Wear-Corrosion Synergism in a CoCrMo Hip Bearing Alloy Is Influenced by Proteins

Background Although numerous in vitro studies report on the tribological performance of and, separately, on the corrosion properties of cobalt-based alloys in metal-on-metal (MoM) bearings, the few studies that take into account the synergistic interaction of wear and corrosion (tribocorrosion) have used canonical tribo-test methods. We therefore developed synergistic study using a test method that more closely simulates hip bearing conditions. Questions/ purposes (1) Is the total material loss during tribocorrosion larger than the sum of its components generated during isolated mechanical wear and isolated corrosion? (2) How is the tribocorrosive process affected by the presence of protein? Methods High carbon CoCrMo alloy discs (18) were subjected to corrosion and tribocorrosion tests under potentiostatic conditions in an apparatus simulating hip contact conditions. The input variables were the applied potential and the protein content of the electrolyte (NaCl solution versus bovine serum, 30 g/ L protein). The output variables were mass loss resulting from wear in the absence of corrosion, mass loss resulting from corrosion in the absence of wear, and the total mass loss under tribocorrosion, from which the additional mass loss resulting from the combined action of wear and corrosion, or synergism, was determined in the presence and absence of protein. Results The degradation mechanisms were sensitive to the interaction of wear and corrosion. The synergistic component (64 mu g) in the presence of protein amounted to 34% of total material loss (187 mu g). The presence of protein led to a 23% decrease in the total mass loss and to a considerable reduction in the mean current (4 mu A to 0.05 mu A) under tribocorrosion. Conclusions Synergistic effects during tribocorrosion may account for a considerable portion of MoM degradation and are affected by proteins. Clinical Relevance The in vivo performance of some large-diameter MoM joints is unsatisfactory. The synergistic component resulting from tribocorrosion may have been missed in conventional preclinical wear tests.