The FPL is the only flexor of the thumb IP joint, providing the grip force for pinch. The anterior interosseous nerve supplies it and FDP-index (and pronator quadratus) — AIN palsy produces the failure of the OK sign from combined FPL and FDP-index weakness. The Gantzer accessory muscle head from the medial epicondyle is present in 45% and can compress the AIN, producing AIN syndrome from intrinsic compression.
| Origin | Anterior radius (mid-shaft) and adjacent interosseous membrane; Occasional head from the medial epicondyle or coronoid (Gantzer's muscle — accessory FPL head in 45%) |
|---|---|
| Insertion | Palmar base of the thumb distal phalanx — the only long flexor of the thumb |
| Nerve Supply | Anterior interosseous nerve (C8, T1) — the most radial AIN-supplied muscle |
| Blood Supply | Anterior interosseous artery |
| Actions | Thumb IP joint flexion — the only muscle capable of this motion; Assists thumb MCP flexion; Assists wrist flexion with all finger joints fully flexed |
|---|
AIN palsy from Gantzer's muscle compression, pronator teres arch compression, or spontaneous inflammatory neuritis produces the OK sign failure — the patient cannot form a perfect circle with thumb and index because FPL and FDP-index are paralysed. The diagnosis is confirmed by nerve conduction studies showing isolated AIN involvement. Most recover spontaneously within 6-12 months; surgical exploration for refractory cases.
FPL is tested by isolated thumb IP flexion with the MCP joint stabilised in neutral. It is not palpable as a distinct tendon in the forearm but its tendon is palpable in the thumb flexor sheath at the MCP level.
FPL and FDP-index paralysis from anterior interosseous nerve compression producing OK sign failure managed with observation for 6 months then surgical exploration.
