Variations in the Course of the Thenar Motor Branch of the Median Nerve and Their Relationship to the Hypertrophic Muscle Overlying the Transverse Carpal Ligament

Abstract

Previous anatomical and clinical studies classified the course of the thenar motor branch of the median nerve into 3 types: the extraligamentous, subligamentous, and transligamentous courses. The presence of a truly preligamentous course has been questioned. Furthermore, there has been recent interest in the presence of a hypertrophic muscle over the transverse carpal ligament (TCL). The current study has 2 aims: to determine the prevalence of the various types of thenar motor branch with particular attention to the existence of the preligamentous type, and to investigate whether the hypertrophic muscle is associated with certain thenar motor branch types. One hundred consecutive patients undergoing open carpal tunnel releases were studied prospectively. The following data were documented: type of thenar motor branch, presence of hypertrophic muscle over the TCL, and number of thenar branches. In 100 consecutive patients undergoing open carpal tunnel releases, extraligamentous, subligamentous, transligamentous, and preligamentous types were seen in 56, 34, 9, and 1 patient, respectively. All transligamentous and preligamentous branches were associated with the presence of a hypertrophic muscle over the TCL. Twenty-six of the extraligamentous branches were also associated with the hypertrophic muscle. The extraligamentous type seems to be the most prevalent thenar motor branch type in most previous studies of various races, including our Middle Eastern population, whereas the preligamentous type is rare. Finally, it is important for the surgeon to know that transligamentous and preligamentous types are associated with the presence of hypertrophic muscle origin over the TCL and that the motor branch will be within this hypertrophic muscle. Incision of this muscle should be done carefully and on the ulnar side to avoid injury to the thenar motor branch.