Anatomical variations of the intrinsic muscles of the thumb.

The observation of thirty dissections of intrinsic muscles of the thumb provides some information differing from earlier anatomical studies. The abductor pollicis brevis and the opponens pollicis have been divided into muscles having six or two fascicles (or bellies), respectively. The adductor pollicis muscle has been separated into nine fascicles. Many variations were observed: absence of fascicles, presence of atrophied or supplementary fascicles, or fascicles with varying insertions sites. Neither head of the flexor pollicis brevis muscle seemed to have any fascicles. The distal insertion of its superficial head observed in this work is not in accordance with the usual description found in the literature. A new classification of the thenar muscles is presented based on the present observations. Many variations were observed: some fascicles were absent, others were supplementary or atrophied, and some had variable insertions. The importance and the individual variations of the fascicles are discussed.