Surface damage evaluation and computational modelling of clinically failed knee liners

ABSTRACT Five retrieved knee liners made of cross-linked, ultra-high molecular weight polyethylene were investigated. Each liner was examined by five investigators and in vivo damage was evaluated using four methods (Wasielewski, Brandt, Lombardi, and Hood). Both optical and confocal microscopy techniques were used to quantify damage modes. Oxidation index, crystallinity and hardness parameters were calculated for material characterization. Gait profiles of two subjects (one male and one female) for three different activities were considered in finite element analysis (FEA) simulations. The main hypotheses of this study were that 1) the wear tracks on the liners may be represented by maximum von Mises stress areas, 2) a single composite index may be used to represent 10 different damage modes. Significant reduction in tensile strength was observed with an average value of 27.7 MPa at 50KGy due to oxidation in all the liners. Correlations were made between FEA results and surface evaluation results. Gait analysis of the liners showed males typically have a higher stress level in vivo than females, leading to increased wear. Attempts were made to relate the damage patterns on the liners with the stress development and the damage evaluation methods and our computational simulation validates the methods that we used. Higher stress areas were compared against the surface evaluation results, showing those regions experiencing the high stress also had a high damage index. Even though the liners were mechanically intact, they failed clinically and more likely due to abrasive wear (due to third-body wear particles).