17 th European Conference on Fracture 2-5 September , 2008 , Brno , Czech Republic Corrosion , Fatigue and Corrosion Fatigue Behaviour of Metal Implant Materials , Especially Titanium Alloys

In the last years, implant materials have gained growing importance in all areas of medicine. In the human body, materials may be exposed to high mechanical loads which comprise friction and wear besides multiaxial, multistep fatigue loading. Furthermore, the body electrolyte is very corrosive containing proteins, enzymes and cells besides salts. Basically, the success of an implant depends on the interface which develops between the implant surface and the biosystem and is determined by the surface properties of the implant material. Besides geometrical and morphological characteristics, electrochemical properties are extremely important. Generally, corrosion and fatigue tests on implant materials are performed in physiological salt solutions under strongly simplified (loading) conditions. Concerning the fatigue behaviour, it is hardly known, however, in which way the complex environmental and loading conditions in the human body influence the cyclic deformation and crack growth behaviour. Introduction Over the last decades, implants have become more and more important in all areas of modern medicine. First of all, the used materials have to be biocompatible which was defined as the “ability of a material to perform with an appropriate host response in a specific application” [1]. There is always some sort of interaction between the physiological environment and the foreign body. This response is strongly influenced by the surface properties of the implant material. In order to achieve long term success of an implant, a functional interface between the physiological environment and the implant has to be formed and maintained. Therefore, not only the material has to sustain the conditions in the body but also, and first of all, the physiological environment must tolerate the implant material and structure in an adequate manner. Under physiological conditions, load bearing implants are subjected to a complex interaction of mechanical and chemical-biological loading components (comp. [2]). Cyclic loads and media containing salts in rather high concentrations besides proteins, enzymes, and cells are one aspect of the in vivo loading scheme. Generally, the mechanical properties of implants and implant materials are characterised under strongly simplified loading and environmental conditions. How much the higher complexity of the in vivo conditions comprising the mechanical loading pattern as well as the chemical-biological environment influences the fatigue life, the cyclic deformation behaviour as well as crack formation and growth – a state that must never be reached – has hardly been investigated yet. Fatigue loading and the resulting microstructural changes on the surface may strongly affect the interface to the living tissue. Despite their physiological and mechanical relevance, microstructural investigations of the cyclic deformation behaviour of implant alloys under variable amplitude loading in the crack free fatigue state in quasi-physiological environment have scarcely been performed up to now. Especially the influence of proteins is only known roughly. The main question that arises is, how good the physiological conditions have to be simulated to achieve reliable information on the long-