Extensor digitorum brevis manus muscle in association with a metacarpal boss.

Awareness of the existence of accessory muscles in the hand, such as the extensor digitorum brevis manus (EDBM) muscle, is important when making a differential diagnosis and considering the treatment of mass lesions with similar appearances. Cases of EDBM with associated dorsal wrist ganglion have been reported in earlier published reports. This report describes an unusual case of the EDBM muscle in association with carpometacarpal (CMC) boss. To the best of the authors' knowledge this has not been previously reported.