The FPL is the sole flexor of the thumb IP joint and the primary force generator for precision pinch. It occupies its own fibro-osseous sheath beneath the thenar muscles and through the carpal tunnel radially. The A1 pulley at the thumb MCP is the site of trigger thumb — the thickened FPL nodule catches as it passes through the constricted pulley. FPL laceration in the thumb flexor zone (no man's land equivalent) requires meticulous repair.
Thumb IP joint flexion — the only thumb IP flexor; key pinch and grip force generation
Trigger thumb from FPL nodule catching at the A1 pulley produces painful thumb locking in flexion. Adult trigger thumb: corticosteroid injection resolves 60-70%; A1 pulley release for refractory cases. Paediatric trigger thumb: observation for spontaneous resolution until age 3, then A1 pulley release. FPL laceration in zone II (beneath the thenar muscles) is technically demanding — the sheath is narrow and the tendon must be retrieved with a paediatric feeding tube.
FPL tendon nodule catching at the A1 pulley producing painful thumb locking managed with corticosteroid injection or A1 pulley release.
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