The flexor pollicis longus and its relation to the anterior and posterior interosseous nerves.

Sixty paired cadaver upper extremities were dissected to study the anatomy of the flexor pollicis longus in the forearm and its relation to the median and anterior interosseous nerves. An accessory head was noted in 33 (55%) of 60 specimens. The accessory head was noted to pass anterior to the anterior interosseous nerve in all specimens. The accessory head was noted to pass posterior to the median nerve in 57 specimens, and anterior to the nerve in three. Tendon or muscle anomalies were noted in eight specimens (13%), seven of which involved an anomalous attachment between the FPL and the flexor digitorum profundus of the index.