To investigate the occurrence of microcracks in the canals containing broken instruments in the middle and apical thirds after instrumentation with various systems. 2,7,15,22,26,27, However, complete cracks in the current study were absent in the control group that was not instrumented with files, while they were present in the rotary file groups, and this indicates that the mechanical instrumentation caused complete cracks at both the middle and apical level in accordance with previous studies. 22 Many studies reported that sectioning procedures may cause new microcracks, while other studies attributed the occurrence of microcracks to the preexisting undetected dentinal defects, but most studies have agreed that microcracks occur more frequently after mechanical instrumentation. Despite the great tendency to the microcomputed tomography imaging technique used to detect the potential dentinal damage in comparison with the sectioning technique, 21 a recent study showed that there was no significant difference between micro-CT and the sectioning method adopted in this study to evaluate the occurrence of microcracks. SEM showed more dentinal microcracks while no microcrack was observed with CBCT. There were no statistically differences between micro-CT and stereomicroscopy. No microcrack was observed using the CBCT method. Instrumentation with the PTU system up to F2 and F4 files significantly increased the number of microcracks compared with preoperative samples according to micro-CT imaging (p. Statistical data analysis of instrumentation steps was performed using Friedman and Wilcoxon tests, and the data of imaging methods were analyzed using Kruskal-Wallis and Mann-Whitney U tests (p =. Finally, imaging of the sections was done by scanning electron microscopy (SEM). The sections were imaged under a stereomicroscope. All roots were sectioned horizontally at 2, 4, 6, 8, and 10 mm from the apices of the specimens. After instrumentation stages, the roots were scanned again with micro-CT and then with CBCT in same parameters. Root canal instrumentation was performed up to PTU F2 and F4 files. A total of 30 mandibular molars' mesial roots were scanned with microcomputed tomography (micro-CT) and cone beam computed tomography (CBCT) imaging methods before instrumentation. The aim of the study was to compare different imaging methods in the diagnosis of microcracks on root dentin and to evaluate the frequency of dentinal microcracks observed after root canal preparation using the ProTaper Universal (PTU) system of different sizes.

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