Objective: The purpose of this study was to evaluate the shear bond strength of orthodontic bracket when bonded to pre-conditioned and intact enamel using a self-etching primer within 6 hours and after thermal cycling.
Material and methods: One hundred and twenty freshly-extracted human teeth were divided into four groups according to how the buccal surface to be bonded had been pre-conditioned: 1) acid-etched with 37% phosphoric acid, 2) sand-blasted with 50 microns aluminum-oxide, 3) matted with diamond burr, and 4) intact enamel used as control. Orthodontic metal brackets were bonded to the teeth using the same composite resin (Transbond XT) and self-etching primer (Transbond Plus Self-Etching Primer). Brackets were debonded within 6 hours or after thermal cycling for 2500 times (5 degrees C--37 degrees C--55 degrees C). Shear bond strength was measured on a testing machine at a crosshead speed of 3 mm/min. The bracket-failure interface was quantified according to the modified adhesive remnant index score (ARI). Data were analyzed using the two-way ANOVA test, Scheffé confidence interval of differences of means, and the chi-square test (p < 0.05).
Results: All the pre-conditioned groups showed significantly higher shear bond strength before and after thermal cycling than the control group. There was no significant correlation between thermal cycling and shear bond strength. The ARI scores revealed that the bond failed primarily on the adhesive-enamel interface in all groups before and after thermal cycling, with the exception of the acid-etched group, whose bonds failed mainly on the adhesive-bracket interface after thermal cycling.
Conclusion: The authors recommend that the enamel be preconditioned before applying the self-etching primer when greater shear bond strength is desired.