Hydroxyapatite and related calcium phosphates (CaPs) are similar in composition to the mineral part of bones and teeth. Synthetic CaPs are successfully used for filling bone defects in various clinical indications as they are considered bioactive and osteoconductive, guiding the bone healing process. Nevertheless, most of the synthetic bone substitutes lack the osteoinductive property for regenerating bone tissue over large defects. This chapter reviews the biological properties of CaP materials in relation to their chemistry, crystallographic structure, and solubility. Recent studies have shown that some macro and micro porous CaP ceramics have led to ectopic bone formation when implanted in muscles of animals. Although the interactions of these CaP materials with body fluids, cells, and tissues have been investigated at both the microscopic and ultrastructural levels, there is still a lack of understanding of the possible mechanisms leading to osteoinduction. Both the study of cell–material interactions in vitro and immunochemistry techniques after implantation may provide valuable information. These osteoinductive bone substitutes may be satisfactorily used in future as an alternative to autologous or allogenous bone grafts.
