Carbonate apatite bone replacement

Although bone apatite is the carbonate apatite which contains 6–9 mass% carbonate in its apatite structure, sintered hydroxyapatite free from carbonate has been used as artificial bone substitutes since carbonate apatite cannot be sintered due to the thermal decomposition at high temperature required for sintering. We have proposed a two-step fabrication method for fabrication of carbonate apatite block. First step is the fabrication of a precursor block such as calcium carbonate and tricalcium phosphate. Then the precursor block is immersed in solution in which missing elements for the fabrication of carbonate apatite is supplied to the precursor from the solution. Based on the dissolution-precipitation reaction, the composition of the precursor is transformed to carbonate apatite, the inorganic component of the bone. Carbonate apatite thus fabricated was found to upregulate osteoblastic cells’ differentiation process. Osteoclasts resorbed carbonate apatite whereas no resorption pits were observed in the case of sintered hydroxyapatite. As a result, carbonate apatite showed much higher osteoconductivity than sintered hydroxyapatite. Furthermore, carbonate apatite is replaced to the bone when implanted in the bone defect, whereas sintered hydroxyapatite remained in the bone defect keeping its original shape. Therefore, carbonate apatite fabricated based on dissolution-precipitation reaction using a precursor is thought to be one of an ideal bone replacements for the next generation.