The Flexor Pollicis Longus Tendon Does Not Lie Parallel to the Thumb Metacarpal

Background: Repair of the flexor pollicis longus (FPL) tendon by tendon retriever can be challenging because of uncertainty of the optimal direction. This study aims to describe the FPL tendon path in the thenar eminence. Methods: In 18 hands of 9 cadavers, the angle of the FPL tendon was, after dissection, measured in relation to the axis formed between the metacarpophalangeal (MCP) joint of the thumb and the dorsal carpometacarpal joint (MC1-axis) and the axis formed between the second MCP joint and the distal edge of the trapezium (MC2-axis). Results: The FPL does not follow the angle of the thumb metacarpal. The FPL runs ulnarly from the MC1-axis at a median of 32.5° in abduction and 30.6° in adduction. In relation to the MC2-axis, it runs at a median of 47.5° in abduction and 25° in adduction. Conclusions: The FPL tendon path can be preoperatively marked using: (1) the MC1-axis from which it runs ulnarly approximately at an angle of 30° in both abduction and adduction; or (2) the MC2-axis from which it runs radially at an angle of 47.5° in abduction and at an angle of 25° in adduction. We conclude that the FPL tendon path can be preoperatively marked using these axes and does not parallel the metacarpal axes of the thumb