Constitutive Model for Polyethylenes in Joint Components

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DESCRIPTION (Verbatim from the Applicant): Wear damage to the articulating surfaces of ultra high molecular weight polyethylene (UHMWPE) joint replacements continues to be recognized as a significant clinical problem limiting the longevity of total joint replacements. It has been shown that the cyclic large strain mechanical behavior of UHMWPE affects the damage mechanisms of hip and knee components; however, there is still a lack of quantitative understanding regarding how changing the state of deformation affects the mechanical response of either conventional UHMWPE, or the highly cross-linked UHMWPEs recently introduced into clinical use for total hip arthroplasty and under consideration for use in total knee arthroplasty. There is a need for better predictions of damage and wear from numerical analyses of UHMWPE components. However, to do so, a constitutive model for UHMWPE that accounts for multiaxial and cyclic behavior must first be developed, validated and implemented. Our global hypothesis is that a physically based constitutive theory will more accurately describe the large deformation mechanical behavior of UHMWPE structures under multiaxial and cyclic loading conditions than the material models in current use. It is proposed to: (1) model three UHMWPE materials (virgin, gamma radiation sterilized in nitrogen and gamma radiation cross-linked) using physically-based constitutive theories for polymers and compare the results to the current material model (isotropic plasticity); (2) determine which constitutive theory provides the best model for each UHMWPE material by determining which theory best predicts experimental results; and (3) implement the best constitutive theory into hip and knee implant finite element models to predict the time-dependent multiaxial stress and strain states. The next step will be to utilize the developed tools from this study to predict wear, surface damage (and gross damage) from in vitro hip and knee simulator studies, and ultimately, from in vivo use. The goal is to improve the long-term performance of UHMWPE joint components, regardless of UHMWPE formulation, through significantly improved numerical modeling of components prior to implantation.