Multiphasic calcium orthophosphate (CaPO4) bioceramics and their biomedical applications

Due to the chemical similarity to the inorganic constituents of calcified tissues of mammals, biologically relevant calcium orthophosphates (CaPO4) have been applied as artificial bioceramics suitable for reconstruction of various types of bone defects. Since none of the known individual types of CaPO4 appears to be able to mimic both the composition and the properties of natural bones, various attempts have been sought to overcome this problem and a multiphasic (polyphasic) concept is one of the reasonable solutions. In general, this approach is determined by advantageous formulations consisting of homogeneous blends of two (biphasic), three (triphasic) or more (multiphasic) individual CaPO4 phases possessing diverse solubility and, therefore, bioresorbability, while the optimum ratios among the phases depend on the definite applications. Therefore, all currently known multiphasic CaPO4 formulations are sparingly soluble in water and, thus, after being implanted they are gradually resorbed inside the body, releasing calcium and orthophosphate ions into the biological medium and, hence, seeding a new bone formation. They have already demonstrated a proven biocompatibility, osteoconductivity, safety and predictability in vitro, in vivo, as well as in clinical trials. More recently, in vitro and in vivo studies have shown that some of them might possess osteoinductive properties. Hence, in tissue engineering, multiphasic CaPO4 bioceramics represent promising formulations to construct various scaffolds capable of carrying and/or modulating the behavior of cells. This review summarizes the available information on biphasic, triphasic and multiphasic CaPO4 bioceramics including their biomedical applications. New formulations have been proposed as well.