Effect of Combined CO₂ Laser Irradiation and Sodium Fluoride Varnish on Enamel Surface Morphology and Surface Elemental Composition: An In Vitro Split-Tooth Study

<ns5:p> Background Fluoride administration plays a critical role in the management and prophylaxis of dental caries. Research indicates that laser irradiation can enhance the uptake of fluoride into dental enamel, an effect that is particularly pronounced when used as an adjunct to topical fluoride treatments. The present study aimed to evaluate the impact of CO <ns5:sub>2</ns5:sub> laser irradiation on fluoride uptake in the enamel of permanent teeth. Methods Six human upper premolars, extracted for orthodontic reasons, were selected for this study. The roots were separated from the crowns, and each crown was then sectioned mesiodistally to create mesial and distal halves. These halves were assigned to either an experimental or a control group (n = 6 per group). All specimens received an application of 5% sodium fluoride (NaF) varnish. Subsequently, the samples in the experimental group were subjected to irradiation with a 1 W CO <ns5:sub>2</ns5:sub> laser for a duration of 15 seconds. To assess the effects of the treatments, fluoride uptake was measured and the topographic characteristics of the enamel surface were examined using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The data obtained from the split-tooth design (paired observations from the same tooth) were analyzed using non-parametric methods. The Wilcoxon Signed-Rank Test was employed to compare the chemical element concentrations (F, Ca, P, C) between the experimental (mesial) and control (distal) halves. Effect sizes (r) and 95% confidence intervals (CI) were calculated for each paired comparison. Statistical significance was set at p &lt; 0.05. All analyses were performed using SPSS software version 26 (IBM, USA). Results Scanning electron microscopy (SEM) structural analysis revealed that the enamel surfaces in the fluoride alone group maintained a typical etched pattern with visible micropores. In contrast, the experimental group subjected to both sodium fluoride varnish and CO <ns5:sub>2</ns5:sub> laser irradiation demonstrated distinct surface melting, fusing of enamel crystals, and a smoother recrystallized morphology. Energy-dispersive X-ray spectrometry analysis showed a significant increase in the percentage of fluoride with the application of the laser, The results of the mean ranks showed that the fluoride and CO <ns5:sub>2</ns5:sub> laser group recorded significantly higher ranks in the fluoride element (8.75 vs. 4.25), reflecting a higher concentration and confirmed statistical significance. Statistically, the Wilcoxon Signed-Rank test revealed a significant increase in fluoride concentrations in the laser-treated group compared to the control group (P value = 0.026), which indicates the effectiveness of using CO <ns5:sub>2</ns5:sub> lasers in enhancing fluoride absorption within the tooth structure. Conclusion The combination of CO <ns5:sub>2</ns5:sub> laser irradiation and sodium fluoride varnish resulted in observable enamel surface morphological modifications and increased surface elemental concentrations of fluoride, calcium, and phosphorus in vitro. However, these surface alterations do not constitute direct evidence of enhanced acid resistance, fluorapatite formation, or clinical caries prevention. Further long-term clinical studies are mandatory to validate any potential therapeutic benefit. </ns5:p>