Flexor Pollicis Longus, Anterior View

Description

Running along the anterior forearm, the flexor pollicis longus (FPL) is shown arising from the anterior surface of the radius and adjacent interosseous membrane, then narrowing into a tendon that courses distally toward the thumb. Medially, the FPL belly lies deep to flexor digitorum superficialis and lateral to flexor digitorum profundus, while proximally it sits inferior to the elbow flexor mass and pronator teres. At the wrist, the tendon passes deep to the flexor retinaculum within the carpal tunnel, then continues across the palmar aspect of the thumb to insert on the base of the distal phalanx, anterior to the metacarpals and phalanges shown in the open hand. Its line of pull is clear. Clinically, this anterior perspective clarifies why isolated loss of thumb interphalangeal flexion points you toward anterior interosseous nerve palsy, since the AIN supplies FPL (and often the radial half of FDP and pronator quadratus) and its deficit produces the classic pinch weakness seen in the OK sign. The course of the FPL tendon through the carpal tunnel also matters in flexor tenosynovitis, where thickening of the tendon sheath can cause painful triggering at the thumb and may coexist with median nerve compression symptoms. Surgeons planning volar approaches to the distal radius also use these relationships, because the FPL tendon lies immediately superficial to the volar radius and can be irritated or ruptured by prominent volar plate hardware. Use this artwork for upper limb anatomy teaching blocks, hand surgery lecture slides, and publisher-ready figures on carpal tunnel contents, AIN syndrome, or thumb flexion biomechanics. It also fits patient education materials explaining trigger thumb releases and tendon irritation after distal radius fixation. Anatomical accuracy verified by SciePro's Medical Advisory Board.