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 Table of Contents  
ORIGINAL ARTICLE
Year : 2017  |  Volume : 29  |  Issue : 1  |  Page : 53-59

To evaluate and compare canal transportation, canal-centering ability, and vertical root fracture resistance of teeth prepared with three different rotary file systems: An in vitro study


Department of Conservative Dentistry and Endodontics, SGT University, Gurgaon, Haryana, India

Date of Web Publication25-May-2017

Correspondence Address:
Gargi Mitra
Sector B, Pocket 5 and 6, 4567, Vasant Kunj, New Delhi - 110 070
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/endo.endo_33_17

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  Abstract 

Aim: The aim of the study was to determine the file system with best canal-centering ability (CA) and least canal transportation (CT) and which provides maximum resistance to vertical root fracture (VRF).
Materials and Methods: Forty mesiobuccal roots of extracted maxillary molars were randomly divided into three experimental groups and one control group, with ten samples in each group. Group 1 samples were prepared with ProTaper file system, Group 2 samples were prepared with WaveOne single-file system, Group 3 samples were prepared with self-adjusting file (SAF) system, and Group 4 had no instrumentation. Pre- and post-instrumentation cone-beam computed tomography scans were done. CT and CA were evaluated at 3 mm, 6 mm, and 9 mm. Samples were subjected to load under Instron universal testing machine to determine VRF resistance. Data were analyzed using statistical analysis.
Results: There was no statistical significance for CT, CA, and VRF resistance at all the three levels among the three groups; however, SAF system showed least CT and was more centered.
Conclusion: SAFs produce less CT and more centered preparation compared to ProTaper file system and WaveOne file system.

Keywords: Centering ability; cone-beam computed tomography; ProTaper; self-adjusting files; transportation; WaveOne.


How to cite this article:
Mitra G, Sharma V, Sachdeva J, Singla M, Taneja K, Bhatnagar A. To evaluate and compare canal transportation, canal-centering ability, and vertical root fracture resistance of teeth prepared with three different rotary file systems: An in vitro study. Endodontology 2017;29:53-9

How to cite this URL:
Mitra G, Sharma V, Sachdeva J, Singla M, Taneja K, Bhatnagar A. To evaluate and compare canal transportation, canal-centering ability, and vertical root fracture resistance of teeth prepared with three different rotary file systems: An in vitro study. Endodontology [serial online] 2017 [cited 2019 Sep 21];29:53-9. Available from: http://www.endodontologyonweb.org/text.asp?2017/29/1/53/207003




  Introduction Top


Successful root canal treatment depends on the chemomechanical methods used for shaping of the root canal system into a continuously tapering preparation and removal of both organic and inorganic substrates from the canal.

Maintaining the original canal anatomy by uniformly preparing all the surfaces of the canal is one of the most important principles of shaping the root canal system to prevent procedural errors such as zipping, ledging, perforations, and apical transportation that can occur along with the loss of working length.[1] However, prevention of these undesirable consequences is a great challenge, especially in severely curved canals.

The introduction of greater taper nickel titanium (NiTi) files has revolutionized root canal preparation by virtue of their superelasticity which enable canals to be prepared in a relatively safe manner with a reduced incidence of canal aberrations than with stainless steel instruments.[2]

The ProTaper rotary system (Dentsply Maillefer, Ballaigues, Switzerland) was introduced in 2001, and it works in the continuous rotating motion for preparation of canals. It consists of three Shaping files – Sx, S1, and S2 – and three finishing files – F1, F2, and F3.

WaveOne NiTi file system (Dentsply Maillefer) was introduced in early 2011. It works in a reverse balanced force action.[3] It consists of three single-use files: small (ISO 21 with taper of 6%) for fine canals, primary (ISO 25 with taper of 8%) for the majority of canals, and large (ISO 40 with 8% taper) for large canals.

Despite the popularity of NiTi rotary system, most of these techniques often appear to leave substantial amount of uninstrumented dentin areas. To overcome this disadvantage, a newer system called the self-adjusting file (SAF) system was introduced in the market. SAF system (SAF; ReDent-Nova, Ra'anana, Israel) was introduced in 2010. It consists of a hollow metal file composed of lattice threads that are lightly abrasive and allow for dentin removal with a back and forth grinding motion.[4] It is available in three standard lengths (21 mm, 25 mm, and 31 mm) and two widths (1.5 mm and 2 mm).

Recently, the use of cone-beam computed tomography (CBCT) has been suggested for assessing the canal walls as it is a nondestructive technique which provides three-dimensional (3D) reproduction of the tooth and allows better pre- and post-operative evaluation as the images are provided in orthogonal planes as well as in oblique planes for better assessment of root curvatures and canal transportation (CT).[1]

It has been reported that incidence of vertical root fracture (VRF) is greater in root canal-treated teeth than in vital teeth due the endodontic rotary file systems, which decreases the tooth resistance due to the excessive loss of dentin tissue and decrease in moisture content which might jeopardize the mechanical integrity of the remaining tooth structure.[5],[6],[7]

Hence, the present in vitro study was conducted to compare the canal-centering ability (CA) and CT of three different rotary file systems using CBCT and evaluate the VRF resistance of teeth prepared with these files using Instron universal testing machine.


  Materials and Methods Top


Forty mesiobuccal roots of freshly extracted human maxillary first molars were collected for the study from the outpatient Department of Oral and Maxillofacial Surgery. All the extracted teeth were washed free of blood and mucous in running tap water. Tissue fragments and calcified debris were removed by ultrasonic scalers. The teeth were then stored in 0.1% thymol. The teeth with fully formed apices were included. The teeth with open apices, calcified canals, morphological defects, and fractured teeth were excluded from the study.

Standardization of specimen

All the samples were decoronated at the level of cementoenamel junction with the length of roots adjusted to 14 mm for standardization using vernier calipers. The teeth were randomly divided into three experimental groups and one control group, with ten samples in each group. The control group was used to determine the VRF resistance.

The patency of the root canal was maintained with size #10 K file. A size 15-K file was placed in the canal until; it was visible at the apical foramen, and the working length was established 0.5 mm short of this length. The apices of the roots to be evaluated were then sealed with wax.

Preinstrumentation evaluation

The specimens were then embedded in autopolymerizing acrylic resin. Each sample was inserted in such a manner that the long axis of each specimen was parallel to the long axis of the mold to ensure standardization of the specimens for tomography images pre- and post-root canal preparation. The preinstrumentation images of the specimens were recorded using CBCT in 3D views. Three sections from each tooth were scanned at 3 mm, 6 mm, and 9 mm from the apex to evaluate the apical third, middle third, and coronal third, respectively, of each tooth [Figure 1], [Figure 2], [Figure 3].
Figure 1: Mean canal-centering ability at all three levels of Group 1 (ProTaper), Group 2 (WaveOne), and Group 3 (self-adjusting file)

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Figure 2: Mean canal transportation at all three levels of Group 1 (ProTaper), Group 2 (WaveOne), and Group 3 (self-adjusting file)

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Figure 3: Mean vertical root fracture resistance - force (Newton)

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The biomechanical preparation was done by crown-down technique. Each specimen was prepared up to #20 K hand file after the initial scans.

Preparation of samples

Group 1 samples were prepared using ProTaper file system (Dentsply Maillefer, Ballaigues, Switzerland) up to F2 (25/0.8) according to the manufacturer's instructions. SX file was used to prepare the cervical two-third of each root canal. Subsequently, the S1, S2, F1, and F2 files were employed up to the working length.

Group 2 samples were prepared using WaveOne rotary instruments according to the manufacturer's instructions. Canals were prepared using primary files up to ISO 25 with 8% taper in up and down motion.

Canals of both Group 1 and Group 2 were irrigated with 2 ml of 4% sodium hypochlorite, followed by 4 ml of 17% EDTA and final irrigation by normal saline.

In Group 3, the SAF was operated in each canal for 4 min with continuous irrigation. An in and out manual motion was continuously performed, and irrigation with sterile saline was done through the hollow file by VATEA peristaltic irrigation pump, throughout the 4 min operation and at a rate of 5 ml/min.

In Group 4, no canal preparation was done (positive control group).

Evaluation

Postinstrumentation, all the specimens of all the groups were scanned under the same conditions as the initial scans. Pre- and post-instrumentation images were then superimposed to determine CA and CT [Figure 1], [Figure 2], [Figure 3].

CA is determined by the formula, ([a1 − a2] − [b1 − b2]).

Where a1 and a2 are the shortest distance from the mesial edge of the root to the mesial edge of the uninstrumented and instrumented canal, respectively, and b1 and b2 are the distance from distal edge of the root to the distal edge of the uninstrumented canal and instrumented canal, respectively. The result of “0” indicates no CT and other than “0” means that transportation has occurred.[6]

CT is determined by the formula, (a1 − a2)/(b1 − b2) or (b1 − b2)/(a1 − a2). If the numbers are not equal, the lower figure was considered as the numerator and a result of “1” indicates perfect centering.[6]

Evaluation of vertical root fracture resistance

The mounted specimens were placed on a metal base under the Instron universal testing machine for testing the fracture resistance. A spherical tip of radius 2 mm was used to apply a vertical force at a crosshead speed of 1 mm/min, and this force was applied perpendicular to the long axis of the tooth at the canal orifice. The force was delivered until the root fractured and was measured in Newton.


  Results Top


The data were analyzed using the one-way ANOVA test and post hoc Bonferroni test. P < 0.05 was considered statistically significant and confidence interval of 95% was taken [Table 1],[Table 2],[Table 3].
Table 1: Group 1 - ProTaper

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Table 2: Group 2 - WaveOne

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Table 3: Group 3 - self-adjusting file

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Canal-centering ability

There was no significant difference seen in the mean CA at 3 mm, 6 mm, and 9 mm. The intergroup comparison of mean CA was done using the post hoc Bonferroni test. There was no significant difference in mean CA at all the three levels.

At all the three levels, Group 3 showed maximum CA with least by Group 1 at 3 mm and 6 mm. At coronal third, Group 1 and Group 2 showed the same amount of centering ability [Table 4] and [Figure 1].
Table 4: Comparison of mean values of canal centering ability

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Canal transportation

There was no significant difference in mean CT at 3 mm, 6 mm, and 9 mm. The intergroup comparison of mean CT was done using the post hoc Bonferroni test. There was no significant difference in the mean CT at all the levels.

The least CT was seen with Group 3, at all the three levels. Group 1 showed the maximum CT at all the three levels [Table 5] and [Figure 2].
Table 5: Comparison of mean values of canal transportation

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Vertical root fracture resistance

The maximum force required for VRF was observed in Group 4 (control group) where no preparation was done was 1794 N. The least force required was for Group 3 with 1438 N. The maximum force required for VRF in Group 2 (WaveOne) was 1818 N and Group 3 (SAF) was 2052 N [Table 6] and [Figure 3].
Table 6: Comparison of mean values of vertical root fracture resistance

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No significant difference was found in mean force required for VRF between all the four groups.


  Discussion Top


Chemomechanical preparation is one of the most important stages in root canal treatment protocol. This step of canal shaping in the endodontic treatment holds significance as it influences the subsequent phases of canal irrigation, obturation, and hence the outcome of the treatment itself.[6],[8],[9]

According to Schilder, root canal preparation should present a flare shape from apical to coronal, preserving the apical foramen and not alter the original canal curvature.[6],[10] However, the achievement of an “ideal” preparation is a cumbersome task as the curvature of the canal is considered to be the preeminent risk factor for procedural errors such as ledging, zipping, perforation, and transportation.[6]

The advantage of NiTi instruments over the stainless steel files in root canal preparation is well documented; however, Peters [9 have reported that rotary NiTi files in the curved canals failed to adequately and reproducibly prepare all the inner surfaces. Apical CT is one of the major inherent problems with rotary-NiTi files that have two major drawbacks: first, the apical part of the canal on the inner side of the curvature remains untouched and full of debris, and second, it may lead to ledging or even a subsequent perforation.[11],[12]

The recently introduced WaveOne system is a single-file system that works on the principle of reversed balanced force and in a linear motion. It is manufactured using the M-wire technology, and has shown to improve the fatigue lifespan, flexibility, and torsional resistance, and improved the beneficial shaping result by reducing the screw-in effect.[13],[14]

Another new system, the SAF, is an innovative instrument that consists of a hollow metal file composed of lattice threads that are lightly abrasive and allow for dentin removal with a back-and-forth grinding motion. The SAF is designed as a compressible file with the ability to adapt itself to the root canal cross section.[13]

For assessment of canal instrumentation, several methods have been advocated, such as scanning electron microscope, radiographic evaluation, photographic assessment, and cross sections using the technique of Bramante et al.[15] However, these methods are invasive in nature and labor intensive.[12] Recently, CBCT (a noninvasive technology) has been advocated which utilizes a cone-shaped X-ray beam and an area detector that captures a cylindrical volume of data in one acquisition. CBCT can render cross-sectional slices and produce 3D images that are highly accurate, have high resolution, have lower radiation dose, are fully quantifiable, and provide repeatable results.[16],[17]

The two different concepts presently employed for functioning of NiTi file systems are continuous rotation and reciprocation or discontinuous rotation.[6]

The reciprocating working motion consists of an unequal counterclockwise and clockwise motion. The greater angle of the counterclockwise rotation ensures apical advancement of the file while the clockwise motion disengages the file. The problem of taper lock is reduced by continually reversing the direction of rotation and, hence, leading to minimal torsional and flexural stresses on the instrument.[6] Hence, reciprocating motion is expected to produce a beneficial shaping result by reducing the screw-in effect.[18]

Moghadam et al.[19] emphasize that apical transportations that are >0.3 mm can jeopardize the outcome of treatment which decreases the sealing ability of root filling material and thus are an important factor to be considered. The amount the instrument remains centered in the root canals is also an important factor for the evaluation of possible deviations arising from the instrumentation.[20]

Group 3 (SAF) showed the least CT at all the levels. This could be due to the properties of flexibility and pliability of the SAF file which does not impose its shape on the canal but rather conforms to the original shape of the canal. SAF is used to enlarge the curved canal that tends to keep the apical part of canals closer to its original shape.[12]

ProTaper showed maximum value for root CT at all the three levels which may attribute to progressive taper along the cutting surface and reduction in instrument flexibility. Guelzow et al.[21] found ProTaper to show the highest degree of straightening. At the middle and coronal levels, transportation was seen which can be mainly attributed to progressive taper along the cutting surface in combination with the sharp cutting edges.[15]

Group 2 (WaveOne) showed more CT than Group 3 (SAF) at all the three levels. Tambe et al.[17] conducted a study in which WaveOne system showed less CT and better centering ability as compared to other systems tested. The preparation of manual glide path with K file could have had an impact on the CT. In an earlier study comparing the canal shaping efficacy of WaveOne and Reciproc, it was concluded that establishment of a glide path larger than #15 K file before using WaveOne file exhibited better maintenance of canal anatomy, which was in conformation with the study conducted by Berutti et al.[16],[17],[22]

The least CT at all the levels was observed in Group 3 (SAF). Peters et al.[23] and Paque [24] have shown that the SAF system is capable of touching all of the walls of the root canal system, thereby keeping the apical part of curved canals closer to its original anatomy. A study conducted by Burroughs et al.[25] also showed that SAF had the least mean CT as compared to Profile Vortex and Typhoon Files.[26] The slightly more CT at 9 mm could be due to the use of ProTaper Sx used for coronal flaring. Overall, CT is likely a cumulated effect of coronal flaring, glide path preparation, and the action of the SAF.[15]

VRF is one of the challenging complications that occur during the instrumentation of teeth. The diameter of the prepared canal is one of the potential factors that could affect tendency of VRF. Excessive removal of dentin due to excessive taper can lead to weakening of the root.[27]

It was seen that the use of rotary NiTi files may result in nonuniform thickness of the remaining dentin wall, which may be a predisposing factor for reduction in VRF resistance.[12] However, a significant difference was found in mean force required for VRF resistance for all the four groups.

Group 3 (SAF) showed the highest resistance to VRF among the instrumented groups. Metzger et al.[12] showed that SAF removes a uniform layer of dentin from the canal walls, thus resulting in a relatively uniform remaining dentin thickness of wall. Several studies have demonstrated less tendency of SAF to cause less dentinal cracks when compared to other rotary NiTi files. These craze lines may spread to the root canal surface when the forces are applied leading to vertical fracture.[27] In a study performed by Yoldas et al.,[28] no cracks were found in teeth prepared with SAF, this was attributed to its vibrating in and out motion that may result in less dentin damage.[27]

Group 1 (ProTaper) showed the least resistance to VRF. The rotational forces applied during instrumentation can create microcracks or craze lines in root dentin.[29] Wilcox et al.[30] and Hegde et al.[31] conducted studies which concluded that more the root dentin was removed, the more likely a root was to fracture.[31] Furthermore, larger the taper, the more dentin is removed which has a greater risk of initiating root fracture.[17],[26] The study conducted by Uyanik et al.[32] showed that the amount of dentin removed by ProTaper was significantly greater than HeroShaper.[33]

The decrease in root fracture resistance in Group 2 (WaveOne) was also observed. Kim et al.[34] suggested that file design affected apical stress and strain concentrations during instrumentation, which were linked to an increase in dentinal defects and canal deviations.[29] Triangular or modified triangular cross section results in low cutting efficiency, reciprocal motion seems to enhance the torsional force by extruding the debris toward the apex, hence crack formation at the apical level resulting in less fracture resistance.[35]


  Conclusion Top


The results of the present study indicate that all the tested rotary instruments produced some amount of CT. However, the innovative technology of SAFs renders it to be more centered and produce less CT. This file also shows the least amount of removal of dentin, making the samples prepared with it more resistant to vertical fracture. Although the results were not statistically significant, further research will facilitate extrapolation of the findings of the present study to clinical use.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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[PUBMED]  [Full text]  


    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]



 

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