Mechanistic Study of the Wear of Ceramic Heads by Metallic Stems in Modular Implants

Abstract

Titanium-based and cobalt–chrome alloys as well as some ceramics have been widely used in orthopaedic applications as these materials can significantly enhance the quality of human life as implant materials. However, the in vivo performance of some large-diameter metal-on-metal joints is unsatisfactory, and concerns have been expressed over the wear behaviour of the materials and released metal ions affecting local tissue and more distant organs. The longevity of these materials is highly influenced by their mechanical properties and this has driven the development of alternative ceramic components with greatly improved tribological performance. Even these novel materials are not immune to damage, for instance in some devices alumina-based ceramic components articulate with titanium alloy counterfaces (e.g. in the taper connections of titanium alloy stems and zirconia-toughened alumina femoral heads in modern modular designs) and damage has been reported of the harder ceramic surface by the softer titanium alloy component. In such contacts, the chemically inert ceramic component is not expected to corrode, so the electrochemical damage mechanisms often suggested for metal–metal contacts are not appropriate. This study attempts to understand why this wear might occur by investigating bulk and surface mechanical properties (such as hardness and Young’s modulus) of a number of hip implants and test samples using a Hysitron Triboindenter. AFM images were also obtained to determine the contact area and hence, pile-up correction factors for the metallic material. It was found that the alumina ceramic heads were generally subject to chemomechanical softening after exposure to water for an extended period whilst titanium alloy oxidised preferentially generating a hard oxide surface which was not softened by water. Furthermore, the oxidised titanium showed significantly higher hardness values therefore damaging the chemomechanically softened alumina material.