TiF4 varnish protects the retention of brackets to enamel after in vitro mild erosive challenge

Abstract The effect of fluoride agents on the retention of orthodontic brackets to enamel under erosive challenge is little investigated. Objective: The aim of this study was to evaluate the effect of titanium tetrafluoride (TiF4) and sodium fluoride (NaF) agents on the shear bond strength of brackets to enamel and on the enamel microhardness around brackets under erosive challenge. Methods: Brackets were bonded to bovine incisors. Five groups were formed according to fluoride application (n=10): TiF4 varnish, TiF4 solution, NaF varnish, NaF solution and control (without application). The specimens were submitted to erosive challenge (90 s cola drink/2h artificial saliva, 4x per day for 7 days). Solutions were applied before each erosive cycle and varnishes were applied once. Vickers Microhardness (VHN) was obtained before and after all cycles of erosion and the percentage of microhardness loss was calculated. Shear bond strength, adhesive remnant index and polarized light microscopy were conducted after erosion. The data were analyzed by ANOVA, Tukey, Kruskal-Wallis and Mann-Whitney U tests (α=0.05). Results: The %VHN had no statistically significant differences among the experimental groups. However, considering the comparisons of all groups with the control group, TiF4 varnish showed the highest protection from enamel demineralization (effect size of 2.94, while the effect size for the other groups was >2.4). The TiF4 varnish group had significantly higher shear bond strength compared to other groups. There was no difference among groups for adhesive remnant index. Polarized light microscopy showed higher demineralization depth for the control group. Conclusions: Application of NaF and TiF4 agents during mild erosive challenge minimized the enamel mineral loss around brackets, however only the experimental TiF4 varnish was able to prevent the reduction of shear bond strength of brackets to enamel.


Introduction
Dental erosion is the teeth mineral loss due to a chemical process, by exogenous or endogenous acids, without bacterial involvement 18 . The prevalence of dental erosion among 12 to 21 years-old-students in the world population varies approximately between 15 to 75%, with mild erosion being the most prevalent condition 1,7,14 . Currently, the most important acid sources come from dietary habits due to the increased consumption of soft drinks by the population 18 . The NaF has shown positive results in the reduction of enamel erosion progression 6,13 and its protective effect is associated with the precipitation of calcium fluoride material on eroded dental surfaces, especially when used in high concentration and acidic formulation 13,18 . increases the uptake of fluoride because of its acidic pH and can form a glaze-like surface layer that acts as an acid-resistance diffusion barrier 4,19,23 .
Although some studies have shown that the retention of orthodontic brackets to enamel is decreased when subjected to erosive challenge 12

Specimen preparation
Fifty freshly extracted lower bovine incisors were used in this study. The teeth were cleaned and the buccal surfaces were ground flat with SiC paper discs (400, 600 and 1200 grids) to expose the enamel bonding. The specimens were allocated into five groups The pH of solutions was measured by electrodes and the pH of varnishes were informed by the manufacturer. The composition of materials is described in Figure 1.    Table 1.

Application of brackets
For all treatment groups, the %VHN was significantly lower (with a large effect size) compared with the control group. For other comparisons, no statistically significant differences were found, and the 95% CI either crossed the null hypothesis value or were very close to it (Table 1).
Considering the different sizes of the positive effect of each treatment relative to the control group, the hypothesis that there was correlation between effect size (scalar data) and treatment type (ranked data) was tested. For that, the group ranks were: TiF 4 varnish, rank=5; TiF 4 solution, rank=4; NaF solution, rank=3; NaF varnish, rank=2, and control group,   Table   2). The largest difference was between TiF 4 varnish group and the control group.      This study found that after fluoride application and mild erosive challenge, the treatments had significant effect on the reduction of VHN values. While all treatment groups had significantly lower reduction in VHN values compared to the control group, they did not differ from each other (Table 1) However, the reduction in enamel erosion by TiF 4 agents is primarily attributed to the precipitation of a metal-rich layer on a tooth surface 16,19 .
Titanium ions may substitute calcium in the apatite lattice and show a strong tendency to complex with phosphate groups, forming a new compound (hydrated hydrogen titanium phosphate) or organometallic complexes 4,23 . This reaction forms a glaze-like surface layer that can act as an acid-resistance diffusion  Table 2). The glaze-like surface layer formed probably protected the demineralization of enamel, as explained before, and was able to prevent the decrease of shear bond strength caused by erosion. Fidalgo, et al. 11 (2012) found that NaF fluoride treatments improved the shear bond strength of brackets to enamel after cariogenic challenge, because NaF forms fluoride hydroxyapatite, which is more resistant than hydroxyapatite 11 . However, our study showed that most likely the reaction of titanium ions with enamel apatite caused more protection from shear forces than the NaF reaction.