Development of fused metrology methods for the analysis of hip implant tribology

In order to advance the study of wear phenomena involved in total hip implants a fused metrology system has been designed and constructed. The novel fixture system has been designed and built to facilitate large area surface measurement for hip implant bearing wear. The system allows coordinate measurement machine (CMM) wear map data to be used for precise positional targeting of areal surface metrology using high spatial resolution optical interferometry. The complete measurement process allows Coordinate Measuring Machine measurement and surface metrology datasets to be ‘fused’ thus facilitating carefully positioned wear scar analysis. The fixture utilises two digital rotary stages, in a gimble configuration, to precisely position the bearing component during CMM measurement and surface metrology. To test the effectiveness of the fixture system a cohort of explanted large metal-on-metal (MoM) femoral heads were assessed. Application of the fixture system allowed a set of grouped surface measurements were taken within the wear area, the wear area boundary region, and at unworn locations across the femoral heads. Additionally, a series of stitched surface measurements are taken through the entire wear area and combined into a single surface measurement. The ‘fusion’ method, allowed areas of roughening (or smoothing) to be estimated and overlayed on the corresponding CMM wear map. The developed fixture system allows for better understanding of hip implant performance. Combined with areal surface metrology parameters such as areal average surface roughness Sa, developed surface area ration Sdr and areal surface skewness, Ssk the system could be utilised understand the wear mechanisms for both explanted, in-vitro and in-vivo wear testing and also detailed quality inspection of newly manufactured components. The significance of the system deployment is that wear location and detailed mechanism can be studied simultaneously, thus delivering understanding of typical wear mechanisms and hence the fixture becomes a tool for developing increased implant life through understanding surface interactions.