INTRODUCTION: Root canal preparation is an important process in endodontic therapy.Nickel-titanium (NiTi) rotary file system can be used in single length technique (simultaneous technique) without early coronal enlargement, as well as in crown-down method. The purpose of this in vitro study was to compare single length with crown-down methods’ shaping ability using Mtwo NiTi files.MATERIALS & METHODS: Fifteen acrylic-resin blocks containing simulated canals were divided into two experimental groups. In group A, single length technique was used and in group B root canals were prepared by crown-down technique. Pre-and post-preparation canals were photographed in a standardized manner and were superimposed. The inner and outer walls of canal curvature were evaluated at three points (apical, middle and coronal) to determine the greatest change. The data was statistically analyzed using the Student t-test by Statistical Analysis System (SAS) software.RESULTS: Statistical analysis revealed that in group B, dentine was equally removed within the canal coronal to the curvature, whereas in group A, the inner wall was predominantly removed (P<0.01). The two groups had no significant difference at the apical and middle points of the canal curvature.CONCLUSION: Our in vitro study revealed no significant difference between the single length method and crown-down technique using Mtwo for preparation of apical and middle portion of canal curvature.

Background and Aim: Effective cleaning and shaping of the root canal system is an essential part of a successful root canal treatment and is considered as one of the basic features of rotary instruments. The present study aimed to compare the efficacy of two types of nickel-titanium rotary files namely BioRaCe and Mtwo, in reduction of Enterococcus faecalis (E. faecalis) count in the root canal system. Materials and Methods: In this in vitro study, 50 extracted human single-rooted teeth were assigned to two groups plus a positive and a negative control group (20 samples in each group). First, the root canals were sterilized and were then inoculated with E. faecalis (ATCC: 11700). Subsequently, all the root canals were instrumented with BioRaCe and Mtwo nickel-titanium rotary instruments. Then, samples were collected from inside the root canals and cultured in a special medium to evaluate the remaining colonies. The Chi-square goodness of fit test was used to assess the difference in bacterial colony count after root canal preparation with the two rotary systems (alpha=0.05).  Results: Complete bacterial growth was observed in both the BioRaCe and Mtwo groups. There was no statistically significant difference between the two groups regarding bacterial elimination from the root canal system (P>0.05). Conclusion: Under the limitations of the current study, the two rotary systems exhibited no superiority over each other in terms of their ability to eliminate microorganisms from the root canal system.

Introduction: The objective of this in vitro study was to evaluate whether cervical preparation with Mtwo files in a crown-down technique influences instrumentation time and the cyclic fatigue resistance of these instruments. Methods and Materials: Two instrumentation techniques were evaluated (manufacturer and crown-down). Each group consisted of 10 kits containing four Mtwo instruments (10/0. 04, 15/0. 05, 20/0. 06, and 25/0. 06), which were used to prepare three standard simulated curved resin canals. The mean instrumentation time and the corresponding number of cycles for each instrumentation (NCI) were recorded. The instruments were rotated at a constant speed of 300 rpm in a stainless-steel canal (diameter of 1. 5 mm) at a 90° angle of curvature and 5-mm radius. The center of the curvature was 5 mm from the tip of the instrument. The cyclic fatigue resistance of the files was determined by counting the number of cycles to failure (NCF). Data were analyzed by the Mann-Whitney test. Results: The mean instrumentation time and NCI of files 10/0. 04 and 15/0. 05 were significantly lower (P<0. 05) when the crowndown technique was used compared to the manufacturer’ s method for the same tip size/taper file. There was no significant difference in the mean NCF between the two techniques. Conclusion: The crown-down technique did not interfere with resistance to cyclic fatigue. However, the shorter instrumentation time of files 10/0. 04 and 15/0. 05 could reduce the fracture risk in the case of reuse of these instruments.

Introduction: The aim of this study was to compare the formation of dentinal crack and craze lines in the root dentin during root canal preparation with three different NiTi endodontic systems, naming Reciproc (RCP), ProTaper Universal (PTU) and Mtwo.Methods and Materials: One hundred extracted mandibular premolars with single canals were selected and decoronated. The teeth were randomly divided into four groups of 25 each (n=25). In groups 1, 2 and 3 the teeth were prepared using Mtwo, PTU and RCP, respectively. While in group 4 (control group) the samples were left unprepared. After preparation, all specimens were sectioned perpendicular to the long axis of root at 3, 5 and 9-mm distances from the apex.The sections were then individually observed under 12× magnification using stereomicroscope. The data was analyzed using the chi-square and Fisher’s exact tests. The level of significance was set at 0.05.Results: No cracks were observed in the control group.All engine-driven systems caused dentinal cracks. Mtwo and PTU caused cracks significantly more than RCP (P<0.05). There was no significant difference between RCP and control group (P>0.05).Conclusion: All three engine-driven systems created dentinal defects. Reciproc caused less cracks than Mtwo and ProTaper Universal.

Introduction: File fracture is one of the main procedural mishaps in endodontic treatment. The aim of this in vitro study was to compare the fracture rate of three NiTi rotary systems; Hero 642, Mtwo and FlexMaster in artificial canals.Methods and Materials: In this study, bovine long bone was used. After primary preparation of bones, longitudinal sections with 4-cm diameter were cut and encoded. Subsequently, semicircular sections were prepared. A total number of 500 canals were created in the same way; the upper 3 mm of the canals were initially prepared with orifice shapers and then canals were filed with FlexMaster files sizes 25/0.02 and 25/0.04 to 13 mm of canal length. The prepared canals were assigned into 3 groups of the following systems: Hero 642, Mtwo and FlexMaster. Six selected instruments were used from each system; the files were applied 13 mm along the canals for 10 sec with manufacturer’s suggested speed and torque. The number of the canals prepared by each file before its separation was recorded; finally the data was analyzed with ANOVA test. Results: Mean number of prepared canals in Mtwo, FlexMaster and Hero groups before file separation was 15, 25 and 32, respectively.Conclusion: Results of this study showed that the number of prepared canals by Hero 642 was more than FlexMaster and Mtwo systems.