Wear of Artificial Joint Materials II

Abstract

A twelve-channel wear machine was designed for the screening of candidate materials for use in prosthetic joints, and for investigating the influence of parameters such as load, cycling rate, surface finish, sterilization dosage, and others, on the wear of conventional materials. The rationale for the design of the machine is discussed In detail with respect to the practice of using data obtained with a simplified specimen geometry to predict wear under the widely varying contact geometries in actual prosthetic joints. Three sets of experiments are described: (a) a comparison of polyethylene wear properties in the various lubricants used in previous studies, (b) tests of polyethylene against conventional prosthetic alloys to evaluate the accuracy and repeatability of the wear measurement method, and (c) a comparison of the wear properties of several polymers tested in the screening machine to the clinical performance of these materials, to evaluate the clinical significance of the laboratory test protocol. The wear machine was found to be a very reliable apparatus for providing reproducible data. The simplicity of design and its inherent reliability enabled long-term, multi-specimen tests with a variety of materials to be conducted in a reasonably short time period. Of the lubricants tested, only bovine blood serum produced wear of the same type as that observed on removed prostheses. This was considered to be an essential factor in laboratory wear tests of prosthetic materials. Wear was determined by direct weighing of the polymer specimens with correction for the effects of fluid absorption. The data thus obtained had a much higher repeatability than has been typical of previous studies of wear properties of prosthetic materials. Nevertheless, tests lasting two to three million cycles were necessary to establish accurately the very low wear rate of the polyethylene specimens, in the order of one micron per year's equivalent use of a prosthesis. This was in contrast to the range of 100-200 microns per year that has been commonly reported in the literature. However, consideration of the potential error, especially due to polymer creep, in previous wear measurement methods suggests that the value of one micron per year may in fact represent a minimum for prostheses under optimal conditions. Polyester, PTFE and Delrin polyacetal all exhibited significantly higher wear rates than polyethylene. A comparison of these results with the available data on prior clinical performance of these polymers strongly indicates that the laboratory wear model developed here provides a first-stage quantitative indication of the potential clinical performance of candidate materials.