FPL is the only flexor of the thumb interphalangeal joint. Its AIN innervation makes it the muscle most commonly tested in AIN palsy — inability to flex the thumb IP joint with a normal pinch posture (Kiloh-Nevin sign) indicates AIN pathology.
| Origin | Anterior surface of the radius — middle half, and the adjacent interosseous membrane |
|---|---|
| Insertion | Base of the distal phalanx of the thumb |
| Nerve Supply | Anterior interosseous nerve (AIN, C7, C8, T1) |
| Blood Supply | Anterior interosseous artery |
| Actions | Flexes the thumb IP joint — the only flexor of the thumb IP joint; Assists thumb MCP flexion |
|---|
AIN palsy (from nerve compression at the pronator teres or spontaneous in Parsonage-Turner syndrome) selectively weakens FPL and FDP to the index, producing the Kiloh-Nevin sign (inability to make an O with the thumb and index — the OK sign). FPL tendon rupture in rheumatoid arthritis occurs from attritional rupture over the scaphoid (Mannerfelt-Norman lesion).
Tested by resisted thumb IP joint flexion with the MCP held extended.
Attritional FPL rupture from scaphoid osteophyte erosion in rheumatoid arthritis (Mannerfelt-Norman lesion) producing sudden loss of thumb tip flexion, requiring tendon graft or FDP transfer.
