The Six Dorsal Extensor Compartments of the Wrist

The retinaculum. The extensor retinaculum is a thickening of the deep antebrachial fascia over the dorsum of the wrist. It attaches radially to the lateral margin of the radius and ulnarly to the triquetrum and pisiform, and loosely to the ulnar styloid. It holds the extensor tendons down, preventing bowstringing during wrist and finger extension while still permitting smooth excursion.

Six tunnels. The retinaculum sends septa down to the dorsal ridges of the radius and ulna. These create six osseofibrous compartments, each lined by synovium, through which the extensor tendons glide, numbered 1 to 6 from the radial (thumb) side to the ulnar side.

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Lister's tubercle. The tubercle on the dorsal distal radius is the landmark. Compartment 2 lies radial to it and compartment 3 (EPL) lies ulnar to it, the EPL using the tubercle as a pulley that turns it toward the thumb.

The contents. Learn the six in order with their tendons, the one relation that locates each, and the pathology it is known for. The sections below take each in turn.

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The snuffbox. 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.

• Volar (radial) border: compartment 1, abductor pollicis longus and extensor pollicis brevis
• Dorsal (ulnar) border: compartment 3, extensor pollicis longus, turning around Lister's tubercle toward the thumb
• Floor: the scaphoid and trapezium
• Proximal apex: the radial styloid

What crosses it. The radial artery runs across the floor, where its pulse is palpable, and the cephalic vein begins over it. 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 syndrome (cheiralgia paraesthetica).

Using it clinically. Where the finding sits tells you which structure is at fault:

• Tenderness in the snuffbox is the classic sign of a scaphoid fracture
• Pain along the volar border on a thumb-tuck (Finkelstein or Eichhoff) points to compartment 1 and de Quervain's
• Loss of the EPL dorsal border with absent thumb retropulsion signals EPL rupture

Palpating the rest. Lister's tubercle is the palpable dorsal ridge between compartments 2 and 3. The ECU of 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 these signs sits in scaphoid fractures, extensor pollicis longus rupture, Wartenberg's syndrome and snapping ECU instability.

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The count is only the default. The "2,2,1,2,1,1" tendon count is the default, and the dorsal compartments are among the most variable regions in the body. The variations are examinable because they cause failed surgery, complicate transfer planning and present as dorsal masses.

Compartment 1. Besides the septum, the abductor pollicis longus commonly has multiple slips, typically two to four, inserting variably on the trapezium and thenar fascia. The extensor pollicis brevis may be absent or have its own sub-sheath. Both matter when recognising and fully releasing de Quervain's, and an unreleased EPB sub-sheath explains a proportion of failed releases.

Compartment 4: the donor. The EIP is the usual EPL transfer donor, but accessory extensors exist, an extensor medii proprius (to the middle finger) or an extensor indicis et medii communis, and the EIP is rarely absent. The surgeon must therefore confirm independent index extension before harvesting it. In any transfer plan, confirm an independent donor and a remaining motor to the index or little finger; donor selection is developed in tendon transfers for nerve palsy.

Compartment 4: the lump. The extensor digitorum brevis manus is an accessory muscle belly arising in the fourth compartment. It presents as a soft, benign dorsal wrist swelling that is mistaken for a ganglion or tenosynovitis.

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Compartment 5. The extensor digiti minimi is frequently double (two slips), which is useful to know when assessing little-finger extension and in Vaughan-Jackson ruptures.

Compartment 1: de Quervain's. De Quervain's tenosynovitis is stenosing tenosynovitis of the first dorsal compartment (APL and EPB). It causes radial-sided wrist pain reproduced by Finkelstein's or Eichhoff's test.

The septum. A frequent intracompartmental septum 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, so recognising and releasing all sub-compartments is essential.

How often. A systematic review and meta-analysis found a septum in 67 percent of the de Quervain surgical population against 35 percent of general cadaveric series. In the patient in front of you who has already failed an injection, a separate EPB sub-sheath is therefore more likely than not. The condition itself is set out in de Quervain's tenosynovitis.

Compartments 1 and 2: intersection syndrome. Proximally the first-compartment muscle bellies cross over the second compartment, and that crossover is the site of intersection syndrome. It lies on the dorsoradial forearm about 4 cm proximal to Lister's tubercle; pain there, not at the wrist, separates it from de Quervain's. The crossover is developed in intersection syndrome.

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Compartment 3: the EPL watershed. The extensor pollicis longus makes a sharp turn around Lister's tubercle, and the segment around the tubercle has a relatively avascular watershed. The tendon is therefore vulnerable to attritional or ischaemic rupture, which is seen:

• Classically as a delayed complication of a non-displaced distal radius fracture, where the intact dorsal cortex and haematoma compromise the tendon
• In rheumatoid arthritis
• After volar plating with prominent dorsal screws

Reconstruction. The ruptured ends retract and fray, so direct repair usually fails. The standard reconstruction is an extensor indicis proprius (EIP) to EPL tendon transfer.

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Compartment 4: the PIN and the donor. The posterior interosseous nerve lies on the floor of the fourth compartment. It is the target for dorsal wrist denervation and is 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: Vaughan-Jackson. The EDM overlies the DRUJ. In rheumatoid arthritis a dorsally subluxed, eroded ulnar head ruptures the ulnar-sided extensors first, the Vaughan-Jackson sequence: EDM and the EDC of the little finger, then the ring finger.

Compartment 6: ECU instability. The ECU sits in its own groove on the ulnar head within a subsheath. Disruption of the subsheath causes ECU instability or subluxation, with painful ulnar-sided snapping on forearm rotation.