Background: Clinical studies using extensile approaches for carpal tunnel release (CTR) report a fairly high incidence of thenar motor branch (TMB) variants. As mini-open and endoscopic CTRs have become commonplace, the likelihood of encountering one of these variants in current practice is unknown. The purpose of the present study was to assess prospectively the frequency with which TMB variants are encountered during routine surgery. Methods: All patients who underwent a primary CTR between August 2014 and April 2015 by 11 hand fellowship-trained, orthopedic surgeons were prospectively evaluated. All surgeons performed releases in their usual technique and notified the lead investigator of any median nerve variations encountered. A total of 890 primary CTRs in 795 patients were performed during the study period. Results: Four TMBs seen were transligamentous variants (4/890 of procedures = 0.45%; 4/795 of patients = 0.50%). Three were identified during open CTR, and 1 during endoscopic CTR. In 2 cases, the transligamentous TMB originated from the volar aspect of the median nerve and penetrated the midportion of the transverse carpal ligament. One TMB originated from the volar and ulnar aspect of the median nerve. One TMB originated from the ulnar aspect of the median nerve proximal to the carpal tunnel. There were no cases of TMB injury during the course of the study. Conclusions: TMB variations are encountered infrequently during routine CTR. The most commonly encountered variant during routine mini-open or endoscopic CTR in our study was a transligamentous branch.
Keywords: carpal tunnel release; carpal tunnel syndrome; median nerve anomalies; thenar motor branch; transverse carpal ligament.