A virtual model for the osteosynthesis fixation strength analysis of cancellous screws considering the insertion effect in sawbones with experimental validation.

Abstract

Although the finite element method (FEM) is a valuable computational tool for analyzing factors that influence bone-screw fixation strength in osteosynthesis, it faces challenges in capturing the effects of screw insertion prior to pullout simulation due to mesh distortion and element erosion. To address these limitations, this study introduces an orthopedic computational model based on the Smoothed Particle Galerkin (SPG) method, offering an enhanced approach for simulating bone-screw interactions. The Smoothed Particle Galerkin (SPG) method is an advanced mesh-free numerical technique capable of simulating large deformations and material removal while avoiding common mesh-related issues in FEM. In this study, the SPG method is used to model the Sawbones material during screw insertion and pullout. A bond-failure model is incorporated into the SPG framework to represent material removal, employing two failure criteria: the critical effective shear strain and the critical effective plastic strain. This modeling approach allows for accurate reproduction of thread formation in the bone during screw insertion, capturing the appropriate contact geometry and residual stress conditions for subsequent pullout simulations. To validate the accuracy of the proposed simulation model, experimental tests were performed using Sawbones specimens composed of grade 15 PCF polyurethane foam, serving as an analog for human cancellous bone. The nonlinear material properties of the Sawbones were characterized following ASTM D1621 for compression and ASTM D1623 for tension. Parameters of the bond-failure model were calibrated through a combined screw insertion and pullout simulation using a non-fluted screw with a pilot hole. For the predictive analysis, three test cases were modeled, each combining different pilot-hole sizes and screw types, with and without cutting flutes. The proposed simulation model successfully reproduces thread formation, a feature that is difficult to capture using conventional FEM approaches. The results demonstrate that screw insertion induces residual stress, which strongly affects the pullout force. In addition, both pilot-hole size and screw design are shown to significantly influence residual stress and pullout performance. Comparison of pullout forces between experiments and simulations across three prediction cases, showing average errors of +4.0 %, -11.8 %, and -6.0 %, indicates that the proposed model is a promising tool for analyzing bone-screw fixation strength while accounting for the screw insertion effect, a capability not available in existing simulation frameworks.