The EPB is the second tendon of the first extensor compartment alongside the APL, sharing the tenosynovitis of de Quervain disease. Its insertion on the thumb proximal phalanx extends the MCP joint specifically, complementing the APL's CMC action. A septum divides the first compartment into separate APL and EPB compartments in 30 percent of people, requiring both to be opened during de Quervain release.
| Origin | Posterior radius distal to APL and adjacent interosseous membrane |
|---|---|
| Insertion | Dorsal 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 proximal phalanx at the MCP joint; Assists in thumb CMC extension and abduction |
|---|
EPB extension of the MCP joint positions the thumb for key pinch (lateral pinch) where the pulp of the thumb presses against the lateral index finger. Without EPB function, the thumb MCP joint cannot be stabilised in extension during this pinch pattern.
EPB tendon rupture is rare and produces MCP extension lag while APL function is preserved. It occurs in inflammatory arthritis from tenosynovium invasion or in rheumatoid deformity. EPB tenodesis to the first metacarpal is occasionally performed for persistent MCP hyperextension deformity.
The EPB tendon runs alongside the APL in the first extensor compartment and is palpable between the APL cord (volar) and EPL cord (dorsal) in the anatomical snuffbox.
Stenosing tenosynovitis of EPB and APL in the first extensor compartment, managed with corticosteroid injection and surgical compartment release.
