Effect of layering strategy and prolonged water aging on masking ability of composite resins

Statement of problem: Layered composite resins may adequately mask discolored substrates. However, whether color changes that occur over time affect masking ability is unclear.Purpose: The purpose of this in vitro study was to investigate the effect of layering and water aging on the masking ability of 2 composite resins.Material and methods: Disk specimens (1.5 mm-thick and shade A1) from Charisma Diamond (CD) and Filtek Z350XT (Z350) were made according to the composite resin shade combination used in the layering strategies: bilayer for CD (enamel/dentin) and for Z350 (enamel/body and enamel/dentin) and trilayer for Z350 (enamel/body/dentin) (n=5). Color measurements were obtained with a spectrophotometer over an A3.5 ceramic substrate, and the whiteness index for dentistry (WID) was calculated. The specimens were aged in water at 37 degrees C and evaluated at different times: 24 hours, 1 week, 1 month, 2 months, 6 months, 12 months, 18 months, and 24 months. CIEDE2000 color differences (Delta E-00) and WID differences (Delta WID) were calculated and interpreted by 50:50% color and whiteness perceptibility and acceptability thresholds. Changes among strategies and aging times were analyzed using the Kruskal-Wallis test (alpha=.05). Contrasts were made using the Mann-Whitney U test with Bonferroni correction (alpha=.001).Results: Delta E-00 decreased from 24 hours to 2 months of aging. From 2 months onwards, Delta E-00 increased for all layering strategies and times, without significant changes in lightness for CD (P>.004). Overall, for the CD restorative system, an increase in Delta WID was observed after 1 month of aging (P<.001), while for the Z350 restorative system, Delta WID decreased after aging (P<.001), except for the Z350 bilayer strategy (enamel + dentin) at 1 week and 2 months (P>.004). For all layering strategies and aging times, color differences were higher than the 50:50% perceptibility threshold for Delta E-00 and, after 6 months, higher than the 50:50% acceptability threshold for Delta E-00 and the 50:50% whiteness acceptability threshold.Conclusions: The ability of the tested composite resins to mask a discolored substrate was affected by the layering approach applied and by prolonged water aging.