Extensor Compartments of the Wrist
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Direct answer
Six fibro-osseous tunnels under the extensor retinaculum sort every extensor tendon crossing the wrist: from radial to ulnar, the first holds abductor pollicis longus and extensor pollicis brevis, the second extensor carpi radialis longus and brevis, the third extensor pollicis longus as it turns around Lister's tubercle, the fourth extensor digitorum with extensor indicis, the fifth extensor digiti minimi, and the sixth extensor carpi ulnaris. Each compartment owns its pathology — de Quervain's tenosynovitis in the first, intersection syndrome where the first crosses the second a few centimetres above the wrist, attrition rupture of extensor pollicis longus over Lister's tubercle after distal radius fracture, and rheumatoid extensor digiti minimi rupture. The retinaculum's septa, not the tendons, create the tunnels — so synovitis respects compartment boundaries.
What you must remember
- Compartment contents, radial to ulnar: 1 — abductor pollicis longus, 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.
- Lister's tubercle (dorsal tubercle of the radius): the pulley around which extensor pollicis longus angles about 45 degrees toward the thumb — the landmark for the third compartment and for surgical approaches.
- First compartment disease: de Quervain's stenosing tenosynovitis — pain over the radial styloid, positive Finkelstein's test (flexing the thumb into the palm with ulnar deviation of the wrist), common in new mothers and overuse.
- Intersection syndrome: friction where the first compartment tendons cross the second 4-6 cm proximal to the wrist — crepitus and swelling above de Quervain's territory, confused with it.
- Third compartment rupture: extensor pollicis longus attrition over Lister's tubercle weeks after a distal radius fracture (or in rheumatoid arthritis), losing thumb retropulsion — the thumb cannot lift off a flat table.
- Fourth and fifth in rheumatoid disease: synovitis erodes extensor digitorum and digiti minimi tendons over the ulnar head, producing sequential finger drop (Vaughan-Jackson syndrome); extensor indicis lies ulnar to the index digitorum slip, and extensor digiti minimi is its own compartment — a surgical spare part and a separate citizen.
- Sixth compartment: extensor carpi ulnaris in its groove behind the ulnar styloid, subluxing in rheumatoid and injured in racket sports; its sheath shares the ulnar groove with the triquetrum floor.
Why extensor pollicis longus ruptures after a Colles fracture
The mechanism is attrition, not laceration. In a distal radius fracture the dorsally displaced fragments and callus roughen the groove beside Lister's tubercle; the tendon, already bent sharply around the tubercle and poorly vascularised at the turn, frays over the ridge during the weeks of immobilisation. The patient, cast removed, suddenly cannot extend the interphalangeal joint of the thumb or lift the thumb off the table with the palm flat — a painless loss days to weeks after injury. Treatment borrows from the neighbouring compartments, transferring extensor indicis proprius to the distal stump, because direct repair of a frayed tendon in scarred bed does not hold.
The same neighbourliness drives rheumatoid reconstruction. Synovitis distends the fourth and fifth compartments over the ulnar head (caput ulnae syndrome), erodes the distal radioulnar ligaments, and saws the tendons across the sharp ulnar head — little finger first, then ring, then middle. The surgeon's answers — synovectomy, tendon transfers, distal ulnar excision — are all compartment anatomy in action, and the exam answer names the compartment numbers as fluently as the ward rounds do.
Where candidates slip
The memory failure is direction: the list is learned radial to ulnar, and examiners enjoy asking ulnar-first. The second slip is boundary borrowing — putting extensor pollicis longus in the snuffbox story without crediting the third compartment, or assigning extensor indicis its own compartment (it shares the fourth). The third is Finkelstein's test described backwards: the wrist is ulnarly deviated (or the thumb adducted) to stretch the first compartment tendons; reversing the movement tests nothing. And remember the retinaculum prevents bowstringing — a fact that explains why its division (or synovial swelling under it) changes extensor mechanics at the wrist.
Frequently asked questions
Which tendons occupy the first extensor compartment?
Abductor pollicis longus and extensor pollicis brevis — the compartment involved in de Quervain's stenosing tenosynovitis.
What is the anatomical role of Lister's tubercle?
It acts as a pulley for extensor pollicis longus, which turns around it in the third compartment toward the thumb — and the attrition point for its delayed rupture after distal radius fracture.
What is intersection syndrome?
Inflammation where the first compartment tendons cross the second (extensor carpi radialis longus and brevis) about 4-6 cm proximal to the wrist, producing crepitant swelling above the radial styloid.
Why does the little finger extensor rupture in rheumatoid arthritis?
Extensor digiti minimi in its own fifth compartment is eroded over a prominent, arthritic ulnar head — the first of the sequential Vaughan-Jackson ruptures.
How is a ruptured extensor pollicis longus reconstructed?
By transfer of extensor indicis proprius to the distal stump, sacrificing independent index extension to restore thumb retropulsion.