Retrieval studies on calcium phosphate-coated implants.

The search for the improvement of implant fixation in bone resulted several years ago in the introduction of calcium phosphate coatings on orthopaedic (and dental) implants. Since then many animal experiments, especially on hydroxylapatite coatings, have been performed. By now the first promising results from clinical studies are also available. In this review, the plasma-spray process is described and some of the current techniques for the analysis of retrieved implants are discussed. It is now widely understood that the calcium phosphate hydroxylapatite is osteoconductive. Various experimental animal studies revealed the capability of calcium phosphate coatings to enhance implant fixation, especially under specific or less optimal conditions such as a with a porous implant, loose fit, micromotion or experimental arthritis. Clinical studies, including the analysis of retrieved hydroxylapatite coated hip implants, confirmed these promising results and as a consequence, hydroxylapatite coated total joint prostheses are now used on a large scale. Two questions that are closely related with each other are very actual, e.g. the strength of the coating-substrate interface and how stable or crystalline should hydroxylapatite coatings be for a proper clinical function? There does not yet seem to be a consistent answer to these questions.