In Vitro Comparative Study of Fretting-Corrosion Resistance of Ti6Al4V and Co28Cr6Mo in a Taper Joint Subjected to High Bending Moment

Co alloy modular necks have been introduced in orthopaedics to address mechanical failure of the neck-stem junction (NSJ) observed in Ti alloy dual-modular hip stem. However, following the introduction of Co alloy neck, unexpected adverse soft tissue reactions have been reported. This study investigated the effect of material combination on fretting-fatigue and fretting-corrosion behavior of the NSJ. Fretting-fatigue tests were performed in saline solution with a stepwise load increase approach to determine the fatigue strength of Ti alloy/Ti alloy and Co alloy/Ti alloy junctions. Fretting-corrosion tests were performed in FeCl₃ solution to investigate the damage phenomena occurring within the junctions. Replacement of Ti alloy with Co alloy neck both increased the junction fatigue strength by 43% and reduced by half the relative micromotions at the neck engagement level in undamaged junctions. However, after the fretting-corrosion test, the ratio between micromotion values measured on the lateral side was roughly reversed, i.e., fretting-corrosion damage markedly increased rocking micromotion in mixed alloy junctions. Additionally, the disassembly load of Co alloy/Ti alloy and Ti alloy/Ti alloy junctions was decreased by 90% and 15%, respectively. The weight loss of Co alloy neck was an order of magnitude greater of that measured for the Ti alloy neck. Co and Cr ion concentration in FeCl₃ solution was about two orders of magnitude greater than Ti ion found in solution collected from Ti alloy/Ti alloy junction. In summary, the adoption of Co alloy neck reduces the risk of mechanical failure of the junction, but increases the risk of adverse soft tissue reaction resulting from fretting-corrosion products spread out from the junction.