The Impact of Countersinking Headless Compression Screws on Fracture Compression

Abstract Scaphoid fractures are commonly fixated with headless compression screws. These devices generate intrinsic compression that is dependent on the bone-screw interface at the proximal and distal fragments as well as depth of insertion. To minimize the risk of prominent hardware, screws are countersunk beneath chondral surfaces, but the biomechanical implications of this practice have not been explored. The purpose of this study is to characterize fracture compression with and without countersinking in three fracture arrangements for a fully threaded headless compression screw. Tests were performed on three Sawbones models and two insertion techniques. The fracture models representing proximal, waist, and distal fractures were composed of a top foam sheet of varying thickness (10, 14, and 18 mm) and a bottom sheet (25 mm) spaced 2 mm apart. Both insertion techniques utilized a 30 mm fully threaded screw inserted with a force of 35 N at 30 rpm. Flush insertion concluded when the screw head was level with the top block, and countersunk insertion was concluded when the screw head was 2 mm beneath the top block surface. This process was completed five times in each configuration. Comparisons between groups were performed using a Wilcoxon Rank-Sum test with significance set at p = 0.05. Fracture compression was similar when countersinking screws in models of proximal or waist fractures but dropped significantly in the model of distal fracture. Driving torque was consistent regardless of screw position or model. There is a higher risk of compression loss when countersinking headless compression screws if the distal fragment is small. When using variable pitch headless compression screws, it is advisable to pick a trajectory where the smaller fragment is entered first to minimize the risk of fixation loss during countersinking.