Simulation of macroscopic surface damage mechanisms to ultra-high molecular weight polyethylene components in total knee replacement

Damage to the surfaces of ultra-high molecular weight polyethylene (UHMWPE) components limits the life of contemporary knee replacements. Clinical retrieval studies, in vitro experiments, and mechanical analyses suggest material degradation and fatigue as mechanisms of macroscopic damage to UHMWPE. However, the mechanisms of damage to UHMWPE are poorly understood. This dissertation examines the contributions of kinematics, inelastic material behavior, and fracture to surface damage to UHMWPE in knee replacements.;Two-dimensional, plane-strain, large deformation finite element simulations of flat UHMWPE components cyclically loaded by translating rigid indenters were performed. Stresses from non-linear finite element analyses were used to drive surface fatigue cracks in linear elastic fracture mechanics simulations. Cyclic moving indenter simulations were extended to three-dimensions using both regular UHMWPE and a reduced modulus UHMWPE. A micro-mechanically inspired constitutive model was fit for UHMWPE and its numerical implementation was formalized.;Plane-strain simulations showed that large deformation and non-linear, history dependent response of UHMWPE result in surface tensile residual stresses that influence cyclic stresses associated with surface damage to implants. Fracture mechanics simulations resulted in crack propagation trajectories that were consistent with observed clinical damage. It was found that linear elastic fracture mechanics alone cannot reproduce clinically observed crack propagation rates. Three-dimensional simulations confirmed the results from two-dimensional Simulations and also predicted large, obliquely oriented, tensile stresses at the mediallateral edges of indenter contact, which were consistent with damage observed in in vitro experiments. A reduced modulus UHMWPE resulted in lower stresses than regular UHMWPE. It was shown that a micro-mechanical approach can be used to model UHMWPE and a framework for improved characterization of the material in knee replacement applications was established.;Relative kinematics of articulation, non-linear inelastic constitutive behavior, and propagation of fatigue cracks are interrelated factors in surface damage to UHMWPE in knee replacement. Computational mechanics simulations of surface damage mechanisms to UHMWPE may be used to extend the life of knee replacements.