The EPB occupies the first extensor compartment alongside the APL at the radial wrist, forming the anatomical snuffbox radial boundary. Together with APL it is involved in de Quervain's tenosynovitis. The EPB has a separate subcompartment within the first compartment in 25% of individuals — failure to identify and release this subcompartment during de Quervain's surgery causes persistent symptoms.
Thumb MCP extension; thumb abduction at the CMC joint (with APL); the primary thumb abductor in the extended position
The Finkelstein test (Eichhoff manoeuvre — the examiner ulnar-deviates the wrist with the thumb enclosed in the fist) reproduces de Quervain's pain. Ultrasound confirms tenosynovitis and identifies EPB subcompartment presence before injection or surgery. The EPB subcompartment is the most common cause of failed de Quervain's release.
First extensor compartment tenosynovitis with EPB subcompartment involvement producing radial wrist pain managed with injection and compartment release.
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