Intramedullary Headless Screw Fixation of Metacarpal Fractures: A Radiographic Analysis for Optimal Screw Choice

Background: The purpose of this study was to investigate variations in radiographic metacarpal anatomy as it relates to intramedullary (IM) fixation of metacarpal fractures and to compare this anatomy with available headless screw dimensions. Methods: We radiographically analyzed posteroanterior and lateral (LAT) radiographs of 120 metacarpals across 30 patients without structural abnormalities. Primary outcomes included IM isthmus diameter, isthmus location, metacarpal cascade, and head entry point collinear with IM canal. Measurements were compared with a list of commercially available headless screws used for IM fixation. Results: The average largest isthmus diameter was in the small metacarpal (3.4 mm), followed by the index (2.8 mm), long (2.7 mm), and ring (2.7 mm) metacarpals. The average cascade angle between long and index, long and ring, and long and small metacarpals was 0°, 24°, and 27°, respectively. The appropriate head entry point ranged between 25% and 35% from the dorsal surface of the metacarpal head on a LAT view. The retrograde isthmus location of the index and long finger was 39.2 and 38.1 mm, respectively. Twenty-five screws from 7 manufacturers were analyzed with sizes ranging from 1.7 to 4.5 mm. Only 8 of 17 screws between 2.3 and 3.5 mm had a length range above 35 mm. Conclusions: Metacarpal head entry point and cascade angle can help identify the appropriate reduction with the guide pin starting point in the dorsal 25% to 35% of the metacarpal head. Surgeons should be mindful to choose the appropriate fixation system in light of the variations between metacarpal isthmus size, isthmus location, and available screw lengths.