Fretting-crevice corrosion of cemented metal on metal total hip replacements

The development and use of 2nd generation Metal-on-Metal (MoM) Total Hip Replacements (THRs) recently became popular due to the advancement of new materials and manufacturing processes promising improved performance compared to traditional metal on metal total hip replacements. The Ultima TPS MoM THR was designed and developed as a 2nd generation MoM THR specifically aimed at younger more active patients due to the anticipated low wear rates and increased longevity of MoM THRs. In 2010, published clinical data highlighted the early failure of the Ultima TPS MoM due to fretting-crevice corrosion at the stem-cement interface. Since 2010 similar observations have been reported by other clinical centres implicating competitor products as well as the Ultima TPS MoM THR. In this thesis a systematic study has been completed in order to investigate and understand the role of fretting-crevice corrosion, and system variables, on the degradation mechanisms occurring at the stem-cement interface. Static and dynamic experimental approaches were adopted utilising in-situ electrochemistry and extensive post-test analysis. A full failure analysis of the retrieved cohort was also conducted in which the surface morphology and surface chemistry were quantified. The review of the retrieved cohort demonstrated distinct directionality and plastic deformation of the metallic surfaces, characteristic of fretting-crevice corrosion demonstrating that complex fretting, electrochemical and tribo-chemical reactions exist at the stem-cement interfaces. The formation of thick deposited layers was also seen to occur within these interfaces, with films being seen to consist of Cr2O3 bound with organic materials. Experimental investigation indicated that surface roughness, PMMA bone cement chemistry and galvanic coupling influenced the fretting-crevice corrosion rates at the stem-cement interface. It was found that by increasing the surface roughness the pure and wear enhanced corrosion rates were significantly reduced. This was due to the combination of two influencing factors. An increase in the apparent volume at the stem-cement interface and the formation of a SiO2 film created due to processing procedures. In an attempt to replicate the entire MoM system, fretting-crevice corrosion tests subjected to galvanic coupling were also conducted. Galvanic coupling was seen to significantly increase the rates of oxidation under static and dynamic conditions. This was due to the large potential differences developed across the system between active and passive areas, increasing the rates of oxidation and metallic ion release. Sulphate containing PMMA bone cements were also seen to decrease a metals’ resistance to corrosion. Cyclic loading was seen to further influence the surface morphology and chemistry of the femoral stem; along with the chemistry of the bulk environment. Distinct directionality of the femoral stem surfaces, characteristic of fretting wear, was seen in the proximal regions whilst localised crevice corrosion was seen in the distal regions. Thick Cr2O3 films were observed within the stem-cement interface resulting in a cobalt rich bulk environment. This complimented the retrieval analysis, along with other clinical studies which have associated cobalt rich environments with fretting-corrosion at the stem-cement interface. From the findings presented in this thesis it has been possible to understand the factors and system variables pertaining to the early failure of the Ultima TPS MoM cohort. Furthermore, the findings and hypothesis developed by the candidate have been used to inform recommendations for current and future product combinations along with new design rules and experimental techniques for the development of future hip prostheses.