Influence of coatings on ion release from large diameter metal-on-metal hip bearings

Abstract

The reduction of wear debris and metal ions from metal-on-metal (MoM) cobalt chromium molybdenum (CoCrMo) hip replacements is needed to help reduce the incidence of adverse tissue responses that cause the high clinical failure rate of these previously popular devices. In addition, infection following primary and revision hip joint replacement surgery is a major complication and has a serious impact on a patient's quality of life whilst putting an economic strain on the health care system; silver (Ag) has been used successfully in medicine as an antibacterial. A range of surface engineered CoCrMo surfaces were investigated for wear and ion release using up to 4 million cycles (Mc) of hip simulator wear testing. Wear testing was performed using both standard acetabular orientation (35°) and more clinically adverse orientation (60°). Bearings were further challenged halfway through a wear test by subjecting the bearings to repeated head and cup luxation-reposition cycles (partial dislocation followed by reduction), and then the effects on subsequent wear and ion release investigated. Electron beam physical vapour deposition (EBPVD) was used to deposit chromium nitride (CrN) and Ag-doped CrN (CrN-Ag; 17, 41 and 51 wt.% Ag) coatings onto large diameter CoCrMo heads and cups. A number of CoCrMo hips were triode plasma nitrided (TPN) or treated using a duplex process of TPN followed by CrN coating. A total of 27 hip bearings were tested, which included current, clinically available MoM controls. All coatings acted as a barrier to Co dissolution using a static immersion model. CrN coating reduced wear rates by 58% - 100% compared to conventional MoM hips. Most notably there was a 99% reduction in Co release into the simulator lubricant when tested at both cups inclinations following 2.00 Mc, and a reduction of 89% when tested following a severe coating damage protocol. The MoM bearings showed signs of the selfpolishing phenomenon often associated with these types of bearings. Wear of the CrN-Ag bearings was similarly lower than the MoM controls in all wear phases, and the CrN-Ag coatings reduced Co release. Approximately 70% of the total Ag released over duration of the test was released over the first 0.17 Mc cycles of wear testing and was predominantly contained within wear particles rather than as ionic Ag. Adverse head and ���� ���� ���� iii cup damage resulted in catastrophic failure of the 17 wt.% Ag coating, but the 51 wt.% Ag coating was the lowest wearing, indicating that the self-lubricating properties of Ag played a role in the tribological contact. The TPN bearings wore almost twice as much as the MoM controls and released twice the amount of Co; the duplex bearings wore almost 5-times more than the CrN coating and did not act as a barrier to Co release. The poor wear and ion release characteristics of the bearings that had been nitrided was attributed to nitrogen (N) having a stronger affinity for Cr than Co. Although the results of the present thesis indicate that both wear and ion release can be reduced by utilising EBPVD CrN-based surface coatings, it is likely that, given the current observations of high rates of failure of MoM implants, that surface engineering of MoM will not be embraced in the near future. However, there may be opportunities to exploit this technology in alternative orthopaedic implant applications.