Initial engagement and axial force model for self-tapping bone screws

Abstract

Abstract Correctly torquing bone screws is important for implant fixation longevity and strength. A model-based smart screwdriver has been proposed to regulate torque, however current models ignore axial force and the initial engagement of the screw. This was addressed here by deriving a model based on the concept of a net axial force generated by a difference in contact areas on the inward and outward sides of the screw threads. This force is opposed by the shear strength of the material around the threads. The results of the derivation were able to predict the effects of different axial forces during insertion in relation to the hole material strength. The results may be used to compensate for initial thread breakage in future model-based smart screwdrivers to improve their accuracy.