The dissolution of single crystals of hydroxyapatite in citric and lactic acids

The dissolution of hydroxyapatite single crystals is highly anisotropic. Parallel to the c-axis the dissolution is fast, while perpendicular to the c-axis dissolution is negligible. In 5 M citric acid solutions at 37°, the acid penetrates into the single crystals parallel to the c-axis with a velocity of approximately 60 nM/sec. Scanning electron microscopy shows that the single crystals are twisted along the c-axis. Both the details of the dissolution behaviour as well as the surface morphology observed indicate the presence of screw dislocations in the single crystals with the dislocation line parallel to the c-axis.