The roles of carbonate content and laser irradiation on the demineralization of apatite systems and dental enamel.

Apparent solubilities (ion activity products, Kiap's) of synthetic carbonate apatites (CAP) were measured in acidic buffers (with and without fluoride) using a rotating pellet dissolution method (Levich hydrodynamics). Slurry density studies were performed to independently demonstrate that these Kiap values were indeed true measures of the driving force for dissolution of these metastable CAP's. Carbonate common ion experiments at different pH values under one atmospheric CO2 pressure showed that CAP (Na+ x (Ca2+) [10-x] (PO4{3-}) [6-x] (CO3{2-}) x (OH2) dissolution was governed by an ion activity product, Khap = a10Ca2+a[6]PO43-1[2]OH-, instead of Kcap = axNa+a[10-x]Ca2+a[6-x]PO4[3-a[x]]CP3[2-a[2]]OH-. In the presence of solution fluoride, Lfap = a10Ca2=a[6]PO4[3-a[2]]F- was found to govern the dissolution of CAP. Most importantly, when the pKhap and pKfap values were plotted against the carbonate content of CAP preparations, the slopes of both plots were found to be identical. These findings strongly support the hypothesis that CAP dissolution in acidic medium involves a rate-limiting surface complex, Ca2+10(PO4[3-])6(OH-)2; the activation energy for which is dependent upon the carbonate level in the CAP preparation, whereas Ca2+[10](PO4[3-])6F-2 is the rate-limiting surface complex for CAP in the presence of solution fluoride. Selective partial saturation, common ion, and rotating pellet dissolution experiments on carbon dioxide laser irradiated hydroxyapatite (HAP), CAP, and human enamel suggested that laser irradiation converted HAP which possessed the two-site dissolution character to a HAP of lower effective solubility (i.e,, site #2 only with Khap = 10[-130±1]). As for CAP, a part of the mineral was decarbonated and transformed by excessive laser energy into new more soluble phases. Initial dissolution of laser irradiated (65 J/cm2) human enamel was also found to be completely inhibited by chemical inhibitors as expected. All these results supported two hypotheses; (1) laser irradiation converted the surface of human enamel to site #2 HAP and (2) there was significant synergism between laser treatment and chemical dissolution rate inhibitors (again consistent with site #2 HAP).