Deflection of Cancellous Bone Screws Under a Cantilever Bending Load

Abstract

Surgical bone screws can be subjected to cyclic bending loads when plating constructs are used in the fixation of weight bearing members. While extensive research has been conducted on axial loading that leads to screw pull-out, there is a gap in our understanding of how asymmetric bending loads contribute to screw fracture. The focus of this research was to examine the effect of screw length (20 mm and 40 mm) and cancellous bone density (0.48 g/cm3 and 0.24 g/cm3) on the relative stiffness of 6.5 mm cancellous bone screws subjected to a cantilever bending load. It was hypothesized that longer screws in higher density cancellous bone would result in less screw deflection, supporting clinical practice. For testing, synthetic composite bone was used to simulate the characteristics of natural bone while subjecting screws to quasi-static loading with a universal testing machine. Contrary to the hypothesis, neither screw length nor cancellous bone density resulted in a statistically significant difference (p > 0.05) in deflection for loading up 450 N. The cortical shelf appeared to support the majority of the bending load through compression, rather than acting as a fulcrum. When the 3.0 mm cortex was removed, there was a significant difference in deflection due to both screw length and cancellous bone density.