The Effect of Pitch Variation and Diameter Variation on Screw Pullout.

INTRODUCTION: There are many screw and thread designs commercially available to surgeons for bone fixation. There is a paucity of literature on comparative mechanical properties of various screw and thread designs including variable pitch screws, and tapered screws. This purpose of this study was to test whether varying a screws thread pitch and/or tapering a screws core diameter alters the mechanical performance of screws.

METHODS: A mechanical pullout test was performed on 4 different screw designs, including a variable pitch screw, a constant pitch screw, and variations of these in a straight and tapered screw design. Three-dimensional printing technology was used to manufacture the metal screws in order to control for as many variables as possible.

RESULTS: The pullout strength of the constant pitch screws (304.9 ± 25.3 N, P < .001) was significantly greater than the variable pitch screws (259.7 ± 23.4 N). The pullout strength was also significantly greater for screws with a tapered diameter (305.4 ± 24.1 N) than a constant diameter (259.1 ± 23.5N, P < .001). Tapered diameter variable pitch screws had the largest stiffness overall, which was statistically significant against all other groups ( P ⩽ .001).

CONCLUSION: The pullout strength is significantly greater for screws with a tapered diameter than a constant diameter and greater for screws with a constant pitch than for a variable pitch. Results of stiffness testing is 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.

LEVELS OF EVIDENCE: Level V: Mechanical study.