Flexor Pollicis Longus Under the Skin, Anterior View

Description

Running along the volar (anterior) forearm, the flexor pollicis longus (FPL) belly occupies the deep anterior compartment on the radial side, arising from the anterior surface of the radius and adjacent interosseous membrane. Its tendon courses distally toward the wrist deep to the superficial flexor mass, lying lateral to the flexor digitorum profundus and closely related to the flexor carpi radialis tendon and radial neurovascular plane. At the carpus the FPL tendon passes beneath the flexor retinaculum within the radial bursa, then continues into the thumb to insert on the base of the distal phalanx, anterior to the first metacarpal and phalanges shown in the hand. Skin is retained as a superficial layer, so the highlighted muscle reads as a deep structure under an intact integument. Functionally, this is the prime flexor of the thumb interphalangeal joint, the movement you test when asking a patient to pinch tip to tip. Anterior interosseous nerve palsy (a median nerve branch) selectively weakens FPL and the radial half of flexor digitorum profundus, producing the classic inability to form a neat “OK” sign, a bedside clue that hinges on knowing where this muscle sits relative to the deep flexor plane. Hand surgeons also track the FPL tendon through the carpal tunnel and into the thumb during flexor tendon repair, trigger thumb release, and assessment of synovitis in the radial bursa that can coexist with carpal tunnel syndrome. Use this artwork in upper-limb anatomy teaching blocks to contrast superficial and deep anterior forearm layers, in hand therapy materials explaining pinch mechanics, or in surgical education covering volar wrist and thumb tendon approaches. Clear relationships at the wrist help when annotating carpal tunnel contents for textbooks and clinical slide decks. Anatomical accuracy verified by SciePro's Medical Advisory Board.