A novel thread design for bone screw reducing lateral stress concentration while increasing pullout resistance

Abstract

Bone screw is used commonly in orthopaedic surgery. Thread profile is the most important structure that determines the mechanical performance of bone screw. Buttress thread is currently the standard thread for bone screw for its good pullout strength. Even with good clinical result, the number of reported screw failures is high, especially in patients with poor bone quality. As the exclusive standardized method, pullout test has been used widely in developing new screw designs and validating existing screws. However, by applying pullout tests in the axial direction, the clinical situation is not realistic because it rarely simulates most of the physiological loading conditions. Therefore, the typical buttress thread design based on the axial pullout test is not necessary to have better fixation stability. Which biomechanical test is more relevant with the fixation stability of the bone screw remains unclear. Thus, this project aims to investigate the correct evaluation test for bone screw and develop novel thread design based on this correct evaluation test. Firstly, we investigated the screw loosening mechanism of diaphyseal plate fixation by finite element analysis (FEA) and screw loosening X-ray feature analysis. We found that the radial stress of the screw plays a larger role in screw loosening than the axial stress. Bone resorption triggered by high radial stress of screws is indicated as the mechanism of screw loosening. Secondly, we investigate how the pullout strength and lateral migration resistance of bone screw are related to screw stability of locking plate (LP) and dynamic compression plate (DCP) fixation. We found that lateral migration resistance is the primary contributor to locking screw fixation stability when applied to an LP in resisting both craniocaudal and torsional loading. For compression screws applied to a DCP, lateral migration resistance and pullout strength work together to resist craniocaudal loading, while pullout strength is the primary contributor to the ability to resist torsional loading. And then, we explored the thread features that can improve the axial pullout strength and lateral migration resistance of bone screws respectively by FEA and biomechanical test. We found that a proximal flank angle larger than 90° can improve the pullout strength of the bone screw, a flat thread crest and an undercut feature can improve the lateral migration resistance of the bone screw. Finally, we developed locking screw with a novel thread design that possesses a flat crest and a head-facing undercut feature and compression screw with a novel thread design that possesses a proximal flank angel larger than 90°, a flat crest and a tip-facing undercut feature and compared their fixation stability with typical buttress thread screw in the FEA model and biomechanical test. We found that both of the novel locking screw and compression screw show superior performance when compared with their counterparts of typical buttress thread. In conclusion, lateral migration resistance should be considered in the bone screw evaluation. Novel thread with a proximal flank angel larger than 90°, a flat thread crest and an undercut feature can improve bone screw fixation stability.