In Vitro Bond Strength and Failure of Brackets on Porcelain

Objectives: To investigate and compare in vitro the tensile bond strength to porcelain and failure patterns at debonding when different combinations of bracket, adhesive and light curing source are used. Methods: Tensile tests were performed using: one ceramic bracket versus one metal bracket two orthodontic composites; type Bis-GMA and Urethane dimethacrylate (UDMA), and four light curing units of the same range of emission spectrum but various light intensities: three light emitting diode units (LED) and one halogen-based unit. 160 glazed porcelain (fluorapatite glass-ceramic) samples were randomly divided into sixteen equal groups. The porcelain surface was conditioned with 9% hydrofluoric acid before silane application. The composite was photo-polymerised 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 analyzed at 0.05 significance level, using R language; linear model with constant variance for the bond strength and multinomial distribution for the failure mode. Results: Bond strength in all groups was sufficient to withstand orthodontic treatment (> 6 MPa). No statistical difference between the adhesives, but comparing bracket:light interaction it was significantly higher with the ceramic bracket. No significant differences between the metal bracket's groups, but for the ceramic bracket the results were significantly higher with the LED light of higher intensity. No significant difference between the halogen-based light and the LED of lower intensity. No fracture in porcelain with the metal bracket neither when light is the halogen-based, but it occurred in 35% of the ceramic bracket samples and the risk was 6 times higher when using UDMA composite and 10 times lesser when the light is of the higher intensity. Conclusion: As the bond strength is higher with ceramic bracket a careful debonding is advised to avoid porcelain fracture.