Upper Limb Anatomy · Clinical

Carpal Tunnel Contents

The carpal tunnel transmits nine tendons and one nerve beneath the flexor retinaculum. Anything that increases tunnel pressure — synovial thickening, fluid retention, mass lesion — compresses the median nerve, producing the most common entrapment neuropathy in clinical practice.

✦ The Mnemonic

"Four Scouts, Four Deputies, Plus One Flag-bearer, Needs the Map"

4 × FDS, 4 × FDP, 1 × FPL, Median nerve — nine tendons plus one nerve

4 FDS × 4 Flexor Digitorum Superficialis — four tendons, arranged in two planes

4 FDP × 4 Flexor Digitorum Profundus — four tendons, posterior to FDS

1 FPL × 1 Flexor Pollicis Longus — sole long flexor of the thumb IP joint

N Median Nerve Most vulnerable structure; gives LOAF muscles in the hand

📚 Clinical Breakdown

The carpal tunnel is bounded by the carpal bones (floor and sides) and the flexor retinaculum (roof). The FDS tendons are arranged with the middle and ring finger tendons anterior to the index and little finger tendons. The FDP tendons lie posterior as a single plane.

The median nerve lies most superficially, immediately deep to the flexor retinaculum. This position makes it the most susceptible structure to compression. At the distal edge of the retinaculum, the nerve gives the recurrent (thenar) motor branch, which supplies the LOAF muscles: Lumbricals 1&2, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis.

The flexor retinaculum attaches medially to the pisiform and hook of hamate, and laterally to the scaphoid tubercle and trapezium ridge. The ulnar nerve and ulnar artery pass superficial to the retinaculum in Guyon's canal — these structures are NOT within the carpal tunnel.

Tunnel boundaries Flexor retinaculum (roof); carpal bones (walls + floor)

Nerve compressed Median nerve (most superficial content)

Hand muscles (LOAF) Lumbricals 1&2, Opponens pollicis, Abd. poll. brevis, FPB

Outside tunnel Ulnar nerve & artery pass in Guyon's canal

⭐ Clinical Pearl

Phalen's test: maximum wrist flexion for 60 seconds reproduces symptoms — sensitivity ~75%. Tinel's sign: percussion over the tunnel produces paraesthesia in the median nerve distribution — sensitivity ~50%. A positive Phalen's is more reliable. The classic sensory deficit spares the thenar eminence (palmar cutaneous branch exits proximal to the tunnel).

⚠ Exam Trap

The palmar cutaneous branch of the median nerve arises 5 cm proximal to the flexor retinaculum and crosses superficially — it is NOT compressed in carpal tunnel syndrome. This is why thenar skin sensation is PRESERVED in CTS, despite thenar muscle wasting. This distinction has appeared in many final examinations.

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