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.
An expanded entry for Flexor Pollicis Longus.
| 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%) |
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| 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 |
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It is the only muscle that flexes the tip of the thumb, making it essential for pinch grip, writing, and any task that presses the thumb pad against an object.
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.
Other versions of this entry: Flexor Pollicis Longus · Flexor Pollicis Longus Detail
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