Anomalous Thenar Musculature Associated with Aberrant Median Nerve Motor Branch Take-off: An Anatomic and Clinical Study

ABSTRACT Introduction Iatrogenic laceration of the median nerve motor branch during carpal tunnel release is a devastating complication that has been reported with both open and endoscopic techniques. The purpose of this study is to highlight a previously underreported relationship between an aberrant course of the median nerve motor branch and an anomalous thenar muscle that places the motor branch at high risk. Materials and methods This was a two part study. The first part was a retrospective review of over 500 cases of carpal tunnel release over a 7 year period. There were 530 carpal tunnel releases performed and 20/530 cases were found to have a characteristic anomalous thenar muscle associated with a consistent aberrant course of the motor branch of the median nerve. Part two was an anatomic study in which 42 cadaveric wrists were dissected to determine median nerve branching patterns, dimensions of the transverse carpal ligament, and thenar musculature dimensions. Results Twenty patients (3.8%) in the clinical arm were found to have anomalous musculature. In each case, the characteristics of the muscle were similar; the muscle was triangular in shape, was distal to the FPB, and had minimal fascial covering. In 100% of the cases the motor branch was found to be more anterior/central or ulnar in its take-off. In the anatomic study, two hands (5%) had aberrant musculature extending distal to the transverse carpal ligament (TCL). Both were associated with an anterior/central or ulnar median motor branch take-off with recurrent course. Larger thenar musculature dimensions and anomalous thenar muscle were associated with more anterior and ulnar motor branch take-off. Conclusion There appears to be a high association between anomalous thenar musculature and an aberrant course of the motor branch of the median nerve placing it a greater potential risk for iatrogenic injury. Lourie GM, Gaston RG, Peljovich AE, Marshall JJ, Patterson L. Anomalous Thenar Musculature Associated with Aberrant Median Nerve Motor Branch Take-off: An Anatomic and Clinical Study. The Duke Orthop J 2012;2(1):18-22.