RESUMO
This in vitro study investigated the influence of using different combinations of bracket, adhesive, and light-curing source on the tensile bond strength to porcelain and on failure patterns at debonding. Tensile tests were performed using: one ceramic bracket versus one metal bracket, two orthodontic composites; type bisphenol A-glycidyldimethacrylate and urethane dimethacrylate (UDMA), and four light-curing units with the same range of emission spectrum but various light intensities: three light-emitting diode (LED) units and one halogen-based unit. One hundred and sixty porcelain samples were randomly divided into 16 equal groups. The porcelain surface was conditioned with 9 per cent hydrofluoric acid before silane application. The composite was photo-polymerized for 40 seconds. After storage in water at 37°C for 24 hours, the samples were subjected to tensile force until bond failure. Bond strength and bond failure mode were recorded; results were analysed (α = 0.05) using R language; linear model with constant variance for the bond strength and multinomial distribution for the failure mode. The bond strength in all groups was sufficient to withstand orthodontic treatment (>6 MPa). There was no statistical difference between the adhesives, but comparing bracket × light interaction, it was significantly higher with the ceramic bracket. No significant differences were seen between the metal bracket groups, but for the ceramic bracket, the results were significantly higher with the LED light. No fracture was observed in porcelain with the metal bracket but it occurred in 35 per cent of the ceramic bracket samples and the risk was higher when using UDMA composite and lower with LED high intensity light.