Reduction of the fracture risk of modular taper interfaces in orthopedic implants by enhancing their resistance towards fatigue and fretting-corrosion by the application of targeted surface modifications

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From a clinical point of view, the modular interfaces of revision hip implants are subject to an increased risk of fracture, which can result in costly follow-up operations for patients. The aim of the project is to improve the durability of these modular interfaces and to significantly reduce the risk of fracture. This is to be realized by means of targeted surface modifications at the modular taper interface. The machining processes aim at reducing the susceptibility of the interface to material fatigue under combined mechanical-corrosive loading. The focus lies thereby on the targeted induction of residual stresses (RS) in the near- surface, because residual stresses are known to have a high impact on the fatigue properties of metallic materials. In the joint project, different mechanical surface treatment processes (deep rolling, shot peening, turning) will be used to modify the outer cone surface, resulting in different residual stress depth distributions and different surface roughness. Aside from optimizing the interface, a better understanding of the effects of mechanical surface treatments on the fatigue behavior in a corrosive environment is sought because little is known about the effect of residual stresses in loading situations which comprise mechanical and corrosive stress. A systematic approach is intended to investigate the influence of the factors residual stress state of the joining partners, cone topography and cone geometry on the durability of the interface. For all variations of the cone interface, the frictional fatigue behavior under combined dynamic-mechanical loading will be characterized in a physiological framework. Furthermore, the stability of the RS during the dynamic loading is analyzed to evaluate the effectiveness of the RS over the course of the fatigue tests. Subsequently, corrosive loading is added to the loading scenario to evaluate whether the failure of the cone interfaces is fatigue-driven or corrosion-driven. Finally, long-term dynamic tests with a constant load level will serve to evaluate the effects of the different surface modifications under combined mechanical and fretting-corrosive loading. The knowledge gained during the project will allow implant manufacturers to address the hitherto unsolved problem of frictional-corrosive wear and the resulting increased risk of implant fracture. Ultimately the historically established taper interfaces will be specifically adapted to the demands of modern implant systems. Finally, the project aims on minimizing material- and design-related implant failures and on a significant reduction of the burden on patients and the healthcare system.