半水石膏顆粒の硬化反応に基づく炭酸アパタイト連通多孔体の創製と組織学的評価

概要

Carbonate apatite [CO3Ap: Ca10-a (PO4)6-b (CO3)c] has been used as a bone substitute due to its high osteoconductivity and bioresorbability. The architecture of CO3Ap, particularly the interconnectivity of pores, is also a factor that significantly affects the outcome of bone regeneration. In this study, interconnected porous CO3Ap blocks were fabricated by the setting reaction of calcium sulfate hemihydrate [CSH: CaSO4·1/2H2O] granules followed by dehydrating heat treatment and two-step dissolution– precipitation reaction.

　When CSH granules were immersed in water, calcium sulfate dihydrate [CSD: CaSO4·2H2O] crystals were formed and precipitated on the surface of the granules. The interlocking of CSD crystals from the adjacent granules bridged the granules to each other, resulting in the formation of an interconnected porous block. However, unrestricted growth of CSD crystals clogged significant portions of the pore network, negatively affecting the formation of a highly interconnected porous structure. Controlling the water content, using either filter paper or centrifuge, resulted in successful fabrication of highly interconnected porous CSD blocks. CaCO3 blocks as intermediate precursor were fabricated by immersing CSD blocks in a Na2CO3 solution. However, the measured mechanical strength was low. To overcome this issue, the mechanical strength of CaCO3 blocks was greatly improved when calcium sulfate anhydrous [CSA: CaSO4] blocks were prepared by the heat treatment of CSD blocks before carbonation. The CaCO3 blocks were then converted to CO3Ap via dissolution–precipitation reaction in a Na2HPO4 solution.

　The effect of pore interconnectivity on bone regeneration was evaluated by reconstructing the rabbit distal femur defect using CO3Ap blocks with 15% and 30% of interconnected pores. The new bone formation was evaluated histomorphometrically after 4 and 12 weeks of implantation. At 4 weeks, new bone deposition was abundant in the central area of CO3Ap with 30% interconnected pores. In contrast, CO3Ap with 15% interconnected pores had limited new bone formation in the central region. At 12 weeks, a large portion (> 90%) of CO3Ap with 30% interconnected pores was replaced with cancellous-structured bone. In comparison, even after 12 weeks post-operative, CO3Ap with 15% interconnected pores remained in the defect and the boundary with the host bone was still clear. In conclusion, introducing a highly interconnected pores can significantly elicit rapid bone regeneration by allowing early bone ingrowth, followed by replacement of the material with new bone.