Fluoride action on acid resistance of unaltered human surface enamel

Unaltered surface enamel of extracted human teeth was subjected to tests of resistance to dissolution in 10 mM acetic acid at pH 4.0 and 10 mM EDTA at pH 7.4 in a miniature continuous flow system. Fluoride in EDTA had no effect on enamel dissolution. Inclusion of 0.25, 0.50 and 5.0 mM NaF in the acid at pH 4.0 greatly increased the resistance of the enamel to dissolution. The 0.5 mM NaF in the acid almost instantly reduced enamel dissolution and the effect persisted through the 1 h experiments. These results suggested that the acid labile fluoride in plaque and tooth minerals may provide the primary beneficial effect of all types of enamel fluoridation.