The Six Dorsal Extensor Compartments of the Wrist

Compartments 1 and 3 are not only listed - they are the two borders of the anatomical snuffbox, which is where both are examined at the bedside.

• Borders of the snuffbox. The volar (radial) border is compartment 1 - abductor pollicis longus and extensor pollicis brevis; the dorsal (ulnar) border is compartment 3 - extensor pollicis longus (turning around Lister's tubercle toward the thumb). The floor is the scaphoid and trapezium, and the proximal apex is the radial styloid.
• What crosses it. The radial artery runs across the floor of the snuffbox (pulse palpable here), the cephalic vein begins over it, and branches of the superficial radial nerve cross the roof and compartment 1 - so the nerve is at risk in de Quervain's release and gives the dorsal radial sensory numbness of Wartenberg's (cheiralgia paraesthetica) syndrome.
• Using it clinically. Tenderness in the snuffbox is the classic sign of a scaphoid fracture; pain along the volar border on a thumb-tuck (Finkelstein/Eichhoff) points to compartment 1 / de Quervain's; and loss of the EPL dorsal border with absent thumb retropulsion signals EPL rupture.
• Palpating the rest. Lister's tubercle is the palpable dorsal ridge that separates compartment 2 (radial) from compartment 3 (ulnar); the extensor carpi ulnaris (compartment 6) is felt in its groove on the dorsal ulnar head and tested for snapping in forearm rotation.

The work-up behind each of those signs sits in scaphoid fractures, de Quervain's tenosynovitis, extensor pollicis longus rupture, Wartenberg's syndrome and snapping ECU instability.

The dorsal compartments are among the most variable regions in the body, and the variations are examinable because they cause failed surgery, complicate transfer planning, and present as dorsal masses.

• Compartment 1. Besides the intracompartmental septum, the abductor pollicis longus commonly has multiple slips - typically two to four (inserting variably on the trapezium and thenar fascia), and the extensor pollicis brevis may be absent or have its own sub-sheath - all relevant to recognising and fully releasing de Quervain's.
• Compartment 4. The extensor indicis proprius is the usual EPL transfer donor, but accessory extensors exist - an extensor medii proprius (to the middle finger) or extensor indicis et medii communis - and the EIP is rarely absent; the surgeon must therefore confirm independent index extension before harvesting it. The extensor digitorum brevis manus, an accessory muscle belly arising in the fourth compartment, presents as a soft dorsal wrist swelling that is mistaken for a ganglion or tenosynovitis.
• Compartment 5. The extensor digiti minimi is frequently double (two slips), useful to know when assessing little-finger extension and in Vaughan-Jackson ruptures.
• Why it matters. These variants explain a proportion of failed de Quervain's releases (an unreleased EPB sub-sheath), influence tendon-transfer donor choice (confirm an independent donor and a remaining motor to the index/little finger), and account for benign dorsal masses (extensor digitorum brevis manus). Donor selection is developed in tendon transfers for nerve palsy.

De Quervain's tenosynovitis is stenosing tenosynovitis of the first dorsal compartment (APL and EPB), causing radial-sided wrist pain reproduced by Finkelstein's / Eichhoff's test. A key surgical point is the frequent intracompartmental SEPTUM that creates a separate sub-sheath for the EPB: if it is not recognised, a corticosteroid injection or a surgical release that only opens the APL sub-sheath fails. A septum was found in 67 percent of the de Quervain surgical population against 35 percent of general cadaveric series in a systematic review and meta-analysis - so in the patient in front of you who has already failed an injection, a separate EPB sub-sheath is more likely than not. Recognising and releasing all sub-compartments is essential; the condition itself is set out in de Quervain's tenosynovitis, and the proximal crossover in intersection syndrome.

The extensor pollicis longus runs in the third compartment and makes a sharp turn around Lister's tubercle, which acts as its pulley. The segment around the tubercle has a relatively avascular watershed, so the EPL is vulnerable to attritional/ischaemic rupture - classically a delayed complication of a NON-DISPLACED distal radius fracture (the intact dorsal cortex and haematoma compromise the tendon), and also in rheumatoid arthritis and after volar plating with prominent dorsal screws. Because the ruptured ends retract and fray, direct repair usually fails, so the standard reconstruction is an extensor indicis proprius (EIP) to EPL tendon transfer.

• Compartment 4 (EDC + EIP): the posterior interosseous nerve lies on its floor - the target for dorsal wrist denervation, and at risk in dorsal exposures. The EIP (which lies ulnar and deep to the EDC of the index) is the standard donor for EPL reconstruction.
• Compartment 5 (EDM): overlies the DRUJ; in rheumatoid arthritis a dorsally subluxed, eroded ulnar head ruptures the ulnar-sided extensors first - the Vaughan-Jackson sequence (EDM/EDC little then ring finger).
• Compartment 6 (ECU): the ECU sits in its own groove on the ulnar head within a subsheath; disruption of the subsheath causes ECU instability/subluxation and painful ulnar-sided snapping on forearm rotation.

The first-compartment septum prevalence and ultrasound data come from the cited Abi-Rafeh meta-analysis, and the EPL-rupture incidence/association with non-displaced distal radius fractures from the cited Bogart and Vidlock report - a single case report, so the 0.07 to 5 percent range is a figure it quotes from the wider literature rather than one it measured, and the quoted span is wide enough that it should be given as an order of magnitude rather than a precise risk when consenting. The compartment numbering and tendon contents are standard, universally established anatomy. The anatomical variants (abductor pollicis longus slips, accessory extensors, extensor digitorum brevis manus, double extensor digiti minimi) are established descriptive anatomy; no series was found giving the proportion of failed de Quervain releases attributable specifically to an unreleased extensor pollicis brevis sub-sheath, so no such figure is quoted. See also de Quervain's tenosynovitis, intersection syndrome, extensor pollicis longus rupture, distal radius fractures, snapping ECU instability and rheumatoid arthritis of the hand.