Preparation and characterization of porous apatite ceramics coated with β-tricalcium phosphate

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂;HA) is one of the most biocompatible materials with bones, and porous HA is promising bone substitute materials for clinical applications. While there are reports that β-tricalcium phosphate (Ca₃(PO₄)₂;TCP) has higher resorbability than HA when the material is implanted in a bone defect. In the present study, porous HA coated with β-TCP was prepared by our unique method. The porous HA of about 60% porosity with interconnecting pore structure was soaked in diammonium hydrogen phosphate solution, and then the HA was sintered at 900°C for 3 h. β-TCP was revealed by X-ray diffractometry on the surface of porous HA. It was possible to control the content of surface-formed β-TCP arbitrary by varying the concentration of the solution. The obtained HA coated with 33 wt% β-TCP (33TCP) had about 60% open porosity with the pore size from 150 to 400 μm. The average compressive strength of this porous ceramics was 17.5 MPa. Surface coated HA with β-TCP deprived of the brittleness in handling. The weight of HA implanted into muscles was increased obviously at 4 weeks because of formation of carbonate hydroxyapatite on the surface of HA. The weight of 33TCP was scarcely changed up to 12 weeks, but the weight tended to increase at 24 weeks. The carbonate hydroxyapatite was not formed on 33TCP at 4 weeks, but formed on it at 24 weeks. Therefore β-TCP coated porous HA behaved like β-TCP initially after implantation, and then behaved like HA.