Accuracy of a Superficial Landmark of the Recurrent Branch of the Median Nerve and Anatomical Features of Transverse Muscle Fibres Observed During Carpal Tunnel Release

<b>Background:</b> Surgeons use anatomical landmarks like the scaphoid tubercle, pisiform, trapezial tubercle and hook of hamate, along with Kaplan cardinal line (KCL) to avoid injury to the recurrent motor branch (RMB) of the median nerve during carpal tunnel release. The presence of transverse muscle fibres (TMF) overlying the transverse carpal ligament (TCL) may suggest proximity of the RMB, but their anatomical relationship is unclear. In this study, we evaluated the accuracy of anatomical landmarks to the RMB, TMF origin and insertion, and examined the relationship between TMF presence and RMB running patterns. <b>Methods:</b> We dissected 30 hands from 16 fresh-frozen cadavers. After marking the superficial landmarks, we made a skin incision to confirm the presence of TMF and examined their origins and insertions. We then opened the carpal tunnel, dissected the RMB and recorded each position on a coordinate system using a fluoroscopic imaging system. <b>Results:</b> TMF were observed in 18 hands (60%): 13 were continuous with the abductor pollicis brevis (APB), 2 were continuous with the superficial head of the flexor pollicis brevis (FPB) and 3 were continuous with both. The bifurcation point of the RMB was significantly located 4.5 mm ulnar and 7.5 mm proximal to the superficial landmark at the median. The RMB was classified according to Poisel classification: 24 (80%) were of the extraligamentous type, 4 (13%) of the transligamentous type, 1 (3%) of the preligamentous type and 1 (3%) of the subligamentous type. Amongst these, the transligamentous/preligamentous/subligamentous types are at high risk for RMB injury during TCL incision. No significant association existed between TMF presence and these high-risk RMB types.. <b>Conclusions:</b> The actual RMB may be located ulnar and proximal to the superficial landmark, indicating that surgeons should be cautious about RMB damage even in the absence of TMF.