Neuromuscular anatomy of common fibular nerve with special focus on fibularis tertius muscle

Fibularis tertius muscle (FT) was first described by Vesalius in 1543 and was subsequently studied in detail by Henle and Hyrtl in the nineteenth century. There has been a controversy over whether FT is a separate beings. With regard to the question, it has been reported that the FT is an independent muscle or a part of the adjacent muscles such as the extensor digitorum longus muscle (EDL) and the extensor digitorum brevis muscle. In this study, to clarify the origin of the FT from the perspective of neuromuscular anatomy, gross and stereomicroscopic dissections and modified Shiler's staining were performed on 48 sides of the lower legs from 25 Japanese cadavers. Of the 48 specimens, 43 specimens (89.6%) had a FT muscle belly, while 5 specimens (10.4%) did not. Most FTs were innervated by distal nerve branches from a common trunk with the EDL. The innervations of the FT and EDL suggest that the FT and EDL were brother muscles, and we proposed here a hypothesis that the FT and EDL originate from a common muscle mass and compete in a tug-of-war for muscle belly components during their development.