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AN12.1-15 | Forearm & hand — Part 1

Part 1: Ventral (Anterior) Forearm — Muscles, Nerves, and Vessels

The front of the forearm contains the anterior (flexor) compartment — 8 muscles arranged in three layers, almost entirely supplied by the median nerve, with one important exception.

Superficial Layer (4 muscles — origin: medial epicondyle via common flexor origin):

• Pronator teres — pronates the forearm; nerve: median; two heads (humeral + ulnar) with the median nerve passing between them
• Flexor carpi radialis (FCR) — flexes and abducts the wrist; nerve: median; its tendon is the landmark for the radial artery pulse at the wrist
• Palmaris longus — weakly flexes wrist; nerve: median; absent in ~15% of people; its tendon is harvested for tendon grafts
• Flexor carpi ulnaris (FCU) — flexes and adducts the wrist; nerve: ulnar nerve (C8, T1) — the only superficial flexor NOT supplied by the median nerve

Intermediate Layer (1 muscle):
• Flexor digitorum superficialis (FDS) — flexes the middle phalanges of fingers 2–5; nerve: median nerve; tendons split at the proximal phalanx to allow FDP to pass through

Deep Layer (3 muscles):
• Flexor digitorum profundus (FDP) — flexes distal phalanges (fingers 2–5); nerve: medial half (ring + little) by ulnar nerve; lateral half (index + middle) by the anterior interosseous nerve (AIN, a branch of median)
• Flexor pollicis longus (FPL) — flexes the thumb's interphalangeal joint; nerve: AIN (median)
• Pronator quadratus — pronates forearm at the distal radioulnar joint; nerve: AIN; deepest muscle, directly on radius and ulna

The Anterior Interosseous Nerve (AIN): A purely motor branch of the median nerve. It supplies FDP (lateral half), FPL, and pronator quadratus. Injury causes inability to form an "OK" sign — the index finger and thumb cannot form a circle because terminal flexion is lost.

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Nerves and Vessels of the Forearm:

Nerve	Motor in forearm	Sensory
Median	All flexors except FCU and medial FDP	Lateral 3½ fingers (palm)
Ulnar	FCU, medial FDP	Medial 1½ fingers
Radial (superficial)	None (purely sensory in forearm)	Dorsal lateral hand
Posterior interosseous n.	All posterior compartment muscles	None

The radial artery pulse is palpable at the wrist between the FCR tendon and the radial styloid. The ulnar artery is accompanied by the ulnar nerve in the distal forearm, passing through Guyon's canal at the wrist.

Part 2: Flexor Retinaculum, Carpal Tunnel Syndrome & Muscles of the Hand

The Flexor Retinaculum

The flexor retinaculum (transverse carpal ligament) is a thick fibrous band bridging across the concavity of the carpal bones, converting it into the carpal tunnel.

Attachments: Medially — pisiform and hook of hamate. Laterally — scaphoid tubercle and trapezium ridge.

Contents of the carpal tunnel:
1. Flexor digitorum superficialis (4 tendons)
2. Flexor digitorum profundus (4 tendons)
3. Flexor pollicis longus (1 tendon)
4. Median nerve — the ONLY nerve in the carpal tunnel

Note: The ulnar nerve and ulnar artery pass through Guyon's canal (ulnar canal) — superficial and medial to the retinaculum, NOT through the carpal tunnel.

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Carpal Tunnel Syndrome (AN12.4)

CTS is compression of the median nerve within the carpal tunnel.

Anatomical basis: Any increase in volume within the fixed space of the carpal tunnel (thickened flexor tendon sheaths, oedema in pregnancy, hypothyroidism) compresses the median nerve.

Symptoms: Pain and tingling in the lateral 3½ fingers; worse at night; weakness and wasting of the thenar eminence (especially abductor pollicis brevis — exclusively supplied by the recurrent branch of median nerve just distal to the retinaculum).

Clinical tests: Tinel's sign (tap over carpal tunnel = reproduces symptoms); Phalen's test (sustained wrist flexion for 60 s).

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Small Muscles of the Hand (AN12.5)

Thenar muscles (thumb side): APB (abducts thumb ⊥ palm; median), opponens pollicis (opposes; median), FPB (flexes proximal phalanx; median superficial head + ulnar deep head), adductor pollicis (adducts; ulnar deep branch — NOT a thenar muscle by position).

Hypothenar muscles (little finger side): All supplied by the ulnar nerve (deep branch).

Intermediate muscles:
• Lumbricals (4): Flex MCPJs + extend IPJs; lateral 2 by median, medial 2 by ulnar
• Palmar interossei (3): ADduct fingers (PAD = Palmar ADduct); all ulnar
• Dorsal interossei (4): ABduct fingers (DAB = Dorsal ABduct); all ulnar

Movements of the Thumb (AN12.6)

The carpometacarpal joint (CMCJ) of the thumb is a saddle joint — the most mobile joint in the hand. Opposition (abduction + flexion + medial rotation) is the uniquely human movement that makes tool use possible.

Movement	Key muscles	Nerve
Opposition	Opponens pollicis + FPB + APB	Median
Adduction	Adductor pollicis	Ulnar
Extension	EPL, EPB	Radial

Part 3: Vessels and Nerves of the Hand, Claw Hand, and Palmar Infections

Arterial Supply of the Hand (AN12.7)

The hand is supplied by the radial and ulnar arteries, anastomosing to form two arches:

Superficial palmar arch: Formed mainly by the ulnar artery, completed by the superficial branch of the radial artery. Lies deep to the palmar aponeurosis; gives 3 common palmar digital arteries → proper digital arteries for the medial 3½ fingers.

Deep palmar arch: Formed mainly by the radial artery (via the anatomical snuffbox and 1st dorsal interosseous space), completed by the deep branch of ulnar. Gives palmar metacarpal arteries.

Allen's test checks patency of both arteries before radial artery cannulation or harvest.

Nerves of the Hand:

Nerve	Motor supply	Sensory supply
Median (recurrent + digital branches)	Thenar (APB, OP, FPB-sup), lateral 2 lumbricals	Lateral 3½ fingers (palm + dorsal tips)
Ulnar (deep + superficial)	Hypothenar, medial 2 lumbricals, all interossei, adductor pollicis, FPB-deep	Medial 1½ fingers
Radial (superficial branch only)	None in the hand	Dorsal lateral hand only

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Claw Hand (AN12.8)

Claw hand (ulnar nerve injury) results from:
• Lumbricals and interossei (ulnar) paralysed → cannot flex MCPJs or extend IPJs
• Long flexors (FDP, FDS — intact) → flex the IPJs unopposed
• Extensor digitorum (radial, intact) → extends the MCPJs unopposed
• Result: MCPJs extended + IPJs flexed = claw

Why worse in ring and little fingers? The lateral 2 lumbricals (index + middle) are supplied by the median nerve — they partially correct those fingers. Only medial 2 lumbricals (ring + little) are ulnar.

Paradox: Low ulnar lesion (at wrist) → more pronounced claw than high lesion (at elbow), because in high lesions FDP (medial half) is also paralysed, reducing the IPJ flexion.

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Fibrous Flexor Sheaths and Bursae (AN12.9)

• Fibrous flexor sheaths: Pulley-like tunnels on the palmar surface of each finger, holding tendons against bone and preventing bowstringing
• Ulnar bursa (common flexor sheath): Encloses all tendons entering the carpal tunnel; extends from proximal to the wrist to mid-palm. The little finger's sheath connects continuously with the ulnar bursa.
• Radial bursa: Encloses FPL tendon; extends to the thumb's terminal phalanx

If the two bursae communicate (in ~50% of people), infection of either can spread → horseshoe abscess.

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Fascial Spaces of the Palm (AN12.10)

Two potential spaces deep to the palmar aponeurosis:
• Thenar space (lateral): Infection = swelling of thenar eminence + immobile index finger
• Mid-palmar space (medial): Infection = dorsal + palmar oedema, semiflexed fingers

Infections spread via lumbrical canals to the dorsal web spaces → "collar-stud abscesses."