Biodegradation and bioresorption of calcium phosphate ceramics.

The use of several calcium phosphate (Ca-P) materials for bone repair, augmentation, substitution and as coatings on metal implants has gained clinical acceptance in many dental and medical applications. These Ca-P materials may be of synthetic or natural origin, available in different physical forms (dense or macroporous, particles or blocks) and are used in bulk as coatings for metallic and non-metallic substrates or as components in composites, cements and bioactive glasses. Biodegradation or bioresorption of calcium phosphate materials implies cell-mediated degradation in vitro or in vivo. Cellular activity during biodegradation or bioresorption occurs in acid media; thus the factors affecting the solubility or the extent of dissolution (which in turn depends on the physico-chemical properties) of the Ca-P materials are important. Enrichment of the microenvironment due to the release of calcium and phosphate ions from the dissolving Ca-P materials affects the proliferation and activities of the cells. The increase in the concentrations of the calcium and phosphate ions promotes the formation of carbonate apatite which are similar to the bone apatite. The purpose of this invited paper is to discuss the processes of biodegradation or bioresorption of Ca-P materials in terms of the physico-chemical properties of these materials and the phenomena involved including the formation of carbonate apatite on the surfaces and in the vicinity of these materials. This phenomenon appears to be related to the bioactivity of the material and the ability of such materials to directly attach to bone and to form a uniquely strong material-bone interface.