Effect of fluoride toothpastes on surface roughness of nickel–titanium wires: an in vitro study

Objective: The main objective of this in vitro study was to determine the effect of toothbrushing with fluoride and natural fluoride-free toothpastes on surface roughness changes of nickel-titanium (NiTi) wires as an indirect indicator of corrosion-related surface degradation. Materials and Methods: Thirty rectangular superelastic NiTi wires (0.019 × 0.025”, 20 mm length; Orthometric) were randomly assigned to three groups (n = 10 each): Group 1 – fluoride toothpaste with brushing; Group 2 – fluoride toothpaste without brushing; Group 3 – fluoride-free toothpaste with brushing. Brushing was simulated with an electric toothbrush twice daily for 1 min over 28 days (cumulative 1 h). Initial and final surface roughness (Rz, µm) was measured using a portable roughness tester. Data were analyzed using paired Student’s t-test and ANOVA with Tukey post hoc comparisons (p < 0.05). Results: Significant increases in surface roughness were observed in Group 1 (0.717 ± 0.085 to 2.440 ± 0.225 µm; p < 0.001) and Group 2 (0.791 ± 0.082 to 1.904 ± 0.247 µm; p < 0.001). In contrast, Group 3 showed only a slight and non-significant increase in surface roughness (p = 0.167). Intergroup analysis revealed significant post-treatment differences (p < 0.001), with Tukey’s test distinguishing three subsets: Group 1 > Group 2 > Group 3. Conclusions: Fluoride-containing toothpaste, particularly when combined with brushing, significantly increased surface roughness of NiTi wires, likely due to fluoride-induced degradation of the protective titanium oxide layer, which increases susceptibility to surface deterioration. Clinically, these findings suggest that fluoride-free formulations may represent a safer alternative for orthodontic patients requiring prolonged use of NiTi wires.