Hand-operated and rotary mity instruments in combination with an endodontic handpiece and endodontic motor preparation: A comparison of shaping ability in simulated curved root canals

The aim of this in vitro pilot study was to describe the shaping ability of a rotary nickel-titanium (NiTi) instrument in combination with different torque-controlled endodontic devices and to compare them with NiTi fi les in combination with a conventional preparation by hand in severely curved root canals of plastic blocks. Thirty blocks simulating a severely curved root canal were divided into three groups (n=10 for each). All canals were prepared with NiTi fi les to an ISO size of 40 using an apical-coronal preparation technique. Mity Roto 360° fi les (Loser, Leverkusen, Germany) were used for the automatic systems, while Mity K-Files (Loser, Leverkusen, Germany) were used for the preparation by hand. We evaluated the frequency of instrument failure, the preparation time, loss of working length, loss of weight, elbow-zip effects, and changes in root canal anatomy. The data were analyzed statistically using one-way ANOVA followed by a post hoc Bonferroni test (p < 0.05). A total of three instrument failures were observed: one for Mtwo direct and two for Endo IT. Preparation time was signifi cantly longer when preparing by hand (16.64 min) than for both automatic systems, which did not differ signifi cantly. Critical loss of working length could be detected for all systems (1.3–2.81 mm). Irrespective of the torque-controlled motor, the shape of the canal anatomy was retained better with Mity Roto fi les than hand preparation. In summary, the two automatic systems did not differ signifi cantly. Within the limitations of this study and especially due to the long working length losses, neither the MTwo direct handpiece nor the Endo IT professional motor can be recommended in combination with Mity Roto 360° fi les when an apical-coronal preparation method is used in severely curved root canals. Research Article Hand-operated and rotary mity instruments in combination with an endodontic handpiece and endodontic motor preparation: A comparison of shaping ability in simulated curved root canals Maria Giraki1*, Edith Harapetian2, Stefan Rüttermann1 and Susanne Gerhardt-Szep1 1Department of Operative Dentistry, Center for Dentistry and Oral Medicine (Carolinum), Goethe University Frankfurt, Frankfurt am Main, Germany 2Dental offi ce, Heppenheim, Germany Received: 15 April, 2019 Accepted: 18 June, 2019 Published: 19 June, 2019 *Corresponding author: Dr. Maria Giraki, Department of Operative Dentistry, Center for Dentistry and Oral Medicine (Carolinum), Goethe University, Theodor-SternKai 7, Frankfurt am Main 60596, Germany, Tel: +4969-6301-84495; E-mail: ;


Introduction
In the last years, the technical possibilities for the preparation of curved root canals have been greatly optimized through the development of special fi le systems and the use of torque-controlled endo motors. Factors such as preserving the original anatomy of the root canal while avoiding instrument fractures and iatrogenic preparation errors, such as loss of working length, zipping, or ledging account for the success of endodontic treatments [1,2]. Unlike stainless-steel fi les, nickel-titanium (NiTi) instruments have suffi cient cleaning ability and can preserve root canal anatomy. To minimize the risk of instrument fracture, nickel-titanium rotary instruments with torque-controlled motors not exceeding the recommended speed for the specifi c system should be used [3].
The latest development in torque control is the incorporation of gear systems within the cordless endodontic handpiece that regulates torque depending on the size of the rotary instrument.
This eliminates the need for conventional torque-controlled motors. In addition, the cordless nature of the instruments offers complete portability [4]. The clinical practicability, local fl exibility, and simple adjustment options of endodontic handpieces can simplify the application of automatic systems in endodontic treatment. This supports the need for cordless automated preparation systems for endodontic therapy. An example of a cordless torque-controlled motor is the SiroNiTi endodontic handpiece. Based on own study results, its use cannot be recommended due to several disadvantages, such as longer preparation time and instrument failure, as well as changes to the preoperative cross-sections [5]. The aim of the handpiece and endodontic motor preparation: A comparison of shaping ability in simulated curved root canals. J Dent Probl Solut 6(1): 013-020. DOI: http://doi.org/10.17352/2394-8418.000067 present study was, therefore, to test another cordless handpiece possessing the following advantages: No reading of set values on a card -which simplifi es and quickens the setting of the torque limitation directly at the contra-angle -and its use in the conventional apical to coronal preparation technique. Therefore, this in vitro pilot study aimed to describe the shaping ability of a NiTi instrument (Mity Roto 360°, Loser, Leverkusen, Germany) in combination with a mechanical hightorque-controlled endodontic handpiece (study group with the MTwo direct device, Sirona Dental Systems, Bensheim, Germany), and to compare it with the same rotary NiTi instrument in combination with a conventional electric torquecontrolled motor (control group 1 with Endo IT professional device, VDW, München, Germany), and with conventional NiTi fi les (Mity K-File, Loser, Leverkusen, Germany) used in the hand preparation technique (control group 2). The parameters that were compared include the frequency of instrument failure, the preparation time, loss of working length, loss of weight, elbow-zip effects, and changes in root canal anatomy. For reasons of standardization, the preparation was performed on curved root canal equivalents in transparent plastic blocks (VDW, Munich, Germany).
Our fi rst hypothesis was that there is a difference between the two torque-controlled devices in terms of frequency of instrument failure, preparation time, loss of working length, loss of weight, elbow-zip effects, or changes in root canal anatomy. Our second hypothesis was that there is no difference between the endodontic handpiece and the preparation by hand with NiTi K-Files in terms of frequency of instrument failure, preparation time, loss of working length, loss of weight, elbow-zip effects, or changes in root canal anatomy.

Sample preparation
This study involved n = 30 simulation blocks of a root canal (VDW GmbH, Munich, Germany) that were equivalent to severely curved root canals. The blocks were made of acrylic. The canals of the blocks were standardized, pre-formed, and all were 19 mm in length. Measured from the coronal part of the translucent simulation block, the curve began at 10 mm. The simulation blocks were numbered and randomly divided into three groups: n = 10 for the study group (SG) with handpiece preparation (Mity Roto 360° fi les with MTwo direct), n = 10 for the control group 1 (CG1) with endodontic motor preparation (Mity Roto 360° fi les with Endo IT), and n = 10 for a second control group (CG2) with preparation by hand (Mity K-Files). We evaluated the following parameters: instrument failure, loss of working length, preparation time, loss of weight, the presence of elbow-zip effects, and changes in canal anatomy. Figure 1 is a schematic diagram of the experimental setup.

Root canal preparation
All canals were prepared by one person with an apical to coronal preparation technique using NiTi fi les to an ISO size of 40. For automatic preparation with the two torque-controlled endodontic devices (MTwo direct handpiece and Endo IT motor in combination with a contra angle E-Type handpiece 4:1), Mity Roto 360° instruments were used. The preparation by hand was done with Mity K-Files. Rotation speed of Endo IT and MTwo direct was set at a constant rotation of 300 rpm. For the automatic systems torque values were set manually depending on the fi les used, in accordance to the manufacturers' specifi cations.
Before being used, all instruments were loaded using FileCare ® EDTA (VDW, München, Germany). After each preparation step, each canal was rinsed with 2 ml Aqua dest. (Phönix GmbH, Sundern, Germany). Mity fi les were applied a maximum of two times per canal. Room temperature was constantly 21°C.

Instrument failure
Instrument failures are related only to instrument fractures. In the case of a fracture, the endodontic device and  the simulation block number, as well as the type and the size of the fractured instrument, were documented.

Preparation time
Preparation time including rinsing processes and instrument changes was recorded in minutes and seconds by a microchronometer (Siemens, Munich, Germany).

Measuring the loss in working length
Loss of working length was measured by calculating the difference between the working length before and after preparation (in millimeter). Therefore, the canal length before the preparation was measured using a C-Pilot ISO 010 fi le (VDW, Munich, Germany), and after the preparation the working length of the prepared canal was measured by Guttapercha ISO 40 (Roeko, Langenau, Germany). The working length was defi ned as the canal length of the unprepared canal minus 1 mm.

Measuring the loss of weight
Simulation blocks were stored for 48 hours in a drying cabinet (Memmert GmbH, Schwalbach, Germany) at 37°C and were weighed using an analytical balance (Sartorius GmbH, Göttingen, Germany) before and after preparation to compare the removal by the different instruments.

Elbow-zip effects
Elbow-zip effects were assessed using the tenfold enlarged projections of the root canals on a DIN A4 paper. The effects were recorded as no effect (=0), a weak effect (=1), or a strong effect (=2) ( Figure 2).

Canal anatomy
Root canals were photographed with a digital camera (Leica Microsystems Distribution GmbH, DFC 480, Wetzlar, Germany) before and after preparation with an integrated millimeter scale from the same side in order to determine the changes in canal anatomy. The slides of the root canals were projected onto a DIN A4 paper with a slide projector (Zeiss IKON GmbH, Perkeo R1500 AFS, Jena, Germany) prior to preparation. The projection was adjusted so that one millimeter of the millimeter paper was enlarged to one centimeter (10x magnifi cation). The contours projected on the wall were color coded. Then, with the aid of orientation crosses, the comparison slide (identical root canal after preparation) was projected over the same picture and contours of the prepared root canals were drawn in a different color. Differences or changes in canal anatomy were verifi ed using seven measurement points. These measurement points were determined by using the apex of the unprepared root canal as the center of a circle. From this, circular arcs were hit with the help of circles with radii of one, two, four, fi ve, six, nine, and thirteen centimeters. The two points of intersection of each arc with the inner and outer curvature of the unprepared root canal were connected with a straight line and extended to the outer contour of the prepared canal. At each measurement section, the distance (at 90° angle) of the unprepared to the

Statistical analysis
The study variables (frequency of instrument failure, preparation time, loss of working length, loss of weight, elbow-zip effect, changes in canal anatomy) were statistically analyzed using one-way analysis of variance (SPSS Statistics, Version 2.0, IBM, Ehningen, Germany).
Signifi cant differences were determined using the post hoc Bonferroni test. Means and standard deviations (SD) were also evaluated.

Instrument failure rate
Three instrument failures were recorded during preparation: two instrument failures when using the Endo IT control motor (one at ISO-size 35 and one at ISO-size 40) and one failure when using the MTwo direct handpiece (at ISO-size 30). All fractures affected the apical root third. During hand preparation, there was no instrument failure. Table 1 shows the results for the parameters of preparation time, loss of working length, loss of weight, and elbowzip effect. Signifi cant differences among the three groups exist only for the parameter of preparation time, which was signifi cantly longer for the control group 2 than for the study group and control group 1.

Outside curvature
Overall, a larger change in canal anatomy was observed in the apical region of the root canal for the manual preparation In the coronal area, signifi cantly higher material removal was found for measurement point 7 in the Mity K group (CG2: 3.7 mm) than with the two mechanical systems, which did not differ signifi cantly from one another (SG: 1.19 mm; CG1: 1.53 mm).

Instrument failure rate
Three of the 30 instruments were fractured, all while using the Mity Roto 360° fi les, one with the angle piece, and two with the Endo IT motor. This is a fracture ratio of 10% to 20% per automatic system. Complex reasons based on a variety of factors such as the material [6] and design [7], of the fi les used, the application or drive technology [5,8], dynamic or static testingconditions while preparation [9], environmental temperature [9] and user experience [8], are associated with the breakage of NiTi instruments. When endodontic instruments are rotated, they are subjected to cyclic stresses in curved canals [10]. The intensity of these stresses is also related to the arc length and instrument size [11]. According to Lopes et al. [11], the radius and the position of the canal curvature are the most critical parameters that determine the stress on the instrument.
Higher stress levels are produced by decreasing the radius and moving from the apical to the mid-root position [12]. In addition, performance of mechanized preparation in a dynamic model promotes stress distribution along a wider portion of the instrument and increases the number of cycles to fracture, showing an extended life compared to static model studies [9].
Although we used a dynamic way for preparation of root canals, data are not completely comparable, because in our study the preparations were performed by a human being and we had different environmental conditions (room temperature instead of body temperature), that may have infl uenced the fracture risk of instruments [9]. Furthermore, the fracture risk appears to increase with increasing instrument size [13]. In the present study, all fractures were located in the apical portion of the fi le with high instrument size (ISO sizes [30][31][32][33][34][35][36][37][38][39][40]) and occurred at the point of curvature of the canal, which is in concert with the above-cited literature. The instrument design or the cross-sectional geometry can also be an important reason for a fracture risk. According to Turpin [14], the resistance of a fi le to fracture or torsion correlates with the geometry of the fi le and its cutting edges. The design of Mity Roto 360° fi les with wide lateral radial lands shows pronounced contact surfaces with the root canal. This leads to high frictional resistance and a strong instrument load due to torsional forces [15,16].
Simulation blocks for a root canal equivalent were used for this study in order to obtain as standardized and controllable conditions as possible [17]. However, the use of artifi cial root canals has a major disadvantage: In the canal, the frictional heat caused by the rotating fi les causes the plastic to soften locally, which can lead to load peaks in torque [17]. These conditions could have been particularly great with Mity Roto 360° and could explain the observed fractures. The stronger attachment of the instrument at the point of greatest curvature in the canal could have resulted in greater tensile and compressive forces acting on the instrument. A stronger bonding of the cutting edges with the plastic could thus also have contributed to the breakage of the Mity Roto 360° fi le [18].

Preparation time
Engine based root canal treatment is a valuable time relief for patients and practitioners [19]. The result of a lower time intensity of mechanical systems compared to preparation with hand instruments demonstrated in this study coincides with other studies [20,21] and is independent of the type of drive (Mtwo direct/Endo IT). Bürklein [22] was able to demonstrate that preparing with an Endo IT motor was signifi cantly faster compared to a Mtwo direct contra-angle hand piece, but used MTwo fi les in the crown-down technique, so that no informative comparison with the present study conditions could be ensured.

Loss of working length
The preparation of the complete working length is a key prerequisite for the success of root canal treatments [23]. Nevertheless, even with the use of NiTi systems, there may be an undesirable loss of working length [24]. In the case of manual preparation with Mity K-Files, an average loss in the working length of 1.30 mm in simulated canals was also found in the present study. In contrast to the fi ndings at hand, Weiger was unable to detect a loss of working length when using manual NiTi fi les [25]. When using the Mity Roto 360° system, compared to manual technology, there were signifi cantly larger working length losses found in our study: 2.44 mm and 2.81 mm on average. The drive type (contra-angle handpiece/ motor) had no impact on the working length loss within the two mechanical systems. Thompson observed only a small loss of working length between 0.05 mm and 0.15 mm for Mity Roto 360° [26]. In contrast to the present study, Thompson's study opted for the crown-down technique. Thus, different preparation techniques could account for the observed large difference in lost working length. In contrast, Gerhardt-Szep found no loss of working length with the Mity Roto 360° instruments with a treatment technique that is comparable to the present study [27]. One reason for the contradictory results could be that the simulation blocks used were made by different manufacturers and, therefore, did not have exactly comparable properties, such as plastic hardness, root canal cross-section, or curvature. Furthermore, the impending loss of the working length for Mity Roto 360° could have unintentionally resulted in increased pressure exerted by the user while preparation. This, in turn, could have led to frequent jamming of the Mity Roto 360° fi les in the plastic canals, which may have been favored by the heating of the plastic. In this study, the use of Mity Roto 360°, an instrument with less cutting effi ciency, may have been disadvantageous in combination with the apicalcoronal preparation technique.

Loss of weight
In addition to the quality of the root canal preparation, the quantitative aspect of material removal is crucial [19,27,28].
The observed weight loss after preparation is a model for examining the cutting performance and the removal of debris from the fi le systems [27]. The measured weight loss is dependent on the loss of working length and the apical "debris accumulation". The three parameters together provide information about the cutting performance of the fi le. The latter is great when the weight loss is high and the working length loss is simultaneously low [27]. In this study, all three systems had comparable weight loss ranging between 4.85 and 5.59 mg.
The sole consideration of the weight loss average of the Mity Roto 360° fi les and the mean of their simultaneously measured largest working length losses seem to be contradictory.
These may be caused by the practitioner's preparation error described above. However, these may also show that Mity Roto 360° does not have the highest cutting performance due to its long working length losses. In addition, the chips remaining in the prepared simulation block for a root canal equivalent within the groups were not quantifi ed further, which limits the signifi cance of this parameter.

Elbow-zip effect
In this study, a total of fi ve weak elbow-zip effects occurred, which did not differ within the groups. In contrast to the present study, Thompson had no elbow-zip effect in the preparation on the simulated root canals with the Mity Roto 360° instruments, using the crown-down technique [29,30]. This "aggressive" measure against the impending loss of the working length could lead to elbow-zip effects, as postulated by Hülsmann [31]. However, the elbow-zip effects observed in the study for Mity Roto 360° confl ict with its design, which, with its rounded tip and radial lands, should allow better centering in the canal [32]. In the root canal equivalent simulation block, the larger guide surface of the Mity Roto 360° fi les could cause the material to stick to the fi le, thus producing an elbow-zip effect. This hypothesis must be reviewed in further studies.

Canal anatomy
One goal of mechanical root canal preparation with respect to canal anatomy is, among other things, to preserve the original root canal shape. Considering the canal anatomy factor, material loss in the present study is rather uneven over the entire canal length, especially for the Mity K-File. If one looks at the apical area, signifi cantly higher material removal on the outside than for the inner curvature are evident for all three groups. The observation of higher material removal in the apical area at the outer curvature could be confi rmed by further investigations using mechanical NiTi instruments, in which increased removal at the outer curvature of the apical third was accompanied by little or no removal at the inner curvature [33]. With regard to outer curvature, the Mity Roto 360° system showed signifi cantly less material removal overall, regardless of the type of motor in the apical section compared to the manual system with the Mity K instruments, which could be explained by its lower cutting effi ciency [26]. The cutting tip of the Mity K instruments may contribute to a stronger "screwin" effect in the outer canal wall. In the middle canal area with the strongest curvature, the highest material removal by the Mity K instruments was found on the inner curvature. This observation is in line with other studies [27,34]. Root canals, which have been prepared to different intensity degrees, can lead to a transportation of the canal axis, which was observed by Gerhardt-Szep in the form of root canal straightening on standardized artifi cial canals [27]. As expected, the Mity Roto In summary, regardless of the drive motor, the Mity Roto 360° fi les produced more even material removal over the entire canal and the best shape retention of the canal geometry compared to the manual instruments. This observation contrasts with a study by Bürklein, who, however, used the aforementioned systems on root canal equivalents of polyester resin rather than plastic, which might explain the differences [22]. The decisive disadvantage of the Mity Roto fi les, however, is the clinically intolerable loss of the working length, averaging

Review of the research hypotheses
The pre-formulated fi rst and second research hypotheses were rejected and the alternative hypotheses were adopted.
These relate primarily to the extended preparation time in manual processing.