Abstract
This research is focused on one of the methods of total hip replacement, namely hip resurfacing. Contemporary hip resurfacings are typified by a large diameter metal on metal bearing. With the renewed interest in these articulations there is an increasing concern regarding elevated metal ions in patients. Despite metal on metal articulations being in clinical use for several years there are numerous aspects which are still not fully understood. The work detailed in this thesis aims to look at several aspects including protein adsorption on similar Co-Cr-Mo alloys, clinical results of resurfacings which use these alloys, their in vivo wear and corrosion and finally the cellular uptake of soluble Co and Cr ions. Several studies have investigated the consequences of post manufacturing thermal treatments upon the Co-Cr-Mo alloy’s microstructure and tribological properties but none have determined if there are any biological ramifications. Any observed differences might suggest that the way in which these alloys integrate with tissues differ. Adsorption isotherms have been plotted using the surface concentration of nitrogen, as a diagnostic of protein uptake, measured by x-ray photoelectron spectroscopy. The data was a good fit to the Langmuir adsorption isotherm up to the concentration at which critical protein saturation occurred. Differences in protein adsorption on each alloy have been observed. This suggests that development of the tissue/implant interface, although similar, may differ between as-cast and heat treated samples. The clinical performance of a small cohort of retrieved heat treated McMinn resurfacings and BHRs was compared. By correlating patient and linear wear data, further insight into the in vivo performance of the two resurfacings might be obtained. A large scale study of the in vivo performance of the articulating surfaces of heat treated McMinn resurfacings and BHRs has been conducted, which looked for evidence of wear and corrosion using electron microscopy. Based on the incomplete and small number of clinical results in this study it has not been possible to draw any conclusions as to whether or not the heat treated McMinn resurfacing has a higher wear rate than the as cast Birmingham hip resurfacing (BHR). This is due to the numerous complex and multi-factorial interactions associated with the clinical performance of resurfacings. This study found significant pitting on the articulating surfaces of both heat treated McMinn resurfacings and BHRs. The prevalence of pitting was found to be slightly higher on the as cast BHR components. The location of the pitting was found to coincide approximately with the principal wear patch which might suggest that the observed pitting is, in part, the result of a mechanically assisted process. The observed corrosion of the articulating surfaces leads on to the analysis of the uptake of metal ions in cells. With the advocacy of resurfacings in young patients, there is potential amongst this patient population for prolonged periods of metal ion exposure both locally and systemically. ToF-SIMS has been used to study the cellular uptake of Cr and Co ions, and is particularly suited to this work as it enables identification and localisation of chemical elements and complex molecules. ToF-SIMS has proven to be useful in the analysis of metal ion uptake in cells; it has shown that Co bonds to the cell membrane and enters the cell. The intracellular uptake and binding of Co to the cell membrane has been shown to be both time and concentration dependant. The results for Cr, on the other hand, suggest that it does not not bind to the cell membrane. There is very little increase in intracellular Cr with increasing Cr concentration or culture time. The results show the potential for Co and Cr corrosion products to induce cell/tissue damage in vivo.