The true nature of the adductor brevis dually innervated by the anterior and posterior branches of the obturator nerve in humans.

To clarify the true nature and the mechanism of the human adductor brevis (specific adductor brevis, SAB) innervated dually by the anterior and posterior branches of the obturator nerve, we have carried out gross anatomical examination of 100 body halves of 50 adult Japanese cadavers. The SAB was found in 23 of the 100 thighs (23.0%), and its anterior and posterior surfaces received respectively the twigs from the anterior branch of the obturator nerve and the filament(s) from the posterior branch. The filament(s) was either indirectly derived from the medial stratum of the posterior branch through the formation of a common trunk with the twigs distributed in the obturator externus (14/23 thighs, 60.9%) or directly originating in the medial stratum of the posterior branch of the obturator nerve (9/23 thighs, 39.1%). In the close examination of the intramuscular distribution of the nerve to the SAB, the region innervated by the anterior branch of the obturator nerve could clearly be distinguished from that innervated by the posterior branch. The obturator nerve received fibers from L1234 (2/23 thighs) or from L234 (21/23 thighs), and the posterior branch of the obturator nerve ran through the obturator externus (18/23 thighs, 78.3%) or ran over the obturator externus (5/23 thighs, 21.7%), and finally emerged into the thigh. In view of the mode of origin of the filament(s), the structural element of the filament(s), and the pattern of entry of the filament(s) into the SAB, the fasciculus of the SAB, which is innervated by the posterior branch of the obturator nerve, was considered to originate in the obturator externus. Thus, the true nature of the SAB was concluded to be a complex product which was formed by a mechanism in which the fasciculus, which had separated from the obturator externus during the process of ontogeny, fused secondarily to the posterior surface of the regular adductor brevis. From findings in our series of studies, it was estimated that the maximum frequency of occurrence of the SAB could be 56%. Furthermore, from a statistical point of view, the segmental composition or course of the obturator nerve is not considered to be related to either the formation or the incidence of this muscle.