Adverse loading effects on tribocorrosive degradation of 28 mm metal-on-metal hip replacement bearings^*

Following the high clinical failure rates of metal-on-metal total hip replacements much work has been undertaken to investigate their poor performance. So called adverse loading scenarios such as acetabular inclination and microseparation have been attributed to indicators for failure of the implants. The ISO hip simulation standards (ISO 14242:1) still rely on gravimetric and ex situ analysis, considering only the total wear during articulation. Live in situ sensing can provide valuable insight into the degradation mechanisms of metallic interfaces under such scenarios. Clinical 28 mm diameter metal-on-metal components were articulated in a full-ISO hip simulator. The bearings were subjected to increasing angles of acetabular inclination and retroversion over short-term periods of articulation. Corrosive degradation was monitored during sliding by means of an in situ three-electrode cell. Changing acetabular inclination from 30° to 50° resulted in greater cathodic shifts in OCP upon the initiation of sliding; from −50 mV to as much as −150 mV. Under anodic polarisation (0 mV vs. Ag/AgCl) the resultant currents at the initiation of sliding also increased significantly with inclination; from approximately 4–10 µA to over 120 µA. Increased retroversion of 20° also resulted in increased anodic currents of 55–60 µA. Changing the nature of articulation demonstrated increased corrosive material loss compared to a standard ISO 14242 profile. The sole use of gravimetric assessment to determine a wear rate for hip replacement bearings under simulation can therefore neglect important degradation mechanisms, such as tribocorrosive loss in devices with metal sliding interfaces.