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 Table of Contents  
ORIGINAL ARTICLE
Year : 2018  |  Volume : 15  |  Issue : 1  |  Page : 19-26

Centering ability and canal transportation of curved root canals after using different nickel–titanium preparation systems


Endodontic Department, Faculty of Dentistry, Tanta University, Tanta, Egypt

Date of Submission28-Aug-2017
Date of Acceptance06-Nov-2017
Date of Web Publication4-Apr-2018

Correspondence Address:
Tokka M Moukhtar
Endodontic Department, Faculty of Dentistry, Tanta University, Algharbeia, Tanta
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/tdj.tdj_45_17

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  Abstract 

Purpose
To evaluate centering ability and canal transportation in curved root canals after using different nickel–titanium (Ni-Ti) preparation systems including two rotary systems [Revo-S and Twisted File (TF)] and hand Ni-Ti-Flex K-files using cone-beam computed tomography.
Materials and methods
Seventy-five extracted human premolars with similar range of canal curvature (25–35°) were used, after crown removal and working length determination, samples were randomly divided into three groups (n = 25) according to the type of instruments used. Group I (Revo-S), group II (TF) and group III (Ni-Ti-Flex). Preinstrumentation and postinstrumentation cone-beam computed tomography images were recorded at several root canal levels (3, 5, 7, 9, and 11 mm) from the apex under the same circumstances, superimposed and then the centering ability and canal transportation were calculated using specific equations.
Results
Revo-S system produced less mean value of canal transportation and more centering ability than TF and Ni-Ti-Flex systems. In addition, the middle canal level (7 mm) recorded the least mean value of transportation with the highest centering ability for all used instruments.
Conclusion
Preparation of curved root canal using rotary systems produced less change in original canal path compared with the hand Ni-Ti files.

Keywords: canal transportation, centering ability, cone-beam computed tomography, Revo-S, Twisted File


How to cite this article:
Moukhtar TM, Darrag AM, Shaheen NA. Centering ability and canal transportation of curved root canals after using different nickel–titanium preparation systems. Tanta Dent J 2018;15:19-26

How to cite this URL:
Moukhtar TM, Darrag AM, Shaheen NA. Centering ability and canal transportation of curved root canals after using different nickel–titanium preparation systems. Tanta Dent J [serial online] 2018 [cited 2018 Dec 12];15:19-26. Available from: http://www.tmj.eg.net/text.asp?2018/15/1/19/229245


  Introduction Top


The ability of the instrument to keep the original path of the canal without deviation through the shaping procedures is influenced by both instrument's design (taper, flexibility, and type of alloy) and the root canal anatomy [1]. Recently, root canal instruments have been manufactured from nickel–titanium (Ni-Ti) alloy that has a lower modulus of elasticity than stainless steel (SS). This property allows Ni-Ti files to be placed in curved root canals with less lateral force exerted against the canal walls [2].

Many types of modified rotary root canal instruments have been introduced and varied in cross-section, blade design, taper or manufacturing technique. Twisted File (TF) is a rotary file with a completely different manufacturing process. It was claimed that the three new manufacturing processes of these files, namely R-phase heat treatment, twisting of the metal, and special surface conditioning, significantly increase the instrument's resistance to cyclic fatigue and flexibility [3]. Its cutting efficiency and flexibility is high compared with other Ni-Ti rotary files. It has 3–4 times greater resistance to torsion and cyclic fatigue relative to ground files. This system is triangular in cross-section with noncutting pilot tip. It is available in five tapers and various tip sizes. These include #25/0.04, 0.06, 0.08, 0.10, 0.12, 30/0.06, 35/0.06, 40/0.04 and 50/0.04 in 23 and 27 mm length. These files are colour-coded for easy identification. The top band shows the taper and the bottom band the ISO tip size [4].

Revo-S is another modified rotary system which facilitates penetration of root canal by a snake-like movement and offers a more uniform root canal shaping [5]. It has inactive noncutting tip and three cutting edges located on three different radiuses: R1, R2 and R3. This system is composed of two instruments for apical penetration which are shaper and cleaner 1 (SC1) has #25/0.06 in 21 mm length and shaper and cleaner 2 (SC2) with #25/0.04 in lengths of 21, 25 or 29 mm. It has a symmetrical cross-section with a 0.04 taper allowing better penetration. In addition, this system includes a recapitulating and cleaning instrument shaper universal which is #25/0.06 in lengths of 21, 25 or 29 mm. Shaper universal smoothes the root canal walls and recapitulates the action of the first two instruments SC1 and SC2 due to its asymmetrical cross-section, thus respecting the tapered shape of the canal [6].

Regarding apical finishing instruments of Revo-S system, they include AS30 (#30/0.06), AS35 (#35/0.06), AS40 (#40/0.06) in lengths of 21, 25 or 29 mm. They are characterized by reduction of the contact lengths of the blade on root canal dentin leading to reduction of stress, better debris elimination and more efficient cleaning owing to its asymmetrical cross-section that minimizes debris packing in the apical region [7].

Numerous methods have been used to evaluate the canal shape before and after instrumentation as serial sectioning technique, superimposition of preinstrumentation and postinstrumentation digital radiographs or images, computed tomography (CT), μ-CT and cone-beam computed tomography (CBCT) [8]. CBCT imaging is a noninvasive technique for analysis of canal geometry and efficiency of shaping techniques by superimposition of preinstrumentation and postinstrumentation images [3],[9],[10].

Although of different modifications applied to the currently available file systems, none of them was able to maintain the original canal curvature without any deviation. Therefore, the present study aimed to evaluate canal transportation and centering ability of two rotary systems (Revo-S and TF) compared with Hand Ni-Ti-Flex K-files using CBCT in curved root canals.


  Materials and Methods Top


Freshly extracted permanent single rooted completely formed human premolars were collected. Teeth were cleaned from soft tissue and calculus using hand scalers, washed with distilled water and stored in sterile normal saline solution at 4°C until used within 2–3 months after extraction [11].

Approval for this study was obtained from Faculty of Dentistry, Tanta University, Research Ethics Committee. The purpose of the present study was explained to the patients and informed consents were obtained to use their extracted teeth in the research according to the guidelines on human research published by the Research Ethics Committee at Faculty of Dentistry, Tanta University.

Seventy-five teeth were selected and all had similar range of canal curvature (25–35°). Each tooth was digitally radiographed and the degree of curvature was measured according to Schneider's method [Figure 1] [12]. All teeth were decoronated leaving standardized root length of nearly 15 ± 1 mm. The apical patency was checked using SS K-hand file #10 (Dentsply–Maillefer, Ballaigues, Switzerland), then a file #15 was introduced into the root canal until its tip became just visible at the apex and the working length was determined by subtracting 1 mm from that length. All roots were selected as a SS K-hand file #20 were used as an initial apical file that fitted snugly within the canal at working length for standardization of internal canal dimension.
Figure 1: Digital radiographic image showing angle of canal curvature.

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The roots were randomly divided into three equal groups (n = 25) according to the instrumentation system used for root canal preparation as follow: group I, root canals were prepared using rotary Revo-S (Micro-Mega, Besancon, France) system; group II, TF (Sybron Endo, Orange, California, USA) rotary system and group III, hand Ni-Ti-Flex K-files system (Dentsply–Maillefer).

The roots of each group were positioned in a custom-made transparent acrylic resin (Acrostone, Cairo, Egypt) specimen holder to facilitate the imaging process and maintain reproducibility of the CBCT images. Preinstrumentation images were obtained using Cranex 3D device (Soredex, Helsinki, Finland) with 8.6 field of view and 4.9 s exposure time, operating at 90 kV and 10 mA. The images were taken with 0.5-mm-layer thickness axially to determine root canal shape at 3, 5, 7, 9 and 11 mm level from the root apex. The preinstrumentation images were saved on magnetic optical disc to be later compared with their corresponding postinstrumentation ones.

Before using the rotary systems, hand SS #10 K-file was used to create a glide path and this step was repeated till #20 K-hand file. Each file was coated with Glyde file lubricant (Dentsply–Maillefer) and used in slow downward movement without pressure. Each file was discarded after preparation of five canals according to the manufacturer's instructions [13].

Group I

Root canals were prepared using Revo-S rotary files system up to master apical file AS30 (#30/0.06) in a crown-down manner at 350 rpm and torque 0.8 Ncm using 20: 1 gear reduction hand piece powered with a torque limited endodontic motor (Endo-e-Class; Marathon, Dalseo-gu, South Korea) following the manufacturer's instructions.

Group II

Root canals were prepared using TF rotary system up to master apical file (#30/0.06) in a crown-down manner at 500 rpm and 0.5 Ncm torque limit with the same hand piece and electric motor used in group I. Apical patency and file lubricant were used as in group I.

Group III

Root canals were prepared using Ni-Ti-Flex K-hand files in crown-down technique up to master apical file #45/0.02. Each file was used for preparation of eight canals and then discarded according to manufacturer's instructions.

Each root canal was flushed with 3 ml of freshly prepared 2.5% sodium hypochlorite (NaOCl) (Clorox Co., 10th of Ramadan, Egypt) solution using plastic disposable syringe with 30-G closed-end needle before and after application of each instrument. After complete root canal preparation, 1 ml of 17% ethylenediaminetetraaceticacid (Essential Dental Systems Inc., South Hackensack, New Jersey, USA) solution was used for each canal for 1 min, then the canals were rinsed with 3 ml of normal saline solution. This irrigation protocol was followed in root canal preparation for all groups.

After root canal preparation, postinstrumentation CBCT images were obtained under the same conditions and parameters used for preinstrumentation ones. Preinstrumentation and postinstrumentation CBCT images for each root canal level of the three groups were superimposed over each other using Adobe Photoshop software (Adobe Systems, Mountain View, California, USA).

To detect the root canal wall differences between both images, the final image was rotated over the initial image until their external contours coincided together. Zoom was increased to 1200% so that one pixel could be visually identified. When the pixels were not coincident, the distance of difference between them was measured as shown in [Figure 2].
Figure 2: Superimposed preinstrumentation and postinstrumentation cone-beam computed tomography images with coincidence between them.

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Then the superimposed images were transferred to Auto-CAD software (Autodesk Inc., San Rafael, California, USA) to calculate centering ability and canal transportation.

Canal transportation was calculated according to the following formula [14] [Figure 3]:
Figure 3: Schematic drawing of root cross.sections showing how canal transportation and centering ability ratios were calculated [14]. Preinstrumented image (left): original canal space represented by dark shaded area. Postinstrumented image (right): light shaded area represents canal's shape after instrumentation.

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where a1 is the shortest distance from the mesial edge of the root to the mesial edge of the uninstrumented canal; a2 is the shortest distance from the mesial edge of the root to the mesial edge of the instrumented canal; b1 is the shortest distance from the distal edge of the root to the distal edge of the uninstrumented canal; b2 is the shortest distance from the distal edge of the root to the distal edge of the instrumented canal.

According to this formula, a result equal 0 showed no canal transportation; while any other result indicated that canal transportation occurred in the canal. Positive value indicated canal transportation toward mesial direction while negative value indicated canal transportation toward distal direction [14].

On the other hand, the centering ability was calculated according to the following formula [14]:



According to this formula a result equal 1 indicates perfect centering. If these numbers were not equal, the lower figure is considered as the numerator of the ratio.

The data of canal transportation and centering ability values were collected, tabulated and their mean and SD were calculated at each tested group and root canal level. Statistical analysis was performed using one-way analysis of variance with SPSS (version 17.00; SPSS Inc., Chicago, Illinois, USA) software to determine significance differences among groups. Whenever a statistical significant difference was recorded, multiple pair-wise comparisons were performed using Tukey's test to compare the data between each two tested groups or two canal levels at a significance level of P value 0.05 or less.


  Results Top


Canal transportation

[Table 1] represents canal transportation of different tested groups at each root canal level. Similar results were obtained at apical (represented by 3, 5 mm levels) and middle levels (represented by 7 mm level) where group I (Revo-S system) showed the lowest mean canal transportation value while the highest value was recorded for group III (Ni-Ti-Flex). One-way analysis of variance revealed a statistical significant difference among the tested groups (P<0.001). On the other hand, at coronal levels represented by 9 and 11 mm, the lowest mean transportation value was recorded for Ni-Ti-Flex group and the highest value for TF group with statistical significant differences between all tested groups (P<0.001).
Table 1: Canal transportation of the three groups (Revo-S, Twisted File and hand Ni-Ti-Flex) at different root levels (3, 5, 7, 9 and 11 mm) with statistical comparisons

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Regarding comparison between tested levels for each instrument, for both rotary systems (Revo-S and TF), coronal levels recorded the highest mean transportation value while the apical levels recorded the least value with a statistical significant difference among the five levels (P<0.001). Tukey's test revealed statistical significant differences between levels 3 versus 9 mm, 3 versus 11 mm, 5 versus 9 mm, 5 versus 11 mm, 7 versus 9 mm and 7 versus 11 mm.

When hand Ni-Ti-Flex group was considered, apical level showed the highest mean value of transportation while the lowest value was recorded for coronal level with a statistical significant difference between each two tested levels except 3 versus 5 mm and 7 versus 9 mm levels.

Centering ability

[Table 2] represents centering ability results. At the apical and middle levels, group I showed the best centering ability with the highest mean value followed by group II and group III with a statistical significant difference among the tested groups.
Table 2: Centering ability of the three groups (Revo-S, Twisted File, Ni-Ti-Flex groups) at 3, 5, 7, 9 and 11 mm levels with statistical comparisons

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On the other hand, at the coronal levels, group III showed the highest mean value of centering ability while group II recorded the least mean value of centering ability with statistically significant difference among them.

Comparing between tested levels, both rotary systems recorded the highest mean centering ability value at the apical levels while the lowest value was recorded at the coronal levels with statistically significant difference among the five levels. Tukey's pair-wise test recorded statistical significant differences between levels 3 versus 9 mm, 3 versus 11 mm, 5 versus 9 mm and 5 versus 11 mm for both groups and 7 versus 11 mm additionally for TF group.

On the other hand, in Ni-Ti-Flex group, the highest mean value of centering ability was recorded at the coronal levels and the lowest value was at the apical levels. Statistically significant difference was recorded among the five levels. Tukey's test revealed statistical significant differences between all levels except between levels 3 versus 5 mm, 7 versus 9 mm and 9 versus 11 mm.

Regardless of the tested levels, Revo-S group recorded the lowest mean value of canal transportation and the highest centering ability mean value while Ni-Ti-Flex group recorded txhe highest transportation mean value and the lowest centering ability value with a statistical significant difference among the three groups (P<0.01) [Figure 4] and [Figure 5].
Figure 4: Bar chart showing canal transportation of the three groups regardless of root levels.

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Figure 5: Bar chart showing centering ability of the three groups (Revo-S, TF and Ni-Ti-Flex groups) regardless of root level.

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In addition, comparison of tested levels regardless of the groups revealed that the least canal transportation and the highest centering ability mean values were recorded at the middle level without any statistical significant difference among them (P>0.05) [Figure 6] and [Figure 7].
Figure 6: Bar chart showing canal transportation of the five root levels regardless of the groups.

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Figure 7: Bar chart showing centering ability at different root levels regardless of the group.

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  Discussion Top


None of the currently available rotary systems were totally effective in performing perfect biomechanical preparation of curved root canals without transportation [15]. So, there are continuous trials to introduce new file systems to overcome the drawbacks of the previous ones. Therefore, this study was performed to evaluate canal transportation and centering ability of curved root canals prepared with two Ni-Ti rotary systems (Revo-S and TF) which are completely different in their design in addition to hand Ni-Ti-Flex system which is the most popular and is considered as a standard method of preparing root canals, so, it was used as a control group in this study [16].

In the present study, human teeth were used instead of resin blocks for evaluation of the shaping ability of curved root canals to simulate clinical conditions. In addition, a resin block is not an optimal material for testing rotary instruments because it does not reproduce the same microhardness of dentin or possible anatomic variations that cannot be easily simulated [17].

For standardization of the tested groups, teeth with similar apical diameters (#20) were selected and prepared in a crown-down instrumentation sequence up to a master apical file #30/0.06 for both Revo-S and TF rotary systems and #45/0.02 for hand Ni-Ti-Flex system to obtain nearly similar apical preparation diameter and standardize the amount of dentin removed in the apical portion for all samples [18].

CBCT imaging was selected in this study as it is noninvasive method that provides detailed three-dimensional observations at a low radiation dose. In addition to the possibility of choosing smaller field of view compared with the medical CT scans. Moreover, the resulting images have higher resolutions, and more accurate with a higher diagnostic capability [19],[20]. CBCT allows measurements before and after instrumentation at five selected levels (3, 5, 7, 9, 11 mm) from the apex that were chosen for comparison. They represent the end point of canal, the apex of the curve, the beginning of the curve, half-way to the orifice in the straight section and the coronal end of canal; in which curvatures exist and are highly susceptible to iatrogenic mishaps [21].

Both rotary systems maintain the original canal shape better than original hand instruments and minimize canal transportation in the curved root canals in the apical area. This may be attributed to the modification of Revo-S sequence as its asymmetric cross-section design facilitates penetration by snake-like movement which in turn offers more conservative precise root canal shaping with less deviation from original canal path. In addition, this design allows more efficient debris elimination with upward removal of dentin debris which in turn decreases debris accumulation within the canal that may hinder the guidance of the file within the canal and as a result enhance the centering ability of the file within the canal [22].

Additionally, TF is manufactured from specific R-phase heat treatment, followed by twisting of the metal and special surface conditioning, which significantly increases the instrument flexibility [23]. Moreover, the modified noncutting tip of both systems allows guidance of the file to the apex with least amount of aberrations [2],[22].

In addition, the aggressive cutting tip of Ni-Ti-Flex hand files is the main cause of its highest degree of transportation and decreased centering ability at the apical area [24]. The results of this study are in agreement with Diemer and Mallet [25] who reported that Revo-S rotary system enables a better shaping quality, with maintaining the original canal path more compared with other rotary systems. In addition, Elsherief et al. [26] and Valleys [27] agreed with the current findings as they concluded that Revo-S rotary system produced a proper root canal preparation with minor deviation from original canal curvature. Furthermore, the study performed by Gergi et al. [3] agreed with the obtained results as they compared canal transportation and centering ability of TF with hand preparation systems and found that TF was more centered apically than hand preparation.

On the other hand, Ni-Ti-Flex files revealed better performance at coronal portion of the canal, with less canal transportation and better centering ability while TF system was associated with more canal transportation and less centering ability. This may be related to the fact that Ni-Ti-Flex system with its ISO taper (0.02) allows less aggressive cutting at the coronal portion and as a result minimizes canal transportation and increase centering ability [24].

In contrary, TF system started with #25/0.08 which has larger taper compared with 2% taper of Ni-Ti-Flex system coronally and it is known that as the taper increased the stiffness of the instrument increase with more susceptibility of transportation [4].

Regarding Revo-S system, it showed more canal transportation and less centering ability at the coronal level compared to Ni-Ti-Flex hand files. This is due to the larger taper of the file (0.06) at the coronal portion than apical one that causes the file to be less flexible coronally [28].

Moreover, at the coronal level, the highest canal transportation associated with TF may be attributed to the triangular cross-sectional design which may allow more engagement of dentin with increasing friction with canal wall and consequently more liability for transportation compared to the asymmetrical one of Revo-S system [22],[29].

Mendes et al. [30] agreed with the obtained results where they compared the centering ability of different rotary systems including TF and concluded that this system caused less centering ability than other systems at the coronal part although there was no statistical significance among the tested systems. In addition, Nagaraja et al. [31] supported the current results as they evaluated shaping ability of hand Ni-Ti instruments with other rotary systems and they found that using hand Ni-Ti K-files produced lesser canal transportation and more centering ability than other rotary systems at middle and coronal third.

The superior results of Revo-S system in preparation of curved canals with less canal transportation and better centering ability over the other two tested systems may be attributed to the previously mentioned design modifications against drawbacks of hand instrumentation of curved canals. However, the superiority of Revo-S over TF systems may be due to its modified asymmetrical cross-section compared with triangular one of TF which cause more aggressive cutting [27].

These findings were in agreement with Glossen et al. [21] and Tasdemir et al. [32] who found that rotary Ni-Ti instruments produced less apical transportation than hand Ni-Ti systems in curved root canals. In contrary, Aydin et al. [23] disagreed with the obtained results as they stated that Revo-S instruments removed more resin material than TF system resulting in less centered preparation. This disagreement may be due to different apical preparation diameter (#25/0.06) in addition to usage of simulated resin canals instead of natural teeth. Furthermore, results obtained with Hashem et al. [33] was in contrast to the obtained results as they found that Revo-S rotary system produced canal deviation more than TF system. This controversy may be due to different type of teeth (mesiobuccal root canals of mandibular molars with different degree of canal curvature (20–40°).

Comparison of canal transportation at root canal levels regardless of the used systems revealed the least canal transportation at the middle portion which may be attributed to that the middle level represents the beginning of curvature which is anatomically different from the other two-thirds. It is considered more straighter portion with less anatomical variations (lateral canals, lateral apical foramen or apparent curvature) compared with apical third; which in turn will be subjected to minor effect of lateral forces transmitted from files used for canal preparation [34].

Considering the highest canal transportation at the coronal portion, this may be attributed to the cross-sectional configuration of the canal which tends to be oval in the coronal rather than circular in middle and apical thirds which tends to give a less uniform canal preparation because more round root canal section is associated with better adaptation to the file motion within the canal with more centered preparation [35].


  Conclusion Top


Rotary Ni-Ti systems produced less canal transportation and more centering ability than hand Ni-Ti files in preparation of curved root canals.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

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