An accessory belly of the abductor digiti minimi muscle: a case report andembryologic aspects

Accessory fasciculi of the hypothenar muscles have been involved in vascula r and nerve compressions. During a routine dissection an accessory belly of the abductor digiti minimi muscle arising from the tendon of the palmaris longus muscle was found in the lower third of the forearm. The accessory fa sciculus ran through Guyon's canal enclosing the ulnar nerve and vessels. I t was attached by means of two tendons where the fibres of the abductor dig iti minimi muscle ended in a single pennate form. This anatomic variation w as associated with a marked reduction of the caliber of the fourth tendon o f the flexor digitorum superficialis muscle and a split of the median nerve . The nerve supply arose from the ulnar nerve. A fibrous band originating f rom this accessory muscular belly was found covering the median nerve. Base d on the development of muscles and fibrous structures within the hand and forearm, as well as on our results, we consider the present anomalies as an unusual persistence of an undifferentiated group of mesenchymal cells. The se belong to the superficial muscular anlagen layer of the hand, just betwe en the flexor digitorum superficialis muscle blastema (which has the capaci ty of migration) and that for the abductor digiti minimi muscle.