Tendon of flexor pollicis longus

The tendon of flexor pollicis longus (FPL) allows the FPL muscle to exert its action as the sole flexor of the thumb's interphalangeal joint. This tendon begins where the flexor pollicis longus muscle belly ends in the deep anterior forearm. It courses distally, positioned laterally to the flexor digitorum profundus muscle. A key anatomical landmark in its path is the carpal tunnel, through which the FPL tendon passes, specifically located lateral to the flexor digitorum profundus tendons.

The tendon's final destination is its insertion onto the base of the distal phalanx of the thumb. This direct attachment enables the flexor pollicis longus to powerfully flex the interphalangeal joint of the thumb, and also assists in flexion at the metacarpophalangeal and carpometacarpal joints of the thumb, as well as the wrist.