Osteointegration of orthopaedic devices.

The different properties of bone must be considered in order to understand the relation between orthopaedic devices and bone. The epi-/metaphyseal areas are defined by their rigidity, their high vascularity and their quick remodelling process. In contrast, the diaphyses of bone are rather elastic and built of dense, scarcely vascularised bone presenting slow remodelling. Implants can integrate by pure mechanical contact without real affinity to bone or, alternatively, they can favour ongrowth of bone, provided that they are osteoconductive. Amongst different bone substitutes, only some of them are absorbable. Only derivates of bone may present the property of osteoinduction, which is the power to create new bone in any region of the body. Orthopaedic devices are characterised by their shape, their stiffness or elasticity and by the characteristic properties of material. They may be osteoconductive such as titanium alloys and some ceramics, allowing integration in bone. Alternatively, other materials such as steel, CoCr alloys and PMMA cements remain separated from bone by a tiny layer of collagen. The surface structure influences the quality of integration. The integration of implants depends on the mutual interaction of the material with the tissue on the implantation site. All implants undergo fatiguing which can lead to fracture of the implant. All implant-bone contacts are threatened by granulation tissue mainly formed because of wear products, infection and other reasons.