Towards a micromechanical insight into the visco-dynamic behaviour of UHMWPE for the modelling of knee joint replacement systems

Abstract

Considerable progress has been made in understanding implant wear and developing numerical models to predict certain aspects of wear for new orthopaedic devices. However, any model of wear could be improved through a more accurate representation of the biomaterial micromechanics, including time-varying dynamic and inelastic behaviour such as viscous and plastic deformation as well as any history-dependent evolution of its microstructural properties.

Under in-vivo conditions, the contact surface of the UHMWPE tibial insert evolves as a result of applied loads and complex multidirectional motions of the femoral component against it. Overt time, severe inelastic deformations and damage mechanisms occur and ultimately lead to wear. This process is accompanied by the release of UHMWPE debris in the surrounding tissues with the direct consequences of triggering an inflammatory response that leads to osteolysis and subsequently periprosthetic implant loosening. In that case a revision surgery is required. Motivated by these facts, the current research effort has been motivated by the need to gain a mechanistic insight into the micromechanical mechanisms associated with wear of UHMWPE in knee arthroplasty. To this end, two main lines of focus have been followed in this work.

One line of focus concerns the inelastic mechanisms of deformation such as creep and plasticity since they are critical in altering the contact properties of the articulating surface of UHMWPE components, leading to damage and formation of wear debris. Therefore, the relative contributions of elastic, creep, and plastic deformations on the contact area, and so contact pressure has been investigated through different numerical techniques. Additionally, contact pressure is a critical input parameter of computational wear algorithms, and it is therefore essential to establish the nature of and quantify the interplay between contact pressure, contact area, creep and plastic deformations. What are the consequences of neglecting creep deformations on wear predictions?

A first approach to investigate these aspects consisted in conducting a series of physicallybased finite element analyses replicating the mechanical characteristics and operating conditions of an AMTI Knee Simulator. Experimental creep testing on a unicondylar knee replacement system in a physiologically representative context was simulated. In both studies, linear elastic, plastic and time-varying visco-dynamic properties of computational models were benchmarked using literature data to predict contact deformations, pressures and areas.

Results indicate that creep deformations have a significant effect on both experimental and simulated contact pressures at the surface of the UHMWPE tibial insert. The use of a purely elastoplastic constitutive model for UHMWPE lead to compressive deformations of the insert which were in general smaller than those predicted by a creep-capturing viscoelastic model. At high compressive loads, inelastic deformation mechanisms dominate the mechanical response of UHMWPE components by altering the surface geometry (i.e. contact area), and therefore the contact pressure.

The second line of focus concerns the study of the role of transient and permanent polymer chain realignment during multidirectional sliding, and its potential correlation to wear. The main working hypothesis is that the evolution of the UHMWPE microstructure during multidirectional pin-on-disk (POD) tests can provide information on possible correlations between wear, sliding track characteristics and the mechanics of UHMWPE. Therefore, finite element-based POD tests were used to investigate the effects of motion paths in simulated multidirectional sliding motions on metrics related to the mechanical response of UHMWPE, with particular attention to evolution of molecular chain realignment. For this purpose, the concept of anticoaxiality as a measure of molecular chain realignment (or anisotropy) has been introduced.

The concept of anticoaxiality as a measure of molecular chain realignment (or anisotropy) was introduced to quantify the deviation from mechanical isotropy of UHMWPE microstructure. Results from these metrics support the hypothesis that multidirectional sliding as well as long sliding distances produced microstructural changes in UHMWPE, resulting in an enhanced likelihood of material damage, and so wear.

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