Effect of Nd: YAG laser-fluoride application sequence on enamel demineralization: a micro-CT in vitro study

To evaluate, in an in vitro model, whether pulsed Nd: YAG laser irradiation combined with 5% sodium fluoride varnish inhibits bovine enamel demineralization more effectively than fluoride alone, and whether the order of application and the laser power influence the outcome. One hundred and ten bovine enamel specimens were randomly allocated to five groups: Control (C), Fluoride (F), Laser (L), Fluoride-then-Laser (FL), and Laser-then-Fluoride (LF). Irradiated groups received a pulsed Nd: YAG laser at 10 Hz and 300 µs at average powers of 0.5, 0.75, and 1.0 W, with 10 specimens per power subgroup. After treatment, specimens underwent a 48-hour chemical demineralization challenge. Surface loss, measured as lesion step-height in micrometres, and the difference in relative radiodensity between treated and protected enamel were quantified by micro-computed tomography. Data were compared by one-way ANOVA with Tukey's post hoc test, α = 0.05. The control group showed the greatest surface loss, 89.8 ± 24.6 μm, and the largest radiodensity difference, 1756 ± 746 grayscale units. All treatments except FL0.5 significantly reduced surface loss relative to control. The Laser-then-Fluoride sequence produced the lowest surface loss at every power, from 11.2 to 15.1 μm, but these values did not differ significantly from fluoride alone, which yielded 33.8 ± 23.9 μm. Surface loss did not differ significantly among the three laser powers. The lowest radiodensity differences were recorded in LF1 and F, at 317 ± 401 and 376 ± 292 grayscale units, which did not differ significantly from each other. Applying the laser before fluoride varnish outperformed the reverse sequence, whereas laser power did not influence the outcome. Under the conditions of this in vitro model, the anti-demineralization effect of the best laser-assisted protocol was comparable to that of fluoride varnish alone. Whether laser pretreatment confers a measurable advantage in clinical conditions remains to be determined.