The significance of sample preparation when testing surface coatings for orthopaedic implants

In the UK there are over 50,000 hip replacement operations annually, of which approximately 10% are revision surgeries, most commonly necessitated by aseptic loosening. By improving implant design the number of risky and costly revision surgeries can be significantly reduced. Aseptic loosening is mainly caused by osteolysis, a condition that occurs when the body attempts to break down wear particles produced from the articulation of the artificial joint; the chemicals used by the body have limited effect on the wear particles but do start to cause the bone to reabsorb. By reducing the production of wear particles and making those that are produced as biocompatible as possible, the incidence of aseptic loosening can be reduced. Surface coatings are prime candidates for achieving this. By modifying the surface of the implant using coatings, it is possible to modify the tribology of the joint while retaining the bulk properties. This not only enables the increase of wear resistance of the joint but introduces a barrier between the blood and the metal components which are causing increased concern among medical professionals who are worried about the toxic effect of metal ions being released into the body. For orthopaedic implants to reach the market, stringent testing is required. Joint simulators are used for testing, machines that attempt to mimic the movement and loading expected in the body. Unfortunately tests can take months to complete; this is unsuitable for initial development of coatings numerous iterations of which can be produced in a few days. Consequently preliminary testing is carried out using other techniques, such as pin on disc, which sacrifices elements of the simulation of the joint movement for speed. Initial pin on disc tests used to identify potential coating candidates identified a need to accurately control the geometry and surface characteristics of the test samples used for short tests (<1 day) so as to be able to accurately measure the small amounts of wear created by the measurement techniques described within this paper. Control of surface finish was also important to ensure that there were no significant surface defects which may act as stress concentrators and result in premature failing of the coating. By using a 7 axis CNC Zeeko polishing machine, it has been possible to accurately control the surface roughness and form of samples and characterise their effect on the wear of surface coatings that are being developed for orthopaedic applications.