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AN12.1-15 | Forearm & hand — Part 2

Part 4: Dorsal Forearm — Muscles, Nerves, Vessels, and Wrist Drop

The Posterior (Extensor) Compartment (AN12.11)

The posterior forearm contains 12 muscles in two layers, all supplied by the posterior interosseous nerve (PIN = deep branch of radial nerve), EXCEPT brachioradialis and extensor carpi radialis longus (ECRL) — which are supplied by the radial nerve proper.

Superficial Layer (7 muscles, from lateral epicondyle):
Brachioradialis, ECRL, ECRB (extensor carpi radialis brevis), extensor digitorum (ED), extensor digiti minimi (EDM), extensor carpi ulnaris (ECU), anconeus.

Deep Layer (5 muscles, from posterior radius/ulna/interosseous membrane):
Supinator, abductor pollicis longus (APL), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), extensor indicis (EI).

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Nerves and Vessels of the Back of the Forearm (AN12.12)

Posterior interosseous nerve (PIN):
• Deep branch of the radial nerve
• Enters the posterior compartment by passing through the supinator muscle (winding around the lateral neck of radius)
• Supplies ALL posterior compartment muscles except brachioradialis + ECRL
• Terminates as the posterior interosseous nerve of the wrist (sensory to dorsal wrist joint)

Compression at the supinator → radial tunnel syndrome (pain without sensory loss) or PIN palsy (finger drop without wrist drop, since ECRL intact).

Superficial radial nerve: Purely sensory; runs under brachioradialis; supplies dorsal lateral hand and proximal 3½ fingers (dorsal).

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Wrist Drop (AN12.13)

Wrist drop = radial nerve injury in the spiral groove of the humerus (mid-shaft fracture).

Muscles paralysed: All wrist extensors (ECRL, ECRB, ECU), extensor digitorum, EDM, EI, APL, EPL, EPB.

Triceps often spared (its nerve branches arise proximal to the spiral groove).

Level of injury	Wrist drop	Triceps	Finger drop
Axilla	Yes	Yes (paralysed)	Yes
Spiral groove	Yes	Usually spared	Yes
Lateral epicondyle (PIN)	No	Spared	Yes

Mnemonic: BEST muscles lost in wrist drop = Brachioradialis, Extensors, Supinator, Triceps (sometimes).

Part 5: Extensor Retinaculum, Anatomical Snuffbox, and Extensor Expansion

The Extensor Retinaculum (AN12.14)

The extensor retinaculum is a thickened band of deep fascia on the dorsum of the wrist, running from the lateral radius to the triquetrum and pisiform medially. It sends vertical septa creating 6 osseofibrous compartments:

Compartment	Tendons	Clinical note
1 (lateral)	APL + EPB	De Quervain's tenosynovitis here
2	ECRL + ECRB	—
3	EPL	Winds around Lister's tubercle
4	ED + EI	—
5	EDM	—
6 (medial)	ECU	—

De Quervain's tenosynovitis: Inflammation of the 1st compartment (APL + EPB). Pain at the radial wrist aggravated by thumb movement. Positive Finkelstein's test (ulnar deviation of wrist while thumb is flexed across palm).

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The Anatomical Snuffbox

The anatomical snuffbox is a triangular depression visible on the dorsolateral wrist when the thumb is extended and abducted.

• Medial (posterior) boundary: Tendon of EPL
• Lateral (anterior) boundary: Tendons of APL and EPB
• Floor: Scaphoid and trapezium (also the radial artery and branches of the superficial radial nerve)
• Roof: Superficial radial nerve, cephalic vein

Clinical importance: Tenderness in the anatomical snuffbox after a FOOSH injury = suspect scaphoid fracture even with a normal X-ray. The radial artery passes through the floor to reach the palm and form the deep palmar arch.

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Extensor Expansion (Dorsal Digital Expansion) (AN12.15)

The extensor expansion (extensor hood) is a triangular fibrous aponeurosis on the dorsum of each finger.

Formation (mnemonic: ELI):
• E = Extensor digitorum tendon (fans out over dorsum of proximal phalanx)
• L = Lumbricals (contribute via lateral bands on each side)
• I = Interossei (contribute via lateral bands on each side)

Insertions:
• Central slip → base of middle phalanx
• Lateral bands (convergence of lumbrical + interosseous contributions + ED) → base of distal phalanx

Pathology:
• Boutonnière deformity: Rupture of the central slip → middle phalanx herniates through lateral bands → PIPJ flexed + DIPJ extended
• Swan-neck deformity: Volar plate laxity at PIPJ → lateral bands shift dorsally → PIPJ hyperextended + DIPJ flexed