4.5 Arm, Forearm, Wrist & Hand

Skeleton of the Arm and Forearm

The humerus has a head (articulates with the glenoid), an anatomical neck (fracture risks the axillary nerve and posterior circumflex humeral artery) and surgical neck (most common fracture site, also risks the axillary nerve), greater and lesser tubercles separated by the intertubercular (bicipital) sulcus (transmits the tendon of the long head of biceps), the radial (spiral) groove on the posterior shaft (the radial nerve and profunda brachii artery run here — a mid-shaft humerus fracture risks wrist drop), the medial and lateral epicondyles, the trochlea (articulates with the ulnar trochlear notch), the capitulum (articulates with the radial head), and the olecranon fossa (receives the olecranon in extension).

The radius is lateral in the anatomical position; its head articulates with the capitulum and the radial notch of the ulna, the radial tuberosity receives the biceps brachii tendon, and the styloid process projects laterally at the wrist. The ulna is medial; its olecranon (palpable at the elbow, receives the triceps tendon), trochlear notch, coronoid process, and styloid process are key landmarks. The interosseous membrane connects the shafts and transmits force from the radius to the ulna.

Arm Compartments

Two osseofascial compartments of the arm:

• Anterior (flexor) compartment — biceps brachii (flexes the elbow and supinates the forearm when flexed; musculocutaneous nerve C5–C6), brachialis (primary elbow flexor, lies deep, musculocutaneous, with a small radial supply), and coracobrachialis (flexes and adducts; musculocutaneous). The musculocutaneous nerve pierces coracobrachialis and emerges as the lateral cutaneous nerve of the forearm.
• Posterior (extensor) compartment — triceps brachii (three heads: long, lateral, medial; extends the elbow; radial nerve C6–C8) and anconeus (elbow extension, radial nerve).

Forearm Compartments

The forearm has anterior and posterior compartments separated by the interosseous membrane and the lateral intermuscular septum.

• Anterior compartment (flexors/pronators) — superficial layer: pronator teres, flexor carpi radialis (FCR), palmaris longus (often absent, useful as a tendon graft donor), flexor carpi ulnaris (FCU), and flexor digitorum superficialis (FDS); deep layer: flexor digitorum profundus (FDP), flexor pollicis longus (FPL), and pronator quadratus. Innervation: the median nerve supplies all except FCU and the medial half of FDP (to digits 4–5), which are supplied by the ulnar nerve. This is the basis of the ulnar paradox: a high ulnar lesion looks less clawed than a low one because FDP is also paralyzed and cannot flex the proximal and distal interphalangeal joints, so the fingers are less flexed at rest.
• Posterior compartment (extensors) — brachioradialis (flexes the elbow in midprone; radial nerve), extensor carpi radialis longus and brevis (ECRL/ECRB), extensor digitorum, extensor digiti minimi, extensor carpi ulnaris (ECU), and the deep group: supinator, abductor pollicis longus (APL), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), extensor indicis (EI). All are innervated by the radial nerve (the posterior interosseous nerve branch supplies the deep group).

Major Nerves in the Upper Limb

• Median nerve (C5–T1) — supplies most anterior forearm muscles and the thenar muscles and lateral two lumbricals in the hand (the LOAF group: Lumbricals 1–2, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis superficial head). It enters the hand through the carpal tunnel (deep to the flexor retinaculum, with the FDS, FDP, and FPL tendons). Compression there → carpal tunnel syndrome: thenar weakness and atrophy, numbness of the lateral 3½ digits, positive Tinel and Phalen signs.
• Ulnar nerve (C8–T1) — supplies FCU, medial FDP, all interossei, lumbricals 3–4, the hypothenar muscles, adductor pollicis, and the deep head of FPB. It enters the palm superficial to the flexor retinaculum via Guyon's canal and is vulnerable at the medial epicondyle (the "funny bone"). Injury → claw hand (hyperextension of the MCP joints with flexion of the IP joints of digits 4–5, hypothenar atrophy, loss of finger abduction/adduction — recall "PAD-DAB": Palmar interossei Adduct, Dorsal interossei Abduct).
• Radial nerve (C5–T1) — supplies all extensors; injury at the spiral groove (mid-shaft humerus fracture) or in the axilla (crutch palsy) produces wrist drop, with loss of finger and thumb extension (and elbow extension if the lesion is proximal). The posterior interosseous nerve branch (within the supinator) supplies the deep extensors; a pure PIN lesion spares ECRL and brachioradialis, so there is no wrist drop — only loss of finger and thumb extension.

Wrist and Hand

The carpal bones (eight, in two rows) are memorized lateral-to-medial with the mnemonic "Some Lovers Try Positions That They Can't Handle": proximal row scaphoid, lunate, triquetrum, pisiform; distal row trapezium, trapezoid, capitate, hamate (with its hook-like hook of hamate). The scaphoid is the most commonly fractured carpal bone (fall on an outstretched hand, snuffbox tenderness) and has a retrograde blood supply from the dorsal carpal branch of the radial artery entering distally — a proximal-pole fracture risks avascular necrosis of the proximal fragment.

The carpal tunnel is bounded by the carpal bones (arch) and roofed by the flexor retinaculum (transverse carpal ligament); it contains the median nerve and the nine flexor tendons (FDS ×4, FDP ×4, FPL ×1). The flexor retinaculum also gives attachment to the thenar and hypothenar muscles.

The extensor tendons cross the wrist under the extensor retinaculum, which forms six fibro-osseous compartments: I — APL + EPB (site of de Quervain tenosynovitis), II — ECRL + ECRB, III — EPL, IV — EIP + extensor digitorum, V — EDM, VI — ECU.

Intrinsic Hand Muscles

Three groups: thenar (abductor pollicis brevis, flexor pollicis brevis superficial head, and opponens pollicis — median nerve, except the deep head of FPB by the ulnar nerve), hypothenar (abductor, opponens, and flexor digiti minimi — ulnar nerve), and the midpalmar muscles: lumbricals (1–2 median; 3–4 ulnar) flex the MCP and extend the IP joints; interossei (three palmar adduct, four dorsal abduct — "PAD-DAB"; ulnar nerve); and adductor pollicis (ulnar nerve).