Calcium hydroxyapatite synthetic methods

Based on observed tissue response, synthetic bone-graft substitutes can be classified into inert (e.g., alumina, zirconia), bioactive (e.g., hydroxyapatite, bioactive glass), and resorbable substitutes (e.g., tricalcium phosphate, calcium sulfate). Of these, resorbable bone-graft substitutes are preferred for bone defect filling because they can be replaced by new natural bone after implantation, p-tricalcium phosphate (Ca3(PO )2, p-TCP) is one of the most widely used bone substitute material, due to its faster dissolution characteristics. Preparation of magnesium-substituted tricalcium phosphate ((Ca, Mg)3(PO )2, p-TCMP) has been reported by precipitation or hydrolysis method in solution. These results indicate that the presence of Mg stabilizes the p-TCP structure (LeGeros et al., 2004). The incorporation of Mg also increases the transition temperature from p-TCP to a-TCP and decreases the solubility of p-TCP (Elliott, 1994 Ando, 1958). 

The other ceranfic widely used are phosphate salts of calcium, with the chosen phase usually being hydroxyapatite. This material is conventionally prepared by thermal methods at temperatures well in excess of 1000°C. As a result of their preparation at high temperatures, the salts are carbonate free and are made up of much larger and more perfect crystals than those found in biological apatite minerals including bone. The imperfect crystalline structure of bone mineral leads to the natural material being soluble and reactive with respect to body fluids. In contrast, the synthetic materials are much less reactive than those found in living tissue and problems with biocompatibility can arise. 

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