Shear Bond Strength and Effect of Fluoride Varnishes on Reducing the Step Formed Between Sound and Carious Enamel Around Orthodontic Brackets

Objectives: This study aimed to evaluate the shear bond strength and the potential of different fluoride varnishes to reduce the step formed between sound and carious enamel around orthodontic brackets.
Methods: Bovine teeth (n = 40) were sectioned and included in acrylic resin. A polished window of 8x8mm enamel was exposed, and a 2mm of sound enamel was covered with nail varnish. Edgewise brackets were bonded on the exposed area. The specimens were placed in BHI + 2% sucrose with 1x10⁶cells/mL of Streptococcus mutans, Streptococcus sobrinus, and Streptococcus salivarius (ATCC) for 2 days. Then, the specimens were randomly divided into a control (n=10) and three different varnishes groups (n = 10): NaF, Xilitol, and CPP-ACP. The varnishes were applied on the exposed enamel and the specimens were maintained in relativity humidity at 37^o C. After 24 hours, the varnishes were removed and samples were stored in remineralizing artificial saliva (pH 7.0) for 24 hours at 37^o C. The shear bond strenght was evaluated in an universal testing machine (0.5 mm/min). After that, the nail varnish was removed, and a non-contact profilometer evaluation of the gap formed between sound and carious enamel was perfomed. Data were analysed by Shapiro-wilk, Kruskal-wallis and Mann-Whitney test; p < 0.05.
Results: No differences were found in shear bond strength between control (6.55 MPa ± 3.93), NaF varnish (7.89 MPa ± 3.64), Xilitol varnish (8.95 MPa ± 4.70), and CPP-ACP varnish (10.60 ± 5.83) groups (p>0.05; Kruskal-Wallis). Also, there was no difference the gap formed in control (6.34 ± 2.77), NaF varnish (5.89 ± 2.28), Xilitol varnish (7.98 ± 3.74), and CPP-ACP varnish (4.91 ± 1.00) groups (p > 0.05; Kruskal-Wallis).
Conclusions: Based on the experimental conditions it is concluded that the tested varnishes did not influence shear bond strength, neither minimized the gap formed between sound and carious enamel.