Anatomical Relationships between Muscles Overlying Distal Transverse Carpal Ligament and Thenar Motor Branch of the Median Nerve

BACKGROUND: The purpose of the current study is to investigate anatomical relationships between the muscle overlying the distal transverse carpal ligament (TCL) and the thenar motor branch of the median nerve. METHODS: Of the 192 wrists that underwent open carpal tunnel release, a muscle belly overlying the TCL was observed on the distal margin of TCL in 25 wrists and ligament exposure could not be achieved without transection of it. We recorded surgical findings of these 25 wrists. The origin of the recurrent motor branch arising from the major median nerve was marked on the axial and coronal section diagrams of the wrist. RESULTS: The presence of muscle overlying the TCL was seen in 25 wrists (21 patients, 13%). The locations of origin were distributed not only on the radial side but anterior or ulnar side of the major median nerve. Abnormal branches originated from the unusual side in 14 cases (56% of those with a muscle overlying the TCL): central-anterior side in eight cases, ulnar-anterior side in five cases, and ulnar side in one case. These anomalous branches were frequently associated with the muscle belly overlying the TCL in our study regardless of the origin site. The branches were prone to cut if careless midline incision along the third web space was performed. Unusual origin and aberrant pathway of the recurrent thenar motor branch were associated with the presence of a muscle overlying the TCL. CONCLUSIONS: A thorough knowledge of the standard and variant anatomy of the muscle belly and recurrent motor branch in the carpal tunnel is fundamental to prevention of complications such as muscle wasting or atrophy by iatrogenic motor branch injury during carpal tunnel release.