Simulating contact using the elastic foundation algorithm in OpenSim.

Modeling joint contact in OpenSim is not well understood. This study systematically investigated the variables associated with the elastic foundation contact model within OpenSim by performing a series of controlled benchtop experiment and concomitant simulations. Four metal-on-plastic interactions were modeled, including a model of a total knee replacement (TKR). Load-displacement curves were recorded during cyclic loading between 100 and 750 N. Geometries were imported and into OpenSim and contact mechanics were modeled with the on-board elastic foundation algorithm. A hybrid optimization algorithm determined that stiffness and dissipation coefficients for TKR implants were 1.52 × 1010 N/m and 57.7 Ns/m, respectively. Estimations of contact forces were 10.2% of blinded experimental data (average root mean square error: 76.82 ± 11.47 N). In the second portion of this study, freely available eTibia TKR renderings were used to test the ubiquity of the tuning parameters. They were also used to perform a sensitivity analysis of material stiffness and mesh density with regard to penetration depth and computational time. When a stiffness of 1 × 1010 was applied to an eTibia model with 5000 faces, a 100 kg load caused 0.259 mm of penetration. Under the same conditions, the tuned model experienced 0.300 mm of penetration. Material stiffnesses between 1 × 1013 and 1 × 1015 N/m increased computation time by factors of 12-23. This study provides much needed clarity regarding the use of the OpenSim EF algorithm. It also demonstrates the utility of OpenSim to model deformable materials and complex geometries, and this approach can be adapted to make reasonable estimations for both natural and surgically modified joints.