Plasma-Sprayed Apatite Coatings: Review of Physical-Chemical Characteristics and Their Biological Consequences

Hydroxyapatite (HA) is widely used as a coating on orthopedic medical implants to improve anchoring to bone tissue and facilitate bone repair. The use of plasma-sprayed HA coatings on metal implants has raised as much controversy as interest over the last decade. Although faster and stronger fixation and enhanced bone growth have been reported, the long-term performance of HA-coated implants has been questionned This ambivalent behaviour is related to the thermal decomposition of HA during the plasma spraying of HA powder, which has several observable consequences for the physical-chemical and biological properties of the coatings. This review presents the current knowledge on the characteristics of plasma-sprayed HA coatings and their biological consequences. Various schemes of chemical reactions and physical-chemical processes that can occur during HA-particle plasma spraying, HA-coating formation, or after implantation of HA coatings are presented. Standard and emerging methods developed for the characterization of HA coatings are reviewed, with a focus on Raman spectroscopy, which allows high-quality imaging. Finally, several promising trends that have been explored in recent years to improve the mechanical resistance of coatings and their adhesion on the substrate, to improve the coating microstructure and crystallinity, or to confer a therapeutic activity (e.g., antibacterial activity) on plasma-sprayed HA coatings are reviewed.