The Vertebral Column: Bones, Joints, Ligaments and Muscles

The vertebral column is the chiropractor's home ground, and Spinal Anatomy carries more weight on Part I than any other domain. This chapter builds the spine from its bones up: the features that identify every region, the joints and ligaments that allow and limit motion, the muscles that move and stabilize it, and the developmental story that explains the variants you will meet on every second radiograph.

Section 1Osteology of the axial skeleton

The typical vertebra

The adult column usually has 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral and about 4 coccygeal (3 to 5). Every typical vertebra has two functional parts. The vertebral body anteriorly is a short cylinder of cancellous bone inside a thin cortical shell that bears compressive load; its superior and inferior surfaces are covered by cartilaginous endplates and edged by a raised ring apophysis. The vertebral (neural) arch posteriorly is formed by two pedicles, which project back from the body, and two laminae, which meet in the midline. Body and arch enclose the vertebral foramen; stacked, these form the vertebral canal that houses the cord, meninges and cauda equina.

Seven processes arise from the arch: one spinous process where the laminae meet, two transverse processes at the pedicle-lamina junction, and four articular processes (two superior, two inferior) carrying the facets of the zygapophyseal joints. The region of lamina between the superior and inferior articular processes is the pars interarticularis, the site of spondylolysis. The superior and inferior vertebral notches above and below each pedicle form, with the adjacent vertebra, the intervertebral foramen.

Regional features

The atypical vertebrae

C1, the atlas, has no body and no spinous process. It is a ring of an anterior arch (with an anterior tubercle and a facet for the dens on its posterior surface), a posterior arch (with a posterior tubercle and, on its upper surface, a groove for the vertebral artery and the C1 nerve), and two lateral masses. The superior articular facets are concave, kidney shaped and face up and medially to cradle the occipital condyles; the inferior facets are flat to slightly concave and nearly circular. In some people a bony bridge over the vertebral artery groove forms a posterior ponticle (arcuate foramen, Kimmerle anomaly).

C2, the axis, carries the dens (odontoid process), which projects up from the body to articulate with the anterior arch of the atlas, held by the transverse ligament. Its superior articular facets sit on the body and pedicles, lateral to the dens, rather than behind the pedicles; this means the C1 to C2 joints lie anterior to the C2 to C3 joints. C2 has a large, bifid spinous process, the first prominent spinous process palpable below the occiput, and its pars region (isthmus) is where a hangman fracture occurs.

C7, the vertebra prominens, has a long, non-bifid spinous process that is usually the most prominent in the lower neck (though T1 is sometimes more prominent; flexing and extending the neck helps, because C6 slides forward under the palpating finger while C7 stays put). Its transverse foramina are small and usually transmit only accessory vertebral veins, because the vertebral artery normally enters the transverse foramen of C6. The anterior tubercle of C6 is large and called the carotid tubercle, against which the common carotid artery can be compressed. A cervical rib may articulate with C7.

The L5 vertebra has the largest body, which is wedge shaped (taller anteriorly), contributing to the lumbosacral angle. Its stout transverse processes arise from the pedicles and the body and anchor the iliolumbar ligaments. Its inferior facets face forward more than other lumbar facets (more coronal), which resists forward slip of L5 on S1.

The sacrum and coccyx

The sacrum is formed by five fused vertebrae and transmits body weight from L5 to the pelvis through the sacroiliac joints. Its base faces upward and carries the S1 body, whose projecting anterior lip is the sacral promontory, and the two alae (fused transverse and costal elements). Its concave anterior (pelvic) surface has four pairs of anterior sacral foramina for the ventral rami; the dorsal surface has four pairs of posterior sacral foramina for the dorsal rami. Down the dorsal midline runs the median sacral crest (fused spinous processes); lateral to it are the intermediate crests (fused articular processes) and, beyond the foramina, the lateral crests (fused transverse processes). The laminae of S5 (and often S4) fail to fuse, leaving the sacral hiatus, flanked by the sacral cornua and used for caudal epidural injection. Laterally, the ear-shaped auricular surface (spanning S1 to S3) articulates with the ilium, and behind it the rough sacral tuberosity anchors the interosseous ligament. The dural sac ends at about S2.

The coccyx has three to five rudimentary segments; the first has coccygeal cornua that articulate with the sacral cornua. It anchors the gluteus maximus, coccygeus, levator ani (anococcygeal body) and the sacrotuberous and sacrospinous ligaments.

Ribs, sternum and the skull base

There are 12 pairs of ribs: true ribs 1 to 7 join the sternum by their own costal cartilages; false ribs 8 to 10 join the cartilage above; floating ribs 11 and 12 end free. A typical rib (3 to 9) has a head with two facets (for its own vertebra and the one above, separated by a crest that the intra-articular ligament anchors to the disc), a neck, a tubercle articulating with the transverse process of its own vertebra, an angle and a costal groove sheltering the intercostal vein, artery and nerve (VAN, from above down). Rib 1 is short, broad and flat, with a single head facet, a scalene tubercle for anterior scalene, and grooves for the subclavian vein in front and subclavian artery behind. Rib 2 has a tuberosity for serratus anterior. Ribs 10 to 12 have a single head facet, and 11 and 12 lack necks and tubercles.

The sternum has a manubrium (jugular notch at about T2 to T3, clavicular notches, rib 1 cartilage), a body (ribs 2 to 7, with the sternal angle at rib 2) and a xiphoid process (xiphisternal joint at about T9).

The upper cervical spine is inseparable from the occipital bone. The occipital condyles flank the foramen magnum, which transmits the medulla, the vertebral arteries, the anterior and posterior spinal arteries, the spinal roots of the accessory nerve and the tectorial membrane. The hypoglossal canal pierces the base of each condyle, and the jugular foramen (CN IX, X, XI, internal jugular vein) lies lateral to it. The basiocciput (clivus) receives the apical ligament and tectorial membrane; the external occipital protuberance (inion) and the superior and inferior nuchal lines anchor the nuchal ligament and posterior neck muscles; the mastoid process anchors SCM, splenius capitis and longissimus capitis.

Section 2Arthrology of the spine

Adjacent vertebrae from C2 to S1 are joined at three points: an anterior intervertebral disc (a secondary cartilaginous joint, or symphysis) and two posterior zygapophyseal (facet) joints (synovial plane joints). Together with the ligaments they form the motion segment (functional spinal unit), the smallest unit that shows the mechanical behavior of the whole column. In the cervical spine, small uncovertebral joints add two more contacts.

The intervertebral disc

There are 23 discs, the first between C2 and C3 and the last between L5 and S1; there is no disc between the occiput and C1 or between C1 and C2. Discs make up roughly a quarter of the length of the presacral column, which is why height decreases through the day as they lose water under load. They are thickest in the lumbar region and thinnest in the upper thoracic region; cervical and lumbar discs are thicker anteriorly, which helps create the lordoses.

The disc is innervated only in the outer third of the annulus. Posteriorly the supply is the recurrent meningeal (sinuvertebral) nerve, which also supplies the posterior longitudinal ligament, the anterior dura and the vertebral periosteum; laterally and anteriorly fibers come from the ventral rami and gray rami communicantes. Painful internal disc disruption and the ingrowth of nerves along radial fissures in degenerated discs explain discogenic pain without nerve root compression.

Zygapophyseal joints

The facet joints are true synovial joints with hyaline cartilage, a fibrous capsule lined by synovium, and intra-articular meniscoid folds (fibroadipose wedges at the joint poles) that can become entrapped and are one proposed basis for acute locked back. Each joint is supplied by the medial branch of the dorsal ramus from the level above and the level of the joint, the basis of medial branch blocks and radiofrequency neurotomy. Their orientation is the main determinant of regional motion.

Uncovertebral joints

The uncinate processes rise from the lateral margins of the superior surfaces of C3 to C7 (and sometimes T1) bodies and contact the beveled inferior lateral edges of the body above. The cleft that develops here in childhood (around age 10) is the uncovertebral joint (joint of Luschka). Whether it is a true synovial joint or a degenerative fissure in the disc is debated, but its clinical significance is clear: the uncinates limit lateral translation and lateral flexion, guide flexion and extension, and form the anteromedial boundary of the cervical intervertebral foramen. Uncovertebral hypertrophy is a leading cause of cervical foraminal stenosis, best seen on oblique films.

The intervertebral foramen

Each foramen is bounded above and below by pedicles, anteriorly by the posterolateral vertebral bodies and the disc (and the uncinate in the neck), and posteriorly by the zygapophyseal joint capsule and the lateral ligamentum flavum. It transmits the spinal nerve (or its roots) with its dural sleeve, the dorsal root ganglion (in the foramen in most of the lumbar spine), the spinal branch of a segmental artery, the intervertebral veins linking internal and external plexuses, the recurrent meningeal nerve, lymphatics and fat. The nerve occupies about a third to a half of the foramen. Foraminal size falls with extension and ipsilateral lateral flexion, the basis of the Spurling maneuver.

Cervical nerves C1 to C7 exit above the vertebra of the same number, and C8 exits between C7 and T1; from T1 down, each nerve exits below its numbered vertebra. Because the lumbar roots cross the disc above the foramen they exit through, a typical posterolateral L4 to L5 herniation compresses the traversing L5 root, while a far lateral (foraminal) herniation at the same disc compresses the exiting L4 root. A C5 to C6 cervical disc compresses the C6 root.

The upper cervical joints

The atlanto-occipital joints are paired synovial condyloid (ellipsoid) joints between the convex occipital condyles and the concave superior facets of the atlas. Their main motion is flexion and extension (nodding, the "yes" joint, about 15 to 25 degrees), with a few degrees of lateral flexion and negligible rotation.

The atlantoaxial articulation comprises three synovial joints: a median pivot (trochoid) joint between the dens and the anterior arch of the atlas in front and the transverse ligament behind (with a bursa between ligament and dens), and two lateral atlantoaxial joints between the lateral masses, which are planar and biconvex in their cartilage. Their chief motion is rotation (the "no" joint), about 40 degrees to each side, roughly half of all cervical rotation. Because the lateral joint surfaces are biconvex, the atlas descends slightly as it rotates, which tightens the capsules and helps explain why end-range rotation stresses the vertebral arteries.

Costovertebral joints

Each typical rib makes two synovial joints with the spine. The joint of the head of the rib links the two head facets with the demifacets of adjacent bodies and the disc between, divided into two cavities by the intra-articular ligament and reinforced by the radiate ligament (ribs 1, 10, 11 and 12 have a single cavity). The costotransverse joint links the tubercle with the transverse costal facet (ribs 1 to 10 only), reinforced by the costotransverse, lateral and superior costotransverse ligaments. The axis through the two joints determines rib motion: in the upper ribs it is near coronal, giving pump-handle elevation of the sternum; in the lower ribs it is closer to sagittal, giving bucket-handle lateral expansion.

The sacroiliac joint

The sacroiliac joint joins the auricular surfaces of the sacrum and ilium. Its anterior and inferior portion is a synovial joint whose sacral surface is covered by thicker hyaline cartilage and whose iliac surface by thinner fibrocartilage-like cartilage; the posterior and superior portion is a syndesmosis held by the massive interosseous ligament. The surfaces are irregular, with an interlocking ridge and furrow that increase with age, providing form closure; the ligaments, thoracolumbar fascia and muscles (gluteus maximus, latissimus via the fascia, biceps femoris via the sacrotuberous ligament, multifidus) compress it and provide force closure.

Motion is small, a few degrees of rotation and one or two millimeters of translation. Nutation is the sacral base moving anteriorly and inferiorly while the coccyx moves posteriorly (occurs when standing, in trunk flexion and in late pregnancy); it tightens the sacrotuberous, sacrospinous and interosseous ligaments and is the more stable, close-packed position. Counternutation is the reverse and is resisted by the long dorsal sacroiliac ligament. Sacroiliitis is an early feature of the seronegative spondyloarthropathies, especially ankylosing spondylitis, and affects the synovial (lower) part first.

Curves and regional motion

The fetal column is one C-shaped curve concave forward. The thoracic and sacral kyphoses are primary curves retained from this posture. The cervical lordosis develops as the infant holds up the head (around 3 to 4 months) and the lumbar lordosis as the child sits, stands and walks (about 9 to 18 months); these are secondary curves, produced mainly by wedge-shaped discs. Typical adult values: cervical lordosis about 30 to 40 degrees, thoracic kyphosis about 20 to 45 degrees, lumbar lordosis about 40 to 60 degrees. The lumbosacral (Ferguson) angle, between the top of the sacrum and the horizontal on a standing lateral film, averages about 41 degrees; an increased angle increases shear at L5 to S1.

Section 3Syndesmology: the ligaments of the spine and pelvis

The long ligaments

The anterior longitudinal ligament (ALL) is a broad, strong band running down the anterior surfaces of the vertebral bodies and discs from the basiocciput and anterior tubercle of the atlas (where it continues as the anterior atlanto-occipital membrane) to the pelvic surface of the sacrum. It is widest in the lumbar spine, firmly attached to the bodies and their margins and less firmly to the discs, and it limits extension.

The posterior longitudinal ligament (PLL) runs inside the vertebral canal on the posterior surfaces of the bodies and discs from C2 (above which it continues as the tectorial membrane) to the sacrum. It is broad in the cervical region but becomes narrow in the lumbar region, with a serrated (denticulate) outline that flares over each disc and narrows over each body. It is attached more firmly to the discs than to the bodies, leaving room behind the bodies for the basivertebral veins. It limits flexion, is richly innervated by the sinuvertebral nerve, and its narrowness in the lumbar spine leaves the posterolateral annulus unreinforced, which is why most lumbar herniations are posterolateral. Ossification of the PLL (OPLL), especially in the cervical spine, causes myelopathy.

The short ligaments

The craniovertebral ligaments

The occiput, atlas and axis are held together by an internal set of ligaments that, from anterior (deep) to posterior (superficial) inside the canal, are the apical ligament, the alar ligaments, the cruciform ligament and the tectorial membrane, with the dura behind them.

The ligaments of the pelvis

The iliolumbar ligament runs from the tip of the L5 transverse process (often also L4) to the posterior inner lip of the iliac crest. It is the key ligamentous link between the lumbar spine and pelvis, limiting forward slip, lateral flexion and rotation of L5 on S1, and it is a recognized pain source in low back pain over the iliac crest.

The greater sciatic foramen is divided by piriformis. Above it pass the superior gluteal nerve and vessels; below it pass the sciatic nerve, inferior gluteal nerve and vessels, posterior femoral cutaneous nerve, nerves to obturator internus and quadratus femoris, and the pudendal nerve and internal pudendal vessels. The lesser sciatic foramen transmits the obturator internus tendon and readmits the pudendal nerve and internal pudendal vessels to the perineum.

Section 4Myology of the axial skeleton

The back muscles fall into three layers. The superficial (extrinsic) layer (trapezius, latissimus dorsi, levator scapulae, rhomboids) moves the upper limb and is supplied by ventral rami or the accessory nerve. The intermediate layer (serratus posterior superior and inferior) is respiratory and supplied by intercostal nerves. The deep (intrinsic, true) back muscles move and stabilize the column and are all supplied by dorsal rami of spinal nerves, a rule worth memorizing because it lets you answer innervation questions about any of them.

The suboccipital region

Four small muscles lie deep to semispinalis capitis between the occiput, atlas and axis. They are all supplied by the suboccipital nerve (dorsal ramus of C1), which is purely motor. They have a high density of muscle spindles and act mostly as fine postural adjusters and proprioceptive sensors of head position.

The suboccipital triangle is bounded by rectus capitis posterior major (superomedially), obliquus capitis superior (superolaterally) and obliquus capitis inferior (inferiorly). Its roof is semispinalis capitis, and its floor is the posterior atlanto-occipital membrane and posterior arch of the atlas. It contains the third part of the vertebral artery in its groove on the atlas and the suboccipital nerve. The greater occipital nerve (dorsal ramus of C2) emerges below obliquus capitis inferior, curves up across the triangle and pierces semispinalis capitis and trapezius to supply the posterior scalp; entrapment produces occipital neuralgia. The third occipital nerve (C3) supplies the C2 to C3 facet joint, a common source of cervicogenic headache.

The deep back muscles

The splenius capitis and cervicis form a bandage over the deeper muscles of the neck, arising from the nuchal ligament and spinous processes of C7 to T6 and inserting into the mastoid process and superior nuchal line (capitis) or the transverse processes of C1 to C3 (cervicis). Acting alone, they rotate the head and neck to the same side.

The erector spinae is the large intermediate column lateral to the spinous processes. It arises by a broad common tendon from the posterior iliac crest, dorsal sacrum, sacroiliac ligaments and lumbar spinous processes and divides into three columns, lateral to medial: iliocostalis (to the angles of the ribs and cervical transverse processes), longissimus (to transverse processes and, as longissimus capitis, the mastoid process) and spinalis (spinous to spinous, best developed in the thorax); "I Love Spaghetti". Bilaterally it extends the column and controls flexion eccentrically; unilaterally it laterally flexes to the same side. It falls electrically silent at full flexion (the flexion relaxation phenomenon), when ligaments and fascia carry the load; loss of this silence is a marker of chronic low back pain.

The transversospinalis group runs obliquely upward and medially, so acting on one side it rotates the column to the opposite side; bilaterally it extends. The deepest segmental muscles are the interspinales (spinous to spinous, cervical and lumbar), the intertransversarii (transverse to transverse; the anterior cervical and lateral lumbar parts are supplied by ventral rami) and the levatores costarum (transverse processes C7 to T11 to the rib below), which elevate the ribs.

Prevertebral and scalene muscles

The prevertebral muscles lie on the front of the cervical spine and are supplied by ventral rami: longus colli (from T3 to the atlas, the main deep neck flexor and a cervical lordosis stabilizer), longus capitis (C3 to C6 transverse processes to the basiocciput, flexes the head), rectus capitis anterior (C1 lateral mass to basiocciput, flexes the head) and rectus capitis lateralis (C1 transverse process to the jugular process, laterally flexes).

The roots and trunks of the brachial plexus and the subclavian artery pass between anterior and middle scalenes through the interscalene triangle, while the subclavian vein passes in front of the anterior scalene. Scalene hypertrophy or a cervical rib narrowing this space produces neurogenic or arterial thoracic outlet syndrome (Adson test).

Abdominal wall, quadratus lumborum, psoas and fascia

The aponeuroses of the three flat muscles form the rectus sheath. Above the arcuate line (about a third of the way from umbilicus to pubis), the internal oblique aponeurosis splits around the rectus; below it, all three aponeuroses pass in front, leaving only transversalis fascia behind the muscle.

The thoracolumbar fascia in the lumbar region has three layers. The posterior layer arises from the spinous processes and supraspinous ligaments and covers the erector spinae; the latissimus dorsi and, through it, the contralateral gluteus maximus attach to it, creating a diagonal "posterior oblique sling" across the sacroiliac joints. The middle layer attaches to the tips of the transverse processes and separates erector spinae from quadratus lumborum. The anterior layer covers the front of quadratus lumborum. The posterior and middle layers fuse at the lateral raphe, where transversus abdominis and internal oblique attach. Tension from these muscles and from the expanding erector spinae within the fascial envelope (the hydraulic amplifier effect) stiffens the lumbar spine and pelvis for load transfer.

Section 5Development and variants of the spine

Somites, sclerotomes and resegmentation

During the third and fourth weeks, the paraxial mesoderm on each side of the neural tube and notochord segments into paired blocks called somites, beginning in the occipital region around day 20 and reaching about 42 to 44 pairs. Each somite differentiates into a ventromedial sclerotome, which forms the vertebrae and ribs, and a dorsolateral dermomyotome, which gives the dermis of the back (dermatome) and skeletal muscle (myotome). The epaxial part of the myotome forms the intrinsic back muscles (hence their dorsal ramus supply), and the hypaxial part forms the limb and body wall muscles (ventral rami).

Sclerotome cells migrate to surround the notochord and neural tube. Each sclerotome then splits into a loosely packed cranial half and a densely packed caudal half, and in the process of resegmentation the caudal half of one sclerotome fuses with the cranial half of the next to form one vertebral body. This explains three adult facts: each vertebra is intersegmental, built from two adjacent somites; the spinal nerves, which grew out to their own myotomes, now pass between vertebrae through the intervertebral foramina; and the segmental myotomes span two vertebrae, so the deep muscles can move one bone on the next. Because the first cervical sclerotome half joins the occiput, there are eight cervical nerves but only seven cervical vertebrae.

The notochord regresses where it is surrounded by the developing bodies but persists between them, expanding as the nucleus pulposus; the surrounding mesenchyme of the dense sclerotome half forms the annulus fibrosus. Notochordal remnants also form the apical ligament of the dens, and rare malignant chordomas arise from such remnants, most often in the sacrococcygeal region and clivus.

Chondrification and ossification

The mesenchymal (membranous) vertebra chondrifies from about the sixth week, and ossification begins around the eighth week. A typical vertebra has three primary ossification centers: one for the centrum and one for each half of the neural arch. Note that the centrum is not the whole adult body: the posterolateral corners of the body come from the arch centers, joined at the neurocentral synchondroses, which fuse between about 3 and 6 years. The two halves of the arch fuse posteriorly in the first year or two (in the lumbar region first). At puberty, secondary centers appear at the tips of the spinous and transverse processes, at the mammillary processes in the lumbar spine and as the ring apophyses on the rims of the bodies; these fuse by about age 25.

Failures of formation and segmentation

• Block vertebra (failure of segmentation): two or more bodies fused because the intervening disc did not form. Congenital blocks, most common at C2 to C3 and C5 to C6, show a rudimentary disc remnant, a narrowed anteroposterior "wasp waist" at the old disc level and often fused posterior elements, which distinguish them from acquired fusion (infection, ankylosing spondylitis, surgery). Motion concentrates at adjacent segments, accelerating their degeneration.
• Klippel-Feil syndrome: multiple congenital cervical fusions with the classic (but often incomplete) triad of short neck, low posterior hairline and restricted motion; associated with Sprengel deformity (undescended scapula, sometimes with an omovertebral bone), renal anomalies, hearing loss and scoliosis.
• Hemivertebra (failure of formation): one lateral half of a body fails to develop, leaving a wedge that causes congenital scoliosis; a posterior hemivertebra causes congenital kyphosis.
• Butterfly vertebra: failure of fusion of the two lateral chondrification centers, with a sagittal cleft giving a butterfly shape on the frontal film; usually incidental.
• Agenesis or hypoplasia of a pedicle, with compensatory sclerosis of the opposite pedicle, which must be distinguished from a lytic metastasis destroying a pedicle (the "winking owl" sign).

Neural tube and arch defects

Spina bifida occulta is failure of the laminae to fuse in the midline, most often at L5 or S1, with intact skin and normal neural tissue; it is seen in a large minority of lumbosacral films and is usually incidental, though a hair tuft, dimple or lipoma over it may signal an underlying tethered cord. Spina bifida cystica includes the meningocele (meninges and CSF herniate through the defect) and the far more serious myelomeningocele (cord or roots in the sac), with paralysis, sphincter dysfunction and often Chiari II malformation and hydrocephalus. These neural tube defects are reduced by periconceptional folic acid, and screened for by maternal serum alpha fetoprotein.

Transitional segments and other variants on film

Spondylolysis, a defect of the pars interarticularis, is not a true congenital anomaly but a fatigue fracture in a genetically susceptible pars, most often at L5 (about 85 to 90% of cases), common in athletes who repeatedly hyperextend (gymnasts, linemen). Seen on an oblique film as the collar on the "Scottie dog" neck. Bilateral defects allow spondylolisthesis, graded by the Meyerding system in quarters of the sacral endplate (grade 1 up to 25%, through grade 4 up to 100%, and spondyloptosis beyond). The dysplastic (congenital) type arises from deficient upper sacral or L5 facets and carries a higher risk of progression and of cauda equina symptoms.

Words worth knowing