Fabrication of a carbonate apatite granules sponge as a new bone substitute and its histological evaluation at rat calvarial bone defects

Carbonate apatite (CO₃Ap)-granules bone substitute exhibits excellent osteoconductivity and replaced by a new bone based on bone remodeling process. However, no improvement has been made so far with respect to the drawbacks of granules; i.e., intraoperative scatter and postoperative migration. The aim of this study was to evaluate the feasibility of fabricating a CO₃Ap-granules sponge to overcome these drawbacks. The CO₃Ap granules sponge was successfully fabricated by mixing 300–600 µm CO₃Ap granules with 3 % atelocollagen, followed by lyophilization at − 70 °C for 24 h and a dehydrothermal treatment at 150 °C for 24 h. The CO₃Ap-granules sponge and CO₃Ap-granules were implanted into rat calvarial bone defects for 2 and 4 weeks. No significant intraoperative scatter and postoperative migration were evident when the bone defects were reconstructed with the CO₃Ap-granules sponge, whereas significant both intraoperative scatter and postoperative migration were observed when the bone defect was reconstructed with CO₃Ap granules. Although less osteogenesis was anticipated for the CO₃Ap-granules sponge due to the coverage of CO₃Ap-granules surface with atelocollagen, no significant differences were observed in bone formation between the CO₃Ap-granules sponge and CO₃Ap-granules cases because the 3 % atelocollagen and lyophilization kept the CO₃Ap granules surface only partially covered. Consequently, the CO₃Ap-granules sponge and CO₃Ap-granules exhibited similar bone formation at both 2 and 4 weeks after surgery. Thus, the CO₃Ap-granules sponge improved the handling performance and retention ability at the bone defects without scarifying the osteogenesis of CO₃Ap-granules.