Upper limb & breast · Free article
Wrist anatomy for MRCS Part A with a 3D cross-section: flexor retinaculum, carpal tunnel contents, Guyon's canal, the six extensor compartments and snuffbox.
The eight carpal bones lie in two rows. The proximal row, radial to ulnar, is scaphoid, lunate, triquetrum and pisiform; the distal row is trapezium, trapezoid, capitate and hamate. Together they form an arch that is concave on the palmar side, deepened at its radial edge by the tubercle of the scaphoid and the ridge of the trapezium and at its ulnar edge by the pisiform and the hook of the hamate. Across these four bony points stretches the flexor retinaculum (transverse carpal ligament), a tough band about 2.5 cm square that converts the arch into the carpal tunnel. Proximally it is continuous with the deep fascia of the forearm; distally it blends with the palmar aponeurosis, into which palmaris longus inserts across its front. The thenar muscles arise from its radial half with the scaphoid and trapezium, the hypothenar muscles from its ulnar half with the pisiform and hook of hamate. The point to hold on to is that the tunnel has bone on three sides and an unyielding ligament on the fourth, so anything that swells its contents or narrows its walls raises the pressure on the one structure inside that cannot tolerate it, the median nerve.
| Structure | Where it runs | Why it matters |
|---|---|---|
| Palmar cutaneous branch of the median nerve | Arises about 5 cm above the wrist crease, runs between flexor carpi radialis and palmaris longus and crosses the retinaculum superficially to the skin of the thenar eminence and central palm | Palm sensation is spared in carpal tunnel syndrome; a painful neuroma follows if a radially placed incision cuts it |
| Ulnar nerve and artery | Guyon's canal, between the pisiform and the hook of hamate: floor the flexor retinaculum and pisohamate ligament, roof the palmar carpal ligament and palmaris brevis. The nerve divides into a superficial sensory and a deep motor branch that hooks round the hamate | Compression paralyses the intrinsics but spares the dorsum of the hand; the artery lies radial to the nerve and continues as the superficial palmar arch |
| Dorsal branch of the ulnar nerve | Leaves the nerve about 5 cm above the wrist and winds beneath flexor carpi ulnaris onto the dorsum | Dorsal ulnar sensation is spared in a Guyon's canal lesion and lost in a lesion at the elbow |
| Flexor carpi radialis | In its own tunnel within the radial part of the retinaculum, in the groove on the trapezium | The landmark tendon: radial artery on its radial side, median nerve on its ulnar side and deeper |
| Palmaris longus | Superficial to the retinaculum into the palmar aponeurosis; absent in roughly one person in seven | The median nerve lies directly beneath and slightly radial to it; a useful tendon graft |
| Radial artery | Between flexor carpi radialis and brachioradialis, then dorsally beneath APL and EPB across the snuffbox to form the deep palmar arch; a superficial palmar branch crosses the thenar muscles | The pulse and the arterial line; Allen's test checks that the ulnar artery can supply the hand alone |
| Ulnar artery | Radial to the ulnar nerve, through Guyon's canal to become the main supply of the superficial palmar arch | The arch lies distal to the retinaculum, at about the level of the outstretched thumb, so decompression stops short of it |
| Compartment | Tendons | Landmark | Clinical association |
|---|---|---|---|
| 1 (most radial) | Abductor pollicis longus, extensor pollicis brevis | Radial border of the snuffbox, over the radial styloid | De Quervain's tenosynovitis, with pain on Finkelstein's test; intersection syndrome where these tendons cross the second compartment 4–6 cm above the wrist |
| 2 | Extensor carpi radialis longus and brevis | Radial to Lister's tubercle | Longus is supplied by the radial nerve above its division, so wrist extension survives a posterior interosseous palsy with radial deviation |
| 3 | Extensor pollicis longus | Hooks round the ulnar side of Lister's tubercle; ulnar border of the snuffbox | Attrition rupture a few weeks after an undisplaced distal radius fracture: the thumb can no longer lift off a flat table |
| 4 | Extensor digitorum, extensor indicis | In line with the third metacarpal | The terminal branch of the posterior interosseous nerve lies on the floor and supplies the wrist joint; the usual dorsal approach to the wrist |
| 5 | Extensor digiti minimi | Over the distal radio-ulnar joint | Ruptures in rheumatoid synovitis of the joint (Vaughan-Jackson), the little finger dropping first |
| 6 (most ulnar) | Extensor carpi ulnaris | In the groove on the back of the distal ulna, held by its own subsheath | Part of the triangular fibrocartilage complex; subluxes out of its groove in forced supination |
The extensor retinaculum runs obliquely from the radius to the pisiform and triquetrum rather than to the ulna, so the forearm can rotate beneath it, and sends septa to the bone that make the six tunnels, each with its own synovial sheath. The superficial branch of the radial nerve crosses the first compartment in the subcutaneous tissue here, which is why tight watch straps and handcuffs numb the back of the thumb web.
The scaphoid is the commonest carpal bone to fracture, usually across its waist after a fall on the outstretched hand, and its blood supply explains why the fracture matters. Branches of the radial artery enter through the dorsal ridge near the distal pole and run proximally inside the bone, so the proximal pole depends on flow across the waist; a displaced waist or proximal pole fracture cuts it off, with avascular necrosis and non-union to follow. The signs are tenderness in the snuffbox and over the scaphoid tubercle and pain on axial compression of the thumb, and the first radiographs are often normal. NICE guidance on non-complex fractures advises considering MRI as the first-line investigation when a scaphoid fracture is suspected and the films do not show it. One other injury belongs here: the lunate, in the floor of the tunnel, can dislocate forward into it after a hyperextension injury and present as an acute carpal tunnel syndrome.
Carpal tunnel syndrome is the commonest entrapment neuropathy. The patient, typically a middle-aged woman, is woken by tingling and pain in the thumb, index and middle fingers and shakes the hand to relieve it; holding a phone or a steering wheel provokes it and the ache may spread up the forearm. Later the thumb becomes clumsy, the thenar eminence flattens as abductor pollicis brevis wastes, and objects are dropped. The sensory loss stops at the wrist crease: the palm, supplied by the palmar cutaneous branch, is normal, which separates the tunnel from a lesion in the forearm. Tapping the nerve at the crease sends tingling into the fingers (Tinel's), a minute of full wrist flexion reproduces the symptoms (Phalen's) and so does pressure over the tunnel (Durkan's); nerve conduction studies confirm a prolonged latency across the wrist. The causes are anything that fills the tunnel or narrows it: pregnancy, hypothyroidism, rheumatoid tenosynovitis, diabetes, acromegaly, dialysis amyloid, obesity, a malunited distal radius fracture, a ganglion. Night splinting and a steroid injection help many patients; the definitive treatment is division of the flexor retinaculum, open or endoscopic.
Test yourself
A 45-year-old man has anterior shoulder pain after a fall. He cannot lift the dorsum of the hand away from the lumbar spine when the arm is placed behind the back, and passive external rotation of the shoulder is increased compared with the other side.
Which structure is most likely injured?
On the ulnar side, the pisiform and the hook of hamate; on the radial side, the tubercle of the scaphoid and the ridge of the trapezium. It bridges the carpal arch to form the carpal tunnel and gives origin to the thenar and hypothenar muscles.
Nine tendons and the median nerve: four of flexor digitorum superficialis, four of flexor digitorum profundus and flexor pollicis longus. The ulnar nerve and artery, palmaris longus, flexor carpi radialis (in its own tunnel) and the palmar cutaneous branch of the median nerve all pass outside it.
From radial to ulnar: 1, abductor pollicis longus and extensor pollicis brevis; 2, extensor carpi radialis longus and brevis; 3, extensor pollicis longus; 4, extensor digitorum and extensor indicis; 5, extensor digiti minimi; 6, extensor carpi ulnaris.
Because the palmar cutaneous branch of the median nerve leaves the main trunk about 5 cm above the wrist and crosses the flexor retinaculum superficially, outside the tunnel, so it is not compressed. Numbness over the palm points to a lesion higher in the forearm.
Written for revision in the MRCS Part A style under our editorial standards. It is not clinical advice: always confirm against current guidance. Spotted an error? Tell us.
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