Plasma-Sprayed Osseoconductive Hydroxylapatite Coatings for Endoprosthetic Hip Implants: Phase Composition, Microstructure, Properties, and Biomedical Functions

This contribution attempts to provide a state-of-the-art account on the physicochemical and biomedical properties of plasma-sprayed hydroxylapatite (HAp) coatings that are routinely applied to the surfaces of metallic endoprosthetic and dental root implants, designed to replace or restore lost functions of diseased or damaged tissues of the human body. Even though the residence time of powder particles of HAp in the plasma jet is extremely short, the high temperature applied introduces compositional and structural changes of the precursor hydroxylapatite that severely affect its chemical and physical properties and in turn, its biomedical performance. These changes are based on the incongruent melting behaviour of HAp and can be traced, among many other analytical techniques, by high-resolution synchrotron X-ray diffraction, vibrational (Raman), and nuclear magnetic resonance (NMR) spectroscopies. In vivo reactions of the plasma-sprayed coatings with extracellular fluid (ECF) can be assessed and predicted by in vitro testing using simulated body fluids as proxy agents. Ways are being discussed to safeguard appropriate biological performance of hydroxylapatite coatings in long-term service by controlling their phase content, porosity, surface roughness, residual stress distribution, and adhesion to the implant surface.