The present study was designed to investigate whether synthetic octacalcium phosphate (OCP) combined with collagen (OCP/collagen) can repair a critical-sized defect in dog skull. OCP/collagen has been shown to biodegrade and to tend to be replaced by newly formed bone if implanted in rat calvaria defects. Materials and Methods: An OCP/collagen disk was prepared from pepsin-digested atelocollagen isolated from porcine dermis and synthetic OCP. Two critical-sized defects (20mm in diameter) were made in a dog skull. Ten disks of OCP/collagen or collagen (control) were implanted in the bone defects and resected with surrounding tissues at 3, 6, or 12 months after the implantation. The specimens were analyzed radiographically, crystallographically, histologically, and histomorphometrically. Results: X-ray diffraction and FTIR analyses showed that OCP tended to convert to a poorly crystallized hydroxyapatite, similar to that of biological apatite, by 3 months. Radiographic and histologic analyses showed that the implantation of OCP/collagen disks initiated new bone formation in the defects at 3 months after implantation. However, there was no promotion of bone formation by control collagen disks even with prolonged implantation up to 12 months. Histomorphometric analysis revealed that the percentage of newly formed bone in the defect implanted with OCP/collagen increased significantly, from 30.91±6.65 at 3 months to 51.22±5.99 at 12 months, although the value tended to reach a plateau at 6 months (44.49±3.34). On the other hand, the percentage of remaining OCP was estimated at approximately 10% at 3 months and remained nearly unchanged thereafter. Conclusion: The results suggest that bone regeneration of a critical-sized bone defect of dog calvaria by OCP/collagen can be enhanced for 3 to 6 months and that OCP/collagen holds potential as a bone substitute material.
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