The aim of the present study will be to evaluate the cyclic and torsional fatigue resistence of heat treated NiTi instruments after instrumentation of the root canals of mandibular molars. In addition, the thickness of dentin and areas touched and not touched by these instruments will be analyzed. 30 Vertucci type IV extracted human lower molars will be selected, with 2 mesial channels and volumes of these similar. The teeth will be digitized for confirmation of standardization and stored in 0.1% thymol solution. After selecting the teeth, these will be divided into 3 groups (n = 10) according to the selected NiTi instruments: group I (control): Reciproc M-Wire; group II: Sequence Rotary File; Group III: X1 Blue File. A new instrument will be used for each tooth, so 30 instruments will be needed. For root canal instrumentation, conventional coronary opening procedures will be performed. The CT will be established by the files k10 and 15, 1mm short of the apical foramen and the instrumentation will be performed with a VDW motor according to the manufacturer's recommendations. The teeth will be instrumented in a vessel simulating body temperature (37 ° C). Irrigation will be performed with 2.5% sodium hypochlorite solution at each instrument change. After the instrumentation the files will be submitted to the fatigue tests and later analyzed in SEM. The mesial roots of the molars will be digitized for the analysis of the remaining dentin thickness and the visualization of the areas touched and not touched by the instruments in the whole channel and in 3 cross sections using the Micro CT. The data obtained will be tabulated and for each test the D'Agostino and Pearson test will be performed to analyze the normality. According to the results will be applied parametric and non-parametric tests with significance level of 5% (± = 0.05).
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