Topographical Anatomy and the Appendicular Skeleton

Every palpation, every orthopedic test and every film you read rests on a shared map of the body: its planes, its regions, its surface landmarks and the bones and joints of the limbs. General anatomy carries a fifth of the Part I weight, and the appendicular skeleton supplies a large share of those items because it is so easy to test with a single landmark, a single fracture or a single ligament. Learn this chapter well and the muscles, nerves and vessels of the next chapter will have somewhere to attach.

Section 1Planes, terminology and the body's regions

The anatomical position and the cardinal planes

All anatomical description assumes the anatomical position: the body stands erect, the head and eyes face forward, the upper limbs hang at the sides with the palms facing anteriorly, and the feet point forward and sit slightly apart. Whatever position a patient actually occupies on the table, terms such as anterior, medial and proximal are always read as if the patient stood this way. That is why the radius is lateral and the thumb is lateral, even when the forearm is pronated.

Three planes, each at right angles to the other two, slice the body. A sagittal plane is vertical and runs from front to back, dividing right from left. The single sagittal plane through the midline is the median (midsagittal) plane; any other is a parasagittal plane. A frontal (coronal) plane is also vertical but runs from side to side, dividing anterior from posterior. A transverse (horizontal, axial) plane divides superior from inferior; this is the plane of the axial CT and MRI slice, conventionally viewed from the patient's feet so the patient's right lies on the viewer's left.

Each plane has a perpendicular axis about which movement in that plane occurs. Flexion and extension happen in the sagittal plane about a coronal (frontal, mediolateral) axis. Abduction and adduction happen in the frontal plane about a sagittal (anteroposterior) axis. Rotation happens in the transverse plane about a vertical (longitudinal) axis.

Terms of direction and movement

Directional terms come in opposing pairs: superior and inferior (cranial and caudal), anterior and posterior (ventral and dorsal), medial and lateral, proximal and distal, superficial and deep, and ipsilateral and contralateral. Palmar (volar) and plantar replace anterior and inferior in the hand and foot.

Movement terms pair the same way. Flexion decreases the angle between segments in the sagittal plane and extension increases it; at the ankle these are dorsiflexion and plantarflexion, and the thumb flexes in the frontal plane because it sits rotated 90 degrees from the fingers. Abduction moves a part away from the midline (for fingers, away from the middle finger; for toes, away from the second toe) and adduction returns it. Medial and lateral rotation turn a limb about its long axis, and circumduction traces a cone. Pronation crosses the radius over the ulna to turn the palm back; supination uncrosses it. Inversion and eversion turn the sole medially and laterally. The scapula adds elevation, depression, protraction, retraction and upward and downward rotation, and the thumb adds opposition.

Regions and surface landmarks

Topographical anatomy links what you can see and feel at the surface to what lies beneath. The board expects you to know the vertebral level of the commonly used landmarks, because those levels let you locate viscera, vessels and spinal segments from the outside.

Head and neck. The external occipital protuberance (inion) marks the midline at the back of the skull, and the mastoid process lies behind the ear. The sternocleidomastoid divides the neck into an anterior triangle (bounded by the midline, the mandible and the anterior border of sternocleidomastoid; subdivided into submental, submandibular, carotid and muscular triangles) and a posterior triangle (bounded by sternocleidomastoid, trapezius and the middle third of the clavicle; it contains the spinal accessory nerve, the trunks of the brachial plexus and the third part of the subclavian artery). In the anterior neck, the hyoid lies at C3, the thyroid cartilage at C4 to C5 (the common carotid bifurcates at its upper border, about C4), and the cricoid cartilage at C6, where the larynx becomes the trachea and the pharynx becomes the esophagus. The anterior tubercle of the C6 transverse process (the carotid tubercle) lies deep to the cricoid.

Back. The spinous process of C7 (vertebra prominens) is usually the first clearly palpable spinous process, though T1 is often equally prominent; flexion and extension of the neck help separate them because C7 glides forward under the palpating finger with extension while T1 stays fixed. With the arms at the sides, the root of the spine of the scapula lies at T3, the inferior angle at T7 and the superior angle at T2. A line joining the highest points of the iliac crests (the supracristal or intercristal line) crosses the L4 spinous process or the L4 to L5 interspace, the classic landmark for lumbar puncture. The skin dimples over the posterior superior iliac spines mark S2, the level of the termination of the dural sac and the middle of the sacroiliac joints.

Thorax. The sternal angle (angle of Louis), where the manubrium meets the body of the sternum, is the most useful landmark in the chest: the second costal cartilage articulates here, so rib counting starts here, and a horizontal plane through it reaches the T4 to T5 disc. That plane marks the tracheal bifurcation, the beginning and end of the aortic arch, the junction of the superior and inferior mediastinum and the entry of the azygos vein into the superior vena cava. The jugular (suprasternal) notch lies at T2 to T3, the xiphisternal junction at T9, and the male nipple at the fourth intercostal space, in the T4 dermatome. The apex beat lies in the left fifth intercostal space at the midclavicular line.

Abdomen. Two vertical midclavicular lines and two horizontal planes (subcostal above, intertubercular below) divide the abdomen into nine regions: right and left hypochondriac, epigastric, right and left lumbar (lateral), umbilical, right and left inguinal (iliac) and hypogastric (pubic). Clinicians more often use four quadrants. The transpyloric plane, halfway between the jugular notch and the pubic symphysis, lies at L1 and crosses the pylorus, the neck of the pancreas, the duodenojejunal flexure, the hila of the kidneys, the origin of the superior mesenteric artery and approximately the end of the spinal cord. The subcostal plane (lowest point of the tenth costal cartilage) lies at L3. The umbilicus lies at the L3 to L4 disc in a slim adult and sits in the T10 dermatome. The aorta bifurcates at L4, just below and to the left of the umbilicus. McBurney point lies one third of the way from the anterior superior iliac spine to the umbilicus and overlies the usual base of the appendix.

Pelvis and perineum. The anterior superior iliac spine (ASIS), the iliac crest and its tubercle, the pubic tubercle and the pubic symphysis are palpable anteriorly; the inguinal ligament runs from ASIS to pubic tubercle. Posteriorly the ischial tuberosities bear weight in sitting and the coccyx can be felt in the natal cleft. The diamond-shaped perineum is bounded by the pubic symphysis, the ischiopubic rami, the ischial tuberosities, the sacrotuberous ligaments and the coccyx. A line between the ischial tuberosities divides it into an anterior urogenital triangle and a posterior anal triangle.

Section 2The integument

The skin is the body's largest organ, roughly 15 percent of body weight and close to two square meters in an adult. Together with its appendages (hair, nails and glands) it forms the integumentary system. It has two layers of its own, the epithelial epidermis and the connective tissue dermis, and rests on the hypodermis (subcutaneous tissue, superficial fascia), which anatomists do not count as skin but which shares its blood supply and receptors.

The epidermis

The epidermis is keratinized stratified squamous epithelium. It is avascular and is nourished by diffusion from capillary loops in the dermal papillae, which is why a shallow scrape that does not bleed has not reached the dermis. Cells are born in the deepest layer and migrate toward the surface over roughly four weeks, flattening, filling with keratin and finally dying.

Four cell types matter. Keratinocytes are the majority and produce keratin. Melanocytes, derived from neural crest, sit in the basal layer and transfer melanin in melanosomes to surrounding keratinocytes, where it caps the nucleus against ultraviolet damage; people of all skin tones have similar numbers of melanocytes, and differences in color come from the amount, type and distribution of melanin. Langerhans cells are bone marrow derived antigen presenting cells. Merkel cells pair with sensory nerve endings to form slowly adapting touch receptors.

Thick skin (palms and soles) has a stratum lucidum and many sweat glands but no hair; thin skin carries hair follicles and sebaceous glands.

The dermis and hypodermis

The dermis has two layers. The superficial papillary dermis is loose areolar connective tissue that projects upward as dermal papillae, interlocking with epidermal ridges to resist shear; it holds capillary loops and Meissner corpuscles. The deeper and much thicker reticular dermis is dense irregular connective tissue rich in type I collagen and elastic fibers; it gives skin its strength and elasticity and houses hair follicles, glands, larger vessels and nerves. The predominant orientation of collagen bundles produces cleavage (tension) lines, often called Langer lines; incisions made parallel to them gape less and scar more neatly.

The hypodermis is loose connective tissue and adipose tissue that anchors skin to deep fascia, insulates and stores energy. It contains the deep Pacinian (lamellated) corpuscles and the superficial veins and cutaneous nerves that clinicians meet during venipuncture and nerve blocks.

Appendages

Hair grows from a follicle, an epidermal down-growth into the dermis. The hair bulb at its base surrounds a vascular dermal papilla, and its matrix cells divide to form the shaft. The arrector pili is a band of smooth muscle from the papillary dermis to the follicle; its contraction, driven by sympathetic fibers acting on alpha-1 receptors, raises the hair and dimples the skin ("goose bumps").

Sebaceous glands open into hair follicles and secrete sebum by holocrine secretion (the whole cell disintegrates). They are androgen sensitive, which explains acne at puberty. Eccrine sweat glands are coiled tubular glands distributed over almost the entire body and densest on palms, soles and forehead; they open directly onto the skin, secrete a watery fluid by merocrine exocytosis and are key to thermoregulation. Their innervation is a classic exception: they are supplied by sympathetic postganglionic fibers that release acetylcholine onto muscarinic receptors. Apocrine sweat glands in the axilla, areola and anogenital region open into hair follicles, become active at puberty and secrete a protein-rich product that bacteria break down into body odor.

Nails are plates of hard keratin made by the nail matrix, visible as the pale lunula. Clubbing, spooning, pitting and splinter hemorrhages are findings worth noticing on every hand you touch.

Cutaneous receptors

The skin surface and the spinal nerves

Every patch of skin is supplied by a spinal (or trigeminal) nerve, so the skin surface is a readable map of the spinal cord. A dermatome is the area of skin supplied by the sensory fibers of a single spinal nerve root. Dermatomes overlap considerably, so section of one root alone produces a smaller deficit than the map suggests; testing the center of the dermatome (the autonomous zone) is most reliable. A dermatome is not the same as the territory of a peripheral nerve: the median nerve, for example, carries fibers from C6, C7 and C8, and a lesion of it produces a peripheral nerve pattern, not a dermatomal band. Distinguishing the two patterns is the heart of the neurologic exam.

Skin as an organ

The skin is a barrier (physical, chemical and immunologic), a thermoregulator through sweating and sympathetically controlled dermal blood flow, a sensory organ, and an endocrine organ: ultraviolet B converts 7-dehydrocholesterol to cholecalciferol (vitamin D3), which the liver and kidney then hydroxylate.

Burns illustrate depth: superficial (epidermis, red and painful), partial thickness (into dermis, blistered) and full thickness (dermis and nerve endings destroyed, leathery and painless). The adult rule of nines gives 9 percent to the head, 9 to each upper limb, 18 to each side of the trunk, 18 to each lower limb and 1 to the perineum.

Section 3Osteology of the upper limb

Each upper limb contains 32 bones: the pectoral girdle (clavicle and scapula), the humerus, the radius and ulna, eight carpals, five metacarpals and fourteen phalanges. The girdle attaches to the axial skeleton only at the sternoclavicular joint; everything else is held by muscle. That design buys a huge range of motion at the price of stability, a trade-off that explains most shoulder pathology.

Clavicle

The clavicle is an S-shaped strut, convex forward in its medial two thirds and concave forward in its lateral third. It holds the limb away from the trunk and transmits force to the axial skeleton. The underside carries the conoid tubercle and trapezoid line for the two parts of the coracoclavicular ligament, and a groove for subclavius. The clavicle is the first bone to begin ossifying (fifth to sixth fetal week), largely by intramembranous ossification, and its medial epiphysis is the last to fuse, in the mid twenties. It is the most commonly fractured bone in the body, usually at the junction of its middle and lateral thirds after a fall on the shoulder or outstretched hand. Sternocleidomastoid lifts the medial fragment while the limb drags the lateral fragment down.

Scapula

The scapula is a flat triangular bone lying over ribs 2 to 7. The subscapular fossa faces the ribs; posteriorly the spine separates a small supraspinous fossa from a larger infraspinous fossa; the spine ends laterally as the acromion, the bony roof of the shoulder. The hooked coracoid process projects anteriorly and gives attachment to pectoralis minor, coracobrachialis and the short head of biceps, as well as the coracoclavicular, coracoacromial and coracohumeral ligaments. The shallow glenoid cavity faces laterally and slightly upward and forward; above it the supraglenoid tubercle anchors the long head of biceps, and below it the infraglenoid tubercle anchors the long head of triceps. The superior border carries the suprascapular notch, bridged by the superior transverse scapular ligament: the suprascapular artery passes above the ligament and the suprascapular nerve passes below it. The spinoglenoid notch at the lateral base of the spine is a second entrapment point where only infraspinatus is affected.

Humerus

The anatomical neck is the groove just around the head, where the capsule attaches; the surgical neck is the narrower region below the tubercles and is the common fracture site. The greater tubercle lies laterally and receives supraspinatus, infraspinatus and teres minor; the lesser tubercle lies anteriorly and receives subscapularis. Between them the intertubercular (bicipital) groove carries the tendon of the long head of biceps under the transverse humeral ligament, with pectoralis major inserting on its lateral lip, teres major on its medial lip and latissimus dorsi in its floor. Midshaft laterally lies the deltoid tuberosity, and posteriorly the radial (spiral) groove carries the radial nerve and profunda brachii artery.

Distally the bone flattens into medial and lateral supracondylar ridges ending in the medial epicondyle (prominent; common flexor origin; the ulnar nerve runs in a groove behind it) and the lateral epicondyle (common extensor origin). The articular surfaces are the rounded capitulum laterally for the radial head and the spool-shaped trochlea medially for the ulna. The coronoid and radial fossae (anterior) and olecranon fossa (posterior) receive processes in flexion and extension.

Radius and ulna

The ulna is the stabilizing bone of the elbow and is larger proximally. Its olecranon forms the point of the elbow and receives triceps; the coronoid process projects anteriorly below it and receives brachialis at the ulnar tuberosity just below; the trochlear notch between them grips the trochlea; and the radial notch on the lateral side of the coronoid receives the radial head. Distally the small ulnar head and styloid process are separated from the carpus by the triangular fibrocartilage.

The radius is the rotating bone and is larger distally. Its disc-shaped head articulates with the capitulum above and spins within the annular ligament against the radial notch. Below the neck lies the radial tuberosity for biceps. Distally the broad end carries the styloid process (which reaches about 1 cm further distally than the ulnar styloid), the ulnar notch for the distal radioulnar joint, and the dorsal (Lister) tubercle, around which the extensor pollicis longus tendon turns. The interosseous membrane transfers load from the radius to the ulna.

Two eponymous distal radius fractures are favorites. A Colles fracture follows a fall on an outstretched hand, usually in an older woman with osteoporosis; the distal fragment is displaced dorsally and radially, producing the "dinner fork" deformity. A Smith fracture is the reverse, with volar displacement after a fall on a flexed wrist.

Carpals, metacarpals and phalanges

The eight carpals sit in two rows. From lateral (thumb side) to medial, the proximal row is the scaphoid, lunate, triquetrum and pisiform, and the distal row is the trapezium, trapezoid, capitate and hamate. A common memory aid is "Some Lovers Try Positions That They Can't Handle." The proximal row (minus the pisiform) articulates with the radius and the articular disc; the distal row articulates with the metacarpals. The pisiform is a sesamoid bone within the flexor carpi ulnaris tendon. The capitate is the largest carpal and the first to ossify (about age 1); the pisiform is the last (about age 9 to 12).

The five metacarpals each have a base, shaft and head; the heads form the knuckles. The thumb has two phalanges and each finger three (proximal, middle and distal), giving fourteen.

Palpable landmarks of the upper limb

Palpable landmarks include the clavicle, acromion and AC joint line, the coracoid in the deltopectoral groove, the scapular spine, medial border and inferior angle, the greater tubercle below the acromion, the intertubercular groove during rotation, the epicondyles and olecranon (a straight line in extension and a triangle in flexion; a disrupted relationship suggests dislocation), the radial head spinning distal to the lateral epicondyle, both styloids, Lister tubercle, the scaphoid in the snuffbox, the pisiform and the hook of the hamate.

Section 4Osteology of the lower limb

Each lower limb has 31 bones: the hip bone (os coxae), femur, patella, tibia, fibula, seven tarsals, five metatarsals and fourteen phalanges. Where the upper limb is built for reach and dexterity, the lower limb is built for weight bearing and locomotion, so its bones are heavier, its joints deeper and its ligaments stronger.

The hip bone and the pelvis

The hip bone forms from three bones, the ilium, ischium and pubis, which meet at a Y-shaped (triradiate) cartilage in the acetabulum and fuse at about 15 to 17 years. The ilium and ischium each supply about two fifths of the acetabulum and the pubis about one fifth. The acetabulum's articular lunate surface is horseshoe shaped; its inferior gap, the acetabular notch, is bridged by the transverse acetabular ligament, and the nonarticular acetabular fossa holds a fat pad and the ligament of the head of the femur.

The ilium carries the iliac crest, the ASIS (sartorius, inguinal ligament), the AIIS (rectus femoris, iliofemoral ligament), the posterior superior and inferior spines, the iliac fossa and the auricular surface for the sacrum, with the greater sciatic notch below.

The ischium has a body, a ramus, the ischial spine (sacrospinous ligament; the pudendal nerve crosses it, the landmark for pudendal block) and the ischial tuberosity (hamstrings, sacrotuberous ligament, the hamstring part of adductor magnus; the bone we sit on). Between spine and tuberosity is the lesser sciatic notch. The sacrospinous and sacrotuberous ligaments convert the notches into the greater and lesser sciatic foramina.

The pubis has a body with the pubic crest and pubic tubercle (inguinal ligament), a superior ramus with the pectineal line (pecten pubis), and an inferior ramus that joins the ischial ramus as the ischiopubic ramus. The two pubic bodies meet at the pubic symphysis. The pubis and ischium enclose the obturator foramen, whose canal transmits the obturator nerve and vessels.

The two hip bones, sacrum and coccyx form the pelvis. The pelvic brim separates the greater (false) pelvis from the lesser (true) pelvis. Sex differences reflect adaptation for childbirth.

Femur

The femur is the longest and strongest bone. Its head forms about two thirds of a sphere and carries a small pit, the fovea capitis, for the ligament of the head. The neck joins the shaft at an angle of inclination of about 125 degrees in the adult (about 150 degrees at birth). A smaller angle is coxa vara and a larger one coxa valga. The neck is also twisted forward relative to the line of the femoral condyles, an angle of anteversion (torsion) of about 10 to 15 degrees in adults.

The greater trochanter receives gluteus medius and minimus, piriformis, obturator internus and the gemelli; the lesser trochanter receives iliopsoas. The intertrochanteric line (anterior) marks the capsule and iliofemoral ligament, and the intertrochanteric crest lies posteriorly. The linea aspera on the back of the shaft anchors the adductors, vasti and short head of biceps, continuing up as the gluteal tuberosity. Distally the medial and lateral condyles are separated by the intercondylar fossa, and the adductor tubercle receives the hamstring part of adductor magnus.

The blood supply of the femoral head comes mainly from the medial circumflex femoral artery through retinacular branches that run up the neck under the synovium, with a smaller contribution from the lateral circumflex femoral artery and from the artery of the ligament of the head (a branch of the obturator artery that matters chiefly in children). A displaced intracapsular neck fracture tears the retinacular vessels and risks avascular necrosis of the head, whereas extracapsular intertrochanteric fractures do not. A hip fracture in an older adult classically leaves the limb shortened and laterally rotated.

Patella

The patella is the largest sesamoid bone, embedded in the quadriceps tendon. Its posterior surface has a larger lateral facet and a smaller medial facet that glide on the femoral patellar surface. The Q angle, between a line from ASIS to the center of the patella and a line from the patella to the tibial tuberosity, is normally about 13 degrees in men and 18 degrees in women; a larger angle increases lateral pull, which helps explain why lateral patellar dislocation and patellofemoral pain are more common in women.

Tibia and fibula

The tibia is the weight-bearing bone of the leg. Its proximal end has medial and lateral condyles whose flat upper surfaces (the tibial plateau) are separated by the intercondylar eminence with its medial and lateral tubercles, and by anterior and posterior intercondylar areas for the cruciate ligaments and menisci. The tibial tuberosity receives the patellar ligament. Gerdy tubercle on the anterolateral condyle receives the iliotibial tract. The shaft is triangular, its anterior border (shin) and medial surface subcutaneous; posteriorly the soleal line marks the soleus origin. Distally the medial malleolus projects downward and the fibular notch receives the fibula.

The slender fibula bears little body weight (roughly a sixth or less) but anchors muscles and forms the lateral wall of the ankle mortise. Its head receives biceps femoris and the lateral collateral ligament; the common fibular nerve winds around its neck, where it is exposed to fracture, tight casts and leg crossing. The lateral malleolus extends further distally and lies more posteriorly than the medial malleolus, which is why inversion is freer than eversion.

Tarsals, metatarsals and phalanges

Seven tarsals form the hindfoot and midfoot. The talus sits between the malleoli; its trochlea (body) articulates with the tibia and fibula, its head with the navicular, and its undersurface with the calcaneus. It has no muscle attachments, and its blood supply enters mainly through the neck and runs backward, so neck fractures risk avascular necrosis of the body. The calcaneus is the largest tarsal; its tuberosity receives the calcaneal (Achilles) tendon and its sustentaculum tali supports the talar head, with flexor hallucis longus grooving its underside. The navicular lies medially in front of the talus; its tuberosity receives most of tibialis posterior. The cuboid lies laterally and has a groove for the fibularis longus tendon. The three cuneiforms (medial, intermediate, lateral) articulate with the first three metatarsals; the recessed intermediate cuneiform locks the base of the second metatarsal in place like a keystone.

The five metatarsals have bases, shafts and heads. Under the first metatarsal head lie two sesamoids within the flexor hallucis brevis tendons. The tuberosity at the base of the fifth receives fibularis brevis; an avulsion fracture here follows inversion injury, whereas a Jones fracture at the metaphyseal diaphyseal junction a little further distal heals poorly because of a watershed blood supply. The second and third metatarsal shafts are the usual sites of stress (march) fractures. The phalanges number fourteen, two in the great toe and three in each lesser toe.

Palpable landmarks of the lower limb

Routinely palpated are the iliac crests (compared for pelvic obliquity), ASIS and PSIS, the pubic tubercle (inguinal hernias lie above and medial to it, femoral hernias below and lateral), the ischial tuberosity, the greater trochanter (level with the pubic tubercle and hip joint center), the femoral condyles and joint line, the tibial tuberosity and Gerdy tubercle, the fibular head and neck, both malleoli, the sustentaculum tali, the navicular tuberosity and the base of the fifth metatarsal.

Section 5Arthrology and syndesmology of the appendicular skeleton

Arthrology is the study of joints and syndesmology the study of ligaments: shape sets which movements are possible, while ligaments, capsule and muscle decide how much is allowed.

Classifying joints

Structurally, fibrous joints are united by dense connective tissue: sutures, syndesmoses (distal tibiofibular joint, interosseous membranes) and gomphoses (teeth). Cartilaginous joints are either synchondroses (primary; hyaline cartilage, usually temporary, such as the growth plate and first sternocostal joint) or symphyses (secondary; fibrocartilage, midline, such as the pubic symphysis and intervertebral discs). Synovial joints have a cavity. Functionally these correspond roughly to synarthroses (immovable), amphiarthroses (slightly movable) and diarthroses (freely movable).

A synovial joint has hyaline articular cartilage (fibrocartilage in the TMJ and SC joint), a fibrous capsule, a synovial membrane lining everything except cartilage, and synovial fluid rich in hyaluronic acid and lubricin that nourishes the avascular cartilage. Accessory structures include ligaments, discs, menisci, labra and bursae. Hilton law: the nerves to the muscles moving a joint also supply the joint, which is why hip disease can present as knee pain.

Two clinical ideas recur. The close-packed position is the position of maximal congruence and ligament tension, where the joint is most stable and least able to move further (glenohumeral: abduction with lateral rotation; hip: extension with medial rotation and abduction; knee: full extension; ankle: dorsiflexion). The capsular pattern is the characteristic proportional loss of motion when the whole capsule is inflamed or contracted (glenohumeral: lateral rotation lost most, then abduction, then medial rotation; hip: medial rotation, flexion, abduction; knee: flexion more than extension; elbow: flexion more than extension).

The shoulder complex

The shoulder complex has four articulations. The sternoclavicular (SC) joint is saddle-shaped with an articular disc; the disc and the strong costoclavicular ligament are its main stabilizers. It is the only bony link between upper limb and trunk.

The acromioclavicular (AC) joint is a plane joint. The AC ligament gives horizontal stability and the coracoclavicular ligament (lateral trapezoid and medial conoid parts) gives vertical stability, suspending the limb. In a complete AC separation both are torn and the clavicle appears to step up.

The glenohumeral joint is a ball and socket joint with a large head on a shallow glenoid, deepened by the glenoid labrum. Its loose capsule is reinforced by the superior, middle and inferior glenohumeral ligaments and the coracohumeral ligament; the coracoacromial arch roofs it, with the subacromial bursa between arch and supraspinatus. Stability is mainly dynamic, from the rotator cuff. The weak anteroinferior region explains why more than 95 percent of dislocations are anterior, risking the axillary nerve, an anteroinferior labral tear (Bankart) and a posterolateral head impaction (Hill-Sachs). Posterior dislocation follows seizures or electric shock.

The scapulothoracic articulation is physiologic, not anatomic. In elevation the glenohumeral joint and scapula move in about a 2 to 1 ratio (scapulohumeral rhythm): of 180 degrees, about 120 occur at the glenohumeral joint and 60 from scapular upward rotation, and the humerus must rotate laterally so the greater tubercle clears the acromion.

The elbow and forearm

The elbow combines the humeroulnar (hinge), humeroradial and proximal radioulnar (pivot) joints in one capsule. The ulnar collateral ligament resists valgus stress and is injured in throwers; the radial collateral ligament resists varus and blends with the annular ligament around the radial head. A sudden pull on a young child's hand slips the head out of the annular ligament (nursemaid's elbow). The carrying angle is about 5 to 15 degrees, greater in women; a malunited supracondylar fracture can reverse it (cubitus varus). Pronation and supination also use the distal radioulnar joint, stabilized by the triangular fibrocartilage complex.

The wrist and hand

The radiocarpal joint is condyloid: the radius and the articular disc of the triangular fibrocartilage complex (TFCC) receive the scaphoid, lunate and triquetrum; the ulna does not touch the carpus. The midcarpal joint adds much flexion and radial deviation.

The first carpometacarpal joint is a saddle permitting opposition and is the hand's commonest site of osteoarthritis. Metacarpophalangeal joints are condyloid; their collateral ligaments are tight in flexion, so hands are splinted with MCPs flexed. Interphalangeal joints are hinges with a palmar (volar) plate that blocks hyperextension.

The hip

The hip is a deep ball and socket joint stabilized mainly by bone and ligament. The acetabular labrum and transverse acetabular ligament complete the socket. Three capsular ligaments spiral so that they tighten in extension: the iliofemoral (Y ligament of Bigelow, AIIS to intertrochanteric line), the strongest in the body, limiting extension; the pubofemoral, limiting abduction; and the ischiofemoral, limiting medial rotation. The ligament of the head adds little stability but carries a small artery. Most dislocations are posterior (dashboard injury), leaving the limb shortened, flexed, adducted and medially rotated and endangering the sciatic nerve.

The knee

The knee comprises the medial and lateral tibiofemoral joints and the patellofemoral joint. It is a modified hinge with poor bony fit, so it depends on menisci, ligaments and muscle.

The menisci are fibrocartilage wedges that deepen the tibial surfaces and spread load; only their peripheral third is vascular. The medial meniscus is C-shaped and bound to the capsule and MCL, so it is less mobile and more often torn. The lateral meniscus is nearly circular, more mobile and free of the LCL.

The cruciate ligaments are intracapsular but extrasynovial and named for their tibial attachments. The ACL runs from the anterior intercondylar area to the medial side of the lateral femoral condyle and prevents anterior translation of the tibia; noncontact pivoting tears it, and the Lachman test is the most sensitive sign. The stronger PCL runs from the posterior intercondylar area to the lateral side of the medial femoral condyle and prevents posterior translation; a dashboard blow tears it.

The flat MCL, blended with capsule and medial meniscus, resists valgus; the cordlike LCL, from lateral epicondyle to fibular head, resists varus. A lateral blow to a planted knee produces the "unhappy triad" of ACL, MCL and medial meniscus injury.

In terminal extension the tibia rotates laterally on the femur and locks the knee (the screw-home mechanism); popliteus unlocks it by rotating the femur laterally on the planted tibia.

The suprapatellar bursa communicates with the joint, so effusions ballot the patella. The prepatellar bursa inflames with kneeling (housemaid's knee), the superficial infrapatellar with upright kneeling, and a popliteal (Baker) cyst is usually a distended semimembranosus or gastrocnemius bursa.

The ankle and foot

The proximal tibiofibular joint is a plane joint; the distal one is a syndesmosis whose injury is a "high ankle sprain."

The talocrural joint is a hinge in which the talar trochlea sits in the malleolar mortise. The trochlea is wider anteriorly, so dorsiflexion is close-packed and stable, while plantarflexion is loose; most sprains therefore occur in plantarflexion with inversion.

Of the three lateral ligaments, the anterior talofibular (ATFL) is weakest and most often sprained, the calcaneofibular fails next and the posterior talofibular rarely tears. The fan-shaped medial deltoid ligament is so strong that eversion often avulses the medial malleolus instead.

The subtalar joint produces most inversion and eversion; the transverse tarsal (Chopart) joint (talonavicular plus calcaneocuboid) adds to it. The Lisfranc ligament binds the medial cuneiform to the second metatarsal base. The spring (plantar calcaneonavicular) ligament cradles the talar head, the long and short plantar ligaments support the lateral column, and the plantar aponeurosis tightens as the toes dorsiflex (windlass mechanism). Foot pronation combines eversion, abduction and dorsiflexion; supination the reverse.

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