Ultrastructure study of hydroxyapatite precipitation on ceramic surfaces in dog model

Calcium phosphate (Ca–P) minerals were precipitated on porous alpha-tricalcium phosphate (α-TCP) after implantation in dog muscles for four weeks. Scanning electron microscopy (SEM) examinations indicated that the Ca–P precipitates exhibited flattened-hexagonal rod shapes. Electron diffraction revealed that the rod-like precipitates were hydroxyapatite (HA) elongated in the c-axis. High-resolution transmission electron microscopy (HRTEM) was used to analyze the ultrastructures of the precipitates. We found the evidence of octacalcium phosphate (OCP) structure embedding in a few rod-like HA precipitates. The orientation relationship of OCP/HA revealed in the precipitates was consistent with observations of the solid-state OCP/HA transformation in synthetic specimens, i.e., OCP (010)//HA (01¯0) and OCP (001)//HA (001¯).