Carpal Tunnel Anatomy
On this page
Direct answer
Nine flexor tendons and one nerve share the carpal tunnel: the four tendons of flexor digitorum superficialis, four of flexor digitorum profundus and flexor pollicis longus travel with the median nerve beneath the flexor retinaculum, which is strapped from the scaphoid tubercle and trapezium ridge laterally to the pisiform and hook of the hamate medially. The concave carpal bones form the floor and walls; the retinaculum the unyielding roof. Any swelling inside — tenosynovitis, oedema of pregnancy or hypothyroidism, rheumatoid synovium, a lunate dislocation — compresses the median nerve and produces carpal tunnel syndrome: nocturnal paraesthesia of the lateral three and a half digits, thenar wasting and a positive Phalen or Tinel test. The ulnar nerve is never inside; it passes with the ulnar artery through the separate Guyon's canal in front of the retinaculum.
What you must remember
- Boundaries: roof — flexor retinaculum; floor — the carpal concavity (trapezium, trapezoid, capitate, hamate); lateral wall — scaphoid and trapezium; medial wall — pisiform and hamate with its hook.
- Contents: median nerve (most superficial, just under the retinaculum, on the lateral side of the tendons), FDS and FDP tendons within the common ulnar bursa, and flexor pollicis longus within the radial bursa — nine tendons, one nerve, two synovial sheaths.
- Sparing rule: the palmar cutaneous branch of the median nerve leaves the trunk in the forearm and passes superficial to the retinaculum, so sensation over the thenar eminence is spared in carpal tunnel syndrome — the single best discriminator from a proximal median lesion.
- Motor consequences: the recurrent (motor) branch arises at the distal edge of the retinaculum and supplies abductor pollicis brevis, flexor pollicis brevis (superficial head) and opponens pollicis — wasting of the thenar eminette with preserved sensation elsewhere clinches the level.
- Guyon's canal, next door: ulnar nerve and ulnar artery in front of the retinaculum between its superficial layer, the pisohamate ligament and the hook of hamate; compression here (cyclist's palsy, ganglion) affects the ulnar nerve with a normal median examination.
- Associations worth quoting: pregnancy, hypothyroidism, diabetes, rheumatoid arthritis, acromegaly, dialysis-related amyloidosis, and repetitive vibration exposure — the viva expects at least four.
- Treatment logic: night splints and steroids for mild disease; division of the flexor retinaculum (open or endoscopic) decompresses the tunnel when wasting or denervation appears.
A night-time case worked through
A woman in the third trimester wakes repeatedly at 3 a.m. with her hand "asleep", shaking it awake — the history alone nearly makes the diagnosis. Nocturnal oedema raises tissue pressure inside a compartment with walls of bone and unyielding ligament; the median nerve, the softest resident, fails first. Examination shows hypoaesthesia of the thumb, index, middle and radial half of ring fingers — but pinch the skin over the thenar eminence and it is normal, because the palmar cutaneous branch travelled outside the tunnel. Phalen's test (full wrist flexion for sixty seconds) reproduces the paraesthesia; Tinel's tap over the tunnel sparks them. Thenar abduction against resistance is weak and the thenar eminence flattens as abductor pollicis brevis wastes. Nerve conduction confirms slowing across the wrist segment. Pregnancy-associated cases mostly resolve after delivery — physiology reversing itself — whereas the postmenopausal or rheumatoid patient with wasting goes for retinacular release, a fifteen-minute operation whose entire rationale is the anatomy above.
Where students slip
Two errors dominate. First, placing the ulnar nerve inside the tunnel: it is not there, and a patient with numb little and ring fingers plus weak interossei has a Guyon's canal or cubital problem, never carpal tunnel. Second, forgetting the palmar cutaneous branch when localising a lesion — a candidate who says "thenar sensation is lost in carpal tunnel syndrome" has just mis-localised the lesion to the forearm or above, and the examiner will follow up accordingly. A third, subtler slip: the retinaculum also gives origin to thenar and hypothenar muscles and serves as a flexor pulley, so its division transiently weakens grip mechanics.
Frequently asked questions
What structures bound the carpal tunnel?
The flexor retinaculum as roof; the carpal bones (trapezium, trapezoid, capitate, hamate) as the floor and walls, with scaphoid and pisiform at the proximal corners.
Name the nine tendons passing through the carpal tunnel.
Four flexor digitorum superficialis, four flexor digitorum profundus, and one flexor pollicis longus tendon — sharing the tunnel with the median nerve.
Why is thenar skin sensation spared in carpal tunnel syndrome?
The palmar cutaneous branch of the median nerve arises proximal to the wrist and passes superficial to the flexor retinaculum, escaping compression.
How do Phalen's and Tinel's tests work?
Phalen's sustains full wrist flexion for sixty seconds to press the nerve against the retinaculum; Tinel's percusses over the tunnel, both reproducing the characteristic paraesthesia.
What is Guyon's canal and which structures pass through it?
A fibro-osseous passage in front of the retinaculum between the pisiform and hook of hamate, transmitting the ulnar nerve and ulnar artery — the site of cyclist's palsy.