In 2005, about 629,000 total joint replacement (TJR) surgeries were performed in the United States alone and the number is expected to increase by 343% by 2030. In addition, the average age of the patient receiving TJR is decreasing. Therefore, there is an immediate need to enhance the material properties of the implants. Fracture of ultra high molecular weight polyethylene (UHMWPE) components used in total joint replacements is a clinical concern. In this work, static and cyclic fracture resistance of conventional and highly crosslinked and post-processed UHMWPE materials were evaluated in ambient air and physiologically relevant environmental conditions.
Applicability of a compliance based automated system for crack length measurement during fatigue crack propagation (FCP) tests was demonstrated for UHMWPE materials. The Standard compliance calibration coefficients were found to accurately predict the fatigue crack growth only in the low da/dN regime (da/dN < 10-4 mm/cycle). New compliance calibration coefficients that can accurately predict the fatigue crack growth were computed for different UHMWPE materials. FCP studies were conducted in ambient air and in 37°C PBS environments to evaluate the cyclic fracture resistance of UHMWPE materials. In a 37°C PBS environment, the resistance to fatigue crack inception and propagation of sterilized and highly crosslinked UHMWPE materials were found to be reduced compared to ambient air. This findings suggests that under in-vivo conditions UHMWPE implants are more likely to be susceptible to fatigue fracture than might be expected from tests conducted in ambient air. The presence of crack closure overestimates the FCP resistance in the near threshold regime. Crack closure was not observed for any of the UHMWPE materials under the testing conditions selected for this study. Under in-vivo conditions, UHMWPE components may be subjected to overloads. On application of an overload, some test specimens exhibited crack retardation while others exhibited crack acceleration on application of a single overload. On application of multiple overloads crack arrest was observed.
To evaluate the static fracture resistance of UHMWPE materials, J-R curves were obtained in ambient air and 37°C PBS environments using a single specimen normalization method. The single specimen method based on power law based deformation function was demonstrated to predict J-R curves accurately for UHMWPE materials. Significantly lower J-R fracture resistance was observed in the 37°C PBS environment as compared to that in an ambient air environment. A novel experimental method based on CTOD was developed to evaluate fracture initiation in UHMWPE materials. This method predicted conservative and more reliable J-initiation fracture toughness estimates as compared to the traditional blunting line approach.