The present study was designed to investigate whether the granule size of synthetic octacalcium phosphate (OCP) and the resultant intergranular spaces between the granules formed by the filling affect its osteoconductive and biodegradable characteristics in a mouse calvaria critical-sized defect up to 10 weeks after implantation. Mercury intrusion porosimetry showed that OCP granules having distinct diameter sizes ranging from 53 to 300 (S-OCP), 300 to 500 (I-OCP) and 500 to 1000 μm (L-OCP) produced distinct intergranular spaces between OCP granules ranging from 28.8 to 176.6 μm. The dissolution rate of OCP, estimated by the phosphate concentration in the culture medium, was the highest in S-OCP, followed by I-OCP and L-OCP, while the specific surface area of OCP decreased. Histological and histomorphometric analyses showed that bone formation around the implanted granules increased significantly with increasing granule size coupled with activating the appearance of TRAP- and cathepsin K-positive osteoclastic cells. The rate of new bone formation formed with L-OCP was two times higher than that formed with S-OCP at 10 weeks after implantation. The results indicated that the osteoconductive and biodegradable properties of OCP can be augmented by increasing the granule size, most probably by thus providing enough spaces between the granules, suggesting that the intergranular spaces formed by the granules may work similarly to pores, as reported in porous ceramic materials. It seems likely that the enhancement of bone formation by OCP is accompanied by simultaneous activation of osteoclastic resorption of OCP.
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