Predicting thenar motor branch anatomy for a safer carpal tunnel release

Anatomic variations of the thenar motor branch of the median nerve (TMB) that place the nerve more ulnarly in the palm can increase the risk for iatrogenic injury to the nerve during carpal tunnel release. This study examines the prevalence of an origin of the TMB that is more ulnar than its typical branching from the radial aspect of the median nerve and explores whether the appearance of the palmar intrinsic muscles at surgery can predict an unusual TMB origin prior to visualizing the TMB itself.

A prospective study of patients undergoing carpal tunnel release surgery was undertaken to document the take-off point of the TMB along the circumference of the median nerve and the presence or absence of intrinsic muscle obscuring the distal transverse carpal ligament.

Forty-one hands were evaluated. Eleven (26.8 %) demonstrated transverse carpal muscle. The most common origin of the TMB was from the radial aspect of the median nerve (56.1 %). In approximately 20 % of nerves, the TMB originated near its anterior midline. There was a statistically significant relationship between the TMB originating more ulnarly and the presence of transverse carpal muscle.

This study introduces a new classification system for further defining anatomic variations of the TMB. When transverse carpal muscle is absent, the TMB is highly likely to arise from the radial aspect of the median nerve. In the presence of transverse carpal muscle, the origin of the TMB is unpredictable and is significantly more likely to arise from the median nerve more ulnarly than is typically seen. Under these circumstances, attempts should be made to identify the TMB prior to completing ligament division.