The normal and pathologic MRI appearance of the tibialis anterior proximal motor branch

Foot drop, the clinical manifestation of tibialis anterior (TA) muscle dysfunction, may be caused by pathologies involving selectively the TA motor nerve branches. Recently, in patients with fibular intraneural ganglion cysts, cystic involvement of the TA motor branch from the articular branch was demonstrated. However, no imaging characteristics of this functionally important TA proximal motor branch have been reported. We hypothesized that this particular TA proximal motor branch could be visualized and characterized on high-resolution clinical magnetic resonance imaging (MRI) in both normal and pathologic cases. Twenty-six consecutive high-resolution knee MRI examinations performed for routine indications such as exclusion of intra-articular pathology and four illustrative cases imaged to evaluate pathology of the common fibular nerve and its branches (specifically the TA proximal motor branch) were retrospectively reviewed. In cases without fibular nerve pathology, the TA proximal motor branch was visualized in 21 of the 26 patients. In the cases in which the nerve was not visualized, the imaging techniques could explain the nonvisualization of the branch. The involvement of the proximal branch to the TA was evident in the four patients with pathology, which included a perineurioma, a fibular intraneural ganglion cyst, a plexiform neurofibroma, and an inflammation of the nerve. In both normal and pathologic cases, the functionally important TA proximal motor branch originating from the articular branch can be consistently visualized on high-resolution MRI. This branch can be selectively affected by a variety of pathologies and its identification on imaging may help elucidate the predominant TA muscle involvement in these patients.