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 optimise tightness when inserting cortical screws, based on bone characterisation and screw geometry. Methods Cortical human diaphyseal tibiae screw holes (n=20) underwent destructive testing to firstly establish the relationship between cortical thickness and experimental stripping torque (Tstr) and secondly to calibrate an equation to predict Tstr. Using the equation’s predictions, 3.5 mm screws were inserted (n=66) to targeted torques representing 40-100% of Tstr, with the compression generated during tightening recorded. Once the target torque had been achieved, immediate pullout testing was performed. Results Cortical thickness predicted the stripping torque (R2=0.862, P<0.001) as did an equation based on tensile yield stress, bone-screw friction coefficient and screw geometries (R2=0.894, P<0.001). Compression increased with screw tightness up to 80% of the maximum (R2=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 Tstr (R2=0.014, P=0.175). Conclusion Screws tightened to between 70 and 80% of the predicted maximum generated optimum compression and pullout forces. Further tightening did not significantly increase compression, made no difference to pullout and increased the risk of the screw holes being stripped. Whilst further work is needed developing intraoperative methods for accurately and reliably predicting the maximum tightness for a screw, this work justifies ensuring insertion torque is considerably below the maximum.