Ultra-morphological study of the interaction of dental adhesives with carbamide peroxide-bleached enamel

Purpose: To evaluate the effects of a carbamide peroxide bleaching agent on interfaces formed by two one-bottle dental adhesives to etched enamel. The null hypotheses tested in this study were that vital bleaching with a commercial 10% carbamide peroxide gel would not (1) increase the concentration of oxygen in the superficial layer of enamel or (2) induce ultra-morphological changes in resin-enamel interfaces. Materials and Methods: Five extracted human incisors were treated with 10% carbamide peroxide (Opalescence) for 4 h/day for 1 week and were compared with non-bleached teeth for oxygen, calcium, and phosphorus relative concentration using EDS. Mean elemental concentrations were analyzed using a t-test (bleached vs. unbleached enamel), one-way ANOVA (for surface location and also for depth) and three-way ANOVA (with bleaching treatment, surface location, and depth as the main factors). For TEM, fifteen extracted human molars were sectioned to obtain two crown halves. After roughening the occlusal surface, one half was bleached with Opalescence while the other half was stored in artificial saliva for 1 week. Enamel was etched for 15 s with a 35% phosphoric acid and bonded with one of three adhesives (Prime & Bond 2.1, Syntac Single-Component, or Scotchbond Multi-Purpose Adhesive - control) and a composite resin (Protect Liner F). Small enamel/resin sticks with a cross-section of 1.0 mm × 1.0 mm were removed and the specimens were processed for TEM observation. Results: Vital bleaching with 10% carbamide peroxide caused no significant changes in relative oxygen concentration of enamel. For calcium and phosphorus, bleaching resulted in significant decreased relative concentrations. Bleaching also resulted in morphological alterations in the most superficial enamel crystallites. Some altered crystallites exhibited electron-lucent cores and reduced thickness of material around the core.