Effect of Bleaching Agents on Enamel, Composite-resin and Tooth-restoration Interface

Objectives: To evaluate in vitro the nanohardness of tooth enamel (TE), composite resin (CP), dental adhesive (DA) and enamel hybrid layer (EHL) exposed to 35% hydrogen peroxide-based bleaching agents and analyze the tooth-restoration interface using the scanning electron microscopy (SEM).
Methods: Forty bovine incisors crowns were embedded in epoxy resin and a 2x2x2mm cavity was prepared in the medial third of the flattened buccal surface of each tooth. The cavities were restored (two-step etch-and-rinse Adper Single Bond 2 + nanocomposite resin Filtek Z350 XT), polished and the specimens were divided into 4 groups (n=10), corresponding to each bleaching agent used (TB: Total Blanc Office/ pH=7.22-6.33; HPB: Whiteness HP Blue Calcium/ pH=8.89-8.85; HP: Whiteness HP/ pH=6.65-6.04; PO: Pola Office/ pH=3.56-3.8), which was performed according to manufacturers’ recommendations. The nanohardness of the substrates was measured before, immediately after the bleaching procedure and after 7-day storage in artificial saliva (37°C), with Ultra-Microhardness Tester (DUH-211S, Shimadzu), using 100mN load for TE and CR, and 10mN for DA and EHL. Epoxy replicas were prepared for SEM analysis.
Results: For nanohardness test, two-way ANOVA and Tukey test (p<0.05) revealed that the bleaching agent with the lowest pH value (PO) was the only one to decrease the nanohardness of TE and EHL immediately after its application, however, after 7-day storage in artificial saliva the nanohardness levels of these tissues returned to their original values. No significant statistical difference was observed for nanohardness of CR and DA exposed to all bleaching agents in all evaluation times. SEM analysis revealed gaps between TE and DA after the exposure to all bleaching agents, however, the more evident gap in tooth-restoration interface was observed immediately after application of agent PO.
Conclusions: The bleaching agent with the lowest pH value decreased the mechanical properties of dental hard tissues and induced gap formation on tooth-restoration interface.