Bone formation on HA implants: a commentary.

The present paper reviews bone formation on dense hydroxyapatite (HA) implants. Calcification of bone matrix formed on HA is different in areas where collagen fibers are dense and scattered with matrix vesicles than in those interfacial layers containing few or no collagen fibers and matrix vesicles. Calcification of collagen-coated areas begins with crystallization within the matrix vesicles. In contrast, calcification of the interfacial layer is initiated by epitaxial crystal growth on the HA. Crystallization within the matrix vesicles near the HA and those on the HA start simultaneously. Calcification of the interfacial layer is the most important feature of HA as a biomaterial. Such calcification is never observed on titanium implants. The recent postulation that the rate of bone formation on calcium-phosphate (CP) ceramics correlates with the solubility of CP ceramics is improbable, as in vitro immersion tests have been unable to establish correlation between osteoblast differentiation and the solubility of the CP ceramics. With regard to HA-plasma spray-coated implants, future research should focus on the purity of HA and the structure of the ceramics in order to increase the in vivo chemical and mechanical durability of HA coating.