Structural features of biologically involved phosphates

Long-term changes of hydroxyapatite-coated dental implants

There are many controversies about the long-term prognosis of hydroxyapatite (HA)-coated implants. Failure may be related to compositional and structural changes of the coating occurring during implantation. Two retrieved and two unused HA-coated blade-type implants were examined by stereomicroscopy, secondary electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and electron probe microanalysis. The objective was to investigate the HA morphology, composition, and structure, and to characterize the changes that occurred in the retrieved implant coatings. Retrieved implants presented partial loss of the coating, especially at the apical and mesiodistal edges. Remaining HA was thick and flattened in the cervical and central areas and gradually thinner and rougher towards the apical and mesiodistal edges. Increase of Cl and Mg, decrease of OH, and X-ray diffraction peak broadening were found in the retrieved implant coatings, in comparison with the unused implants. Morphological changes of the retrieved implants seem to depend on stress values in the surrounding bone and on implant mobility. Compositional changes and increased amount of lattice imperfections appeared in the retrieved implant coatings, as a result of ion substitutions in the apatite lattice. However, the present study could not confirm the influence of these changes on implant failure.

The stoichiometric solubility of calcium orthophosphates

On the assumption that the calcium orthophosphates dissolve stoichiometrically, the solubility of Hap, Fap, DCPD, OCP and TCP, as well as of CaCO3 and CaF2, have been calculated in terms of grams of solid phase per liter under equilibrium conditions. As exemplified in the text, these calculated solubilities facilitate the understanding of the complex behavior of the orthophosphates when in solution.