The objective of this work has been to study the origin of surface failures in UHMWPE tibial bearings of total knee replacements. Earlier examination of hundreds of retrieved prostheses, along with analysis of the properties of UHMWPE material from retrieved bearings, had shown that a large number of the bearings suffered from subsurface oxidation. This oxidation was related to the gamma irradiation used to sterilize the tibial bearings. Mechanical properties of the polymer were significantly deteriorated in the oxidized region, with the most severe reduction of strength and ductility occurring about 1 mm beneath the contact surface. In this work the contact stress distribution in the bearings was analyzed, and tribotesting of the bearing materials was carried out under simulated service conditions. Fatigue cracks and delamination developed in bearings tested in a knee simulator and in rolling/sliding test specimens, and the damage was similar to that found in retrieved tibial bearings. The fatigue cracks invariably initiated in the embrittled oxidized layer, and the depth of that layer determined the depth at which the cracks began. The stress analysis showed that the maximum shear stress and von Mises equivalent stress reached high levels in the subsurface oxidized zone where the delamination and contact fatigue failures initiated. [S0742-4787(00)04401-5]

Coatings are increasingly used to improve the mechanical and tribological behavior of surfaces. It is necessary to develop models to guide the initial choice of coating/substrate combinations that can withstand the applied loads. A three-dimensional model of an elastic multilayered body, loaded both normally and tangentially against an elliptical rigid body (partial sliding, rolling/sliding conditions), is presented here. This model is based on linear elasticity theory, integral transforms, Fast Fourier Transform, and unilateral contact analysis with friction. Normal and tangential contact conditions between the two bodies are first determined and then used to calculate the multilayered body stress field. One application is given here: The influence of the mechanical properties of coating and substrate, as well as coating thickness, is studied on contact conditions, internal stresses, and potential failure mechanisms.

Ultra High Molecular Weight PolyEthylene (UHMWPE) is the polymer bearing material most commonly used for the design of the tibial insert in total knee joint replacement (TKJR). Sterilization by gamma irradiation in air causes oxidative changes in the polymer component that leads to mechanical property variations through its thickness. This may be one of the causes of the component failure. A general three‐dimensional elastic contact model has been therefore developed to predict both the contact stresses and the resulting internal stresses accounting for the plastic component finite thickness and its multilayered structure (constitutive layers of different thickness and mechanical properties) resulting from oxidation. Predictions of the maximum contact pressure and internal stresses have demonstrated that sterilization induces higher contact pressure, stress discontinuities at layer interfaces, possible bending effects and delamination risks.