A critical review of decades of research on calcium phosphate-based coatings: How far are we from their widespread clinical application?

This critical review reports that several decades of calcium phosphate investigations, mostly on hydroxyapatite (HA) coatings, did not reveal a direct link between microstructural and topographical properties of the HA-coated implants of different types (dental, orthopaedic, acetabular cup and so on) and their effects on bone osteogenesis. It should be concluded that most of the research revealed positive and favourable effects of the HA-coated implants, irrespective of the coating fabrication procedure; however, there are no universal criteria by which to compare the published works. Moreover, most of the research has been devoted to in vitro characterisation of the bioactive performance of the HA coatings, mostly for dental and orthopaedic implants, that is, a vast majority of them are used for a simple simulated body fluid solution, which does not provide a direct link for evaluating the success of in vivo and clinical trials. All studies use different approaches for preparing bioactive HA-based coatings, as well as for surface characterisation and evaluating in vitro or in vivo performance. Thus, a comparison of the published studies is rather difficult, and future standardised approaches of HA coating preparation procedures and characterisation, as well as their application, are required.