The usage of fluoride varnish for a moderate to low caries-risk group has not been well validated. This study aimed to evaluate the preventive and therapeutic efficacies of fluoride varnish on the initiated root caries.

Ten premolars were sectioned into quarters, further divided into two windows, one of which was painted with Fluor Protector (1,000 ppm fluoride, Ivoclar Vivadent). An initial lesion with a well-preserved surface layer was produced by pH cycling. Scanned line analysis using energy dispersive spectrometry determined the weight percentages of Ca and P in the demineralized layer. Scanning Electron microscopy and confocal laser scanning microscopy (CLSM) evaluated the varnish-applied root surfaces.

The mean lesion depth (SD) was 12.3 (2.6) µm (single cycling) and 19.6 (3.8) µm (double cycling). Double cycling extended the lesion depth, but induced no more mineral loss than single cycling (p < 0.05). The mean weight percentages of Ca and P between groups with and without varnish were not significantly different (p < 0.05). A CLSM showed varnish remained within 15 µm of the surface layer.

When a mild acid challenge initiated root tissue demineralization, the application of low-concentration fluoride varnish did not influence the lesion depth or the mineral composition of the subsurface lesion.