Development of a technique for accurately determining clinical wear in explanted total hip replacements

It has long been recognised that wear analysis of total joint replacements is an important means in determining failure mechanisms, functionality and improving longevity of these devices. The standardised method for assessing wear volumes in such joints is through gravimetric means and it has been shown that this method is a useful tool for indicating simulated wear volumes. This method is, however, of little value when considering the case of a clinically explanted bearing surface for which, by definition, there is no ‘pre-wear’ data.The purpose of this study is to present a co-ordinate measuring machine (CMM) based technique for assessing the volumetric material loss in explanted total hip replacement joints and to establish its effectiveness. A 3D CAD model was created of an idealised surface representing the pre-wear surface of the explanted acetabular cup to be measured. Here, two methods are proposed for discovery of a good estimate of the original pre-wear surface, which may differ significantly from the idealised CAD model. The first uses an unworn identical component as a baseline measurement and the second utilises the unworn portion of the explanted acetabular cup to create a more accurate ‘zero-cycle’ CAD entity. From these pre-wear surfaces, it is possible to directly calculate volumetric material loss and to graphically map the wear ‘scar’, i.e. the penetration of the femoral head into the acetabular cup.