Correlation between muscle morphology of the transverse carpal ligament and branching pattern of the motor branch of median nerve

Purpose This was an intraoperative anatomical study to identify the prevalence of variations in the motor branch of median nerve and to correlate these with the presence of a transverse carpal ligament (TCL) with superficial or interposed muscle. Methods Over a period of 12 years in 1400 consecutive patients in whom the carpal tunnel was opened for any reason, observations were made in each hand to (1) determine the presence or absence of muscle fibers lying superficial to or within the TCL and (2) identify the anatomic branching patterns of the motor branch of median nerve. The primary objective was to ascertain whether the appearance of the TCL could be used to predict the anatomy of the motor branch. Results A normal motor branch arising beneath a purely ligamentous TCL was found in 1011 hands (72%). The focus of this paper is on the remaining 386 hands (28%), in which muscle fibers were found lying superficial to or within the TCL; in this group, only 29 hands (8%) had a normal motor branch. An anomalous motor branch was found in 93% of hands with a TCL with superficial or interposed muscle fibers and in less than 1% of hands with a purely ligamentous TCL. Conclusions In exposing the carpal tunnel, special care should be taken by the surgeon to identify and protect the motor branch when muscle fibers are encountered superficial to or within the TCL.