Following higher than acceptable failure rates, the most recent generation of Metal-on-Metal THRs have all but been removed from the market. Many recent studies in the literature attribute their failure to higher wear rates as a result of so-called ‘adverse loading’ scenarios.
In order to investigate the in situ corrosive degradation of 28 mm Metal-on-Metal and Metal-on-Ceramic Total Hip Replacement components under these ‘adverse loading’ scenarios, two hip simulators were instrumented with three-electrode electrochemical cells. Various DC electrochemical analysis techniques, including Linear Polarisation Resistance and Potentiostatic Polarisation, were used to quantify the corrosion currents released from the bearings during sliding.
Under 0.8 mm of Microseparation the corrosion currents were found to increase by a near order of magnitude compared to Standard Gait; which resulted in an increase of estimated corrosive volume loss from approximately 0.03 - 0.05 mm3 to as much as 0.24 mm3. A similar increase was observed for Metal-on-Ceramic bearings whereby the contribution of corrosion to total material loss from the bearing shifted from approximately 4 - 8 % to as much as 17 %. Under potentiostatic polarisation the resultant anodic current transient was found to increase with increased angle of acetabular inclination. The magnitude of peak current increased from approximately 5 - 10 µA at 30° inclination to 80 - 120 µA at 50°. Corrosion at the bearing surface of 28 mm Total Hip Components was found to be a significant source of corrosive material loss and ion release. This was also sensititve to the articulations conditions and did not necessarily scale linearly with total mass loss. Consideration of the mechanisms of degradation is therefore critical to
pre-clinical assessment of devices in order to better predict in vivo performance
