Modeling and Simulation of UHMWPE

A thorough understanding of the mechanics of UHMWPE is important for efforts to improve the performance of orthopedic components. Elastic properties, resistance to plastic deformation, stress and strain at failure, fatigue behavior, and wear resistance of UHMWPE are believed to play roles in the life expectancy of an UHMWPE bearing. There exists a fundamental relationship between a material s intrinsic mechanical properties, akin to state variables, and how a structure made of the material will respond under mechanical stimuli. This material-specific fundamental relationship is referred to as a constitutive model. A validated constitutive model is a required input to a finite element (FE)-based simulahon of a structure made of the material in question. 

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Bergstrom J, Rimnac C, Kurtz S. An augmented hylaid constitutive model for simulation of unloading and cyclic loading behavio-of conventional and highly crossUnked UHMWPE. Biomaterials 2004 25 2171-8. 

Of greater importance is how well the physics-inspired model framework represents the governing micromechanisms, and ultimately, how well the model can predict the behavior of a given material under different loading conditions than that for which the model was originally calibrated. The simulations of the small punch test demonstrate that the HM provides satisfactory and valid predictions of large-deformation multiaxial behavior of conventional and highly crosslinked UHMWPEs. 

Notched monotonic tensile specimens can be a useful approach to begin to understand the effect of structural notches on the behavior of UHMWPE total joint replacement components. Therefore, we developed a testing methodology to characterize the stress strain and fracture behavior of a notched tensile specimen. Additionally, the stress-strain behavior of notched tensile specimens can be used to challenge the Hybrid Constitutive Model for UHMWPE (see Chapter 35) with a multiaxial stress state. Accurate prediction of the behavior of a notched specimen by a simulation utihzing the Hybrid Model would be one vahdation of its accuracy in describing the mechanical behavior of UHMWPEs. 

UHMWPE exhibits a complicated, nonlinear mechanical response that is dependent upon initial processing (radiation and/or thermal treatment), as well as the strain rate and temperature. Due to UHMWPE s complex mechanical behavior, finding an appropriate constitutive model for simulating the behavior of a UHMWPE component can be challenging. For example, the use of a simple elastic or viscoelastic model may yield accurate results for a particular component geometry and material formulation in a specific loading scenario. Small changes to any of these parameters, however, may render this choice of constitutive model inappropriate. 

It is also clear that thermoplastics are very different and typically exhibit a broader range of behavior in comparison with other structural materials, such as metals. The observed behavior is a manifestation of the different microstructures of the two types of materials and the different micromechanisms controlling the deformation resistance. It is therefore not surprising that different material models should be used when simulating UHMWPE compared with metals. 

Hyperelastic models are often used to represent the behavior of crosslinked elastomers, where the viscoelastic response can sometimes be neglected compared with the nonlinear elastic response. Because UHMWPE behaves differently than do elastomers, there are only a few specific cases when a hyperelastic representation is appropriate for UHMWPE simulations. One such case is when the loading is purely monotonic and at one single loading rate. Under these conditions it is not possible to distinguish between nonlinear elastic and viscoplastic behavior, and a hyperelastic representation might be considered. Note that if a hyperelastic model is used in an attempt to capture the... 

One question is whether the diffusion phenomenon can affect the mechanical properties of the polymer and to what extent. Experimental data, obtained in vitro with model compounds, suggests that modifications of mechanical properties induced by diffusion are quite limited [80]. In vivo diffusion leads to a variation in the surface wetabil-ity, then it favors the adhesion to the UHMWPE surface of a monomolecular layer of polar compounds, such as proteins or phospho-lipids [81]. It must be taken into account that in vitro tests (i.e., hip or knee simulators) can accelerate the mechanical processes but, even in the presence of a suitable lubricant, do not correctly reproduce this phenomenon, which is strongly time dependent. 

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