The effect of microseparation on corrosion rates of Metal-on-Metal Total Hip Replacements

The poor performance of Metal-on-Metal (MoM) bearings has to date been blamed on "adverse loading" conditions. Studies have focused on the effect of cup inclination and microseparation on gravimetric wear rates and highlighted the importance of surgical technique when implanting such devices. Up to four fold increase in the wear rates of MoM bearings subjected to microseparation has been reported during the bedding-in period. The contribution of corrosive processes to overall material degradation during adverse loading has not previously been investigated. In the present study 28 mm HC CoCrMo alloy Total Hip Replacements were tested to 1 Mcycles under standard gait and severe microseparation conditions in an electrochemically instrumented hip simulator. An order of magnitude increase in material lost as a result of oxidation was noted (0.234 to 2.044 mm³/Mcycle) during microseparation. Corrosive degradation may therefore be a much more significant contribution to poor bearing performance under adverse loading than previously considered.