The long-term stability of metallic materials for use in joint endoprostheses

In orthopedic implants, metals are permanently incorporated into bones and tissue. Usually the metals are not in the electrochemical equilibrium state immediately after implantation. The metal oxide forms itself only after hours up to weeks. In the meantime the metal releases ions into the tissue, much more than in the equilibrium state. On the other hand a passive layer can interchange mechanically with bones and implanted components. The resulting destruction of the surface leads equally to an increasing release of ions. As a consequence of these facts, perfect metals for application in implants must have a short repassivation period and mechanically indestructible surface oxides. To what extent the applied metals perform these conditions is tested for the stainless steel type 316 L, the multiphase alloy MP-35 N and TiAIV. A comparison of the rates of corrosion resulting from damage to, or destruction of, the passivating oxide layers, with the rates that can occur in connection with local forms of corrosion, shows that in joint implants, it is merely the repassivation properties of the metals employed that determine the concentration of ions in the tissue.