Stripping torques in human bone can be reliably predicted prior to screw insertion with optimum tightness being found between 70% and 80% of the maximum

AIMS: To devise a method to quantify and optimize tightness when inserting cortical screws, based on bone characterization and screw geometry. METHODS: Cortical human cadaveric diaphyseal tibiae screw holes (n = 20) underwent destructive testing to firstly establish the relationship between cortical thickness and experimental stripping torque (T(str)), and secondly to calibrate an equation to predict T(str). Using the equation’s predictions, 3.5 mm screws were inserted (n = 66) to targeted torques representing 40% to 100% of T(str), with recording of compression generated during tightening. Once the target torque had been achieved, immediate pullout testing was performed. RESULTS: Cortical thickness predicted T(str) (R(2) = 0.862; p < 0.001) as did an equation based on tensile yield stress, bone-screw friction coefficient, and screw geometries (R(2) = 0.894; p < 0.001). Compression increased with screw tightness up to 80% of the maximum (R(2) = 0.495; p < 0.001). Beyond 80%, further tightening generated no increase in compression. Pullout force did not change with variations in submaximal tightness beyond 40% of T(str) (R(2) = 0.014; p = 0.175). CONCLUSION: Screw tightening between 70% and 80% of the predicted maximum generated optimum compression and pullout forces. Further tightening did not considerably increase compression, made no difference to pullout, and increased the risk of the screw holes being stripped. While further work is needed for development of intraoperative methods for accurate and reliable prediction of the maximum tightness for a screw, this work justifies insertion torque being considerably below the maximum. Cite this article: Bone Joint Res 2020;9(8):493–500.