Calcium phosphates for bone tissue regeneration and reconstruction

Objectives: Although the autograft is the golden standard for reconstruction of bone defect, it has serious drawbacks such as invasion to the healthy site, limitation of the form of correctable bone, limitation of the correctable amount of bone. Sintered hydroxyapatite is therefore used as artificial bone filler since it shows good osteoconductivity and excellent tissue response. Unfortunately, it would not be replaced with bone. Therefore, dentists claims that autograft is better than sintered hydroxyapatite since autograft would be replaced with bone and bone has not only mechanical but biological function. Methods: Bone apatite is not hydroxyapatite but carbonate apatite that contains 7-8wt% carbonate. Sintering process cannot be applied for the fabrication of carbonate apatite block since carbonate would be liberated upon heating. However, carbonate apatite is the most stable phase thermodynamically, and thus, it could be prepared based on phase-transformation reaction or so-called dissolution-precipitation reaction. Results: Precursor phase have to be made first for the fabrication of carbonate apatite block using phase transfer reaction. One of the candidates may be calcium carbonate block. Calcium carbonate block can be prepared by exposing calcium hydroxide compact to carbon dioxide. Then the calcium carbonate block is immersed in disodium hydrogen phosphate aqueous solution. Upon immersion of calcium carbonate block to phosphate salt solution, carbonate apatite slightly dissolve to supply calcium and carbonate ion, then reprecipiated as carbonate apatite by taking phosphate ion from the solution. The carbonate apatite thus prepared was resorbed with osteoclasts and replaced with bone. Conclusion: Since carbonate apatite block prepared by phase-transformation reaction can be replaced with bone, it will be an ideal artificial bone replacer in the near future.