Variations in the Anatomy of the Third Common Digital Nerve and Landmarks to Avoid Injury to the Third Common Digital Nerve With Carpal Tunnel Release

BACKGROUND The third common digital nerve (TCDN) has been described as the most commonly injured digital nerve during carpal tunnel release (CTR). Anatomic variations of the origin and course of the TCDN from the median nerve may place this structure at risk. Anatomic landmarks may be useful to predict the location of the TCDN to minimize the risk for injury to this structure during CTR. METHODS Twenty cadaveric hands were used to determine the origin and course of the TCDN. The origin of the TCDN from the median nerve was identified in relation to the transverse carpal ligament (TCL), cardinal line, and superficial palmar arch. The course of the TCDN was inspected in relation to the scaphoid tubercle and ring finger. RESULTS Three specific anatomic variations for the origin of the TCDN were identified: type 1 originating proximal to the distal edge of the TCL (3 of 20 patients), type 2 originating distal to the TCL but proximal to the superficial palmar arch (14 of 20 patients), and type 3 originating distal to the TCL and at or distal to the superficial palmar arch (3 of 20 patients). The origin of the TCDN was measured as an average of 5.0 +/- 1.2 mm distal to the cardinal line. The TCDN coursed along an oblique vector from the scaphoid tubercle to the midpoint of the palmar digital crease of the ring finger for type 2 or type 3 variations. Near the cardinal line, the oblique course of the TCDN traverses the vector of the longitudinal incision used for CTR. CONCLUSION The TCDN is one of the most frequently damaged neurological structures during CTR. Iatrogenic injury to this structure can be disabling and even devastating to patients. A detailed knowledge of the carpal tunnel and its underlying structures can prevent inadvertent injury to the TCDN. Anatomic landmarks to predict the origin and the course of the TCDN allow the surgeon to preoperatively predict the possible locations and paths of this important structure. This information can prove to be useful in avoiding injury to the TCDN by clinicians performing CTR in their practice, whether via the open or via endoscopic technique.