A common treatment for individuals who suffer from advanced arthritis of the knee and other joints is to surgically replace the diseased articular surfaces of the natural joint with biocompatible metal and plastic surfaces. In the case of Total Knee Replacement (TKR), unacceptably short service lives of approximately seven years are often observed. It has been hypothesized that premature failure occurs as a result of high contact stresses caused by incongruent contact between metal and Ultra High Molecular Weight Polyethylene (UHMWPE) surfaces.
Contact stresses as high as two times the reported 5% offset yield strength of UHMWPE have been predicted for some TKR designs. This situation is in conflict with standard engineering practices which require operating stresses to be a "factor of safety" below material yield stresses. Even the unsatisfactory seven year service lives reported are surprising since material which is overloaded to this extent is expected to fail catastrophically over a few days.
This discrepancy can be explained in part by considering the assumed material properties of the contacting bodies. Many TKR contact stress analyses consider UHMWPE components to be linearly elastic at all stress levels with a single characteristic Young’s modulus equal to that reported in manufacturer’s literature for UHMWPE at room temperature.
The objectives of this study were: to experimentally determine the stress-strain response of UHMWPE at 23°C and 37°C and the effect of sterilization irradiation on the stress-strain response, incorporate this data into a theoretical contact stress model and to use the modified model to investigate the contact stress state in TKR.
Experimental characterization of UHMWPE at room temperature and TKR service temperature (37°C) indicated that UHMWPE is non-linear through much of the stressstrain range and the Young’s modulus for UHMWPE is approximately 50% lower at 37°C compared to room temperature. Sterilization irradiation did not significantly effect the yield point or the Young’s modulus of UHMWPE.
The non-linear elastic properties were modelled for UHMWPE at 23°C and 37°C; at both temperatures the Young’s modulus has been described as a quartic function of stress. The non-linear material model is suitable for incorporation into finite element and elasticity analyses. A classical Hertzian analysis was modified to analyze TKR articulations in which UHMWPE’s Young’s modulus was characterized by the quartic function determined experimentally.
An experimental validation of this analysis method is presented. The modified analysis predicted contact area radii and contact stresses more consistent with the experimental measurements than did the standard elastic solution. The non-linear elasticity of this material apparently allows a kind of "stress reduction mechanism" to occur which results in tower final stresses due to a decreasing Young’s modulus with applied stress.
A quasi-linear analysis of a typical TKR is presented. The predicted stresses were consistent with the observed lifetime of these joints. Implications for TKR design with examples are presented.
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