Functional plasma-sprayed hydroxylapatite coatings for medical application: Clinical performance requirements and key property enhancement

Functional hydroxylapatite (HAp) coatings deposited by atmospheric plasma spraying on parts of hip and knee endoprosthetic implants, artificial dental roots, spinal implants, and other medical devices serve to aid in osseointegration by providing a biocompatible and osseoconductive/osseoinductive template for bone growth-supporting actions of cytokines and noncollagenous proteins and proteoglycans, mediated by transmembrane proteins such as integrins. To be successful in this task, HAp coatings need to be carefully designed and optimized by closely controlling key properties such as phase composition, degree of crystallinity, crystallographic texture, thickness, adhesion strength to the implant surface, porosity, pore size distribution, surface nanostructure and roughness, residual coating stresses, and dissolution kinetics during in vivo contact with extracellular fluid. As this contribution discusses salient aspects of design, properties, and application of HAp coatings, it attempts to chart ways toward improving their in vivo performance and, thus, may be considered a helping hand and guiding manual of instruction for their successful deposition. In this review, much contextual recourse has been taken to the work of the present author and his research group during the last two and a half decades.