Base of the Third Metacarpal as a Palpable and Reliable Landmark for Identifying the Median Nerve's Thenar Branch

Purpose The objective of this study was to investigate whether the base of the third metacarpal can predict the location of the thenar branch (TB) of the median nerve and the accuracy of palpating the base of the third metacarpal.Methods In 15 patients with ulnar nerve lesions around the elbow, we transferred the opponens motor branch to repair the deep terminal division of the ulnar nerve (DTDUN). Before surgery, we located the TB by palpating the base of the third metacarpal volarly. During surgery, we placed three needles at the following places: one at the entrance of the TB into the abductor pollicis brevis, another at the point where the TB contacted the thenar muscles, and third at the DTDUN's trajectory over the third metacarpal. We obtained fluoroscopic images and measured distances between the needles and structures with image software. We also examined the relationship between the TB, DTDUN, and the volar tubercle of the base of the third metacarpal in cadaver hands. Finally, we invited 22 surgeons to palpate the base of the third metacarpal on volunteer hands and verified their accuracy using fluoroscopy.Results During surgery, after dissection and palpation of the TB, under fluoroscopy, we confirmed that the palpable bone prominence was the base of the third metacarpal. In cadaver dissections, we observed the TB crossing the volar tubercle of the base of the third metacarpal superficially from proximal to distal and from ulnar to radial. The DTDUN was, on average, 14 mm distal to the base of the third metacarpal distal limit. In total, 19 of the 22 surgeons correctly identified the base of the third metacarpal and consequently the trajectory of the TB.Conclusions The palpable base of the third metacarpal can be used to determine the trajectory of both the TB and DTDUN. (J Hand Surg Am. 2023;48(11):1174.e1-e6. Copyright (c) 2023 by the American Society for Surgery of the Hand. All rights reserved.)