EPB runs with APL in the first extensor compartment and may have its own sub-compartment in up to 34% of individuals, requiring separate release in de Quervain surgery.
| Origin | Posterior radius — distal third |
|---|---|
| Insertion | Base of the proximal phalanx of the thumb |
| Nerve Supply | Posterior interosseous nerve (C7, C8) |
| Blood Supply | Posterior interosseous artery |
| Actions | Extends the thumb MCP joint; Assists thumb abduction |
|---|
EPB sub-compartment is the key cause of failed first compartment release. Ultrasound identifies separate sub-compartments pre-operatively. EPB's MCP role distinguishes it from APL during clinical testing.
Palpable at the lateral anatomical snuffbox during resisted thumb MCP extension.
Persistent symptoms after first compartment release due to unrecognised EPB sub-compartment, requiring return to surgery for targeted release.
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