Mechanical preparation of the root canal system (RCS) is among the most important steps of endodontic treatment for cleaning the canal walls and creates a conical form to facilitate access, effective irrigation, and three-dimensional obturation of the RCS. ¹ Irrespective of the technique of cleaning and shaping, this process results in the reduction of radicular dentin. Over-reduction of radicular dentin in one point compared to other points at the same distance relative to the longitudinal axis of the tooth is a procedural error called canal transportation. ² Apical transportation (AT) moves the physiologic canal terminus to a new location on the external root surface and results in the accumulation of debris and residual microorganisms due to inadequate cleaning and shaping of the canal end. ³ , ⁴ Continuation of instrumentation of the root canal in the wrong path by larger files creates a teardrop shape at the apical region and results in AT and zipping of the root canal. ⁵ Such canal preparation does not create a resistant form for gutta-percha compaction and results in poor condensation and overextension of gutta-percha. ³ Advances have been made in root canal preparation techniques to overcome problems such as transportation. The main goal of endodontic treatment is to eliminate pulpal tissue, bacteria, and their products from the RCS and proper shaping of the root canal for obturation. ¹ This goal can be achieved using hand files or NiTi rotary files.

Engine driven is used to save time and prevent fatigue of dentists and patients. ⁶ Rotary instruments have greater flexibility and cutting ability compared to hand instruments (SS), ⁷ , ⁸ and due to their super-elastic property, they preserve the actual tapered shape of the canal during preparation and eventually decrease the risk of canal transportation. ⁹ , ¹⁰ , ¹¹ , ¹² However, high price and risk of file separation and fracture are among the shortcomings of these systems that may result in failure of canal preparation. ¹³ , ¹⁴ Considering these shortcomings, manufacturers have modified these systems to improve their properties and have introduced new systems to the market.

Mtwo is a new generation of rotary files. In contrast to other rotary systems, small size Mtwo files (10/04 and 25/06) reach to the working length and are used at the beginning of root canal preparation to access the apical third. Standard Mtwo rotary system includes four files with 10–25 tip sizes and 4%–6% taper. The number of grooves on the file indicates its taper. The length of these files can be 21, 25, and 31 mm. Helical angle or flute is the angle between the cutting blade of the file and dentinal wall along the file length. This angle is variable in Mtwo files and is a specific property for each file. It increases the cutting efficiency of Mtwo files with larger size and increases the mechanical strength of smaller Mtwo files. Evidence shows that this file does not change the root anatomy. Its risk of fracture in curved canals is low and better cleans the root canal from debris compared to other rotary systems. ¹⁵ , ¹⁶

Recently, a new technique with reciprocating motion, the Reciproc system was introduced to increase the clinical service of NiTi files and their cyclic fatigue resistance compared to files with continuous motion because it is claimed that reciprocating motion decreases torsional stress created by reverse cyclic rotation of file. This system has three sizes for use based on primary canal diameter. The instrument has a variable taper along its length. At the apical 3 mm, R25, R40, and R50 have a taper of 0.08, 0.06, and 0.05, respectively. Reciproc is compatible with endomotor and performs reciprocating motion at a frequency of 10 rpm. Every three reciprocating rotations allow for 360° rotation. The cutting angle of this instrument is larger than the releasing angle, and thus, it allows for forward movement into the canal. ¹⁷ , ¹⁸ Several techniques are available to assess the centering ability of instruments (maintaining the original path of the canal and preventing transportation). Sectioning of the root at different distances from the apex is conventionally used to directly observe the canal shape. However, this method cannot reveal the original canal pathway before preparation. ¹⁹ Previously, scanning electron microscopy, radiography, photography, and computed analyses have been used to assess the efficacy and accuracy of canal preparation. However, some of these techniques are invasive, and comparison of the canal before and after instrumentation is difficult. Some other techniques have the major drawback of traumatizing the specimen. Thus, data obtained through these techniques may be misleading. Recently, some techniques have been introduced for the assessment of the teeth without traumatizing them and have been used for the comparison of canal shape pre- and post-instrumentation. Computed tomography enables three-dimensional (3D) image reconstruction of sections made of the root canal. ²⁰ , ²¹ Cone-beam computed tomography (CBCT) is a new imaging modality that uses a 2D sensor and cone-shaped beam instead of fan-shaped X-ray beam in conventional CT. In this technique, volumetric data of the respective areas are acquired by rotation of the beam and sensor. ²² This technology has been used for the assessment of root canal morphology, fractures, and changes in the canal structure following preparation. ²³ , ²⁴ , ²⁵ , ²⁶ Considering the risk of canal transportation during instrumentation, this study aimed to assess the prevalence of AT following canal preparation with Mtwo and Reciproc R25 using CBCT.

Forty extracted maxillary first molars were collected from the clinics of Babol city and stored in saline solution at 4°C until the experiment. Teeth with almost similar mesiobuccal (MB) root lengths (19–22 mm) and curvature of >40° were selected according to Zhang et al. ²⁷ All teeth had mature apices. MB2 canals were not included in this study. Access cavity was prepared and MB canal was localized. A #10 K-file was introduced into the canal until its tip was visible at the apex. Actual canal length was measured as such, and the working length was considered 1 mm short of the canal length (corresponding to the position of apical constriction). Specimens were mounted in gypsum blocks to simplify the work and achieve a reproducible position for taking CBCT images. Before canal preparation, CBCT images were obtained using Cranex 3D dental imaging system (Soredex, Helsinki, Finland) with a FOV of 6 × 8 and voxel size of 0.1 mm × 0.1 mm from the sections made at 1, 2, 3, 6, and 9 mm distances from the apex (indicative of 1/3 coronal, 1/3 middle, and 1/3 apical) perpendicular to the longitudinal axis of the root. The obtained images were saved in a computer for later comparison with the postpreparation images. The teeth were divided into two groups of 20 each according to the average of the canal curvature. Straight-line access was obtained and a #10 file was used for patency. RC-Prep (VDW, Sweden) and 2 ml of 2.5% NaOCl were used for canal preparation in both groups.

Group 1: Root canals were instrumented using Mtwo system (VDW, Munich, Germany) and handpiece Endo-mate-DT (Nsk-JAPAN) with speed 300 rpm and torque 2 Nm with single length technique and according to the manufacturer's instructions. Instrumentation in apical area up to file No. 25, with tapper 0/06, was done. Sequence using the file was in this way (10/04, 15/05, 20/05, 25/06).

Group 2: Root canals were instrumented using Reciproc R25 system (VDW, Munich, Germany) and handpiece electric motor with contra angle 20:1, according to the manufacturer's instructions.

In each group, instrumentation was performed by one operator who prepared 5 canals during a specific period daily to apply equal force during preparation and prevent the effect of fatigue on the results. Postinstrumentation CBCT images were then obtained under the same conditions and exported to on-demand software. To assess the amount of canal transportation, the technique described by Yin et al. ²⁸ was applied as follows:

Y1: The least distance from the external root surface to the mesial circumference of the unprepared canal in cross sections

To calculate the amount of canal transportation, the following formula was used:

(Y1 − Y2) − (X1 − X2)

The obtained data were placed in the formula. If the resulting number of this formula was <0.1, indicated no transportation, while any other value indicated canal transportation. Negative values indicated canal transportation toward distal while positive values indicated canal transportation toward the mesial. To assess the centering ability using the obtained values, the following formula was used:

[INLINE:1]

The resulting value of one indicated no change in the canal path and that the file remained centered in the canal. The closer the obtained value to zero, the lower the centering ability of the instrument.

Statistical analysis:

The data were analyzed using statistical software SPSS17 (Chicago: SPSS Inc) and t-test and Chi-square tests; P ≤ 0.05 was considered significant.

In this study, the canal displacement was examined at 1, 2, and 3 mm diameters and the ability to maintain the canal centrality was examined at 1, 2, 3, 6and 9 mm distance from apical canals with the curvature extreme (<40°) using Mtwo and Reciproc rotary systems. The mean displacement for the mentioned diameters is listed in Table 1and Figure 1. Based on the statistical results, there was no significant difference between the two systems in terms of the amount and direction of displacement.{Table 1} Figure 1

Distribution of the amount of displacement in the two systems at different diameters.

This study aimed to compare the centering ability of Mtwo and Reciproc, the new single file system, and the amount of canal transportation in severely curved canals prepared with these systems. The selection of severely curved canals according to Schneider et al. was because the iatrogenic errors such as the canal transportation more commonly occur in severely curved canals. The master apical file in both systems was #25 because this size is safe for use in curved canals. In this study, MB canals of the maxillary molars were used because these canals usually have severe curves. Furthermore, these canals are very thin but flat at the same time. These characteristics further complicate preparation of these canals. Our study results showed that both systems caused some degrees of AT at 1, 2, 3, 6, and 9 mm distances from the apex during preparation. However, no significant difference was noted in this respect between the two systems. Moreover, in all sections and in both systems, the amount of AT was <0.1 mm. The amount of canal transportation in a report by Peters was 0.1 mm and thus within the clinically acceptable range. ²⁹

Therefore, both systems were capable of keeping the file centered in the canal and met the required criteria for use in the clinical setting. Occurrence of AT depends on several factors such as the instrument design, physical properties of the alloy, and the preparation technique. ³⁰ Iqbal et al. reported that small AT with the use of preparation systems depends on the centering ability of the file in the root canal, which per se depends on the metal alloy property, design, sharp tip, and conical shape (taper) of the instrument. ³¹

In our study, AT toward the mesial was more frequent than toward the distal in both systems although this difference was not statistically significant. This finding was similar to the results of Junaid et al. ³² They compared AT in curved canals prepared with WaveOne file in a reciprocating motion and a sequence of Twisted Files in a continuous rotating motion and found no significant difference between the two systems. However, the magnitude of transportation in the mesial direction was greater than distal transportation in both systems, which is in accord with the definition of AT (removal of canal wall structure on the outside curve in the apical half of the canal as the result of the innate tendency of the files to return to their original straight shape during canal preparation). Our results were also in accord with those of Stern and Schafer. Stern et al. ³³ evaluated the centering ability and shaping changes in three types of instrumentation techniques with NiTi files including ProTaper, Twisted File (rotary motion), and F2 ProTaper with reciprocating motion. They found no significant difference in the three groups in terms of the amount of transportation, centering ability, and duration of instrumentation between the rotary and reciprocating motions. Schäfer and Schlingemann ³⁴ evaluated the shaping ability of reciprocating single file with Mtwo and ProTaper rotary instruments for preparation of severely curved canals of extracted teeth. They showed that using Mtwo and Reciproc resulted in better cleaning of the apical region compared to ProTaper and WaveOne. Using the reciprocating motion in the Reciproc system, straightening of the canal curve will be minimal, which is in accord with the results of previous studies. This is due to the intermittent rotating motion and high superelasticity of the files, which is in accord with our findings. Madani et al. ³⁵ found no significant difference in AT in different cross sections of the mesial and distal surfaces of the canal on CBCT images following preparation with Mtwo and ProTaper systems; this finding is somehow in line with our findings. In our study, no difference was found between the centering ability of Mtwo and Reciproc systems.

CT scanners due to their practicality and noninvasive nature are routinely used for the assessment of endodontic systems. In this technique, root canal morphology before preparation can be assessed without probing with small hand files; this issue can prevent changing the primary canal anatomy. ² This technique, in comparison with the conventional methods, has higher accuracy and does not traumatize the specimen. The results are highly reproducible, numerous images can be obtained from the canals, and comprehensive information can be achieved about the anatomy of the RCS before, during, and after mechanical preparation.

The present study had several limitations and advantages. The strength points of our study were canal preparation by one operator (high reproducibility of results), using software calculations (high accuracy), and use of CBCT. The in vitro design was among the drawbacks of the study limiting the generalizability of the results to the clinical setting.

Within the limitations of this study, no difference was noted in AT and centering ability of the two systems. Thus, both systems can be used with minimal risk of procedural errors for root canal preparation. We suggest that to increase the accuracy, study conditions must be matched for all teeth. Specimens must be randomly assigned to the groups, and the curve of the roots must be considered and matched for random allocation of the canals to study groups.

Acknowledgments

The authors would like to thank the administrative and secretarial staff of the Department of Endodontics of Dental Faculty, Babol University of Medical Sciences, for their contribution to the maintenance of our patient record without which this project would have been impossible.

Financial support and sponsorship

The medical university of Babol, provided part of financial support in this research.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.