The extensor pollicis brevis travels through the first extensor compartment alongside the abductor pollicis longus, forming the pair of tendons involved in de Quervain tenosynovitis. It inserts on the proximal phalanx of the thumb, extending only the MCP joint rather than the IP joint, distinguishing it from the extensor pollicis longus which reaches the distal phalanx. In approximately 25 percent of people the EPB has its own separate subcompartment within the first extensor compartment, a variation that increases failure of conservative de Quervain treatment.
| Origin | Posterior surface of the radius distal to the APL origin and adjacent interosseous membrane |
|---|---|
| Insertion | Dorsal surface of the base of the proximal phalanx of the thumb |
| Nerve Supply | Posterior interosseous nerve (C7, C8) |
| Blood Supply | Posterior interosseous artery |
| Actions | Extension of the thumb MCP joint; Abduction of the thumb |
|---|
Extending the thumb MCP joint while abducting the first metacarpal, it positions the thumb for opposition and pinch tasks, working with the APL to open the web space for grasping.
The EPB occupies the first extensor compartment alongside the APL, and the Finkelstein test stretches this compartment by ulnar deviating the wrist with the thumb tucked inside the fingers, reproducing de Quervain pain. Surgical decompression of the first compartment must identify both tendon slips and any separate EPB subcompartment to prevent persistent symptoms after decompression.
The EPB tendon is palpable as the more posterior and thinner of the two tendons forming the anterior snuffbox boundary, distinguishable from the APL by resisting MCP thumb extension specifically.
First extensor compartment stenosis involving both APL and EPB tendons producing radial wrist pain and positive Finkelstein test, with surgical decompression needed when injection fails.
