Effects of acidic and neutral fluoride gels on surface microhardness and morphology of composite resins used in pediatric dentistry: an in vitro factorial study

BACKGROUND: Concerns persist regarding the effects of repeated topical fluoride application on the surface integrity of composite resins. This study specifically evaluated how acidic and neutral fluoride formulations and varying exposure durations influence the surface microhardness and morphology of pediatric dental composite resins. METHODS: Four light-cured composite resins (Filtek Z250, FSM Hi-Fill Kids Blue, FSM Hi-Fill Kids Yellow, and Gænial) were evaluated. Specimens were exposed to acidulated phosphate fluoride (APF) or neutral sodium fluoride (NaF) gels for either 4-24 h to simulate short-term and repeated exposure conditions. Surface microhardness was assessed using a Vickers microhardness tester (100 g load, 15 s dwell time), and surface morphology was examined qualitatively using scanning electron microscopy (SEM). Data were analyzed using three-way ANOVA to evaluate the effects of composite type, fluoride type, exposure duration, and their interactions. RESULTS: Composite type and exposure duration significantly influenced surface microhardness (p < 0.05), whereas the main effect of fluoride type was not statistically significant. However, significant interaction effects indicated that the response to fluoride exposure was material-dependent. Overall, APF exposure resulted in greater reductions in microhardness and more pronounced surface changes compared with NaF, particularly after prolonged exposure. Filtek Z250 consistently exhibited the highest microhardness values, whereas Gænial showed the lowest values across most experimental conditions. SEM findings supported these results, revealing more evident surface irregularities and matrix alterations following APF exposure. CONCLUSIONS: Topical fluoride applications affected the surface microhardness of composite resins in a material-dependent manner. Acidulated fluoride formulations induced greater surface alterations than neutral formulations, particularly under prolonged exposure conditions. From a clinical perspective, neutral fluoride formulations may be preferred over acidic agents to minimize surface degradation and preserve the longevity of composite restorations in pediatric patients receiving repeated preventive care.