Evaluation of Microhardness and Bleaching Efficacy of Two Composite Resins

Objectives: The purpose of this in vitro study was to evaluate bleaching efficacy and microhardness of two composite resins, after staining and whitening.


Methods: Sixty discs (10x2mm) were fabricated using two different composite resins, one nanohybrid (Empress Direct) and another nanoparticulate (Filtek Z350XT). The discs were immersed in distilled water (24h) and divided in 3 groups (n=10), according to the solution: distilled water, coffee and wine. The discs were immersed for 2h/day (37°C) during 30 days. Subsequently, four bleaching sessions (45 min) were performed using 37% carbamide peroxide with 48h interval. Color was evaluated quantitatively using a Vita Easyshade spectrophotometer, in standardized area. Microhardness was evaluated in a Microhardness HMV tester, using a diamond indenter.The test was performed three times at the same specimen and a mean value was calculated.Color variation (ΔE) and microhardeness values were measured at three different times: T0) initially; T1) after pigmentation; and T2) after bleaching. The data were analysed by Student T and Mann-Whitney U tests, ANOVA and Kruskal Wallis tests followed by Sidak test (α=0.05).
Results: For microhardness, the nanoparticulate resin showed significantly higher mean values than the nanohybrid resin (p <0,001) for all groups. After bleaching, both composite resins immersed in coffee and wine showed significantly lower microhardness value than water (p<0.001). The color variation after staining with coffee (p=1.0) and wine (p=0.393) was statistically similar between the composite resins. Bleaching was clinically effective for composite resins stained with wine.No difference was observed between nanoparticulate (ΔE=3.54) and nanohybrid (ΔE=5.09) resins (p=0.123). When stained with coffe, bleaching was not clinically effective, however nanohybrid resin (ΔE=2.26) showed higher mean values than nanoparticulate resin (ΔE=1.08) (p<0.001).
Conclusions: Nanohybrid resin presented a greater color variation compared to nanoparticulate resin. Bleaching was not capable of return composite resins initial color and reduced the microhardness values.