Effect of Bleaching Agent on Interfacial Fracture Toughness of Resin Composite-dentin Interfaces

The effect of bleaching on fracture toughness (Kic) of existing composite-dentin interfaces was assessed using the notchless triangular prism (NTP) specimen fracture toughness test. Human molars and premolars (<6 months old) were wet ground on 600 SiC to obtain 4x4x4x4mm triangular prisms with buccal or lingual exposed for bonding. Buccal or lingual dentin surfaces, ground on 600 grit sandpaper immediately before bonding, were bonded using a resin composite (Z250, 3M) and a dentin bonding agent (Single Bond, 3M) to obtain 4x4x4x8mm dentin-composite NTP specimens. Samples were randomly divided into 18 groups (n=9) and exposed to four concentrations of bleaching agent (11,13,16, and 21% carbamide peroxide). Controls were exposed to the same conditions as experimental groups, but were treated with tap water instead of carbamide peroxide. Exposure to the bleaching agent took place in 30-minute increments for 2 hours per day during the first week and 4 hours per day in the following two weeks. Samples were stored in tap water for 1 hour in between treatments, and 7 hours overnight. Testing was conducted after a cumulative exposure of 14, 42, and 70 hours. The maximum force required to fracture specimens, using an Instron 4311 universal testing machine at a cross head speed of 0.1 mm/min, was recorded and used to calculate Kic in order to investigate the effect of concentration and length of exposure to the bleaching agent on interfacial fracture toughness. Results were analyzed using two-factor ANOVA and Bonferroni multiple means comparisons. Increasing length of exposure to the bleaching agent resulted in a significant decrease in fracture toughness (p<.01). No statistical differences in bleaching agent concentration were detected (p=0.78). SEM observations of fractured surfaces revealed differences in fracture path of control and experimental groups.