An Experimental and Computational Mechanical Analysis of Bone Anchors and Substrate Interface

Abstract

Screw loosening is a common concern in orthopedic surgery. Therefore, this study is a computational and experimental evaluation of bone substrate and cortical screw interface. An axial load test was performed in an Instron with titanium alloy screws with 2.7 mm, 3.5 mm, and 4.5mm diameter orthopedic cortical screws inserted into SawBone with densities varying from 10 to 50 pounds per cubic foot (PCF), incrementing in sets of 10 PCF. SawBone was chosen in place of human bone because it has been shown to emulate the same mechanical properties (1). Twelve samples were prepared for each screw size and substrate density combination to ensure statistical significance. The experimental data was collected into average stress-strain plots for each density. Then, each combination was modeled two-dimensionally within Ansys. Each two-dimensional model contained a refined mesh, appropriate material properties, boundary conditions analogous with the experimental setup, and had proper contact definition. The FE models were validated by plotting their stress-strain data against the stress-strain data collected in experimentation. It was found that the substrate density and screw diameter have a direct effect on the interfacial stiffness. Additionally, stress risers were located at the far top edge of the screw thread on the SawBone and the top support on the SawBone.