Anatomical study of distal insertion of the abductor pollicis longus. Concept of a new musculo-tendinous unit: the abductor carpi muscle.

Having performed 100 anatomical dissections we found that in the first dorsal compartment of the wrist besides other tendons there are one or two tendons belonging to a musculo-tendinous unit, not yet described, inserting in the trapezium and acting almost together with the other units going to the first metacarpal. This is present in 71% of cases. Its function is to abduct the carpus. Therefore we suggest to call this unit the "abductor carpi" muscle. If it is missing, which is "abnormal", instability and arthritis of the trapezio-metacarpal joint occur in a higher percentage because the other abducent forces pull the base of the first metacarpal with a shearing effect on the TM joint. Besides other less frequent variants, other "anomalies" such as the presence of a tendon going to the abductor pollicis brevis (59%) or to the opponents muscle (15%) have been found.