IntroductionIn the recent years, tooth bleaching has become increasingly popular due to its optimal efficacy and non-invasive nature. It can be performed in the office or at home by use of over the counter products.1 This treatment is based on the use of hydrogen peroxide or carbamide peroxide, which release unstable free radicals that react with stains and pigments and result in tooth whitening.2 Factors such as the concentration of hydrogen peroxide and the source of energy as driving force of chemical reactions can affect the efficacy of bleaching treatment.3 High-intensity light was first used to enhance the whitening chemical reactions. Thermal lamps and hot spatula were later used as heat source to enhance the whitening reaction. Although effective, these methods carried the risk of pulp hyperthermia. 4 Presently, activating light sources such as plasma arc lamps and laser at different wavelengths have replaced direct heat to enhance the whitening efficacy of bleaching agents.5 Use of lasers such as diode laser at 810 or 980 nm wavelength and Nd:YAG laser at 1060 nm wavelength is increasing for this purpose due to the optimal efficacy of photothermal bleaching. 6On the other hand, there are some concerns regarding the effects of bleaching treatment on dental materials, which are susceptible to wear and degradation. Chemical softening caused by bleaching can potentially affect the physical and mechanical properties of toothcolored restorative materials such as microhardness and roughness. 8,9 Dental materials may show variable reactions to bleaching agents. These reactions may range from alterations in surface morphology to change in their physical and chemical properties. Microhardness is defined as resistance of a material to indentation. As one of the most important properties of restorative materials, microhardness is often measured to This study sought to assess the effect of bleaching combined with irradiation of 810 nm and 980 nm diode laser on microhardness of 2 commonly used self-cure and light-cure glass ionomer cements (GICs) in comparison with conventional bleaching (without laser). Methods: In this in vitro, experimental study, 60 samples were fabricated of A2 shade of Fuji IX and Fuji II LC GICs (n = 30) and each group was divided into 3 subgroups (n = 10). The first subgroups were subjected to bleaching with Opalescence Xtra Boost plus 980 nm diode laser irradiation. The second subgroups were subjected to bleaching with Opalescence Boost plus 810 nm diode laser irradiation and the third subgroups were subjected to bleaching with Opalescence Xtra Boost without laser. Microhardness was measured at baseline and after the intervention using Vickers hardness tester. The data were analyzed using two-way analysis of variance (ANOVA) (P < 0.05). Results: Microhardness decreased in all subgroups after the intervention (P < 0.001) irrespective of the type of GIC (P = 0.201) or surface treatment (P = 0.570). The baseline microhardness of the three subgroups within each group of GIC was not significantly...
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