Overlays or occlusal veneer restorations are reliable techniques that provide minimal removal of dental tissue and can be indicated to restore endodontically treated teeth in order to protect remaining dental tissues. However, it's important to notice that this type of restoration may be less resistant to mechanical forces during chewing process. Furthermore, this characteristic could be affected according to the type of the material and their thicknesses. Objectives: To evaluate the influence of thickness and type prosthetic material on in vitro fatigue behavior, ultimate fracture load, and failure mode of endodontically treated teeth restored with CAD/CAM overlays. Materials and Methods: A hundred and eight (108, n=15) sound human molars will be used in this study. Ninety teeth will be endodontically treated, while the remaining 18 teeth will be kept sound. Overlay preparations will be restored with CAD/CAM indirect materials, which will be milled with the following thicknesses: thin 0.6-0.7 mm and thick 1.4-1.6 mm. Three types of materials will be tested: di- (DL) and lithium disilicate (LD), and indirect composite (CI), forming 6 experimental groups, including 2 thicknesses and three materials. Controls will consist of prepared three (3) sound teeth restored with the same materials. After cementation, the overlays will be submitted a cyclic fatigue in a frequency of 5Hz, with a load of 50N and increasing 25N after each 500 cycles until catastrophic failure. The fracture load of the specimen will be noted as the machine stopped by a failure detection module. The number of loads at failure and endured cycles will be recorded. The data of the accelerated fatigue will be drawn, in a Kaplan Meier survival curve to cycles and life table for load. For all statistical analyses, the level of significance will be set at 95%. Post-hoc pairwise comparison will be used to detect differences among groups.
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