Congenital malposition of flexor pollicis longus--an anatomy note.

Congenital malposition of the flexor pollicis longus (FPL), characterized by partial insertion of its tendon into the extensor tendon of the thumb, has been described by several authors. During treatment of a patient with this problem, we identified significant abnormalities of the more proximal anatomy of the hand, including perforation of the transverse carpal ligament by the FPL, a common muscular origin of the FPL and the flexor digitorum profundus (FDP) tendon to the index finger, and branches of the median nerve both palmar and dorsal to the preaxial border of the thumb. Knowledge of those possible anatomical abnormalities associated with malposition of the FPL will aid in the design of reconstructive procedures.