Volumetric material loss in metal-on-metal total hip arthroplasty: Bearing surface wear versus taper corrosion

Introduction: Corrosion at the taper junction and wear at the bearing surface are both thought to contribute to the high revision rates reported for metal-onmetal total hip replacement (MoM THR). However the relative contribution of the taper junction and bearing surface to overall material loss in MoM THR remains undetermined. Methods: Taper corrosion was qualitatively assessed using a peer-reviewed grading system for 111 failed MoM THR components from five different manufacturers. Components with obvious taper damage were sectioned and examined using scanning electron microscopy (SEM). Then 48 consecutive retrieval cases underwent volumetric wear analysis: the taper interface by using a roundness measuring machine and the bearing surfaces with the use of a coordinate measuring machine. Results: 86% of the components showed evidence of taper corrosion, with at least moderate severity observed in 60%. There was no difference in corrosion seen across manufacturers (p = 0.28). Microscopic analysis showed that the machined thread on the trunnion (of the stem) had been ‘imprinted’ onto the head taper surface, causing considerable material loss. The mean volumetric wear of the taper interface was shown to be 4.9mm3 (range = 0.3-25.2) but greater material loss occurred at the bearing surface (p < 0.01) where the mean volumetric wear was 26.4mm3 (range = 0.6-254.0). Conclusions: Taper corrosion is common in MoM THR and affects all manufacturers equally. Nevertheless, although the taper junction is a clinically important source of material loss and is likely to contribute to the reported high failure rates, the bearing surface is the greater source of material loss. Take-home message: Taper corrosion is common and is likely to contribute to the high revision rates reported in metal-on-metal total hip replacement. However this retrieval analysis indicates that the bearing surface is a larger contributor to the overall volumetric material los