Effect of bleaching on subsurface micro-hardness of composite and a polyacid modified composite

ObjectiveTo investigate the influence of different bleaching techniques on subsurface physical properties of composite and polyacid modified composite tested via determination of micro-hardness.MethodsSpecimens of Tetric Flow, Tetric EvoCeram and Compoglass were light cured (2.5 mm thickness) and stored in artificial saliva for 2 weeks (n = 12/group). The samples were only removed for application of the following bleaching agents in a humid atmosphere: Either Vivastyle (1 h/d), Whitestrips (30 min/d), sodium-perborate–water mixture (once for 72 h), Simply White (1 h/d), or Opalescence XtraBoost (1st and 5th day for 15 min) were applied on the surfaces of the samples. Untreated specimens served as negative controls, samples treated with ethyl alcohol for 1 h acted as positive controls. After the bleaching period, samples were cross-sectioned and the micro-hardness (Knoop) of different subsurface levels (0.1 mm–2.0 mm) was determined.ResultsAll bleaching techniques significantly reduced the Knoop-hardness of the restoratives compared to untreated controls. Thereby, bleaching significantly affected not only superficial but also the deep layers of the specimens: in superficial layers (0.1 mm, 0.2 mm) lowest micro-hardness values amounted to 69.5% and 76.3% of the respective untreated controls (Compoglass/Vivastyle). In deeper subsurface levels, the lowest hardness was observed with Opalescence/Tetric EvoCeram (0.3 mm: 78.3%; 0.4 mm: 80%; 0.5 mm: 80.5%; 1.0 mm: 84.2%; 2.0 mm: 84.4%).SignificanceBleaching with the tested bleaching agents softens the adhesive restorative materials examined. Due to the fact that subsurface layers are also affected, polishing of the surface may not suffice for re-establishing the physical properties of the surface of the fillings.