An unusual pre-ligamentous thenar motor branch of the median nerve

The pre-ligamentous variant of the thenar motor branch (TMB) of the median nerve is extremely rare. In all previously reported cases, the branch arose from the radial or antero-radial aspect of the median nerve in the distal forearm and then pierced the antebrachial fascia to reach the thenar muscles. We report on a case in which the pre-ligamentous TMB not only arose from the ulnar side of the median nerve but it also remained deep to both the antebrachial fascia and the transverse carpal ligament until it reached the thenar muscles. The course of this variant puts the TMB at significant risk of injury during both open and endoscopic carpal tunnel release. Level of Evidence: Level V, risk study.