Some background theory for the cushion bearing concept in joint replacement implants

Abstract

The cushion bearing concept has the potential to be a major engineering advance in joint replacement surgery. Essentially, the concept involves the application of elastomeric surface layers in joint replacement implants in order to reduce friction and wear. A numerical solution procedure was applied to analyze the steady-state lubrication of a nominal line contact configuration of infinite width in which the “column model” gave the surface deflection of a compliant layer. High precision formulae for minimum and central film thickness were developed. The analysis was extended to include the influence of curvature on central film thickness and for a typical case it was shown that this influence was negligible. Following the approach of previous researchers, the formula for central film thickness was adapted to apply to an elastic layer with Poisson's ratio of 0.5 and the predictions for film thickness were compared with the results of a study in which numerical solutions were based on a full elasticity model. The agreement between the two approaches was quite good and thus the work of the previous researchers was supported. As a part of this comparison, a formula was developed for central film thickness, using data based on the full elasticity model.