The Dissolution Behaviour of Synthetic and Biological Apatites

Abstract

It has been proposed that carbonate in human dental enamel enhances the solubility of enamel and consequently influences caries susceptibility, In this study a series of synthetic carbonated and non-carbonated apatites were investigated using infrared and Raman spectroscopy, X-ray diffraction, chemical analysis, high resolution transmission electron microscopy, and their dissolution behaviour was tested under sink conditions, using the rotating disc method. synthetic apatites, were prepared either by aqueous precipitation or by high temperature solid state reaction. These samples and human dental enamel were studied at low temperatures (50 K) using laser Raman spectroscopy and also by infrared spectroscopy. The effect of carbonate, F, Sr and Zn substitutions on apatite crystal lattice imperfection was evaluated using X-ray diffraction. The crystal structures of selected apatite samples were directly observed as n-beam lattice images, resolved to 2.7 Å, a resolution greater than has been previously reported for apatites. The reactivities of the synthetic apatites compressed into pellets, were studied in a dissolution cell using 0.01, 0.05 mol 1-1 acetate buffers at pH 4.5, 5.0. This study demonstrates that carbonate exists in two crystallo-graphically distinct sites in the apatite crystal structure and is consistent with a carbonate-for-phosphate substitution mechanism, where four carbonate groups replace three phosphate groups. Sodium ions and hydroxyl deficiencies are shown to be involved with the substitution of carbonate ions in the apatite structure. High temperature carbonated apatites exhibited domain structures with crystal defects such as grain boundaries, tilt boundaries, single dislocations and stack faults. The frequency of these defects increased with the carbonate content of the samples. Aqueous apatites and dental enamel did not have a domain structure and few dislocations were observed. However crystal lattice imperfection in the form of paracrystalline lattice distortions, was demonstrated in aqueous apatites using X-ray diffraction and this increased with increased carbonate substitution. Trace amounts of fluoride, Sr and Zn decreased crystal lattice imperfection in carbonated apatites. Large differences in the degree of crystal lattice imperfection were observed between bone, dentine and enamel. The initial rates of dissolution of carbonated apatites were directly related to their carbonate content and were more reactive when compared to non-carbonated apatites. This appeared to be due, in part, to a decrease in the apparent activation energy for dissolution for carbonated apatites. Structurally incorporated fluoride in carbonated apatites at the levels found in dental enamel, did not influence the dissolution rates although topical fluoride at a concentration of 1 µg/ml reduced the dissolution rates by 20-30%. The initial dissolution reaction was found to be of the intermediate type being neither entirely diffusion controlled nor surface chemically controlled. After ten minutes, diffusion processes within the apatite pellets were found to influence the overall dissolution rate. It appeared that carbonate induced crystal defects in the apatite crystal structure contribute substantially to the dissolution properties of biological apatites.