Compartments of the Extensor Retinaculum
Overview
The space deep to the extensor retinaculum is divided into six osseofibrous compartments by five septa passing from the retinaculum to the dorsal surface of the radius and ulna.
Compartments are numbered I to VI from lateral (radial) to medial (ulnar) side.
Each compartment contains tendons enclosed in synovial sheaths that reduce friction.
The Six Compartments
| Compartment | Tendons / Structures | Bone Relation | Notes |
|---|---|---|---|
| I | Abductor pollicis longus (APL) + Extensor pollicis brevis (EPB) | Lateral aspect of radius | Roof of anatomical snuff box; site of de Quervain’s tenosynovitis |
| II | Extensor carpi radialis longus (ECRL) + Extensor carpi radialis brevis (ECRB) | Dorsal surface of radius | Medial wall of anatomical snuff box |
| III | Extensor pollicis longus (EPL) | Lister’s tubercle (dorsal radial tubercle) acts as a pulley | EPL wraps around Lister’s tubercle; site of spontaneous rupture in Colles’ fracture |
| IV | Extensor digitorum (ED) + Extensor indicis (EI) + Posterior interosseous nerve + Anterior interosseous artery | Dorsal surface of radius | Largest compartment |
| V | Extensor digiti minimi (EDM) | Over inferior radioulnar joint | |
| VI | Extensor carpi ulnaris (ECU) | Groove on dorsal aspect of ulna head | ECU stabilises ulnar side of wrist |
Mnemonic for Compartment Contents (I–VI)
“2, 2, 1, 2, 1, 1” — number of tendons in each compartment
Or by content:
- I: APL + EPB → “APple” → Thumb abductor + short thumb extensor
- II: ECRL + ECRB → Two radial wrist extensors
- III: EPL → Lone thumb extensor (hooks around Lister’s)
- IV: ED + EI → Digit extensors (+ PIN + AIA)
- V: EDM → Little finger extensor
- VI: ECU → Ulnar wrist extensor
Synovial Sheaths
Each compartment is lined by a synovial sheath extending approximately 2.5 cm proximal and 2.5 cm distal to the extensor retinaculum. The sheaths allow gliding of tendons within the osseofibrous tunnels.
Anatomical Snuff Box
The anatomical snuff box is a triangular depression on the radial (lateral) side of the wrist, visible when the thumb is extended and abducted.
Boundaries:
| Boundary | Structure |
|---|---|
| Anterior (lateral) | Tendons of APL and EPB (compartment I) |
| Posterior (medial) | Tendon of EPL (compartment III) |
| Floor | Scaphoid bone + trapezium |
| Roof | Skin |
| Proximal | Styloid process of radius |
Contents of anatomical snuff box:
- Radial artery (crosses floor from lateral to medial)
- Cephalic vein (crosses superficially)
- Terminal branches of superficial radial nerve (cutaneous, cross superficially)
- Radial nerve branches
Clinical significance:
- Tenderness in the snuff box following a fall on the outstretched hand → suspect fracture of scaphoid
- The scaphoid receives its blood supply via vessels entering the distal pole — a waist fracture risks avascular necrosis of the proximal pole
- Scaphoid fractures may be missed on initial X-ray — if clinical suspicion is high, treat as fracture and re-image at 10–14 days (or MRI)
Lister’s Tubercle
The dorsal radial tubercle (Lister’s tubercle) is a bony prominence on the dorsal surface of the lower end of the radius:
- Acts as a pulley for the tendon of extensor pollicis longus (compartment III)
- EPL wraps medially around this tubercle to reach the thumb
- Important landmark on the dorsum of the wrist — compartment II lies lateral to it, compartment III medial to it