Effects of magnesium and fluoride on ion exchange and acid resistance of enamel

Abstract Labial surfaces of bovine incisor enamel, after weak acid demineralization, were exposed for 24 h in solutions that contained trace levels of calcium as 45 Ca, 0.4 mM NaF, and 1.0 mM MgCl 2 at pH 7.0. The solutions approached saturation with apatites in the absence of NaF, and saturation with fluorapatites in the presence of NaF. NaF greatly increased the exchange of 45 Ca. MgCl 2 decreased 45 Ca exchange, but had no effect on F − exchange in the surface minerals. MgCl 2 decreased, while NaF increased the acid resistance of the exchanged surface minerals on later exposure to 10 mM acetic acid at pH 4.5. These results indicated that magnesium in oral fluids and tooth minerals may promote caries.