Extent of polymerization of dental bonding systems on bleached enamel

Purpose: To analyze the extent of polymerization of four adhesive systems on whitened enamel immediately after bonding or delayed for 24 hours and 2 weeks. The effect of prolonged light-curing was also analyzed. Methods: One adhesive of each class was investigated: Adper Scotchbond Multi-Purpose (three-step etch-and-rinse), One-Step (two-step etch-and-rinse), Clearfil Protect Bond (two-step self-etching) and Xeno III (one-step self-etching). Enamel fragments were treated with Opalescence Xtra Boost for 30 minutes, powdered and pressed into aluminum pans. Specimens were bonded immediately after bleaching, after 24 hours or after 14 days of storage in 100% humidity at 37 degrees C. Unbleached enamel powder was prepared as control. Extent of polymerization of bonded interfaces was obtained with differential scanning calorimetry (DSC) at 20, 40 and 60 seconds and data were statistically analyzed with 3-way ANOVA and Tukey's post-hoc test. Results: The extent of polymerization obtained from DSC exotherms of adhesives applied immediately after bleaching was significantly lower compared to controls. Increased extent of polymerization after storage was confirmed for all adhesives and no difference with controls was found after 14 days. Prolonged irradiation time increased curing rate for all the tested adhesives. This study supported the hypothesis that polymerization of the adhesive is reduced after enamel bleaching and delayed adhesive application reverses the polymerization inhibition. Prolonged polymerization intervals may counteract the inhibition of polymerization due to the bleaching procedure; nevertheless further clinical studies should validate this hypothesis.