputs, not on the nociceptive signal alone.

Later research showed that the specific wiring Melzack and Wall drew was too simple. Inhibition in the dorsal horn involves many interneuron types acting both before and after the synapse, and the substantia gelatinosa contains excitatory as well as inhibitory neurons. But the central ideas proved durable: the spinal cord is an active processor, not a relay; touch input can reduce pain transmission; and the brain can reach down to control what reaches it. The theory explains why rubbing an injury, applying vibration or using transcutaneous electrical nerve stimulation (TENS) can ease pain, and it opened the door to treating pain through the nervous system as well as the tissues. Melzack later extended these ideas in the neuromatrix theory, which describes pain as an output of a widely distributed brain network shaped by genetics and experience.

The descending modulation system

In 1969 a striking experiment showed that electrical stimulation of the periaqueductal gray (PAG) in the midbrain of rats produced enough analgesia to allow abdominal surgery without anesthesia. This stimulation produced analgesia led to the discovery of the body's own pain control system and, in the 1970s, of opioid receptors and the endogenous opioids: enkephalins, β-endorphin and dynorphins.

The descending system works as a chain. Higher centers, including the prefrontal cortex, anterior cingulate, amygdala and hypothalamus, project to the PAG. The PAG projects to the rostral ventromedial medulla (RVM), which includes the serotonin rich nucleus raphe magnus. The RVM sends axons down the dorsolateral funiculus to the dorsal horn. A parallel noradrenergic pathway descends from the locus coeruleus and nearby nuclei in the pons. In the dorsal horn, serotonin and norepinephrine (acting largely through α2 adrenergic receptors) inhibit transmission directly and by activating local inhibitory interneurons that release enkephalin and GABA. Opioids act presynaptically, closing calcium channels so that nociceptor terminals release less glutamate and substance P, and postsynaptically, opening potassium channels that hyperpolarize the projection neuron.

This system is bidirectional. The RVM contains OFF cells, which inhibit nociceptive transmission, and ON cells, which facilitate it. In acute danger the system usually inhibits pain so that the animal can escape (stress induced analgesia). In persistent pain, after nerve injury, with chronic stress or poor sleep, the balance can shift toward descending facilitation, keeping the dorsal horn sensitized.

Everyday modulators of pain

• Conditioned pain modulation. A painful stimulus in one part of the body reduces pain elsewhere, a phenomenon studied in animals as diffuse noxious inhibitory controls. In the lab it is tested by measuring a pain threshold before and during immersion of the other hand in cold water. Many people with chronic pain conditions show weaker conditioned pain modulation, consistent with a shift away from descending inhibition.
• Placebo analgesia. Expecting relief can produce real, measurable relief. Part of this effect is blocked by naloxone, an opioid antagonist, showing that expectation can release endogenous opioids; other parts involve endocannabinoids and dopamine reward circuits. Brain imaging shows reduced activity in pain responsive regions during placebo analgesia.
• Nocebo hyperalgesia. Expecting harm or more pain can increase pain. Anxiety triggers the release of cholecystokinin, which counteracts opioid analgesia and facilitates pain. Warnings delivered in frightening language, labels that suggest damage and anxious body language can all act as nocebos.
• Attention, mood and sleep. Distraction reduces pain, while hypervigilance increases it. Low mood, anxiety and poor sleep amplify pain; one night of sleep restriction lowers pain thresholds the next day.
• Exercise. A single bout of exercise often produces short lived hypoalgesia in healthy people (exercise induced hypoalgesia), although the effect can be weaker or absent in some chronic pain conditions, which is one reason graded, tolerable exercise works better than a sudden large dose.

Manual therapies, including spinal manipulation and mobilization, have been studied for neurophysiological effects of this kind. Laboratory studies report short term changes after manipulation such as raised pressure pain thresholds and reduced temporal summation, which some researchers interpret as engagement of spinal and descending inhibitory mechanisms. These findings are interesting but come mostly from small studies with short follow up, and they do not establish that such effects explain the clinical benefits seen in trials. Similar short term hypoalgesia follows exercise, touch and other interventions, so the effects are probably not specific to any one technique.

Section 4Chronic pain and the biopsychosocial model

When pain persists

Chronic pain is pain that persists or recurs for longer than three months. The time frame is practical rather than biological: most tissues have passed through the inflammatory and proliferative phases of healing by then, so pain that continues usually reflects more than ongoing tissue damage. Chronic pain is common and costly. Large national surveys in the United States estimate that about one in five adults has chronic pain, and that roughly 7 to 8 percent have high impact chronic pain that limits life or work activities on most days. Low back pain is the leading cause of years lived with disability worldwide, and neck pain is not far behind.

The current international classification of diseases divides chronic pain into two groups. Chronic primary pain is pain in one or more regions that is associated with significant emotional distress or functional disability and is not better explained by another condition; examples include fibromyalgia, chronic primary low back pain and irritable bowel syndrome. Chronic secondary pain is linked to another disease as its underlying cause, such as cancer, surgery or trauma, a nerve lesion, a headache disorder, a visceral disease or musculoskeletal disease such as osteoarthritis. In the primary category, pain is regarded as a condition in its own right rather than only a symptom.

The biopsychosocial model

In 1977 the psychiatrist George Engel argued that the dominant biomedical model, which explains illness entirely by measurable biological deviation, could not account for why people with the same pathology suffer so differently. He proposed the biopsychosocial model, in which biological, psychological and social factors interact to shape health and illness. Pain is the clearest example. The model does not say pain is psychological; it says that every pain has biological, psychological and social dimensions, and that the weight of each varies between people and over time. It also does not dismiss biology. A fracture is still a fracture. The point is that treating only the tissue often fails when other factors are keeping the pain system on alert.

Pain catastrophizing deserves special attention. It is a set of exaggerated negative thoughts about pain with three components: rumination (being unable to stop thinking about it), magnification (worrying that something serious is happening) and helplessness (feeling nothing can be done). Catastrophizing is measured with questionnaires and is one of the most consistent predictors of pain intensity, disability and poor recovery after injury and surgery. It is not a character flaw. It is a learned response to threat that can be changed.

The fear avoidance model

The fear avoidance model describes how an episode of acute pain can lead either to recovery or to a self sustaining cycle of disability. After an injury, a person who interprets the pain as manageable and not dangerous continues to move, gradually resumes activity and recovers. A person who catastrophizes about the pain develops fear of pain and of movement, avoids activities believed to be harmful and becomes hypervigilant to bodily sensations. Avoidance prevents the person from learning that movement is safe, leads to deconditioning and loss of valued roles, and feeds low mood and disability, all of which make pain more likely to persist. Each turn of the cycle strengthens the next.

What changes in chronic pain

Persistent pain is accompanied by changes across the nervous system. In the spinal cord, the mechanisms of central sensitization described in Section 2 may persist. In the brain, imaging studies report altered activity and connectivity, with a relative shift in some people from sensory regions toward emotional and learning circuits involving the prefrontal cortex, nucleus accumbens and amygdala as pain becomes chronic. Some studies report altered body maps in the somatosensory cortex and reduced precision in sensing the painful body part, as well as reductions in gray matter in specific regions; several of these changes partially reverse when pain is successfully treated. These findings must be interpreted carefully: they come largely from group averages, they do not diagnose individual patients, and they do not show permanent damage. They do show that chronic pain involves a changed nervous system, which is a reason for hope because nervous systems keep changing.

The motor system adapts too. People in pain often move differently: they brace, guard, move more slowly and co-contract trunk muscles. In the short term this protects injured tissue. In the long term it can increase load, reduce variability of movement and become a habit that persists after the original reason has gone.

Treatment that matches the model

Because chronic pain has several drivers, single treatments rarely solve it. Clinical guidelines for chronic low back pain consistently recommend nonpharmacological care first: education and reassurance, exercise therapy, and psychologically informed approaches such as cognitive behavioral therapy, acceptance and commitment therapy and mindfulness based programs. Many guidelines also list spinal manipulation, massage, acupuncture, yoga and tai chi as options, generally with small to moderate effects. Multidisciplinary rehabilitation, combining physical, psychological and workplace elements, helps patients with more severe disability. Medicines play a supporting role. Nonsteroidal anti-inflammatory drugs have small effects for back pain; duloxetine is an option for some patients; and opioids are not recommended as first line treatment for chronic noncancer pain because their long term benefits are small and their risks, including dependence, overdose and opioid induced hyperalgesia, are significant.

Graded approaches put the fear avoidance model into practice. In graded activity, the patient and clinician set a baseline for a valued activity and increase it on a planned schedule regardless of day to day pain fluctuations. In graded exposure, the patient builds a hierarchy of feared movements (perhaps bending to lift a laundry basket) and confronts them step by step, so that expectations of harm are tested and corrected. Pacing helps people who swing between overactivity on good days and collapse on bad days to spread activity more evenly.

Section 5Explaining pain to patients

Why explanation is treatment

Every patient arrives with an explanation for their pain, whether or not they say it aloud: "my disc has slipped," "my spine is crumbling," "something is out of place," "I must have done serious damage." These beliefs matter because the brain uses them as evidence when it decides how much danger the body is in. A frightening explanation can increase threat, fear, guarding and pain; an accurate, reassuring explanation can reduce them. For this reason, explaining pain is not a courtesy before treatment begins. It is part of the treatment.

Pain neuroscience education (also called therapeutic neuroscience education) is a structured approach to teaching people about the biology of pain, developed by clinicians and researchers from the late 1990s onward. Its goal is reconceptualization: helping a person shift from understanding pain as a measure of tissue damage to understanding it as a protective output of the nervous system that can become overprotective, and that can be retrained. Typical core messages are:

• Pain is real, and it is produced by the brain in every case, including after an obvious injury.
• Pain is a protector. It depends on how much danger the brain perceives, not on how much damage there is.
• Hurt does not always mean harm. After the first few weeks, pain is a poor guide to the state of the tissues.
• The nervous system can become more sensitive over time, like a car alarm that goes off when someone walks past.
• Many things affect the alarm: sleep, stress, mood, beliefs, fitness, the context of movement and the words people hear.
• The system can change. Gradual, safe exposure to movement and activity teaches it to be less protective.

Metaphors and tools that help

Metaphors carry complex ideas quickly and stick in memory. Common examples include the oversensitive alarm system; a volume knob that can be turned up or down by the brain; the protectometer, a gauge whose needle is pushed toward protection by "danger in me" (DIMs) and toward relaxation by "safety in me" (SIMs); and the idea that tissues are like a garden that grows back stronger with sensible loading. Simple drawings of the nervous system, a list of the patient's own DIMs and SIMs, and stories of others who recovered can make the ideas personal.

Explanation works best as a conversation rather than a lecture. Begin by asking what the person believes is wrong and what worries them most. Validate the pain as real. Give a small number of key messages in plain language. Check understanding with teach back ("Can you tell me in your own words what we talked about?"). Link the explanation immediately to action, so that understanding becomes a reason to move, sleep, pace and return to valued activities. Pay attention to health literacy, culture and the person's own goals.

The words we use

Clinicians' words have power, and some common phrases act as nocebos. Describing a spine as "degenerated," a disc as "slipped" or a joint as "bone on bone," or telling a patient never to bend again, can increase fear and disability even when meant kindly. Many of these phrases describe imaging findings that are common in people without pain. The table offers alternatives that are accurate and less threatening.

Imaging findings in people without pain

One of the most useful facts for explaining back pain is how often imaging shows "abnormal" findings in people who have no pain at all. A large systematic review of MRI and CT studies in people without back pain found that disc degeneration and disc bulges are common from early adulthood and become the norm with age. Findings such as disc protrusions, annular fissures and facet degeneration are also common. This is why guidelines advise against routine imaging for low back pain without red flags: it rarely changes management, and labels from the report can increase fear, prompt more procedures and slow recovery.

Does pain education work?

Randomized trials and systematic reviews suggest that pain neuroscience education produces small to moderate short term improvements in pain, disability, catastrophizing and fear of movement in people with chronic musculoskeletal pain. Effects are larger and more lasting when education is combined with active treatment such as exercise or graded activity, and smaller when education is given alone. Education is not a replacement for movement; it is a way to make movement feel safe enough to do. Some patients find the message hard to accept at first, especially if it sounds like their pain is being dismissed, which is why validation and careful wording matter.

Several related approaches build on the same ideas. Cognitive functional therapy combines individualized explanation with guided exposure to feared movements and changes in protective movement habits; a large trial reported meaningful and sustained improvements in disability for people with chronic low back pain. Pain reprocessing therapy teaches patients to reappraise pain as safe and to approach sensations with curiosity rather than fear; an early trial reported that many participants became pain free or nearly so, though the findings need replication. Across approaches, the common thread is that changing what pain means to a person can change how much it hurts and how much it limits life.

Explaining pain in manual and holistic care

Manual therapists, including chiropractors, osteopaths and physical therapists, spend a great deal of time with patients in pain and have many chances to shape their beliefs. Historically, some explanations in these professions emphasized structural ideas such as bones "out of place," misalignment or instability. Contemporary education across the manual therapy professions increasingly favors explanations that are accurate to current neuroscience: hands-on treatment as a way to reduce sensitivity and protective muscle activity in the short term, open a window for movement and build confidence, rather than as a way to put something back in place. This framing respects the experience of patients who feel better after treatment while avoiding messages that create dependence or fear. Holistic practices such as yoga, tai chi, meditation and breathing training fit naturally with pain neuroscience because they work directly with attention, threat appraisal, stress physiology and confident movement, and several have modest supporting evidence for chronic low back pain.

Words worth knowing

Exam Logic and Red Flags

Most people who walk into a clinic with back or neck pain have a problem that is painful but not dangerous, and a small number have something that will not wait: a tumor, an infection, a fracture, a compressed cauda equina or a torn artery. The skill that separates a safe clinician from a risky one is not a memorized list of warning signs but a way of thinking: asking the right questions, choosing tests that actually change the odds, and knowing when to stop treating and send the patient elsewhere. This chapter teaches that way of thinking, from the first question in the history to the wording of a referral letter.

Section 1History and the questions that matter

Why the history comes first

The clinical history is the single most powerful diagnostic tool in spinal care. In general medicine, studies going back decades have found that the history alone points to the correct diagnosis in the large majority of cases, with the physical examination and tests adding confirmation rather than discovery. For spinal pain this is even more true, because most structures that can hurt (discs, facet joints, ligaments, muscles) cannot be reliably told apart by palpation or by imaging. What the history does extremely well is sort the patient into a category and estimate the probability of serious disease before a single test is performed. That estimate, called the pretest probability, governs how every later finding should be interpreted.

A good history has two goals running in parallel. The first is diagnostic: what is likely causing this pain, and is anything dangerous going on? The second is relational: who is this person, what do they fear, what do they expect, and what matters to them? Clinicians who neglect the second goal often miss the information that predicts how the patient will actually do, which is the subject of Section 4.

Diagnostic triage: three bins

International guidelines for low back pain, and increasingly for neck pain, ask the clinician to begin with diagnostic triage, sorting each patient into one of three groups:

1. Serious spinal pathology, also called specific or nonmechanical pathology: malignancy, infection, fracture, cauda equina syndrome, inflammatory arthritis such as axial spondyloarthritis, and visceral or vascular causes referring pain to the back. In primary care this group is small, generally estimated at around 1 percent of acute low back pain presentations, with osteoporotic vertebral fracture the most common member and spinal infection the rarest.
2. Radicular syndromes: pain, and often sensory or motor change, arising from irritation or compression of a nerve root, most often from disc herniation or foraminal or central stenosis. Estimates vary with definitions, but roughly 5 to 10 percent of patients with low back pain fall here.
3. Nonspecific low back pain (or nonspecific neck pain): pain for which no specific pathoanatomic cause can be reliably identified. This is about 90 percent of presentations. The word nonspecific does not mean imaginary or trivial; it means that current tools cannot assign the pain to one tissue with confidence, and that knowing the tissue would not change management much.

The value of triage is that each bin leads to a different plan. Serious pathology needs investigation or referral. Radicular syndromes need a neurological examination, monitoring for progression, and sometimes imaging or surgical opinion. Nonspecific pain is managed with reassurance, advice to stay active, and conservative care such as exercise and manual therapy, while watching for psychosocial barriers to recovery.

The structure of the history

A complete musculoskeletal history has a predictable skeleton. Students who learn the skeleton can adapt it to any complaint:

• Chief complaint in the patient's own words, and the reason they came today rather than last month.
• History of present illness, organized by a mnemonic such as OPQRST (below).
• Past medical history: prior episodes and how they resolved, prior spinal surgery, cancer, osteoporosis, diabetes, inflammatory disease, cardiovascular disease.
• Medications: especially long-term corticosteroids (fracture and infection risk), anticoagulants (bleeding risk with procedures, spinal epidural hematoma), immunosuppressants, and current pain medicines and their effect.
• Family history: inflammatory arthritis, early cardiovascular disease, cancers.
• Social history: occupation and physical demands, smoking, alcohol and drug use (including injection drug use, an infection risk), exercise, sleep, home situation.
• Review of systems: a quick survey of body systems for symptoms the patient may not connect to the back, such as fever, weight loss, urinary changes, abdominal pain or cough.
• Patient perspective: ideas, concerns and expectations (sometimes abbreviated ICE).

OPQRST and what each answer tells you

The history of present illness is easier to remember with a mnemonic. OPQRST is one of the most widely used, and each letter carries real diagnostic weight.

Many clinicians add associated symptoms (numbness, weakness, bladder or bowel change, fever, dizziness) and previous episodes. A useful habit is to finish the OPQRST sequence with an open question: "Is there anything else about this that worries you, or that I have not asked?"

Mechanical versus nonmechanical pain

The most useful single distinction in the spinal history is whether pain behaves mechanically. Mechanical pain varies with posture, movement and load: it is provoked by certain positions and eased by others, it is often worse with prolonged sitting or bending and better with a change of position, and it may be stiff in the morning for a short time before loosening. Nonmechanical pain does not follow these rules. It may be constant, unrelated to movement, progressive over weeks, present at rest and at night, or accompanied by systemic features such as fever, sweats, malaise or weight loss. Nonmechanical pain is not a diagnosis but a prompt to look harder for serious pathology or for pain referred from an internal organ.

Night pain is a classic example of a finding that needs careful interpretation. Many people with ordinary mechanical back pain report waking at night when they roll over; that pain eases when they change position and is mechanical. The worrying pattern is pain that is constant through the night, keeps the patient up regardless of position, and is getting worse over time. Asking "what happens when you change position?" separates the two.

Radicular pain versus somatic referred pain

Students commonly assume that any leg pain from the back is "sciatica" from a pinched nerve. In fact two quite different mechanisms produce pain in the limb. Somatic referred pain arises from a spinal structure such as a disc, facet joint or sacroiliac joint and is felt in a distant region that shares segmental innervation. It is deep, dull, aching and hard to localize, usually stays above the knee, and is not accompanied by neurological deficit. Radicular pain arises from the dorsal root or dorsal root ganglion itself, usually when an inflamed or compressed root fires ectopically. It is sharp, shooting or lancinating, travels in a narrow band, often extends below the knee (or below the elbow in the arm), and is commonly worse in the limb than in the back. When conduction along the root is actually blocked, the patient also has radiculopathy: objective loss of sensation, strength or reflexes in that root's territory. Radicular pain and radiculopathy often occur together, but either can occur alone.

Claudication: neurogenic or vascular?

Leg pain brought on by walking has two classic causes that must not be confused. Neurogenic claudication comes from lumbar spinal stenosis: narrowing of the central canal or lateral recesses squeezes the cauda equina roots, especially in extension, when the canal is smallest. Vascular claudication comes from peripheral artery disease: narrowed arteries cannot supply enough blood to working calf muscles. The history usually separates them before any test is done.

Asking well

The quality of answers depends on the quality of questions. Begin open ("Tell me about your back") and narrow later ("Does the pain go below your knee?"). Avoid leading questions that suggest the answer, and avoid medical jargon: patients often do not know what "radiating" or "incontinence" means, so ask directly and plainly ("Have you had any trouble passing urine, or any leaking you could not control?"). Sensitive questions, such as those about bladder, bowel and sexual function, are best introduced with a brief explanation: "I ask everyone with back and leg pain about this because, rarely, nerves to the bladder can be affected." Normalizing the question makes truthful answers more likely and avoids alarming the patient.

It also helps to ask what the patient thinks is going on. A person who believes their spine is "crumbling" or that a disc has "slipped out" will move and recover differently from one who believes they have strained something that will heal. The patient's own model of the problem is part of the diagnosis.

Section 2Physical examination logic

The examination tests hypotheses

A physical examination is not a ritual in which every test is performed on every patient. It is a set of experiments designed to test the hypotheses raised by the history. If the history suggests a nonspecific mechanical problem, the examination looks for movement patterns, provocation and a normal neurological screen. If the history suggests a nerve root, the examination maps strength, reflexes and sensation and tests neural tension. If the history raises serious pathology, the examination looks for fever, weight loss, a mass, focal bone tenderness or an abnormal neurological picture, and the next step may be a referral rather than further testing.

A typical spinal examination proceeds in a consistent order so nothing important is skipped:

1. General observation and vital signs: how the patient sits, stands and moves, facial expression, gait, temperature, pulse, blood pressure. A fever or tachycardia changes everything that follows.
2. Inspection: posture, alignment, muscle wasting, skin changes, scars, swelling.
3. Palpation: tenderness, temperature, muscle tone, and focal bony tenderness over a spinous process.
4. Range of motion: active first, then passive, noting the amount, the quality, and whether movement reproduces the familiar pain.
5. Neurological examination: strength (myotomes), reflexes, sensation (dermatomes), and screening for upper motor neuron signs.
6. Special tests: neural tension tests, provocation tests, joint stability tests and others chosen for the hypothesis.
7. Related regions: the hip for low back pain, the shoulder for neck pain, the abdomen and peripheral pulses when vascular or visceral causes are possible.

The numbers behind a test: sensitivity and specificity

Every diagnostic test, from a blood test to a straight leg raise, can be described by how it performs against a reference standard (the best available way of knowing the truth, such as surgical findings or MRI). The results fall into a two by two table. Sensitivity is the proportion of people who have the condition and test positive: true positives divided by everyone with the condition. Specificity is the proportion of people without the condition who test negative: true negatives divided by everyone without it.

Two rules of thumb follow. A highly sensitive test rarely misses the condition, so a negative result helps rule it out: SnNout. A highly specific test rarely fires in people without the condition, so a positive result helps rule it in: SpPin. Students often reverse these. The trick is to remember that sensitivity is about the sick (how many of them it catches), so its strength lies in what a negative result means.

Likelihood ratios and pretest probability

Sensitivity and specificity describe a test, but the clinician wants to know about the patient: given this result, how likely is the condition now? Likelihood ratios answer that. The positive likelihood ratio (LR+) is sensitivity divided by one minus specificity; the negative likelihood ratio (LR−) is one minus sensitivity divided by specificity. An LR+ above 10 or an LR− below 0.1 produces large, often decisive shifts in probability. Values of 5 to 10 or 0.1 to 0.2 produce moderate shifts. Values between about 0.5 and 2 barely move the estimate at all, and many orthopedic tests sit in that unhelpful zone.

The result of any test must be combined with the pretest probability, the estimated chance the patient had the condition before the test. This is Bayesian reasoning, and it explains why the same test means different things in different patients. Suppose a test has an LR+ of 5. In a patient with a 1 percent pretest probability, a positive result raises the probability only to about 5 percent: the condition is still unlikely. In a patient with a 40 percent pretest probability, the same positive result raises it to roughly 77 percent. This is why screening low-risk people with imperfect tests produces mostly false positives, and why the history, which sets the pretest probability, is so important.

Reliability and test clusters

A test also has to give the same answer when repeated. Reliability is the agreement between two examiners (inter-rater) or one examiner on two occasions (intra-rater), often reported as the kappa statistic, where 0 is chance agreement and 1 is perfect. Many traditional spinal examination procedures have poor reliability. Static palpation for vertebral position and motion palpation for segmental stiffness generally show low to fair agreement between examiners, while palpation for pain provocation (does pressing here reproduce the pain?) tends to be more reliable. This does not mean palpation is worthless: it guides where a clinician applies treatment and can reproduce symptoms. It means palpation findings should not be treated as precise diagnostic measurements.

Because individual tests are often weak, researchers combine them into clusters. For example, a cluster of sacroiliac joint provocation tests (distraction, thigh thrust, compression, sacral thrust and others) has better diagnostic accuracy when three or more are positive than any single test alone. Clusters also exist for cervical radiculopathy, combining Spurling's test, the upper limb tension test, distraction relief and reduced cervical rotation. Thinking in clusters reflects a general truth: confidence comes from several pieces of consistent evidence, not from one dramatic sign.

Common spinal special tests

The neurological screen

Whenever a patient has limb symptoms, or the history raises any neurological concern, the clinician performs a neurological screen. The logic is simple: each nerve root supplies a characteristic set of muscles (a myotome), a band of skin (a dermatome), and a deep tendon reflex. A lesion of one root produces weakness, sensory change and reflex change that line up with that root. A pattern that does not line up with any single root suggests a peripheral nerve lesion, several roots, or a central problem.

Strength is graded on the 0 to 5 Medical Research Council scale: 0 is no contraction, 1 a flicker, 2 movement with gravity eliminated, 3 movement against gravity, 4 movement against some resistance, and 5 normal strength. Reflexes are graded 0 (absent), 1+ (diminished), 2+ (normal), 3+ (brisk, may spread), and 4+ (very brisk, often with sustained clonus).

Remember that a lumbar disc herniation usually affects the root that exits one level below the disc. The common posterolateral L4 to L5 herniation compresses the traversing L5 root; an L5 to S1 herniation compresses S1. A far lateral herniation, by contrast, catches the root exiting at that same level. The L4 to L5 and L5 to S1 levels together account for the great majority of lumbar disc herniations.

Upper motor neuron signs

Nerve root and peripheral nerve lesions are lower motor neuron problems: they cause weakness, reduced or absent reflexes, reduced tone and, over time, muscle wasting. Lesions of the spinal cord or brain are upper motor neuron problems: they cause weakness with increased tone (spasticity), brisk reflexes, clonus, and pathological reflexes. In spinal practice the most important upper motor neuron condition is degenerative cervical myelopathy, compression of the cervical cord by disc, osteophyte and thickened ligament. It is the most common cause of spinal cord dysfunction in older adults and is frequently missed early. Patients describe clumsy hands, difficulty with buttons or handwriting, numb hands that do not follow a single dermatome, and a vague unsteadiness of gait.

Section 3Red flags: cancer, infection, fracture, cauda equina, vascular

What a red flag is, and what it is not

Red flags are features of the history or examination associated with a higher probability of serious pathology. They are prompts to think, not diagnoses. Research has shown two uncomfortable facts about them. First, red flags are very common: in primary care, most patients with low back pain have at least one, and nearly all of them do not have serious disease. Second, most individual red flags have poor diagnostic accuracy, with likelihood ratios close to 1. Treating every red flag as an alarm would send most back pain patients for imaging and blood tests, generating false positives, cost and anxiety. Ignoring them would occasionally miss a cancer or an abscess.

The solution is to weigh red flags in context. A few flags carry real weight on their own (a history of cancer is the strongest example). Most gain meaning only in combination, or when the pain is nonmechanical, or when the patient fails to improve as expected over several weeks. Good practice also includes safety netting: telling the patient which new symptoms should make them return or seek emergency care.

Malignancy

Spinal malignancy in a patient with back pain is usually metastatic, because the vertebral bodies are rich in red marrow and the vertebral venous plexus (Batson's plexus) is valveless, allowing tumor cells from the pelvis, abdomen and chest to seed bone. The cancers that most often spread to bone are prostate, breast, lung, kidney and thyroid. Multiple myeloma, a primary marrow cancer, also commonly presents with back pain and vertebral fractures in older adults.

• Previous history of cancer is the most informative single flag, with positive likelihood ratios reported in the range of roughly 7 to 15. A patient with new back pain and a history of breast, prostate or lung cancer needs a low threshold for imaging, even years after apparent cure.
• Age over 50, unexplained weight loss, failure to improve after about a month of appropriate care, and constant or progressive pain at night add modestly to suspicion, particularly in combination.
• Thoracic pain deserves extra respect, since the thoracic spine is a common site of metastasis and is less often the site of ordinary mechanical pain.

The main danger of a missed spinal metastasis is metastatic spinal cord compression, which can cause permanent paralysis. Back pain usually precedes neurological signs by weeks; new neurological symptoms in a patient with known cancer are an emergency.

Infection

Spinal infection includes vertebral osteomyelitis, discitis (infection of the disc space, often together as spondylodiscitis) and spinal epidural abscess. Bacteria reach the spine mainly through the blood, and Staphylococcus aureus is the most common organism. Tuberculosis of the spine (Pott disease) remains important worldwide.

Risk factors are the key to suspicion: injection drug use, immunosuppression (including long-term corticosteroids, diabetes, HIV and chemotherapy), a recent bacterial infection such as a urinary tract or skin infection, recent spinal surgery or injection, an indwelling catheter or hemodialysis. Fever is a helpful sign when present but is absent in a substantial minority, so its absence does not exclude infection. The classic triad of epidural abscess, namely back pain, fever and neurological deficit, is present in only a small proportion at first presentation; by the time all three are present, harm may already be done. Severe, unrelenting local pain with focal spinous process tenderness in a patient with risk factors should prompt urgent blood tests (an elevated erythrocyte sedimentation rate and C-reactive protein are typical) and MRI.

Fracture

Vertebral fracture is the most common serious pathology found in primary care back pain. Most are osteoporotic compression fractures of the thoracolumbar spine, which may occur with trivial loading such as bending, coughing or lifting a light object, and some occur with no remembered event at all. The flags that perform best are older age (especially over 70), prolonged corticosteroid use, significant trauma for the patient's age, and a visible contusion or abrasion. In combination, these raise the probability considerably. Women after menopause and anyone with known osteoporosis, a previous fragility fracture or a low body weight carry higher risk.

In the neck, high-energy trauma raises the question of cervical fracture or instability. Two well validated decision rules help decide whether imaging is needed in alert, stable trauma patients. The Canadian C-spine Rule requires imaging for any high-risk factor (age 65 or older, a dangerous mechanism, or tingling in the limbs), then uses low-risk features to decide whether range of motion can be safely assessed, and finally checks whether the patient can actively rotate the neck 45 degrees to each side. The NEXUS criteria allow clinicians to skip imaging only if all five are absent: posterior midline tenderness, intoxication, altered alertness, a focal neurological deficit, and a painful distracting injury. These rules are highly sensitive, meaning a negative rule reliably excludes important injury.

Cauda equina syndrome

The spinal cord ends at the conus medullaris, usually at about the L1 to L2 vertebral level in adults. Below that, the lumbar and sacral roots run down the canal as the cauda equina ("horse's tail"). Massive central compression of these roots, most often by a large central disc herniation at L4 to L5 or L5 to S1, produces cauda equina syndrome. Less common causes include tumor, epidural abscess, hematoma, fracture and severe stenosis. Because the sacral roots S2 to S4 supply the bladder, bowel, sexual organs and the skin of the perineum, the hallmark features are:

• Saddle anesthesia: numbness or altered sensation of the perineum, buttocks and inner thighs (the area that would touch a saddle). Patients may notice it when wiping after using the toilet.
• Bladder dysfunction: difficulty starting urination, a weak stream, loss of the sensation of a full bladder, or painless urinary retention with overflow incontinence. Urinary retention is the most consistent finding in established cases.
• Bowel dysfunction: loss of control or of sensation, reduced anal tone.
• Sexual dysfunction: new erectile dysfunction or loss of genital sensation.
• Bilateral leg symptoms: bilateral sciatica, or progressive weakness in both legs.

Clinicians distinguish an early, incomplete stage (CES-I, with altered urinary sensation or function but preserved voluntary control) from a later stage with retention (CES-R), when the bladder has become insensate and overflows. Outcomes are much better when decompression occurs before retention develops, so any suspicion warrants same day emergency referral for MRI and surgical assessment. Measuring the volume of urine left in the bladder after voiding (post-void residual) is often used in hospital: a large residual supports retention. No clinician should wait for every feature to appear.

Vascular causes

Two vascular emergencies are particularly relevant to spinal practice.

Abdominal aortic aneurysm (AAA) is a dilation of the abdominal aorta, most common in men over 65 who smoke or have smoked, with family history and hypertension adding risk. An expanding or leaking aneurysm can cause deep, constant back, flank or abdominal pain that does not change with movement. A pulsatile abdominal mass, low blood pressure, fainting or pallor suggests rupture, which is immediately life threatening. Back pain that is nonmechanical in an older smoker should prompt an abdominal examination.

Cervical artery dissection is a tear in the inner wall of the vertebral or internal carotid artery that allows blood to track into the wall, narrowing the vessel or forming a clot that can travel to the brain and cause a stroke. It is rare (a few cases per 100,000 people per year) and often occurs in younger adults, sometimes after minor neck trauma or for no clear reason. Early symptoms are usually unusual neck pain or headache, often described as unlike anything experienced before, which may precede stroke symptoms by hours to days. Signs of brainstem ischemia from vertebral artery involvement are often taught as the "5 Ds and 3 Ns": dizziness, diplopia (double vision), dysarthria (slurred speech), dysphagia (trouble swallowing) and drop attacks; plus nystagmus, nausea and numbness, with ataxia of gait. Internal carotid dissection may cause a partial Horner syndrome (drooping eyelid and small pupil), pulsatile tinnitus or cranial nerve signs.

The relationship between cervical manipulation and artery dissection has been debated for decades. Case reports describe dissection and stroke after neck manipulation. Population studies have found that people who later have a vertebrobasilar stroke are more likely to have seen a chiropractor in the preceding days, but similarly more likely to have seen a primary care physician, which suggests that many patients with an early dissection seek care for the neck pain and headache it causes. Current evidence does not establish that manipulation causes dissection, nor can it exclude a small risk. Professional guidance therefore emphasizes careful screening of every patient with neck pain or headache for features of vascular pathology, recognizing that the best protection is identifying a dissection in progress, and immediate emergency referral when it is suspected. Pre-manipulative positional tests of the vertebral artery have poor validity and are no longer recommended as a safety screen on their own.

Section 4Yellow flags and risk of chronicity

Why psychosocial factors predict outcome

Most acute low back pain improves substantially within about six weeks, but recovery is often incomplete and recurrences are common, and a minority of patients develop persistent pain and disability lasting beyond three months. Strikingly, the best predictors of who will develop long-term disability are usually not findings on imaging or examination but psychological and social factors. These are called yellow flags. Unlike red flags, they do not signal danger to the body; they signal risk to recovery. Chapter 6 explained the neuroscience: fear, threat and attention shape how the nervous system processes nociception, and avoidance behavior can turn a healing injury into a long-term problem.

The flag system

Clinicians have extended the color metaphor to describe different kinds of barriers to recovery.

The key yellow flags

• Fear avoidance beliefs: the belief that pain means harm and that activity should be avoided until pain is gone. Avoidance leads to deconditioning, loss of confidence and more pain on return to activity.
• Pain catastrophizing: an exaggerated negative focus on pain, with rumination ("I can't stop thinking about it"), magnification ("something terrible will happen") and helplessness ("there is nothing I can do").
• Kinesiophobia: an excessive fear of movement and reinjury.
• Low expectations of recovery: the patient's own prediction that they will not get better is one of the strongest single predictors of poor outcome.
• Depression, anxiety and distress, including poor sleep.
• Passive coping: reliance on rest, medication and treatments done to the patient rather than active self-management.
• Social and work factors: low job satisfaction, heavy physical work without control over pacing, time off work, and financial strain.

The clinician's own behavior can add or remove yellow flags. Telling a patient their spine is "out," "degenerating" or "unstable," or showing them an MRI full of age-normal findings without context, can create fear that outlasts the original injury. Imaging findings such as disc bulges and degeneration are extremely common in people without any pain and become more common with every decade of age. Language is a clinical tool, and Chapter 6 discussed how to use it well.

Screening tools

Several validated questionnaires help identify patients at risk of chronicity:

• The STarT Back Tool is a brief nine-item questionnaire that classifies patients with low back pain as low, medium or high risk of persistent disabling pain. It combines physical items (leg pain, comorbid pain, function) with a psychosocial subscale (fear, anxiety, catastrophizing, low mood, bothersomeness). In stratified care, low-risk patients receive advice and reassurance with minimal treatment, medium-risk patients receive physical therapy style care, and high-risk patients receive care that combines physical treatment with psychologically informed approaches. A large randomized trial found stratified care improved disability outcomes and was cost effective compared with usual care.
• The Örebro Musculoskeletal Pain Screening Questionnaire covers similar ground, with a strong emphasis on work.
• The Fear Avoidance Beliefs Questionnaire and the Tampa Scale of Kinesiophobia measure fear of activity and movement.
• The Pain Catastrophizing Scale measures rumination, magnification and helplessness.
• The PHQ-9 screens for depression, and its item on thoughts of self-harm must be acted upon if positive.

Section 5When and how to refer

Referral is part of good care

Every clinician works within a scope of practice and a range of competence. Referring a patient is not a failure but a core professional skill, and in spinal care it protects patients in three situations: when serious pathology is suspected, when the patient is not improving as expected, and when another professional can offer something the patient needs, such as surgery, medication, psychological care or specialist rehabilitation. Many patients benefit from co-management, in which the original clinician continues appropriate care while another provider manages a separate aspect of the problem.

Matching urgency to the problem

The most important decision in a referral is how fast it must happen.

An emergency referral means the patient goes directly to an emergency department or calls emergency services; the clinician should call ahead where possible and never send a patient with suspected stroke or cauda equina syndrome to "book an appointment." An urgent referral means contacting the patient's physician or appropriate specialist the same day and making sure the patient is seen within a day or a few days. A routine referral can go through normal channels. When in doubt between two tiers, choose the faster one.

Imaging and laboratory tests

Major guidelines agree that routine imaging is not recommended for acute nonspecific low back pain without red flags. Imaging in this group does not improve outcomes, exposes patients to radiation (for radiographs and CT), finds incidental abnormalities that may lead to further tests or surgery, and can increase fear and disability. Imaging is appropriate when red flags suggest serious pathology, when there are severe or progressive neurological deficits, or when a patient with persistent radicular symptoms is a candidate for an injection or surgery. MRI is the preferred study for suspected infection, malignancy, cauda equina syndrome and nerve root compression; plain radiographs are useful for suspected fracture, deformity and some forms of arthritis.

Common laboratory tests in spinal workups include a complete blood count, erythrocyte sedimentation rate and C-reactive protein (markers of inflammation that are usually raised in infection and often in malignancy), serum protein electrophoresis for myeloma, prostate specific antigen, and HLA-B27 when spondyloarthritis is suspected. These tests are ordered by clinicians whose scope includes them, which is another reason for clear referral pathways.

How to write a referral

A good referral is brief, specific and easy to act on. A widely used structure in health care is SBAR:

The referral should include the clinician's contact details and a clear statement of whether care will continue, pause or stop. Records must document the findings, the reasoning, the advice given and the safety netting instructions. If a patient declines an emergency referral, the conversation, the explanation of risk and the patient's decision should all be recorded, and the clinician should still contact appropriate services where duty of care requires it.

Talking to the patient about referral

Patients can find referral frightening. Explain why it is being made, what the other clinician will do, and how likely serious disease actually is, without either minimizing a real risk or catastrophizing a small one. For example: "Most back pain like yours is not serious, but because of your history I want a physician to check for anything we should not miss. If anything changes before then, here is what to do." Clear safety netting advice is an essential part of every plan, whether or not a referral is made.

Words worth knowing

Posture, Gait and Movement

Posture and movement are where the spine meets everyday life: how we stand at a counter, walk to the car, lift a child or spend eight hours at a desk. Clinicians have long tried to read health from posture, yet the research shows that how a person looks standing still predicts surprisingly little, while how they walk and move tells a great deal. This chapter teaches what normal posture and gait look like, which abnormal patterns carry real diagnostic meaning, how to assess movement and motor control, and what the evidence actually says about ergonomics and healthy movement habits.

Section 1Static posture and its limits as a diagnostic tool

The ideal alignment

Posture is the position of the body and the relationship of its segments at any moment. Clinicians usually start with static standing posture viewed from the side, from behind and from the front. The traditional reference for side view is a vertical plumb line, imagined as a weighted string hanging from above. In the commonly taught ideal alignment, the line passes:

• through the earlobe (external auditory meatus);
• through the bodies of the cervical vertebrae;
• through the acromion (tip of the shoulder);
• roughly through the bodies of the lumbar vertebrae;
• through or just behind the hip joint, near the greater trochanter;
• slightly in front of the center of the knee joint;
• slightly in front of the lateral malleolus (ankle).

This alignment is thought of as efficient because the line of gravity falls close to the joint axes, so little muscle effort is needed to stay upright. Because the line falls just in front of the knee, the knee is held in extension by gravity and its ligaments; because it falls just behind the hip, the hip is pushed into extension and restrained by the strong iliofemoral ligament; and because it falls in front of the ankle, the calf muscles work at a low level to stop the body tipping forward. This is why a person standing quietly sways gently, with calf muscle activity rising and falling.

Classic postural types

Physical therapists in the mid twentieth century described several common deviations from the ideal, and these categories are still taught because they provide a shared vocabulary.

From behind, clinicians look for asymmetries in the level of the shoulders, scapulae, iliac crests and gluteal folds, and for lateral curves of the spine. From the front, they note head tilt, shoulder level, pelvic level, knee alignment (genu valgum or varum) and the arches of the feet.

Why static posture predicts so little

For much of the twentieth century, clinicians assumed that deviations from ideal posture caused pain by overloading tissues, and that correcting posture would relieve it. The evidence has been humbling. Large studies and systematic reviews have generally found that sagittal spinal curves, such as the degree of lordosis or kyphosis, are weakly or not at all associated with back pain in the general population. Adolescents with "slumped" posture do not reliably develop more back pain than their upright peers. People with and without neck pain overlap so much in head posture that measuring it cannot tell them apart. Several reasons explain this:

• Normal variation is wide. Healthy, pain-free people show a broad range of curve sizes, pelvic tilts and shoulder positions.
• Posture is not static. People shift position constantly. A single snapshot in the clinic may not represent how a person sits or stands through the day.
• Tissues adapt. Bones, joints and muscles remodel to the loads they habitually bear, so a long-standing posture is usually well tolerated.
• Pain is multifactorial. As Chapter 6 showed, pain depends on the nervous system's interpretation of threat as much as on mechanical load.
• Measurement is unreliable. Visual posture rating shows only modest agreement between clinicians.

Posture is still worth observing. A marked change can signal pathology: a new list to one side (an antalgic lateral shift) often accompanies acute disc irritation; a rigid, flexed posture may reflect stenosis or spondyloarthritis; a progressive hump may reflect osteoporotic fractures. What posture does not do well is explain ordinary pain or predict who will develop it. A modern summary often taught to students is that "the best posture is the next posture": variety and movement matter more than holding any single position.

Forward head posture and "text neck"

Forward head posture describes a head positioned in front of the trunk, usually measured by the craniovertebral angle: the angle between a horizontal line through C7 and a line from C7 to the tragus of the ear. Smaller angles mean a more forward head. Biomechanical models estimate that flexing the head forward greatly increases the moment the neck extensors must resist; one widely quoted model suggested the effective load rises from about 5 kg in neutral to about 27 kg at 60 degrees of flexion. These calculations are physically reasonable, but the leap from load to injury is not. Neck tissues are designed to tolerate flexion, and studies comparing people with and without neck pain show only small and inconsistent differences in head posture. Long periods without changing position, rather than the angle itself, may matter more for comfort.

Crossed syndromes and muscle imbalance

The Czech neurologist Vladimir Janda proposed that certain muscles tend to become tight and others inhibited in predictable patterns. In the upper crossed syndrome, tight upper trapezius, levator scapulae and pectoral muscles cross with weak deep neck flexors and lower and middle trapezius and serratus anterior, producing forward head, rounded shoulders and scapular winging. In the lower crossed syndrome, tight hip flexors and lumbar extensors cross with weak abdominals and gluteal muscles, producing anterior pelvic tilt and increased lordosis. These models are useful clinical descriptions and have shaped much exercise practice. However, direct evidence that these imbalances cause pain, or that correcting them is the reason patients improve with exercise, is limited.

Leg length inequality and scoliosis screening

Leg length inequality may be anatomical (a true difference in bone length) or functional (an apparent difference from pelvic rotation, muscle tightness, foot pronation or joint contracture). Small differences of a few millimeters are almost universal and are not linked to pain. Larger differences, often cited as greater than about 20 mm, may be associated with back and hip problems. Measurement matters: tape measure methods have only moderate accuracy, and imaging is the reference standard for anatomical difference. Clinicians who identify a small "short leg" during supine leg checks should be cautious about attributing symptoms to it.

Scoliosis is a lateral curvature of the spine combined with vertebral rotation, defined radiographically as a Cobb angle of 10 degrees or more. The most common form is adolescent idiopathic scoliosis, which affects roughly 2 to 3 percent of adolescents, with larger, progressive curves much more common in girls. The Adams forward bend test reveals rotation as a rib hump or lumbar prominence when the patient bends forward; a reading of about 7 degrees or more on a scoliometer is a common threshold for radiographic referral. Curves tend to progress during growth, so skeletal maturity and curve size guide monitoring, bracing or surgery.

Section 2The gait cycle

Defining the cycle

Gait is the manner of walking. Walking is a repeating pattern, so it is described by its basic unit, the gait cycle: the interval from one event of one foot to the next occurrence of the same event in the same foot. By convention the cycle starts at initial contact of one foot (usually the heel in healthy walking) and ends at the next initial contact of the same foot. One gait cycle equals one stride. A step is the interval from initial contact of one foot to initial contact of the other, so one stride contains two steps.

Each limb alternates between two phases:

• Stance phase: the foot is on the ground. In normal walking speed it occupies about 60 percent of the cycle.
• Swing phase: the foot is in the air, moving forward. It occupies about 40 percent of the cycle.

Because each stance phase is longer than half the cycle, there are two periods when both feet are on the ground, called double limb support, each lasting about 10 percent of the cycle (20 percent in total). The rest of stance is single limb support, about 40 percent of the cycle, which exactly matches the swing time of the other leg. As walking speed increases, double support shrinks; at the transition to running it disappears and is replaced by a flight phase when neither foot touches the ground. The presence of double support is the defining feature of walking, and the presence of flight defines running.

The eight phases

The standard terminology used in clinical gait analysis divides the cycle into eight functional phases, five in stance and three in swing. Each has a job.

Spatiotemporal measures

Simple measures of time and distance describe gait well and are easy to collect with a stopwatch and tape.

Speed equals step length times cadence, so a person can walk faster by taking longer steps, quicker steps, or both. Many conditions shorten step length first.

Gait speed is so strongly linked to health that it is sometimes called the "sixth vital sign." In older adults, slower usual gait speed predicts falls, hospitalization, loss of independence and mortality. Values often cited are below about 0.6 m/s associated with dependence in daily activities, below about 0.8 m/s with limited community walking and greater risk, and at least about 1.0 m/s with independent community walking. Crossing a typical street before the signal changes needs about 1.2 m/s.

The three rockers

During stance the body rolls forward over the foot through three pivot points, called rockers, which keep forward motion smooth.

1. Heel rocker (first rocker), in loading response: the body rolls over the rounded heel. The ankle dorsiflexors (tibialis anterior and the toe extensors) contract eccentrically to lower the forefoot gently instead of letting it slap down.
2. Ankle rocker (second rocker), in midstance: the foot is flat and the tibia rotates forward over the ankle. The soleus and gastrocnemius contract eccentrically to control the speed of tibial advance.
3. Forefoot rocker (third rocker, also called the toe rocker), in terminal stance: the heel lifts and the body rolls over the metatarsal heads. The calf muscles now generate the largest burst of power in walking, and the toes extend at the metatarsophalangeal joints, tightening the plantar fascia (the windlass mechanism) to stiffen the foot into a lever.

What the muscles do

Much of walking relies on momentum and gravity, with muscles acting in short bursts, often eccentrically to control and decelerate motion rather than to drive it.

Trunk, pelvis and arms

The body's center of mass rises and falls about 4 to 5 cm in a smooth wave with each step, highest in midstance and lowest in double support, and shifts a few centimeters side to side toward each stance limb. The pelvis rotates forward on the side of the swinging leg, tilts slightly downward on the swing side, and the thoracic spine rotates in the opposite direction. The arms swing opposite to the legs, which helps to counterbalance the rotation of the pelvis and reduces the energy cost of walking. Reduced arm swing, reduced trunk rotation, or a stiff, blocked lumbar spine are all visible clues in gait observation.

Section 3Common gait patterns and what they mean

How to observe gait

Observing gait is part of every musculoskeletal and neurological examination, and it often starts before the formal examination, as the patient walks from the waiting room. A systematic approach helps. Watch from the front, the back and the side, ideally over a long enough distance to see several cycles, and ask the patient to turn, to walk on heels and toes, and to walk heel to toe in a line (tandem gait). Look at:

• the overall impression: speed, symmetry, smoothness, use of aids, and whether the patient looks steady;
• the base of support: narrow, normal or wide;
• step length and timing on each side;
• each joint in turn: foot contact, ankle, knee, hip, pelvis, trunk and arm swing;
• turning, starting and stopping.

Gait abnormalities arise from four broad causes: pain (the body shortens loading on the painful side), weakness (muscles cannot perform their job and the body substitutes another strategy), restricted joint motion (stiffness or contracture forces compensation), and neurological control problems (spasticity, rigidity, incoordination or sensory loss). Recognizing the pattern often localizes the problem quickly.

Antalgic gait

An antalgic gait (from the Greek for "against pain") is a limp in which the patient spends as little time as possible on the painful limb. The stance phase on the affected side is shortened, so the opposite leg swings through quickly and the step taken with the sound leg is short. The pattern is the same whatever the painful structure, from a sprained ankle to an arthritic hip or an irritated lumbar disc. With a painful hip, patients may also lean the trunk over the painful hip during stance, which moves the center of mass closer to the joint and reduces the compressive force the hip abductors must produce.

Trendelenburg gait

In single limb stance, the hip abductors of the stance leg, chiefly the gluteus medius and gluteus minimus, keep the pelvis level. These muscles are supplied by the superior gluteal nerve (L4 to S1). If they are weak, the pelvis drops on the opposite, swinging side: an uncompensated Trendelenburg gait. More often, patients compensate by leaning the trunk over the stance leg, which shifts the center of mass over the hip and reduces the abductor force needed: a compensated Trendelenburg gait (sometimes called a gluteus medius lurch). When the weakness is bilateral, the trunk sways side to side with every step, producing a waddling gait, typical of muscular dystrophies, other myopathies and bilateral hip dysplasia.

The Trendelenburg test examines the same mechanism in the clinic. The patient stands on one leg. Normally the pelvis on the lifted side stays level or rises slightly. If it drops, the test is positive, and the weakness is in the abductors of the stance leg, not the side that drops. Causes include superior gluteal nerve injury, L5 radiculopathy, hip osteoarthritis, hip dysplasia and gluteal tendinopathy.

Steppage gait and foot drop

If the ankle dorsiflexors are weak, the toes cannot be lifted in swing and the forefoot slaps down after heel strike. To clear the hanging foot, the patient lifts the knee and hip unusually high, as if climbing stairs: a steppage gait. Initial contact may be made with the forefoot or the whole foot instead of the heel. Common causes are common fibular (peroneal) nerve palsy at the fibular neck (from leg crossing, casts, prolonged squatting or knee injury), L5 radiculopathy, peripheral neuropathy and some motor neuron diseases. A milder dorsiflexor weakness produces only an audible foot slap in loading response.

Distinguishing fibular nerve palsy from L5 radiculopathy is a classic exercise in localization. Both weaken dorsiflexion and great toe extension. In L5 radiculopathy, ankle inversion (tibialis posterior, a tibial nerve muscle with L5 contribution) and hip abduction (gluteus medius, superior gluteal nerve) are often weak too, because they share the L5 root but not the fibular nerve. In fibular nerve palsy, inversion and hip abduction are spared, and back pain is usually absent.

Central nervous system gaits

The Romberg test separates the two ataxias. The patient stands with feet together, first with eyes open and then closed. Someone with cerebellar ataxia is unsteady even with eyes open. Someone with sensory ataxia can compensate with vision and is fairly steady with eyes open, but sways or falls with eyes closed: a positive Romberg sign. The test therefore assesses proprioception, not the cerebellum.

Mechanical and musculoskeletal patterns

Section 4Movement assessment and motor control

From posture to movement

Because static posture predicts so little, clinicians have increasingly turned to how people move: how they bend, squat, rise from a chair, balance and walk. Movement assessment serves three purposes. It measures function in a way that matters to patients (can they get out of a chair, climb stairs, walk to the shop?), it identifies specific impairments that can be trained, and it provides an outcome measure that can be repeated to track progress. The best movement tests are simple, standardized, reliable and have known normal values.

Functional performance tests

Movement screens

Several standardized screens observe fundamental movement patterns. The Functional Movement Screen scores seven tasks (deep squat, hurdle step, in-line lunge, shoulder mobility, active straight leg raise, trunk stability push-up and rotary stability) from 0 to 3, for a total out of 21. A total of 14 or below was proposed as a marker of increased injury risk in athletes, but later studies have found that the screen predicts injury poorly. The Y Balance Test measures how far a person can reach with one leg in three directions while balancing on the other; asymmetry between sides has been associated with lower limb injury in some sports populations. These screens are useful for describing movement and setting training goals, but they should not be treated as precise injury predictors.

Motor control of the spine

Motor control is the way the nervous system plans and coordinates muscle activity to produce movement and maintain stability. For the spine, a key idea came from studies in the 1990s showing that in healthy people the transversus abdominis contracts slightly before the arm or leg muscles during rapid limb movements: a feedforward (anticipatory) response that prepares the trunk for the coming disturbance. In some people with low back pain this activation was delayed. Other studies found that the lumbar multifidus can show reduced size and altered activity on the painful side after an episode of back pain, sometimes persisting after pain resolves.

Researchers divided trunk muscles into a local (deep) system of muscles attached directly to the vertebrae, such as the transversus abdominis, deep multifidus and pelvic floor, thought to control segmental motion, and a global (superficial) system, such as the rectus abdominis, external obliques and long erector spinae, that moves the trunk and handles large loads. From this model came motor control exercise, often called core stability training, which first teaches isolated activation of deep muscles and then integrates them into functional tasks.

The evidence has refined this story. Changes in muscle timing in back pain are real but vary between individuals: some people with back pain show too little trunk muscle activity, while many show too much, bracing and co-contracting as a protective strategy that may itself become a problem. Systematic reviews have found that motor control exercise reduces pain and disability in chronic low back pain about as much as other forms of exercise, but not more. The practical message is that exercise helps, the specific type matters less than consistency and patient preference, and teaching people to move with confidence rather than with constant bracing is often valuable.

Other movement observations

• Lumbopelvic rhythm: forward bending combines lumbar flexion with hip flexion (anterior pelvic rotation). Early in the movement the lumbar spine contributes more, and later the hips contribute more. People with back pain often move more slowly and with less lumbar motion.
• Flexion relaxation phenomenon: in healthy people, the lumbar erector spinae fall electrically silent near full forward flexion, as passive tissues take over the load. This silence is absent in many people with chronic back pain, reflecting protective guarding, and it often returns with successful rehabilitation.
• Directional preference: in the mechanical diagnosis approach associated with Robin McKenzie, repeated movements in one direction (often extension) may cause symptoms to retreat from the limb toward the spine, called centralization. Centralization is a favorable prognostic sign.
• Cervical joint position error: the patient, blindfolded, returns the head to a neutral starting point after rotating; errors greater than about 4.5 degrees suggest impaired cervical proprioception, common after whiplash.
• Scapulohumeral rhythm: during arm elevation, the glenohumeral joint and scapula move in roughly a 2 to 1 ratio over the full range.

Motor learning

Changing how someone moves is a learning process. The classic model describes three stages: a cognitive stage, when the learner thinks hard about the task and performance is variable; an associative stage, when errors decrease and movement becomes more consistent; and an autonomous stage, when the skill is performed with little conscious attention. Research on motor learning suggests several practical principles:

• An external focus of attention (on the effect of the movement, such as "push the floor away") usually produces better performance and learning than an internal focus on body parts ("tighten your glutes").
• Variable practice, in which the task is practiced in different ways and contexts, transfers better to real life than repeating one exact pattern.
• Feedback helps early but should fade over time so that the learner develops internal control.
• Specificity: improvements are greatest in the tasks actually practiced, so rehabilitation should include the activities the patient wants to return to.

Section 5Ergonomics and movement habits

What ergonomics tries to do

Ergonomics is the study of fitting tasks, tools and environments to the people who use them. In spinal health it covers how workstations, lifting tasks, vehicles and daily routines distribute load on the body. Good ergonomics reduces unnecessary strain and fatigue, but it is important to be honest about the evidence: most single ergonomic changes have modest effects on back and neck pain, and no workstation setup can compensate for staying in one position for hours. The most consistent message from research is that movement variety, regular breaks, overall physical activity and confidence in the body matter more than finding one perfect posture.

Intradiscal pressure: what the numbers say

Much traditional advice about sitting and lifting grew from measurements of pressure inside the lumbar disc. Pioneering studies in the 1960s measured pressure in the L3 disc with needle transducers and reported that sitting produced considerably higher pressure than standing, which led to decades of advice that sitting is especially harmful to the spine. A later study in the late 1990s used a pressure sensor implanted in the L4 to L5 nucleus of a healthy volunteer and measured many everyday positions over a full day. Several of its findings differed from the older work.

Three lessons follow. First, relaxed sitting is not dramatically worse for the disc than standing; the old claim that sitting is "the worst position for your back" is not supported by these measurements. Second, bending forward and lifting, especially with the load held away from the body, produce the highest pressures, which fits with the common experience that bending aggravates disc pain. Third, lying down unloads the disc substantially, and discs absorb fluid overnight, which is why people are slightly taller in the morning and why the disc may be more sensitive to bending early in the day.

It is important not to turn these numbers into fear. Discs are designed to bear load, and loading is necessary for disc nutrition and health. High pressure during a lift does not mean harm. Pressure data explain why some positions feel worse during an episode of disc pain; they do not mean that bending or lifting should be avoided for life.

Sitting and workstation setup

For people who work at a desk, a reasonable workstation reduces awkward sustained positions. Commonly recommended features include:

Sit-stand desks reduce sitting time, and some trials report small improvements in discomfort, but standing still for long periods has its own problems, including leg fatigue and venous pooling. The goal is alternation, not replacing hours of sitting with hours of standing. High total sedentary time is associated with poorer cardiometabolic health independent of exercise, which is another reason to build movement into the working day.

Lifting

Generations of workers have been taught to "lift with your legs, not your back." Biomechanically, keeping the load close to the body is the single most effective way to reduce spinal load, because the moment arm of the load about the lumbar spine shrinks. The posture of the back during lifting is less clear cut. Squat lifting reduces lumbar flexion but brings the knees forward, which can push the load farther from the body and requires more energy; stoop lifting with a rounded back is often more efficient. Systematic reviews have found no convincing evidence that lumbar flexion during lifting, on its own, causes back pain, and no evidence that training workers in "correct" lifting technique prevents back pain. Back belts likewise do not prevent back pain in workers.

What does matter is load tolerance. Tissues adapt to loads they are exposed to regularly and gradually. A person who lifts often and is strong and conditioned tolerates heavy lifting well; problems arise when load exceeds current capacity, as when a sedentary person moves house over a weekend. Occupational guidelines address the task rather than technique alone. The lifting equation published by a United States occupational safety agency uses a load constant of 23 kg as the recommended weight limit under ideal conditions and reduces it for factors such as distance from the body, height of the lift, twisting, frequency and poor grip. Reducing these task factors with lifting aids, team lifts and better layout is the most effective workplace strategy.

Movement habits across the day

• Physical activity: adult guidelines recommend at least 150 to 300 minutes of moderate aerobic activity per week, or 75 to 150 minutes of vigorous activity, plus muscle-strengthening on two or more days. Regular activity is associated with lower risk of recurrent back pain, and exercise is among the best supported treatments for chronic back pain.
• Sleep: no single sleeping posture is proven best for the spine. Medium-firm mattresses have modest support for comfort in people with back pain. Good sleep quality matters more, because poor sleep increases pain sensitivity.
• Backpacks: guidance often suggests keeping school bags below about 10 to 15 percent of body weight and using both straps. The evidence linking backpack weight to lasting spinal problems is weak, but heavy loads can cause discomfort.
• Driving: adjust the seat so the knees are slightly bent with pedals fully pressed, support the lumbar curve, and take breaks on long journeys.
• Variety: the healthiest habit is to change positions often, mix sitting, standing and walking, and stay physically confident. A useful phrase for patients is that "motion is lotion": joints and discs depend on movement for nutrition.

Words worth knowing

Chiropractic Models Through History

Chiropractic began in a single room in Davenport, Iowa, in 1895, and within a few decades it had grown into the largest drug free, hands on profession in the United States. Its history is a story of bold ideas, courtroom battles, family conflict, internal schisms and, more recently, a long effort to test its founding claims with modern research. Knowing that history helps you understand why chiropractors today describe their work in such different ways, and why the word "subluxation" still sparks debate.