Year : 2010 | Volume
: 21 | Issue : 1 | Page : 107--111
The influence of torque and manual glide path on the defect or separation rate of NiTi rotary instruments in root canal therapy
MH Zarrabi1, M Javidi1, M Vatanpour2, H Esmaeili3,
1 Department of Endodontics, Member of Dental Research Center, Dental School, Tehran, Iran
2 Department of Endodontics, Islamic Azad University, Dental School, Tehran, Iran
3 Department of Community Medicine, Mashhad University of Medical Sciences, Mashhad, Iran
Department of Endodontics, Member of Dental Research Center, Dental School, Tehran
Introduction: One of the effecting factors in prognosis of root canal therapy is accidental procedure as broken files that may be unpreventable. Many manufacturers have designed and marketed various electromotors that can control rotational speed and torque. On the other hand, some studies have recommended applying a manual glide path to diminish contact area between the file and canal walls. The purpose of this study was evaluation of the effect of torque and a manual glide path on defects as separation of Nickel-titanium (NiTi) rotary files.
Materials and Methods: This ex vivo randomized controlled trial study was carried out on 160 canals of human«SQ»s matured molars with mild curvature (15-338). After initial preparation of samples and checking for inclusion criteria, in first group, preparation was carried out with air-driven handpiece, and in group two, Endo IT was used as electromotor. In both groups, Mtwo files with simultaneous technique were used for preparation. Then all data were collected and analyzed with Mann Whitny, Mantel Cox, and t-test.
Results: No significant differences between two groups (P < 0.05) were observed. Based on survival analysis, safety probability of files after preparation of nine canals is 64% in group one and 69.9% in group two. There was no significant differences between this safety probability in two groups (P = 0.272).
Conclusion: Usage of torque control handpiece is not an important factor, comparing instrumentation technique.
|How to cite this article:|
Zarrabi M H, Javidi M, Vatanpour M, Esmaeili H. The influence of torque and manual glide path on the defect or separation rate of NiTi rotary instruments in root canal therapy.Indian J Dent Res 2010;21:107-111
|How to cite this URL:|
Zarrabi M H, Javidi M, Vatanpour M, Esmaeili H. The influence of torque and manual glide path on the defect or separation rate of NiTi rotary instruments in root canal therapy. Indian J Dent Res [serial online] 2010 [cited 2021 Mar 1 ];21:107-111
Available from: https://www.ijdr.in/text.asp?2010/21/1/107/62815
Root canal treatment is fundamentally dependent on root canal preparation. This phase of the treatment consists of cleaning and shaping the canals. It is a challenge particularly when shaping curved canals. Stainless steel endodontic instruments, whose characteristics include hardness that increases with size, may set limitations to successful shaping during enlargement of the apical third of a canal. 
Nickel-titanium (NiTi) alloy has developed in manufacturing of endodontic instruments because of its super-elasticity and corrosion resistance. Super-elasticity allows instruments to remain centralized inside the root canal,  which results in satisfactory root canal preparation  and leads to a better prognosis. , This capacity of reversible transformation is called shaped memory, in practical terms translates in to the capacity of the instrument to return to its original form as soon as the force that causes distortion is stopped.  Frequent transformations of these two phases weaken the instrument and reduce its resistance to fracture. 
When the NiTi instrument is rotating around a curvature for a prolonged period of time, it is subjected to repeated tensile and compressive stresses. These stresses result in multiple transformation phases that lead to the production of microvoids and metal fatigue.  This type of metal fatigue leads to an unexpected breakage called flexural (cyclic) fracture, and consequently is not accompanied by deformation. 
Another type of fracture of NiTi endodontic instruments is torsional, which is a result of its engagement inside the root canal.  Due to frictional forces between instrument and canal walls, it is locked into the canal. If the running power is continued, the instrument reaches the ultimate tensile strength point and fracture occurs. Based on different studies, tensional fatigue is the major cause of most defects or fractures of NiTi rotary files. ,, One way to decrease this engagement is reduction of frictional force by a diminished contact area between file and canal walls. Some studies have recommended applying a manual glide path. ,,
Also, usage of torque controlled motors that works under electricity power has been introduced by many manufactures. Although some studies demonstrated that there is no significant difference between various speeds and torques in instrument deformation or fracture, ,, manufactures emphasize on the use of these motors for working precautions. On the other hand, some manufactures have introduced air-driven handpieces for working with different types of rotary systems. Air motors that works under air power do not allow torque control and variation in air pressure could affect the rotational speed and consequently the torque.
As we know file deformity is the first step in the occurrence of file breakage. However, some studies have indicated that breakage may occur with no visible deformity in the working part of the file.
The purpose of this study was to evaluate the influence of torque and a manual glide path on the defect or separation rate of NiTi rotary instruments in root canal therapy.
Materials and Methods
In this ex vivo study, 160 canals of extracted human maxillary and mandibular molar teeth were used in four groups. Inclusion criteria were:
Mature and closed apexModerate canal curvature (between 15-358)Single canal and without major derivation (type I or III)Without obvious defects such as internal resorptionSize of apical foramen
Disinfection of the samples was carried out with 5.25% NaOCl for 1 hour and then, they were preserved in normal saline.
First, an access cavity was prepared using fissure bur #2 and orifices of canals were defined by an endodontic explorer. In all steps, a SS K-type file #10 was used as a patency file.
In this step, the size of apical foramen was checked and working length of all canals was defined as 16 mm by visual manner. All teeth were exposed to determine angle and radius of curvature method of Pruett et al.  All selected canals had moderate curvature (15-358).
Then these samples were allocated randomly into four groups: group 1: Air-driven handpiece, (Anthogyr, France) with applying a manual glide path; group 2: Air-driven handpiece without a manual glide path; group 3: Endo IT electromotor (VDW, Germany) with applying a manual glide path, group 4: Electromotor without a manual glide path.
In all groups, irrigation was used after each file (1 ml 5.25% NaOCl)
RC prep (Premier Dental product; Morristown, PA, USA) was used as a lubricant.Pecking range of the rotary files were 2-3 mmWorking time of each file was five seconds.Assessment of all rotaries was carried out by a magnifier (x8), before and after each use.
In groups 1 and 3, a manual glide path was applied before using rotary files with SS K-type #810 , , and 15 (step-back technique) and mild contact of file #20 with canal walls. Rotary files in all groups were Mtwo and the sequence of use for the simultaneous technique was as the manufacturer instruction: 25/0.07, 25/0.06, 20/0.06, 30/0.05, 40/0.04, and 35/0.04. The rotational speed of files in group 1 and 2 were almost 350 rpm, and in group 3 and 4 were exactly 350 rpm and torque control between 0.8-1.5 gr/cm 2
Finally, all collected data were analyzed with one- and two-way ANOVA, and nonparametric tests such as Kruskal-Wallis and Mann Whitney. Survival analysis was carried out with Kaplan-Meier and Mantel-Cox tests. Statistic analyses were carried out with SPSS, version 13.
In the assessment of rotary file fracture, survival analysis can determine the probability of file fracture based on the effective working time of each file. Mantle- cox test can compare the relative probability of comparison groups, and detect significant difference between them.
Overall, 41 instruments had defects or were fractured. Of these, 78.04% had a structural defect and 21.96% were broken. Mean preparation-working time was the longest in group 1, followed by groups 3, 2 and 4 [Figure 1].
An independent sample t-test showed that without paying attention to the effect of applying a manual glide path, all variables had no significant difference in using an air-driven handpiece or electromotor [Table 1]; but these variables had significant differences with and without applying a manual glide path [Table 2]. The Mann-Whitney test showed no differences between mean rank of fracture files in the two groups with and without a manual glide path (P = 0.07).
Also, two-way ANOVA was carried out to determine the interaction between system types (motor or air-driven handpieces) and manual glide path. Results showed that two factors (system and manual glide path) had no interaction effect on the number of defected or fractured files (P = 0.87) and only applying a manual glide path had an effect (P = 0.004). Descriptive statistics showed that the lowest rate of defect or fracture was in group 1 (mean = 3.17 ± 1.33) followed by groups 3, 2 and 4 [Table 3].
Survival analysis of the files in the 4 groups showed that safety probability of one file in group 1 after 45 seconds of working time (preparation of 9 canals) was 77.78%. This probability after preparation of 17 canals was reduced to 64.81%. Safety probability of files in other groups is showed in [Figure 2]. Mantel-Cox test also showed significant probability differences between these four groups (P = 0.009) [Figure 2].
Safety probabilities among groups, without keeping in mind the effect of system type used (motor or air-driven handpieces) had significant differences [Figure 3] but we found no difference between the use of air-driven handpieces or electromotors [Figure 4].
The purpose of this study was to determine whether there is a significant difference in the use of air-driven handpieces versus electro motors and what the effect of applying a manual glide path is. All selected canals had moderate curvature (15-358) with mean of radius = 8.75 mm. Canal curvatures more than 358 are effective in flexural (cyclic) fatigue, so in these studies selected curvature had to be less than 358. Due to this, our selected canal curvatures were 15-358, as per the Patino et al., Schafer et al. and Li et al. studies. ,,
Since there was group similarity, random allocation of samples was done after their agreement with inclusion criteria. Pecking range of the rotary files used was 3 mm while working rotary files, since this range was the safest. 
Evaluation of defects was performed with a magnifier (x8). Although this magnification is essential, the structure and design of files have a critical effect on the need to use a magnifier. Because of a homogenous structure, Mtwo files show little defects even without a magnifier.
In this study, most files became defected (78.04%) instead of fractured (21.96%). These findings are in agreement with Sattapan et al.  These findings are surprising because it means that a clinician can prevent unwanted breakages by evaluation of a file with or without a magnifier before usage.
Furthermore, our findings show that in all groups, defects are more than fractures. It may be because of other factors such as manufacturing process, or design rather than due to using an electromotor or applying a manual glide path.
Some studies emphasize on the effectiveness of applying a manual glide path in reduction of defect rates. , In agreement with these studies, the current study showed that mean working time of files in the two groups that were prepared primarily by applying a manual glide path (groups 1 and 3) were significantly more than other groups.
Based on other studies, most defects in rotary files happened because of torsional fatigue rather than flexural (cyclic) fatigue. The major cause of torsional fatigue is frictional forces between the file and the canal, in which the file tip becomes wedged in the canal while the upper part of the instrument is still rotating. These forces result in defects or fractures. , Subsequently, reduction in the contact area between the file and canal leads to lower defects because of lower frictional forces.
In the present study there was no significant difference between using the motor or air-driven handpiece.
Bartnick et al.  and Yared and Sleiman  also found no differences between usages of high and low torque control motors and air-driven handpieces. Their results agreed with our findings, as well as the Mesgauez et al. 
Another question to be asked is what is the interaction effect of applying a manual glide path on the motor or air-driven handpiece being used? Our findings showed that these two factors have no effect on each other and this means that even the combination of a manual glide path with electromotor cannot lead to lower defects compared with a manual glide path and air-driven handpiece. This study showed no similar study like the present study in which these two factors have been compared, concurrently.
Another unique result in the present study was survival analysis. Although some authors have recommended single use of rotary files  and others have defined the definite number of use of some rotary systems,  none of them could answer the question of what really is the probability of files remaining intact after a definite time of use. The present study showed this probability with survival curves.
This study showed that applying a manual glide path has more effectiveness than stressing to using a definite motor, so with this protocol, we can reduce the rate of defects or fractures. Also, evaluation of files before using them with or without a magnifier can prevent many unwanted events.
The authors would like to thank the Vice Chancellor for Research of Mashhad University of Medical Sciences for financial support of this study.
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