| Abstract|| |
Aim: To compare the cleaning ability and preparation time of rotary instruments (Mtwo) and conventional manual instruments (K-file) in preparing primary and permanent molar root canals.
Materials and Methods: Access cavities were prepared in 70 primary and 70 permanent teeth and India ink was injected into 120 canals of selected molars. The teeth were randomly divided into two main subgroups (n=20) and three control groups (n=10). In each of these main subgroups, either the manual instrument (K-file) or the rotary system (Mtwo) was used to prepare root canals. After cleaning the canals and clearing the teeth, dye removal was evaluated with the help of a stereomicroscope. In addition, the time needed for root canal preparation was recorded by a chronometer.
Statistical Analysis: Statistical analyses were done using the Kruskal-Wallis, Mann-Whitney and t tests.
Results: With regard to the cleaning ability of root canals, there were no significant differences between the K-file and Mtwo rotary system in primary and permanent teeth in the apical, middle or coronal third of the canals. Moreover, there were no significant differences between primary and permanent teeth prepared with K-files and rotary instruments. In all the groups, shorter times were recorded with the rotary technique. The working time was shorter in primary than in permanent teeth.
Conclusion: The Mtwo rotary system showed acceptable cleaning ability in both primary and permanent teeth, and achieved results similar to those of K-files in less time.
Keywords: Nickel-titanium, permanent teeth, primary teeth, root canal preparation
|How to cite this article:|
Azar MR, Mokhtare M. Rotary Mtwo system versus manual K-file instruments: Efficacy in preparing primary and permanent molar root canals. Indian J Dent Res 2011;22:363
In recent years, the use of nickel-titanium (NiTi) rotary files and automated root canal devices has been increasing in endodontic treatments.  The advantages of rotary NiTi instruments over hand instruments include facilitating canal preparation, , preserving the shape of curved canals ,, and producing smooth surfaces in lesser time than with manual instruments. ,
|How to cite this URL:|
Azar MR, Mokhtare M. Rotary Mtwo system versus manual K-file instruments: Efficacy in preparing primary and permanent molar root canals. Indian J Dent Res [serial online] 2011 [cited 2013 May 18];22:363. Available from: http://www.ijdr.in/text.asp?2011/22/2/363/84283
NiTi rotary instruments are available in different designs and rates of tapering for use in endodontic treatment.  Developmental efforts have focused on increasing the cleaning efficacy of the root canal as well as reducing the time spent on preparation-an especially important factor in primary teeth. As a result, a new generation of NiTi rotary instruments-Mtwo endodontic instruments-was developed  and introduced in 2003.  The standard Mtwo rotary set includes four instruments with variable tip sizes ranging from #10 to #25 and tapers ranging from 0.04 to 0.06-0.07. According to the manufacturer, the minimally invasive Mtwo NiTi instruments should be used in single-length techniques without early coronal enlargement. ,,, This makes the technique simple to use, from apex to crown. Smaller instruments are used before larger ones, as in the step-back technique.  Because of the high flexibility and fatigue resistance of Mtwo instruments, the original canal curvature is preserved and the procedure is efficient, safe  and saves time. , Recently, Mtwo files have also been used to remove gutta-percha from root canals of permanent molars. ,
Several studies have compared the effectiveness of rotary NiTi and hand instruments in cleaning root canals. Most studies have confirmed that NiTi rotary systems are faster than hand instruments,  eliminate problems during the preparation of curved root canals ,,, and result in better conservation of the tooth structure.  With regard to the cleaning ability in permanent root canals, rotary NiTi instruments were better than hand files  or yielded the same results.  Although a few studies have assessed rotary endodontic systems in deciduous teeth, successful pulpectomy has usually been reported with these systems. ,,,,,
Among the rotary systems now available, Mtwo files® (VDW, Munich, Germany) have yielded similar results in comparison to other rotary systems in cleaning and preparing permanent root canals. , Additional advantages of Mtwo instruments over other rotary systems are their better cutting efficacy and their ability to preserve working length and canal curvature and to simplify root canal instrumentation in a shorter time. ,, Because little is known about the cleaning efficacy of the rotary Mtwo system in permanent and primary teeth, the purpose of the present study was to compare the cleaning ability and instrumentation time of rotary (Mtwo) and manual (K-file) instruments in preparing primary and permanent molars.
| Materials and Methods|| |
A total of 140 extracted human mandibular molars with intact and complete roots and without signs of root resorption (especially in primary teeth) were selected for this study. Seventy primary second mandibular molars with 140 root canals and 70 permanent first mandibular molars with 140 root canals were included. Intact distal and mesiobuccal teeth root canals were chosen for canal preparations. At first, all the teeth were immersed in 0.5% sodium hypochlorite for 15 minutes for disinfection and were then stored in distilled water at 37°C. For root canal preparation, we made an access cavity with diamond fissures burs (F868/016, Tees Kavan Ltd., Tehran, Iran) under normal conditions. Standardized radiographs were taken before instrumentation with a size 15 root canal instrument inserted into the canals to check patency and root canal conditions. Teeth without internal or external root resorption and canal calcification were selected for this study. Teeth with sclerotic canals or with an altered apex, severe curvature and root fracture were excluded.
All the teeth were then rinsed and India ink was injected with a 30-gauge insulin syringe into the root canals of 60 primary and 60 permanent teeth. A #15 file was inserted into the root canal to assure penetration of the dye through the canals (from orifice to apex). Then, the teeth were stored in wet conditions and left at room temperature for 48 hours. Ten primary teeth and 10 permanent teeth were used as negative controls in which no dye was injected. The groups of primary and permanent teeth were randomly divided into two main subgroups containing 20 teeth each. One subgroup was instrumented with stainless steel K-files and the other with Mtwo NiTi files (Mtwo). Distal and mesiobuccal roots of each mandibular molar were chosen for endodontic procedures.
In addition, three subgroups containing 10 teeth each were considered as control groups. Root canals of two of these subgroups were filled with ink. In one subgroup, root canals were irrigated with normal saline but not instrumented, and in the other control subgroup (positive group), no irrigation or instrumentation was performed. In a negative control group, no ink was injected and no instrumentation was performed.
All the teeth were prepared by the same operator. The working length was recorded as the length of the initial file at the apical foramen minus 1 mm. Mechanical hand filing with K-files was performed with the step-back technique for apical preparation up to file size 35 and flare up to size 80 for permanent teeth. Primary teeth root canals were prepared manually with K-files up to file 25-30. To increase the similarity between specimens in the main subgroups, primary teeth with root canals of 12 mm length and permanent teeth with root canals of 15 mm length were selected for this study.
Root canal preparation was done with 21-mm-long Mtwo NiTi rotary files driven with a torque-limited rotation Mtwo direct handpiece (Sirona, Bensheim, Germany) with torque control at a maximum speed of 280 rpm. All the files were used to the full length of the root canal, as for the single-length technique. Four Mtwo instruments (10/0.04, 15/0.05, 20/0.06 and 25/0.06) were used in primary teeth. In addition, Mtwo 30/0.05 and 35/0.04 files were used for permanent teeth. Each instrument was used five times and then discarded. During instrumentation, the root canals were rinsed with 5 ml normal saline using disposable syringes. Then, each canal was dried with sterile paper points. The pulp chamber was filled with temporary cement and teeth were then stored in wet conditions.
The time for each canal preparation, including total active instrumentation, instrument changes within the sequence and irrigation, were recorded with a chronometer.
To clear the teeth, we placed the specimens separately in 7% hydrochloric acid for 2 days, and changed the acid solution each day. After decalcification, the teeth were washed under running water. The dehydration process consisted of immersion in a series of ethyl alcohols (Ararat, Tehran, Iran): 70% alcohol for 16 hours (changed after 8 hours) followed by 80% alcohol for 8 hours, 95% alcohol for 8 hours, and 100% alcohol for 8 hours. The dehydrated teeth were cleared in methyl salicylate (Merck, Darmstad, Germany) for 6 hours. Under blind conditions, two observers examined the cleared teeth under a stereomicroscope (Zeiss, Jena, Germany) at ×10 magnification and scored the amount of India ink remaining in the coronal, middle and apical thirds of the canal on a scale of 0-3 as follows:
0 = total clearing, i.e., the canal was completely clean;
1 = almost complete ink removal, with dots of ink in some areas;
2 = partial ink removal, with small patches in some areas (less than 0.5 mm in diameter);
3 = partial ink removal, with larger patches in some areas (between 0.5 and 1.0 mm in diameter).
Statistical analyses were done with nonparametric Kruskal-Wallis, Mann Whitney and t tests.
| Results|| |
In the first of the three control subgroups, the root canals were filled with dye and irrigated with normal saline without instrumentation. We concluded that the dye adhered to the root canals and irrigation alone could not remove it. Positive control group showed that ink can penetrate the whole of the root canals and confirmed that the dye injection was a suitable and an efficient method. Negative control subgroup demonstrated there was no staining inside the canals (before dye injection).
Comparison between two main groups with the first control subgroup (irrigation with normal saline) showed statistical differences in apical, middle and coronal thirds of root canals (P >0.05). [Table 1] and [Table 2] show the frequency distributions of different scores in the coronal, middle and apical thirds of root canals in primary and permanent teeth after K-file and Mtwo rotary file cleaning, respectively. Statistical tests indicated there were no significant differences in the cleaning capacity between the two instrumentation techniques in primary (P >0.05) or permanent teeth (P >0.05).
|Table 1: Comparison of cleaning efficacy with K-files in the coronal, middle and apical third of root canals of primary and permanent teeth|
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|Table 2: Comparison of cleaning efficacy with Mtwo files in the coronal, middle and apical third of root canals of primary and permanent teeth|
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Comparison between primary and permanent teeth showed no significant differences in any of the three root canal regions by using K-files or Mtwo systems (P >0.05).
Mean preparation times for each group of teeth are shown in [Table 3] for both the techniques. In all the groups, shorter times were recorded with the Mtwo technique. Working times were significantly shorter in primary than in permanent teeth (P <0.05). During canal preparation, only one Mtwo instrument and two K-files broke, all in permanent teeth. There was no significant difference between the types of instrument in breakage rate.
|Table 3: Mean preparation time (second) and standard deviation (sd) using K-files and the Mtwo rotary system in primary and permanent teeth|
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| Discussion|| |
The ink penetration and clearing technique is useful for studying the cleaning ability of the instrumentation and the morphology of human teeth because it makes the teeth transparent so that the pulp cavity and root canal walls can be diagnosed. ,
Some studies have reported the advantages of manual instrumentation over rotary files regarding root canal wall preparation; , however, others have reported better results with rotary NiTi systems.  Factors that influence the results may be the kind of rotary instrument and number of instruments used, the working method, irrigation solutions, methods used to assess the cleaned canals and operator experience. ,, In the present study, we found no significant differences in the degree of cleaning capacity between manual and rotary techniques in permanent molar teeth (P >0.05). In agreement with our findings, Schafer and Zapke concluded that manual files and rotary instruments yielded similar degrees of cleanliness.  A study by Sipert et al. revealed no significant differences between preparation in the apical and coronal portions of permanent root canals with hand files or rotary (Race) instruments. However, they reported a significant difference while using hand files in the middle part of the canals.  In addition, several studies have reported the benefits of using NiTi rotary instruments in reducing the risk of problems during root canal preparation. ,,, Kleier and Averbach recommended using NiTi rotary systems to achieve better clinical outcomes with no expert operators. 
In primary teeth, we found no significant differences in the cleaning capacity between the two instrumentation techniques. The benefits of rotary systems in primary teeth have been reported by others. ,, In a clinical trial, Kuo et al. concluded that Protaper NiTi rotary files were safe and efficient for pulpectomy in primary teeth,  in agreement with the findings of Bar et al.  However, Kummer et al. observed that the Hero 642 rotary instruments removed less dentin and led to more uniform root canal preparation in primary teeth than that done by hand files.  Soares et al. found no significant differences between rotary instrumentation with Profile 0.04 and Er, Cr, YSGG laser therapy on the cleanliness of the root canal walls of primary teeth.  Primary teeth have curved roots and narrow canals. Achieving acceptable result by using Mtwo files indicated that this system has cutting efficiency like manual files and could preserve curve of root canals.
In primary and permanent teeth, we found no significant differences in the cleaning ability of Mtwo rotary files between the apical, middle and coronal thirds of the root canals. Moreover, manual K-files yielded the same results in the thirds of the canals in primary and permanent teeth (P <0.05). However, there are differences in anatomy and canal form between primary and permanent teeth this result showed that flexible Mtwo rotary files could preserve and clean the permanent and primary teeth root canals efficiently. Although factors such as operator experience or the forces applied could affect the results of study. Nagarantna et al. obtained similar results in primary and permanent teeth while using Profile rotary files. In addition, they concluded that instrument failures in primary teeth with rotary files were less frequent than in permanent teeth, and recommended using rotary systems for pulpectomy. 
The efficacy of Mtwo rotary systems has been compared to that of other rotary files. Foschi et al. reported that both the Mtwo and Protaper rotary systems produced a clean canal in the coronal and middle thirds, but were unable to produce dentine surfaces free from smear layer and debris in the apical third.  Schafer and Oitzinger found that Mtwo and Race instruments had a greater cutting efficiency than Profile, Flexmaster and Alfa-file rotary NiTi instruments.  Other studies evaluated the preparation of curved canals by comparing Mtwo files against other NiTi rotary devices. The results confirmed that Mtwo NiTi files preserved canal curvatures better than K3, Race , and Protaper instruments.  Other researchers reported fewer preparation errors with Mtwo files, a finding that may be related to the specific design of these instruments. 
The new Mtwo files have an S-shaped cross section and a noncutting tip. Thus, their positive rake angle with two noncutting edges is responsible for effective dentine cutting  and achieving symmetrical root canals.  During the present study, only two Mtwo instruments fractured, an observation consistent with a previous report.  The low fracture rate may be due to the increasing pitch length from tip to shaft, which reduces the tendency of the files to screw-in, thus minimizing the risk of instrument fracture.  In addition, the electric torque-limited handpiece used with Mtwo Direct helps reduce file fractures. However, according to Burkelin and Schäfe, both the Endo IT motor and Mtwo Direct handpiece control motors are equally safe and suitable for preparing curved canals.  Because of the particular design features of Mtwo instruments, file deformation may be significantly more frequent than with other instruments. Thus, the working parts of Mtwo files should be carefully examined after every use.  In the present study, we discarded all files that were found to be permanently deformed after five uses.
During canal preparation, the time needed with Mtwo rotary files was shorter than with K-files in permanent teeth, which was in agreement with the results obtained by Guelzow et al.  According to Schafer et al.  , and Vahid et al.  , Mtwo instruments were significantly faster than other rotary NiTi files. , The greater cutting efficiency of Mtwo compared to other rotary NiTi systems,  together with the use of a single technique which needs fewer instruments, probably contributes to the difference in preparation time.  In the present study, we recorded shorter times with rotary files in comparison to manual K-files in primary teeth, a finding consistent with many with previous studies. ,,,,, However, preparation time in primary teeth with Mtwo or K-files was shorter than in permanent teeth, possibly due to the shorter root length in primary teeth, lower cutting resistance of deciduous dentin  and the need to use fewer instruments. This result is in agreement with several other studies. 
| Conclusion|| |
We found that Mtwo files were as effective as manual K-files for preparing root canals of primary and permanent teeth. Working time was significantly shorter with the rotary instruments. This new system simplified root canal preparation in both permanent and primary teeth and reduced the operating time-both of which are especially important in pediatric dentistry. The Mtwo rotary instruments are therefore suitable for root canal therapy in children as well as adults.
| Acknowledgments|| |
The authors thank the vice-chancellery of Shiraz University of Medical Science, for supporting the research (Grant # 87-4003). This manuscript relevant thesis of Dr Mokhtare and Nikaein. Also the authors thank Dr M Vossoughi from the Center for Development of Clinical Research, Namazee Hospital, for the statistical analysis, and K Shashok (AuthorAID in the Eastern Mediterranean) for help with the English in the manuscript.
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Department of Endodontics, School of Dentistry, Shiraz University of Medical Sciences, Shiraz
[Table 1], [Table 2], [Table 3]