Anatomical study of the accessory head of the flexor pollicis longus and the anterior interosseous nerve in Asians.

Anterior interosseous nerve palsy is known to occur uncommonly due to the compression of the nerve by the accessory head of flexor pollicis longus (AHFPL). This study was conducted to investigate the prevalence and origin of the AHFPL and the topographical relationship between the AHFPL and the anterior interosseous nerve in Asians. The AHFPL was present in 48 of 72 arms examined (66.7%), a majority of which originated from the coronoid process. The anterior interosseous nerve was observed to arise from both the medial and posterior aspects of the median nerve. The topographical relationship between the anterior interosseous nerve and the AHFPL was classified into three types depending on if the anterior interosseous nerve crossed the muscular part, or the tendinous part of the AHFPL, or coursed lateral to the AHFPL. The case in which the anterior interosseous nerve crossed the muscular part of the AHFPL occurred most frequently in the current study. The types in which the anterior interosseous nerve may be compressed were also discussed.