Chemical and physical factors affecting osteoconductivity of octacalcium phosphate bone substitute material

The present article summarizes the factors controlling osteoconductive and biodegradable characteristics of synthetic octacalcium phosphate (OCP) when implanted in bone defects. OCP is a transient precursor, which tends to convert to hydroxyapatite (HA) in physiological environment. We recently confirmed that the subtle change of stoichiometry of OCP from Ca/P molar ratio 1.28 to 1.37, both of which are nonstoichiometric compositions compared to stoichiometric 1.33 of OCP, obtained by partial hydrolysis, makes it reduce the crystallinity and raises the bone formation rate significantly if implanted in marrow space of rat tibia more than those of original OCP and HA obtained via OCP full hydrolysis. The composite, which consists of OCP granules and collagen sponge, is vigorously resorbed by osteoclastic cells if the thick composite is implanted in subperiosteal area of murine calvaria but replaced with newly formed bone if the thin composite or OCP without collagen is used. The results suggest that the physical stress, which might be induced underneath the periosteum, controls activities of osteoblasts and osteoclasts around OCP implant. The osteoconductive characteristics of OCP appear to be controlled by its stoichiometry and the mechanical stimulation induced from surrounding tissue where OCP is implanted.