Effect of high-powered LED-curing exposure time on orthodontic bracket shear bond strength

Objective: To evaluate the influence of light exposure time on the adhesive strength and the failure mode of orthodontic brackets bonded to human enamel. Methods: 100 metal brackets were bonded with Transbond XT to the enamel bucal surface of human premolars. The sample was randomly divided into 5 experimental groups( n = 20) according to the light exposure time (2, 4, 6, 8 and 10 s) and light cured with an LED-curing device (1600 mW/cm(2)). The specimens were thermocycled (5-55 degrees C, 500 cycles), storedin distilled water (37 degrees C, 7 days) and tested in shear, using an Instron universal machine( 1 KN, 1 mm/min). Failure mode was classified according to the Adhesive Remnant Index( ARI) using a stereomicroscope (20x magnification). Shear bond strength (SBS) data were analyzed with one-way ANOVA, followed by Tukey post hoc tests. The failure mode data were submitted to Kruskal-Wallis nonparametric test, followed by Tukey post hoc tests to the ranks. A significance level of 5% was set for all data. Results: The mean SBS values were 5.5 +/- 1.93 MPa (2 s), 7.4 +/- 1.95 MPa (4 s), 8.6 +/- 1.72 MPa (6 s), 9.3 +/- 1.64 MPa (8 s) and 11.6 +/- 2.65 MPa (10 s). Failure mode was mainly classified as Score 2. Both the SBS (p < 0.001) and the failure mode (p = 0.002) were statistically influenced by the exposure time. Conclusion: Reducing the exposure time to less than 10 s decreases the bracket bond strength. The weakest adhesive link was found at the bracket-adhesive interface. (C) 2013 Sociedade Portuguesa de Estomatologia e Medicina Dentaria. Published by Elsevier Espana, S.L. All rights reserved.