Fluoride Dose-Response and Anticaries Effect of Herbal Antibacterial Agents and (Nano-)Hydroxyapatite on Dentin Caries: An in vitro Study

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Introduction: The aim of the study was to investigate the caries-preventive effect of fluoride-free toothpastes, containing either herbal agents or (nano-)hydroxyapatite. Methods: Bovine dentin specimens each having a sound (ST) and a demineralized area (DT) were prepared and randomly allocated to eleven groups (n = 187). Treatments during pH cycling (28 days; 6 × 120 min demineralization/day) were brushing 2×/day with 0 ppm F− [NaF0], 500 ppm F− [NaF500], 1,100 ppm F− [NaF1100], grape seed extract [GSE], (nano-)hydroxyapatite, melaleuca oil [MO1, MO2, MO-CU], and propolis + myrrh [PM1, PM2] containing dentifrices. Dentifrice slurries were prepared with deionized water (1:3 w/w). Differences in integrated mineral loss (∆∆Z) and lesion depth (∆LD) were evaluated before and after pH cycling using transversal microradiography. Results: The correlation between ΔΔZ/ΔLD and F− concentration (NaF0, NaF500, NaF1100) was strong for the DT (rΔΔZ, DT = 0.681; p < 0.001) and very strong for ST (rΔΔZ, ST = 0.861; p < 0.001), indicating a fluoride dose-response for both baseline substrate conditions. For ΔΔZDT and ΔLDDT, only NaF1100 and GSE revealed significant differences compared with NaF0 (p < 0.001; ANOVA). For ΔΔZST and ΔLDST, significant differences could be found for NaF1100 and NaF500 compared to all fluoride-free groups (p ≤ 0.002; ANOVA), without significant difference between fluoride-free groups (p = 1.000; ANOVA). For DT and ST, a hypermineralized surface layer and no surface loss could only be observed when fluoride was present. Conclusion: A dose-response for fluoride concentrations was observed in this mild demineralization pH-cycling model. Fluoride-free dentifrices containing GSE or melaleuca oil showed certain preventive effect against further progression of root caries lesions. However, surface loss was observed for all fluoride-free dentifrices.