THE EFFECT OF PITCH VARIATION AND DIAMETER VARIATION ON SCREW PULLOUT IN 3D PRINTED SCREWS
INTRODUCTION: 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 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+-25.3N, p<0.001) was significantly greater than the variable pitch screws (259.7N+-23.4N). The pull out strength was also significantly greater for screws with a tapered diameter (305.4N+-24.1N) than a constant diameter (259.1N+-23.5N, p<0.001). Tapered diameter variable pitch screws had the largest stiffness overall (mean stiffness 556.6 N/mm), which was greater than all other screw designs (p≤0.001). The difference in stiffness between all screws with either constant pitch or constant diameter was not statistically significant (p=0.060)
CONCLUSION: The pull out strength is significantly greater for screws with a constant pitch than for a variable pitch and greater for a tapered diameter than a constant diameter. The results of stiffness testing are mixed depending on the screw taper. The clinical significance of this study is that it provides data on the effects that thread design and tapering have on the pullout strength of screws providing some evidence that variable pitch screws may have lower pull out strength than constant pitch screws. Utilizing 3D printed screws allowed for the control of all other variables, which may otherwise confound the comparison of different commercially available screws.