The Effect of Pitch Variation and Diameter Variation on Screw Pullout in 3D Printed Screws

Category: Basic Sciences/Biologics Introduction/Purpose: There are many screw and thread designs available to the foot and ankle surgeon for bone fixation. Comparing screws between companies is difficult because properties including core diameter, thread diameter, and pitch confound direct comparisons. There is a paucity of literature on comparative mechanical properties of various screw designs including variable pitch screws and tapered screws. The purpose of this study was to determine the effect of varying thread pitch and/or tapering a screws core diameter. Our hypothesis was that variable pitch screws and tapered screws would have decreased strength compared to screws with constant diameter and constant pitch. Methods: 3D metal printing technology was used to manufacture screws that varied in only the test variables. Four different screws designs were created including variable and constant pitch screws in both straight and tapered diameter versions. These screws were pulled out of synthetic bone at a constant rate while force was measured. Pull out strength and stiffness was calculated for each screw. A generalized linear model and ANOVA were completed to assess the effect on pull out strength and stiffness of the pitch and taper variables. Results: The pull out strength of the constant pitch screws (304.9N+-25.3 N, p<0.001) was significantly greater than the variable pitch screws (259.7N+-23.4 N). The pull out strength was also significantly greater for screws with a tapered diameter (305.4N+- 24.1 N) than a constant diameter (259.1N+-23.5 N, p