2.2 Osteology of the Appendicular Skeleton

Upper limb and shoulder girdle

The pectoral girdle is the clavicle plus scapula. The only synovial joint tethering the upper limb to the axial skeleton is the sternoclavicular joint; the scapulothoracic interface is a physiologic (not synovial) joint. Axial osteology of ribs and vertebrae is a Spinal Anatomy topic; stay on appendicular named features here.

Clavicle. S-shaped strut: convex forward in the medial two-thirds, concave forward in the lateral third. Inferior markings: subclavian groove (subclavius), conoid tubercle and trapezoid line for the coracoclavicular ligaments, and a rough impression for the costoclavicular ligament medially. It is the most commonly fractured bone; the middle third is the usual site, with the medial fragment pulled up by SCM and the lateral fragment pulled down by limb weight. The clavicle is the first bone to ossify and one of the few bones with a mixed intramembranous shaft and endochondral ends.

Scapula. The glenoid fossa is pear-shaped and shallow; supraglenoid and infraglenoid tubercles anchor the long heads of biceps and triceps. The coracoid takes pectoralis minor, coracobrachialis, and the short head of biceps. The suprascapular notch is bridged by the superior transverse scapular ligament: artery over, nerve under (suprascapular artery and nerve). The nerve then turns through the spinoglenoid notch to infraspinatus. The spine ends as the acromion; the spinoglenoid relationship explains isolated infraspinatus atrophy from a paralabral cyst.

Humerus. Distinguish the anatomical neck (immediately distal to the head; capsular attachment) from the surgical neck (metaphysis below the tubercles)—the surgical neck is the common fracture site and the relation for the axillary nerve and posterior circumflex humeral artery. Greater-tubercle facets: supraspinatus, infraspinatus, teres minor (superior to posterior). Lesser tubercle: subscapularis. Intertubercular (bicipital) sulcus: pectoralis major on the lateral lip, teres major on the medial lip, latissimus dorsi on the floor (a lady between two majors). The spiral (radial) groove on the posterior shaft holds the radial nerve and profunda brachii artery—midshaft fracture territory and a wrist-drop stem. Distal humerus: capitulum (radius), trochlea (ulna), coronoid, radial, and olecranon fossae, medial epicondyle (common flexor origin; ulnar nerve immediately posterior), lateral epicondyle (common extensor origin).

Radius and ulna. Radial head is cylindrical (proximal radioulnar joint, held by the annular ligament); radial tuberosity takes biceps; the distal radius has Lister's tubercle (extensor pollicis longus turns around it) and a styloid that sits distal to the ulnar styloid. Ulnar olecranon and coronoid form the trochlear notch; ulnar tuberosity takes brachialis. Nutrient-foramen mnemonic for long bones: arteries grow toward the elbow and away from the knee.

Carpus. Proximal row, lateral to medial: scaphoid, lunate, triquetrum, pisiform. Distal row, lateral to medial: trapezium, trapezoid, capitate, hamate. A useful circling mnemonic is So Long To Pinky, Here Comes The Thumb (proximal row thumb to pinky, then distal row pinky back to thumb). Scaphoid is the most commonly fractured carpal (waist); blood enters distal-to-proximal from the dorsal carpal branch of the radial artery, so a proximal-pole fragment can undergo avascular necrosis. Lunate is the most commonly dislocated carpal (perilunate pattern). Capitate is the largest and the first carpal to ossify (about age 1). Pisiform is a sesamoid in flexor carpi ulnaris. The hook of the hamate bounds Guyon's canal with the pisiform (ulnar nerve and artery).

Metacarpals have bases, shafts, and heads (knuckles are heads). Phalanges: proximal, middle, distal except the thumb (two phalanges).

Lower limb and pelvis

The hip bone (os coxae) is ilium, ischium, and pubis fused at the acetabulum. In the child the triradiate cartilage separates the three bones and fuses in mid-adolescence (often cited around 15–17 years). Acetabular contributions: ilium forms the superior roof, ischium the posterior wall, pubis the anterior wall.

The greater sciatic notch becomes a foramen with the sacrospinous ligament; the lesser sciatic notch with sacrospinous plus sacrotuberous. The obturator foramen is closed by the obturator membrane except at the obturator canal (obturator nerve, artery, vein). Male versus female pelvis: female inlet is wider and more circular (gynecoid), subpubic angle is broader, and the ischial spines are everted—recognition level, not obstetric management.

Femur. The head has a fovea capitis for the ligamentum teres. Angle of inclination between neck and shaft is about 125° in the adult (larger in the infant, so coxa valga of childhood; coxa vara is a decreased angle). Femoral anteversion is about 10–15° in adults. Greater trochanter: gluteus medius and minimus, piriformis, obturator internus and gemelli, obturator externus in the trochanteric fossa. Lesser trochanter: iliopsoas. Anterior intertrochanteric line (iliofemoral ligament, capsular attachment) versus posterior intertrochanteric crest (quadrate tubercle for quadratus femoris). Linea aspera is the posterior shaft ridge; adductor tubercle on the medial condyle takes the hamstring part of adductor magnus.

Adult femoral-head blood supply is dominated by retinacular branches of the medial circumflex femoral artery (posterior femoral neck). Lateral circumflex contributes less. The artery of the ligamentum teres (acetabular branch of obturator) is more important in children and insufficient to save an adult head after a displaced neck fracture—hence AVN risk.

Patella is the largest sesamoid, embedded in the quadriceps tendon, with a lateral articular facet that is larger than the medial facet and carries the body's thickest articular cartilage; it tracks in the trochlear groove.

Tibia and fibula. Tibial plateau (medial concave, lateral convex), Gerdy's tubercle (iliotibial band), tibial tuberosity (patellar ligament; the apophysis of Osgood-Schlatter), soleal line, and medial malleolus. The fibular neck is the relation for the common fibular (peroneal) nerve—foot-drop after a tight cast, fibular-neck fracture, or poorly padded table edge. The lateral malleolus extends more distal and more posterior than the medial malleolus, which is why inversion sprains outnumber eversion injuries and why the mortise is deeper laterally.

Tarsus. Talus (trochlea, neck, head, posterior and lateral processes) has no muscular attachments; ligaments and joint capsules only. Talar-neck fractures threaten the intraosseous blood supply and can cause AVN of the body. Calcaneus is the largest tarsal; sustentaculum tali supports the middle talocalcaneal facet, has a groove for flexor hallucis longus, and gives attachment to the spring (plantar calcaneonavicular) ligament. Navicular tuberosity: tibialis posterior. Cuboid: groove for fibularis longus. Three cuneiforms (medial, intermediate, lateral) complete the distal row. Metatarsals and phalanges parallel the hand; the first metatarsal head has paired sesamoids in flexor hallucis brevis.

Development

Upper limb buds appear in week 4; lower buds follow by a day or two. The apical ectodermal ridge (AER) maintains proximal-to-distal outgrowth (femur then tibia then foot). The zone of polarizing activity in the posterior mesenchyme uses sonic hedgehog to pattern preaxial versus postaxial digits (thumb and hallux are preaxial; radius and tibia are preaxial bones). Dorsal-ventral patterning involves Wnt from dorsal ectoderm.

The upper limb undergoes about 90° of lateral rotation; the lower limb undergoes about 90° of medial rotation. That is why dermatomes spiral on the lower limb, why the flexor (ventral) compartments end up anterior in the arm and posterior in the thigh, and why the knee points forward while the elbow points backward.

Most appendicular bones form by endochondral ossification: a hyaline cartilage model, a primary ossification center in the diaphysis, then secondary centers in the epiphyses. The growth plate (physis) is a synchondrosis that fuses after puberty. Exceptions and timing high-yields:

Primary centers appear in fetal life for the long-bone shafts. Secondary centers appear from late fetal life through childhood in a sequence used for bone-age films. Accessory ossicles (os trigonum behind the talus, bipartite patella superolaterally, os naviculare) must be distinguished from fractures by smooth corticated edges—recognition, not a radiology course.

Histology of bone

Woven bone is immature: randomly oriented type I collagen, more cells, weaker. It is normal in the fetus and in the early fracture callus and is abnormal when it persists in adult lamellar sites (some tumors, Paget disease—pathology chapter). Lamellar bone is mature, with collagen sheets aligned to load.

Compact (cortical) bone is organized into osteons (Haversian systems):

Interstitial lamellae are remnants of old osteons; circumferential lamellae wrap the inner and outer cortex. Cancellous (trabecular) bone has lamellae in plates aligned by Wolff's law, with marrow in the spaces and no true osteons in the smallest trabeculae.

Cells. Osteoprogenitors in the periosteal cambium and endosteum become osteoblasts, which secrete osteoid and are rich in alkaline phosphatase. Osteoblasts trapped in matrix become osteocytes, the mechanosensors that canaliculi connect. Osteoclasts are multinucleated, hematopoietic (monocyte) lineage cells that sit in Howship lacunae, seal a ruffled border, and resorb bone under RANKL signaling from osteoblasts and osteocytes; they stain for tartrate-resistant acid phosphatase (TRAP).

Matrix is organic osteoid (type I collagen, proteoglycans) plus mineral hydroxyapatite. Periosteum has an outer fibrous layer and an inner osteogenic (cambium) layer; Sharpey fibers tether periosteum into cortex. Endosteum lines marrow spaces. Articular surfaces are covered by hyaline cartilage, not periosteum—so they do not heal by periosteal callus.

Sesamoid bones (patella, pisiform, hallux sesamoids, occasional fabella in lateral gastrocnemius) ossify within tendons where they alter the moment arm and reduce friction. They are still lamellar bone histologically.

Muscle attachments and innervation of these bones belong to the next General Anatomy chapter on appendicular myology; foramina and vertebral ossification belong to Spinal Anatomy axial osteology at /study-guides/nbce-part1/spinal-anatomy-axial/axial-osteology.