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Octacalcium phosphate (OCP) is thought to be a precursor of the mineral crystals in biological apatite. Synthetic OCP has been shown to be converted into an apatite structure when implanted in murine calvarial bone, to enhance bone regeneration more than synthetic hydroxyapatite (HA), and to degrade faster than biodegradable beta-tricalcium phosphate. This study was designed to investigate whether OCP implantation enhances the formation and resorption of new bone (remodeling) concomitant with OCP degradation when implanted intramedullary in a rabbit femur for 12 weeks, compared to sintered HA ceramic. Histological and histomorphometric analyses using undecalcified specimens showed that the area of bone apposition was significantly higher on OCP than on HA between 2 and 3 weeks, whereas it subsequently became smaller on OCP than on HA. The area attacked by multinucleated giant cells, including tartrate-resistant acid phosphatase (TRAP)-positive cells, was significantly higher for OCP than for HA at 8 weeks. Radiography revealed resorption of OCP but not of HA. The results disclose some osteoconductive characteristics of synthetic OCP in the bone marrow space: (1) enhancement of bone regeneration at the initial bone apposition stage and (2) stimulation of resorption of the newly formed bone coupled with OCP biodegradation mediated by TRAP-positive osteoclast-like cells. These results suggest that synthetic OCP would be a more useful bone substitute than HA in implant applications where rapid bone formation and concomitant implant resorption are important considerations.

Octacalcium phosphate (Ca8H2(PO4)6 * 5H2O; OCP) has been advocated to be a precursor of biological apatite crystals in bone and tooth. Recent studies, using physical techniques, showed that OCP is present as a transient phase during biological apatite formation in human dentin, porcine enamel and murine bone. However, there is still a controversy regarding the chemical nature of the first mineral formed in the biominerals. A number of studies have demonstrated that synthetic OCP shows bone regenerative and biodegradable characteristics, rather than other calcium phosphate bone substitute materials, such as hydroxyapatite (Ca10(PO4)6(OH)2; HA) ceramic. It seems likely that synthetic OCP may be an alternative to autogenous bone graft. It is known that OCP contains alternative layers of water molecules and an apatite structure, and that the transition of OCP to HA is likely to be spontaneous and irreversible. The conversion process induces modification of local environment adjacent to OCP surface, including the changes in adsorption of serum proteins and concentration of calcium and inorganic phosphate ions. This article reviews the possible application to bone regeneration by synthetic OCP and the mechanism to enhance bone regeneration in relation to biological mineralization in bone and tooth.

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The purpose of this study was to compare the biodegradation rate and quality of regenerated bone among four materials. A short time period of 8 weeks was chosen to examine early bone healing. The rod-shaped implants of commercially available two β-tricalcium phosphate (β-TCP) ceramics with porosity 60% and 71-80%, respectively, laboratory prepared octacalcium phosphate/gelatin composite (OCP/Gel), which has been proven to have a highly osteoconductive and biodegradable property in rat calvarial defect, and gelatin sponge (Gelatin) were implanted in rabbit tibia defect of 6 mm diameter and 7 mm depth for 2, 4 and 8 weeks. Analyses by μCT, histomorphometry and push-in test were carried out to evaluate the extent of the tissue regeneration and the material biodegradation in the long bone. OCP/Gel and Gelatin were completely resorbed but only OCP/Gel induced cortical bone bridge until 8 weeks that has strength compatible to that of the natural bone. β-TCP (71%-80%) and β-TCP (60%) were not completely resorbed and never induced the amount of new bone formation beyond that by OCP/Gel. The results indicate that the new bone having enough strength could be regenerated if the material shows not only higher biodegradation rate but also higher osteoconductivity. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 2833-2842, 2016.