Effect of In-office Bleaching Agents on Microhardness of Composite Resins

Objectives: to evaluate in vitro the effect of different in-office bleaching agents on microhardness values (HV) of microhybrid and nanohybrid composite resins (CR). Methods: Metallic matrixes were used with an internal cavity with 2 mm of thickness and 5 mm of diameter. Five specimens of CR were prepared for each experimental group. In groups G1 to G5 it was used a microhybrid CR (Opallis, FGM), while in groups G6 to G10 used a nanohybrid CR (Brilliant NewLine, Coltène/Whaledent).  ​​ The in-office bleaching agents used were: 35% hydrogen peroxide (HP) (Whiteness HP Max, FGM) and 37% carbamide peroxide (CP) (Whiteness Super, FGM). After the specimens were prepared, the bleaching procedure was realized as follows: G1 and G6: control group without bleaching; G2 and G7: one session with three applications of HP; G3 and G8: two sessions of three applications of HP; G4 and G9: one session with three applications of CP; and G5 and G10: two sessions of three applications of CP. The bleaching procedures were realized in accordance to the manufacturer's recommendations. After it, the HV test was performed using the MMT-3 Digital Microhardness (Shimadzu) with a load of 50 gf during 30 seconds, making four impressions in the top surface of each specimen. Data were statistically analyzed by ANOVA and Bonferroni’s tests (p<0.05). Results: the HV results and standard deviation of each group to the microhybrid CR were: G1- 55.15±2.85, G2- 54.02±2.70, G3- 55.33±3.33, G4- 55.23±1.97, G5- 53.52±2.46; and to the nanohybrid CR were: G6- 49.57±4.78, G7- 51.26±4.48, G8- 48.16±2.75, G9- 50.11±2.07 and G10- 51.19±0.62. Conclusions: the in-office bleaching did not negatively affect the microhardness of the composite resins tested.