Ch09: Upper Limb III — Posterior Arm & Forearm, Hand（臂后区与手）

# Ch09: Upper Limb III — Posterior Arm & Forearm, Hand

Preparatory Mindset

The posterior compartment of the arm & forearm is the extensor side, supplied by the radial nerve. The hand is where the median & ulnar nerves perform their final acts (thenar vs hypothenar, lumbricals, interossei) and where the flexor retinaculum (carpal tunnel) and palmar aponeurosis create clinically relevant spaces.

Exam mindset: Know the radial nerve's winding course (axilla → spiral groove → lateral epicondyle), the anatomical snuffbox, the compartments of the palm (thenar/hypothenar/middle), and the clinical tests for each nerve.

Core Concepts

1. Posterior compartment of the arm

• Muscles: triceps brachii (long, medial, lateral heads) + anconeus
• Innervation: radial nerve
• Action: elbow extension (triceps); anconeus assists
• Radial nerve enters the posterior compartment through the triangular interval (between teres major, long head of triceps, humerus) with the profunda brachii artery, runs in the spiral (radial) groove of the humerus, then passes anterior to the lateral epicondyle into the cubital fossa
• Clinical: radial nerve injury in the spiral groove (midshaft humerus fracture) → wrist drop (see Ch10)

2. Posterior forearm — extensor muscles

• Superficial layer (common extensor origin at lateral epicondyle): brachioradialis (actually flexes elbow — "anomalous extensor"), extensor carpi radialis longus & brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris
• Deep layer: supinator, abductor pollicis longus, extensor pollicis brevis & longus, extensor indicis
• Innervation: radial nerve (deep branch / posterior interosseous nerve)
• Anatomical snuffbox: between extensor pollicis longus (medial) & abductor pollicis longus + extensor pollicis brevis (lateral); floor = scaphoid & trapezium; contains radial artery (pulse) & superficial radial nerve branch
• Mnemonic for snuffbox tendons: "EPL" (medial) vs "APL+EPB" (lateral)

3. Deep fascia & retinacula of the wrist/hand

• Extensor retinaculum (dorsal carpal ligament): holds extensor tendons at the wrist
• Flexor retinaculum (transverse carpal ligament): bridges the carpal arch → forms the carpal tunnel; contents = median nerve + FDS, FDP & FPL tendons (9 tendons + 1 nerve)
• Guyon's canal: ulnar nerve & artery superficial to the flexor retinaculum, between pisiform & hook of hamate — NOT in the carpal tunnel
• Carpal canals: the carpal tunnel (under the retinaculum, for median nerve & long flexors); ulnar canal (Guyon's) for ulnar a./n.; radial carpal canal between the two layers of the retinaculum for FCR

4. Palm of the hand

- Lateral (thenar compartment): thenar muscles (APB, FPB, OP) + vessels/nerves of the thumb - Medial (hypothenar compartment): hypothenar muscles (ADM, FDM, ODM) + vessels/nerves of the little finger - Middle compartment: deep to the palmar aponeurosis — flexor tendons, lumbricals, superficial & deep palmar arches

• Palmar aponeurosis: deep fascia thickening; continuous with palmaris longus tendon
• Compartments (separated by septa):
• Thenar muscles innervation: median nerve (recurrent branch) — APB, OP, superficial head of FPB
• Hypothenar muscles: ulnar nerve
• Lumbricals: lateral 2 (median), medial 2 (ulnar); interossei: all ulnar
• Clinical: Dupuytren contracture (palmar aponeurosis fibrosis → flexion contracture of 4th/5th digits); palmar space infections; Palmaris longus absent in ~15% (donor for tendon graft)

5. Nerves of the hand (final distribution)

• Median nerve: recurrent (thenar) branch + digital branches to lateral 3½ digits (palmar); dorsal branches absent (dorsal lateral hand from radial)
• Ulnar nerve: deep branch (hypothenar, interossei, medial 2 lumbricals, adductor pollicis) + superficial (palmar digital to medial 1½ digits)
• Radial nerve: superficial branch supplies dorsum of hand (lateral 2½ digits, proximal phalanges); no motor in the hand
• Sensory "territory" memory: median = lateral 3½ palmar; ulnar = medial 1½ both; radial = lateral dorsum (thumb side)

6. Palmar arches

• Superficial palmar arch: mainly from ulnar artery (completes with superficial branch of radial) — deep to palmar aponeurosis
• Deep palmar arch: mainly from radial artery — deep to flexor tendons
• Clinical: distal to the arches, digits supplied by digital arteries (end arteries → digit necrosis risk)

High-Yield Points

• Radial nerve course: axilla → triangular interval → spiral groove (with profunda brachii) → anterior to lateral epicondyle → divides into superficial (sensory) & deep (posterior interosseous, motor) branches
• Anatomical snuffbox: EPL (medial) + APL & EPB (lateral); floor = scaphoid (fall on outstretched hand → scaphoid fracture); radial artery pulse
• Carpal tunnel: median nerve + 9 tendons (FDS×4, FDP×4, FPL×1); ulnar nerve is NOT inside
• Thenar muscles = median (recurrent); hypothenar, interossei, adductor pollicis, medial 2 lumbricals = ulnar
• "Ape hand" (median injury at wrist): thenar wasting, loss of opposition; "Claw hand" (ulnar injury): MP hyperextension + IP flexion of 4th/5th, interosseous wasting
• Dupuytren: palmar aponeurosis → 4th/5th flexion contracture
• Palmar arches: superficial = ulnar dominant; deep = radial dominant

Topic Summary

The posterior compartment (triceps/anconeus, radial nerve in the spiral groove with profunda brachii) is the extensor side. The posterior forearm extensors & the anatomical snuffbox (EPL vs APL+EPB; scaphoid floor; radial pulse) follow. The hand is organized by the flexor retinaculum (carpal tunnel: median nerve + 9 tendons; Guyon's canal: ulnar nerve outside the tunnel), the palmar aponeurosis (Dupuytren), and three compartments (thenar/median, hypothenar/ulnar, middle). Motor distribution: median → thenar (recurrent); ulnar → hypothenar, interossei, adductor pollicis, medial lumbricals; radial → none in hand. Palmar arches: superficial (ulnar) & deep (radial).