The FPL is the only flexor of the thumb IP joint and is essential for precision pinch — the pad-to-pad contact between the thumb tip and the index or middle finger. Its anterior interosseous nerve supply makes it an important clinical test for AIN integrity (the OK sign test — failure to form a perfect circle indicates FPL or FDP-index weakness from AIN lesion). The FPL is frequently transferred to restore thumb flexion in high median nerve injuries.
| Origin | Anterior radius and adjacent interosseous membrane (middle half) |
|---|---|
| Insertion | Palmar base of the distal phalanx of the thumb |
| Nerve Supply | Anterior interosseous nerve (branch of the median nerve — C8, T1) |
| Blood Supply | Anterior interosseous artery |
| Actions | Flexion of the thumb IP joint (primary action); Flexion of the thumb MCP and CMC joints |
|---|
Precision pinch requires simultaneous FPL activation for thumb IP flexion and FPB activation for MCP flexion, combined with thumb opposition from the opponens pollicis. Loss of FPL produces an inability to grip small objects between the thumb tip and finger tips.
Anterior interosseous nerve syndrome (Kiloh-Nevin syndrome) specifically produces FPL and FDP-index finger weakness with loss of the OK sign (forming a tear-drop rather than a perfect circle), without sensory loss because the AIN has no cutaneous territory. Most cases resolve spontaneously within 3 to 12 months. FPL tendon rupture in rheumatoid arthritis from the distal radius Vaughan-Jackson erosion is managed with IP joint arthrodesis or FPL reconstruction.
The FPL is not palpable in the forearm as a distinct tendon but its tendon is palpable in the palm of the thumb and its contraction felt during resisted thumb IP flexion.
FPL and FDP-index weakness from AIN compression producing inability to form the OK sign, managed with observation over 3 to 12 months.
