The effect of microstructure of octacalcium phosphate on the bone regenerative property

The present study was designed to investigate whether the microstructure of synthetic octacalcium phosphate (OCP) affects its intrinsic bone regenerative properties as a scaffold and its conversion process into hydroxyapatite (HA). Our previous studies indicated that an agregate of OCP crystals, consisting of randomly oriented plate-like crystals, are capable of enhancing both osteoblastic cell differentiation in vitro and bone regeneration. While the transformation of OCP into HA has been considered in relation to the stimulatory capacity of OCP in bone regeneration, little is known about the effect of the microstructure of OCP granules on these capabilities. Two types of OCP granules, with identical diameters (300-500μm) but composed of crystals with distinct crystal dimensions (4.0 and 26.6μm length), were prepared (hereafter referred to as fine OCP granules [F-OCP] and coarse OCP granules [C-OCP], respectively). The intergranule distances and the porosity, including the intergranule spaces, were 108.5μm and 93.7% for F-OCP, and 67.5μm and 95.7% for C-OCP, as estimated by mercury intrusion. The OCP granules were implanted in mouse critical-sized calvarial defects for up to 14 days. Histological examination demonstrated that osteoblastic cells aligned on the surface of F-OCP at day 7 and formed new bone around the granules up to day 14. On the other hand, cells around C-OCP were sparse at day 7, and resulted in only slight bone formation around the granules at day 14. X-ray diffraction showed that both OCP granules tended to be converted to an apatite structure with similar conversion velocity by the implantation. Adhesion of mouse bone marrow stromal ST-2 cells was markedly inhibited on C-OCP compared to F-OCP in vitro. These results suggested that the microstructure consisting of plate-like crystals of OCP controls cell adhesion on the crystal surfaces and their resultant bone regenerative properties as well as the physicochemical effect associated with the transitory nature of OCP previously reported.