Constitutive Model for Polyethylenes in Joint Components

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DESCRIPTION (provided by applicant): Highly cross linked ultra-high molecular weight polyethylene (UHMWPE) components were introduced into clinical use for total hip and for total knee replacements in 1998 and 2001, respectively. Cross linking improves the wear resistance of UHMWPE, but there is also a loss in ultimate strength, ductility, and resistance to fracture. Development of new designs that use highly cross linked UHMWPEs are limited by the inability to predict fatigue and fracture behavior in vitro and in vivo. Our research for the past 3 years has been successfully aimed at developing and validating a polymer physics-based constitutive model to accurately predict the large deformation mechanical behavior of UHMWPE components under multiaxial and cyclic loading conditions. The continuing global goal of this research is to improve the long-term performance of UHMWPE joint components, for any UHMWPE formulation, through significantly more reliable numerical modeling of components. The hypothesis of this next phase of our research is that a physics-based failure model can be established that will quantitatively predict gross static and fatigue fracture of UHMWPE components. We propose to extend our physics-based constitutive model for UHMWPE to include static and fatigue failure through achievement of the next four specific aims: 1. Extend the constitutive model for conventional and highly cross linked UHMWPEs to incorporate a static failure criterion within a stochastic framework for monotonic uniaxial tension up to failure. 2. Incorporate a fatigue damage failure criterion into the constitutive model. 3. Verify the failure model developed in specific aims (1) and (2) by conducting (and then numerically simulating) tests conducted on notched (triaxial-stress-state) tests under monotonic and cyclic loading conditions. Relevance of Proposed Research to Public Health: There is a need to be able to prospectively predict the propensity for fracture as a failure mode for current and new component total hip and total knee designs that make use of these new, less ductile, UHMWPEs. Recently, there have been four Medical Device Reports of cross linked UHMWPE hip components that failed due to fracture in less than two years. In our own implant retrieval collection, we have one cross linked UHMWPE hip component that failed due to fracture after 8 months. The MDR's and our own implant retrieval experience support that mechanical failure of highly cross linked UHMWPE, albeit in a subset of retrieved cases, is nonetheless a clinically relevant phenomenon.