Variations in innervation of the flexor digitorum profundus muscle.

The composite flexor digitorum profundus muscle has a dual nerve supply from the ulnar nerve (UN) and the anterior interosseous nerve (AIN) but anatomic data regarding the territories of these 2 nerves are limited. In this study, muscles from 20 cadaver forearms were dissected microscopically. The motor nerves were followed to their terminations on individual muscle bellies and the innervation domains mapped. In 75% of cases the AIN supplied the index and middle fingers and the UN supplied the middle, ring, and little fingers; thus, the middle finger had dual innervation. In 20% of cases the AIN went to the index and middle fingers and the UN went to the ring and little fingers. In 5% of cases the AIN went to the index finger and the UN went to the middle, ring, and little fingers. The motor entry points were normalized to the forearm length. The entry points of the UN and AIN branches were at 15% and 30% of forearm length, respectively, distal to the medial epicondyle.