Surgery on the hand is performed under regional anesthesia, often nerve-block anesthesia at the level of the brachial plexus. Tourniquet-produced ischemia provides a dry operative field within which surgery can be performed with precision. Prepare the entire hand and drape it free. Place it comfortably on an operating arm board, with the volar surface of the wrist and hand turned upward.

Outline an incision that curves in the natural skin crease at the base of the thenar eminence, beginning about halfway from the wrist to the web space of the thumb. As the incision approaches the wrist crease, draw it longitudinally across this crease, and then angle the proximal extension of the incision toward the ulnar side of the wrist.

This incision is designed to accommodate the anatomic variations of the median nerve and to provide an adequate release for the carpal tunnel segment of this nerve. If the incision is kept wholly within the skin and the superficial fascia, no motor nerves and no trunks of sensory nerves should be encountered. However, the potential for damage to the motor or recurrent median nerve always exists if one is not cognizant of its presence and its possible anatomic variations. Most frequently, the recurrent (motor) branch of the median nerve is given off the radial division or side of the median nerve distal to the flexor retinaculum and is recurrent (in about 50% of cases). The next most common variant is for the nerve to arise on the radial side of the median nerve in the carpal tunnel but to pass through the tunnel and take a recurrent course to innervate the thenar muscles (in about 33% of cases). The third most common variant (in about 20% of cases) is for the nerve to arise from the radial side of the median nerve in the carpal tunnel, then to pass through fibers of the flexor retinaculum to reach the thenar muscles; in this case, its course is not recurrent. In addition to these variants, in rare instances, the nerve arises from the ulnar side of the median nerve and takes a recurrent course to the thenar muscles. This can be further complicated by the recurrent branch lying on the superficial aspect of the flexor retinaculum. Further variants of note include a high division of the median nerve, so that two nerves lie in the carpal tunnel; accessory branches of the recurrent nerve to the thenar muscles and instances in which the recurrent branch leaves the median nerve proximal to the carpal tunnel and passes through or superficial to the fibers of the flexor retinaculum. One additional anatomic variation of note is the occasional presence of a persistent median artery accompanying the median nerve through the carpal tunnel and, occasionally, an aberrant muscle in the tunnel.