THE NATURE OF THE BONE - IMPLANT INTERFACE

The morphological appearances of the interface between the bone and th e components of arthroplasties depend on multiple factors. Present-day biomaterials being biocompatible when in bulk form, a host reaction c onsequent upon untoward effects of the implants as such is not expecte d. Thus, osseointegration, i.e., the direct apposition of bone to the surface of a foreign material at the light microscopical level, occurs , under favorable biomechanical circumstances, irrespective of the che mical composition of the implant. Osseointegration is a multifaceted p henomenon. First and foremost, it evolves when an initially rigid fixa tion of the component is surgically attained. Interfacial motions are associated with resorption of the bony bed, macrophagic activation and production of wear particles, the close bone-implant apposition is lo st and the formation of an interfacial membrane (IM) ensues. The histo logical features of the IM coincide with the context of its formation and evolution. The quiescent IM is composed of a thin layer of fibrous tissue and its occurrence is compatible with the biofunctionality of the implant. The aggressive or lytic IM (LIM) develops when tissue-irr itating, small, irregularly shaped and edgy breakdown products are dep osited at the interface. The thick LIM consists of an inflamed fibrous tissue, scattered within which are myriad granulomas, and its surface facing the implant displays a synovial-like aspect. The mono-and poly karyonic macrophages, constituting the granulomatous response, ingest and abut on the wear particles. Amongst the intermediary substances of inflammation elaborated by the lymphocytes and macrophages of the LIM , factors which stimulate the osteoclasts play the pivotal role in as much as progressive bone resorption is associated with progressive gro wth of the IM and, hence, with incremental interfacial motion, interfa cial deposition of wear particles and inflammatory-granulomatous respo nse. The ensuing vicious circle culminates in aseptic loosening of the arthroplasty. The morphological features of the LIM, though character ized by a stereotypical reaction pattern, are, in their details, close ly linked with the nature of the diverse components of the composite j oint replacement. The histological appearances of the bone-implant int erface of stable and loose arthroplasties, the tissular reactions to p olymethylmethacrylate, polyethylene, polyacetal, metals and hydroxyapa tite as well as the characteristics of cemented acid cementless porous -coated, press-fit and hydroxyapatite-coated prostheses are described.