Influence of bioactive varnish on the shear bond strength of orthodontic brackets

To evaluate the effect of bioactive varnish application on the shear bond strength (SBS) of orthodontic brackets bonded to demineralized enamel. Bovine incisors were selected and randomly assigned to four groups based on enamel treatment: Group 1 - sound enamel, no treatment (positive control); Group 2 - enamel subjected to a pH-cycling demineralization protocol (negative control); Group 3 - demineralized enamel treated with bioactive varnish (PRG Barrier Coat®, Shofu) and stored in distilled water for 24 h; and Group 4 - demineralized enamel treated with the same varnish and stored in distilled water for 28 days. For SBS and Adhesive Remnant Index (ARI) tests (n = 15 per group), orthodontic brackets (Roth Max® slot 22, Morelli) were bonded to the enamel using the Transbond XT® adhesive system (3 M Solventum). Samples underwent thermocycling (500 cycles between 5 °C and 55 °C), followed by SBS testing using a universal testing machine (0.5 mm/min) and ARI evaluation using stereomicroscopy. Enamel surfaces from each group (n = 3 per group) were also examined using scanning electron microscopy (SEM). SBS data were analyzed using the Kruskal-Wallis test, followed by Dunn's post hoc test. ARI scores were analyzed using Fisher's exact test, with significance set at 5%. The positive control group showed significantly higher SBS values compared to Groups 3 and 4 (p < 0.001). The negative control group showed intermediate values, with no significant differences from the other groups. A significant association was found between groups and ARI scores: in the positive control group, 53.3% of samples had ARI = 0 and 33.3% had ARI = 3; in the negative control group, 93.3% had ARI = 0. In Groups 3 and 4, 100% of the samples showed ARI = 0. SEM analysis revealed an intact surface in the positive control group, a porous surface in the negative control, and a varnish-covered surface in Groups 3 and 4, with Group 3 exhibiting a more homogeneous coating than Group 4. Conclusion: The application of bioactive varnish to demineralized enamel significantly reduced bracket bond strength and resulted in predominantly adhesive failures at the enamel interface. Clinically, this suggests that bonding orthodontic brackets directly to enamel treated with this bioactive varnish may compromise bond integrity, regardless of the waiting period after application.