In Vitro Study of the Effect of Aluminum and the Combined Effect of Strontium, Aluminum, and Fluoride Elements on Early Enamel Carious Lesions

The purpose of this in vitro study was to evaluate the effect of aluminum and of combined strontium, aluminum, and fluoride treatments on enamel demineralization and remineralization. During a 6-day pH-cycling protocol, pre-softened bovine enamel slabs were immersed twice daily for 1 min in the following experimental solutions: (a) distilled water [W] (negative control); (b) 1,000 ppm F [F] (positive control); (c) 1,000 ppm Al [Al]; (d) 1,000 ppm Al, 1,000 ppm F applied interchangeably [Al–F]; (e) 1,000 ppm Al, 1,000 ppm F, applied in sequential order [Al + F]; (f) combined 1,000 ppm Al and 150 ppm Sr [Al + Sr]; and (g) combined 150 ppm Sr and 1,000 ppm F [Sr + F]. Subsequently, the specimens were subjected to a 5-day acid resistance test. Lesions were evaluated quantitatively by performing surface microhardness and qualitatively by using polarized light microscopy. According to the results, solutions [Sr + F] and [Al–F] enhanced remineralization and inhibited demineralization as effectively as the [F] solution and significantly superiorly compared to [Al + Sr] and [Al] solutions. All tested solution groups, except for the [Al + Sr] group, presented significantly increased resistance to acidic attack, compared to [W]. PLM examination revealed that all solution groups, except for the [W] group, developed an acid-resistant zone at lesion surfaces. In conclusion, under the present experimental conditions, the combined strontium–fluoride and aluminum–fluoride treatments presented similar anti-caries efficacy compared to fluoride treatment alone, but they did not show evidence of synergistic activity on pre-softened enamel.