Octacalcium phosphate (OCP) has been suggested as a precursor of biological apatite in bone, dentin, and cementum because its existence explains the nonstoichiometry of apatite crystals in their compositions. Synthetic inorganic calcium phosphate compounds have been used clinically to fill bone defects, and sintered hydroxyapatite (HA) and beta-tricalcium phosphate (beta-TCP), bone substitute materials, are known to be osteoconductive, with beta-TCP also being bioresorbable. Nonsintered synthetic OCP has been shown to enhance bone regeneration accompanied by conversion into hydrolyzed apatitic products in situ and biodegradation. The surfaces of the OCP implant and the converted apatite seem to be continuously exposed to biological constituents, such as extracellular matrices, inorganic biominerals, and cellular components. This article reviews the surface reaction of OCP implants and the biological responses, such as experimentally stimulated bone formation on synthetic OCP, the mechanism of OCP hydrolysis into apatite, and the adsorption of biomolecules onto OCP and the converted apatite, of particular interest in reactive bone induction with synthetic OCP implants.
Octacalcium phosphate (OCP) is resorbable bone regenerative material, but its brittleness makes it difficult to maintain its shape without restraint. We have engineered a scaffold constructed of synthetic OCP and porcine collagen sponge (OCP/Collagen) and investigated whether OCP/Collagen composite could improve bone regeneration. To examine this hypothesis, bone regeneration by the implantation of OCP/Collagen was compared with those by OCP and collagen. Radiographic and histological examination was performed and the percentage of newly formed bone (n-Bone%) in the defect was determined by a histomorphometrical analysis. OCP/Collagen, OCP, or collagen was implanted into the critical-sized defects in rat crania and fixed at 2, 4, or 8 weeks after implantation. OCP/Collagen improved the handling performance than the granules of OCP, and synergistically enhanced the bone regeneration beyond expectation, which were composed of bone nucleation by OCP and cell infiltration by collagen. Histomorphometrical analysis showed that n-Bone% +/- standard error treated with OCP/Collagen (48.4 +/- 5.14) was significantly higher than those with OCP (27.6 +/- 4.04) or collagen (27.4 +/- 5.69) in week 8. The present study suggests that the combination OCP with collagen elicited the synergistic effect for bone regeneration.
Our previous studies have suggested that synthetic octacalcium phosphate (OCP) could be resorbed and replaced by newly formed bone if implanted in rat skull defects. We hypothesized that the implanted OCP is more resorbable than other commonly used bone graft substitutes of calcium phosphate compounds, such as hydroxyapatite (HA) and beta-tricalcium phosphate (beta-TCP). To test the hypothesis, the present study was designed to compare histomorphometrically resorption of the implanted OCP, HA, and beta-TCP, which were kept in the experimental cranial defect of rats for a long term. A full thickness of standardized trephine defect was made in the rat parietal bone, and the same volume of granules of OCP, HA, and beta-TCP were implanted into the defect. Five specimens of each group were fixed 6 months after implantation. The percentage of remaining implants (r-Imp%) and newly formed bone (n-Bone%) in the defect was analyzed histomorphometrically. The statistical analysis showed that the r-Imp% of OCP was significantly lower than that of HA and beta-TCP. In contrast, the n-Bone% of OCP was significantly higher than that of HA and beta-TCP. The present study has shown that the implanted OCP in the rat cranial defect is more resorbable than the implanted beta-TCP and HA, whereas the implanted OCP enhances bone formation more than the implanted beta-TCP and HA.
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