Topographic localization of the motor branch of the median nerve.

Purpose: To redefine the localization of the thenar branch (TB) of the median nerve in relation to the surface landmarks that are in routine use.
Methods: The study was performed in 37 hands of 34 patients who had carpal tunnel release. All of the patients were women and the mean age was 50 years (range, 35-67 y). A radiologic marking technique was used to determine the localization of the TB, the middle finger radial side line, and Kaplan's cardinal line. The TB was marked by circumscribing with a soft radiopaque yarn and the surface landmark lines were shown by taping a K-wire to the hand for each line. An image-intensifier-printed image was obtained for each case and the distances between the markers of the TB and the wires were measured.
Results: The TB had a mean ulnar offset of 12.6 mm (range, 4.0-19.7 mm) from the middle finger radial side line and was located 4.4 mm (range, 0-9.5 mm) proximal to the cardinal line.
Conclusions: During carpal tunnel release surgery the surgeon must pay more attention to the localization of the TB of the median nerve because it was found to be 12.6 mm more ulnar than the location described in the literature.