Simulating Contact Using the Elastic Foundation Algorithm in OpenSim

Abstract

Modeling joint contact is necessary to test many questions using simulation paradigms, but this portion of OpenSim is not well understood. The purpose of this study was to provide a guide for implementing a validated elastic foundation contact model in OpenSim. First, the load-displacement properties of a stainless steel ball bearing and ultra high molecular weight polyethylene (UHMWPE) slab were recorded during a controlled physical experiment. These geometries were imported and into OpenSim and contact mechanics were modeled with the on-board elastic foundation algorithm. Particle swarm optimization was performed to determine the elastic foundation model stiffness (2.14x1011 {+/-} 6.81x109 N/m) and dissipation constants (0.999 {+/-} 0.003). Estimations of contact forces compared favorably with blinded experimental data (root mean square error: 87.58 {+/-} 1.57 N). Last, total knee replacement geometry was used to perform a sensitivity analysis of material stiffness and mesh density with regard to penetration depth and computational time. These simulations demonstrated that material stiffnesses between 1011 and 1012 N/m resulted in realistic penetrations (< 0.15mm) when subjected to 981N loads. Material stiffnesses between 1013 and 1015 N/m increased computation time by factors of 12-23. This study shows the utility of performing a simple physical experiment to tune model parameters when physical components of orthopaedic implants are not available to the researcher. It also demonstrates the efficacy of employing the on-board elastic foundation algorithm to create realistic simulations of contact between orthopaedic implants.