The flexor pollicis longus is the only long flexor of the thumb, supplying the powerful pinch grip strength needed to flex the thumb tip against resistance. Supplied by the anterior interosseous nerve, its weakness is the defining motor deficit of anterior interosseous nerve syndrome, and inability to flex the thumb IP joint produces the characteristic failure of the OK sign pinch when the index DIP is also affected.
| Origin | Anterior surface of the radius and adjacent interosseous membrane; Occasional slip from the coronoid process or medial epicondyle |
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| Insertion | Palmar surface of the base of the distal phalanx of the thumb |
| Nerve Supply | Anterior interosseous nerve (C8, T1) |
| Blood Supply | Anterior interosseous artery |
| Actions | Flexion of the thumb interphalangeal joint; Assists flexion of the thumb MCP and CMC joints |
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It generates the terminal pinch force at the thumb tip that is essential for precision handling, buttoning, writing, and all fine motor tasks requiring thumb tip contact against the fingers.
FPL tendinopathy in a dedicated flexor tendon sheath within the thumb can produce trigger thumb, the most common form of stenosing tenosynovitis in adults, where the FPL tendon catches within the A1 pulley at the thumb MCP level producing a clicking or locked flexed thumb. Corticosteroid injection into the tendon sheath resolves most cases but surgical A1 pulley release is required for locked thumbs.
The FPL tendon is palpable in the thenar eminence as the central tendon coursing toward the thumb tip, becoming firm during resisted thumb IP joint flexion.
FPL tendon catching within the A1 pulley at the thumb MCP joint producing thumb clicking or locking in flexion, managed with corticosteroid injection and surgical pulley release for locked cases.
