Keywords
canal centering - canal volumetric change - microcomputed tomography - single file
- transportation
Introduction
The shaping of the root canals system is an essential and critical step in the process
of endodontic treatment. The importance to mechanically debride and shape the root
canals is not only to remove the vital and necrotic tissue and disrupt the bacterial
biofilm but also to provide a space for the antimicrobial irrigation solutions and
medicaments to perform their action on the bacteria and to disinfect the root canal.[1]
[2]
The revolution in the manufacturing of root canal instruments leads to a great variety
of these endodontic instruments. Although these variations are valuable in providing
different alternatives that can cope with the variations in the root canal system,
however, the performance of these instruments inside each canal is not always predictable.[3]
[4]
[5]
The invention of the NiTi rotary instrumentation was advantageous in minimizing the
procedural errors and time of treatment.[6] A new technique for instrumentation was introduced by Yared, where a single file
was used in a reciprocating motion rather than rotation, overcoming the large sequence
of files used in a rotation motion.[7] Although some studies described the higher performance of the reciprocating motion,
other studies showed that the rotary files produced more centralized preparations
and less transportation.[8]
[9] These controversies have been reported to a large extent, and this dictates that
extensive studies should be carried to compare the performance of these motions and
file designs using the most accurate and reliable tool for analysis.[10]
Microcomputed tomography (micro-CT) is considered a useful and noninvasive technique
that permits the proper visualization of different parts of the canals with higher
resolution and accurate details, thus providing a reliable method to evaluate the
behavior of different endodontic instruments in different parts of the root canal
system.[11] Micro-CT is the recommended and the ideal radiographic scanning technique for in
vitro studies.[12]
The objective of the present study was to evaluate the effect of four different single-file
systems, namely WaveOne Gold, RECIPROC Blue, HyFlex EDM, and One Shape, on canal centering,
canal transportation, and change in the canal volume using micro-CT.
Materials and Methods
A total of 80 mesiobuccal and mesiolingual canals of extracted human permanent mandibular
molars were used for the study after calculating the total sample size using the G*Power
3.1.9.4 software (Franz Faul, Universität Kiel, Germany) with power of 0.84.[13] This study was approved by the Research Ethics Committee of Ain Shams University
(Cairo, Egypt) under approval no. FDASU-RECID 021607. The soft tissue and organic
debris were removed by placing the samples in 5.25% NaOCl for 10 minutes and then
placing in normal saline until the time of the experiment. After the access cavity
was prepared for all the samples, a size 10 K–file was introduced into the canals
to check for the apical patency and maintain a glide path. The length of each canal
was determined by passing a size 10 K–file slightly beyond the apex then adjusting
it to be 0.5 mm from the apex. Only a size 10 K–file was used before canal preparation
with the single-file systems. According to the method described by Pruett, mesial
canals with an intermediate radius of curvature (r > 4 and r ≤ 8 mm) and angle of
curvature ranging between 25°and 35°were selected for this study.[14] The canals were randomly allocated to the four groups according to the file system
used (n = 20) using the GraphPad QuickCalcs random number calculator (https://www.graphpad.com/quickcalcs/randomize2). Then the average canal radius and curvature of each group were checked to confirm
that the groups are comparable statistically with no significant difference between
the canal curvatures or diameters before canal preparation.
For the WaveOne Gold group (Dentsply Maillefer; Ballaigues, Switzerland), the size
25 primary file with a 7% taper was used according to the manufacturer instructions;
the file was introduced into the canal with in-and-out picking motion until reaching
the full working length (WL). In the RECIPROC Blue group (VDW, Munich, Germany), the
R25 with a size of 25 and 8% taper was used in the same manner as WaveOne gold files;
these two files were used in a reciprocating motion. For the Hyflex EDM group (COLTENE/Whaledent
AG, Switzerland), the size 25 primary file with taper 8% was used according to the
manufacturer instructions using a speed of 400 rpm and maximum torque of 2.5 Ncm.
In the One Shape group (Micro-Mega, Besançon, France), size 25 file with taper 6%
was used in a rotation motion at the same speed of the Hyflex EDM files, but the torque
was set at 2.0 Ncm. An endodontic motor with both rotation and reciprocation motion
was used for the four groups (X-SMART PLUS; Dentsply Maillefer, Ballaigues, Switzerland).
To ensure that the same length of the file’s working part used was introduced into
the canals, the samples used have an average tooth length of 19 ± 0.5 mm.
A 30-G long needle was used for irrigation. The irrigating needle was inserted before
preparation to remove any debris that may interfere with the files and preventing
them from reaching the full WL. A total of 5 mL of 0.9% saline (sodium chloride [NaCl])
was carried into the canal by picking motion. Where the insertion of the file inside
the root canal was followed by insertion of the irrigating needle to the canal level
prepared, a 1 mL of irrigation was inserted with picking motion to prevent any blockage
until the file reached the full WL, then a complete flush of the canal was done[15]; 17% ethylenediaminetetraacetic acid (EDTA) gel (Glyde; Dentsply Maillefer, Ballaigues,
Switzerland) was used as a lubricant. Each file was used for only four canals and
then discarded.
Micro-computed Tomography Analysis
The teeth were scanned before and after preparation using high-resolution desktop
micro-CT scanner SkyScan (SkyScan 1174; SkyScan, Bruker, Belgium), then were placed
in a specific mold that holds the teeth in a reproducible position during the whole
time of scanning. The microfocus X-ray tube was set at 50 kV of acceleration voltage,
800 µA of beam current. Scanning was performed at 32.17 µm resolution, and a 0.5 mm
aluminum filter was placed for filtration of the X-ray beam to remove unwanted low
energy beams. The rotation step angle was 0.50°, with approximately 3,800 seconds
of exposure time. Reconstruction of the images was done using NRecon software (SkyScan
1174), the ring artifact correction was set at 10, and beam hardening correction was
set at 15.
Evaluation of Centering Ratio and Canal Transportation
Canal centering ratio and canal transportation were analyzed using the CTAn V.1.18.8.0
(SkyScan 1174) software according to technique developed by Gambill et al, which measures
the distance from the edge of the canal to the periphery of the root (mesial and distal)
on pre- and postinstrumentation images of micro CT ([Fig. 1]). The following formula was implemented for canal centering: (X-X’)/(Y-Y’). According
to this formula, a result of “1” will indicate a perfect centering ability; results
higher than “1” will indicate more mesial movement and results lesser than “1” will
indicate more distal movement.[16] This formula (X-X’) - (Y-Y’) was used for canal transportation where results higher
or lesser than 0 indicates canal transportation.[16]
Fig. 1 Cross-sectional reconstructed images showing the measurements taken on the preinstrumentation
and postinstrumentation images at the apical, middle, and coronal levels. On the figure:
X denotes the shortest distance from the mesial aspect of the root to the periphery
of the prepared canal. X’ denotes the shortest distance from the mesial aspect of
the root to the periphery of the unprepared canal. The same for Y and Y’.
The canals were divided into three parts: apical, middle, and coronal (3, 5, and 7
mm from the apex, respectively). The pre- and postinstrumentation records were done
at the same number of scanned slices from the apex.
Measurements of Canal Volume Percentage Change
The pre and post instrumentation images were superimposed over each other using the
three-dimensional (3D) registration ([Fig. 2]) of the DataViewer software V. 1.5.6.3 (SkyScan 1174), then the superimposed images
were saved. The volumetric changes at three canal levels were then calculated using
the CTAn software V. 1.18.8 (SkyScan 1174) ([Fig. 3]).
Fig. 2 Cross-sectional reconstructed images showing the preinstrumentation, postinstrumentation,
and the superimposed images at the three different canal levels. The red color in
the superimposed images indicate the difference in the canal parameters after instrumentation.
Fig. 3 Representative 3D reconstructed images showing the total canal volume; before instrumentation
in red color, after instrumentation in green color, and the superimposed image of
the pre- and postinstrumentation micro-CT images in two different views: buccal and
mesial. 3D, three-dimensional; CT, computed tomography.
Statistical Analysis
The data passed the normality test and were statistically analyzed using one-way analysis
of variance (ANOVA). Post hoc pairwise comparisons were conducted using the Tukey
test (GraphPad Prism version 7.00 for Windows, GraphPad Software, La Jolla, California,
United States). The statistical significance level was set at p < 0.05.
Results
Canal Centering
The results are shown in [Table 1]. No significant difference was found in the centering ability of the four groups
at the coronal level (p = 0.083) or the middle level of the canals (p = 0.131). At the apical level, the difference was significant (p < 0.05), where the WaveOne Gold group showed the best centering ratio among the other
groups.
Table 1
The mean and standard deviation of canal centering ratio for the four groups in different
levels of the root canal: apical, middle, and coronal
|
Group
|
Canal level
|
|
Apical Mean ± SD
|
Middle Mean ± SD
|
Coronal Mean ± SD
|
|
Abbreviation: SD, standard deviation.
|
|
WaveOne Gold
|
1.458 ± 0.344
|
0.813 ± 0.223
|
0.891 ± 0.349
|
|
RECIPROC Blue
|
2.049 ± 0.479
|
0.919 ± 0.401
|
0.94 ± 0.491
|
|
HyFlex EDM
|
1.64 ± 0.436
|
0.958 ± 0.408
|
0.928 ± 0.309
|
|
One Shape
|
2.226 ± 0.376
|
0.726 ± 0.278
|
0.838 ± 0.343
|
Canal Transportation
There was no significant difference in canal transportation between the four groups
at the coronal level (p = 0.567) or the middle level (p = 0.246). However, WaveOne Gold showed the least canal transportation at the apical
level (p < 0.05). The results are shown in [Table 2].
Table 2
The mean and standard deviation of canal transportation for the four groups in different
levels of the canal: apical, middle, and coronal
|
Group
|
Canal level
|
|
Apical Mean ± SD mm
|
Middle Mean ± SD mm
|
Coronal Mean ± SD mm
|
|
Abbreviation: SD, standard deviation.
|
|
WaveOne Gold
|
0.035 ± 0.033
|
−0.025 ± 0.011
|
−0.028 ± 0.026
|
|
RECIPROC Blue
|
0.068 ± 0.58
|
−0.021 ± 0.019
|
−0.02 ± 0.019
|
|
HyFlex EDM
|
0.052 ± 0.05
|
−0.019 ± 0.017
|
−0.021 ± 0.017
|
|
One Shape
|
0.086 ± 0.065
|
−0.033 ± 0.023
|
−0.03 ± 0.026
|
Change in Canal Volume
The results are shown in [Table 3]. The results showed that there was a significant difference in canal volumetric
changes between the four groups at the coronal level (p < 0.05), where the HyFlex EDM group showed the highest canal volumetric changes.
No significant difference was found among the four groups in the total percentage
of canal volume change (p = 0.616), in the percentage of canal volume change at the apical level (p = 0.655), or the middle level (p = 0.49).
Table 3
The mean and standard deviation of canal percentage volumetric change after preparation
with the examined files at different levels of the canal
|
Group
|
Canal level
|
Total volume change
Mean ± SD %
|
|
Apical Mean ± SD %
|
Middle Mean ± SD %
|
Coronal Mean ± SD %
|
|
Abbreviation: SD, standard deviation.
|
|
WaveOne Gold
|
18.81 ± 16.12
|
14.71 ± 9.51
|
25.3 ± 11.1
|
21.77 ± 11.91
|
|
RECIPROC Blue
|
19.27 ± 10.87
|
16.09 ± 6.82
|
25.59 ± 11.97
|
23.39 ± 12.66
|
|
HyFlex EDM
|
20.42 ± 4.66
|
19.15 ± 5.4
|
37.58 ± 11.39
|
25.47 ± 6.35
|
|
One Shape
|
15.05 ± 11.72
|
13.94 ± 12.19
|
33.37 ± 1.8
|
20.72 ± 8.27
|
No significant difference in canal volumetric changes at the different canal levels
for the same type of file was found for WaveOne Gold group (p = 0.144) and the RECIPROC Blue group (p = 0.058); however, there was a significant difference in the HyFlex EDM group and
the One Shape group (P < 0.05), where the canal volumetric change was highest at the coronal level in both
groups.
Discussion
One of the critical criteria and prerequisites for optimal instrumentation of the
root canals is the preservation of the original canal outline, curvature, and centering.
The ability of the endodontic files to respect the anatomy of the canals is vital
to prevent the procedural errors that may occur in different parts of the canals,
such as transportation in the apical third, strip perforations in the middle third
or weakening of the coronal part by cutting more cervical dentin.[17]
The mesial canals of the mandibular molars usually represent a challenge in the endodontic
treatment owing to their curvature, and anatomical variations and the proper selection
of the endodontic files that will respect these anatomical variations is fundamental.[18] The canals were selected with an intermediate radius of curvature (r > 4 and r ≤
8 mm) and angle of curvature ranging between 25°and 35°for this study to represent
the curvature angle and radius found in the majority of the mesial canals of mandibular
molars as indicated by previous studies.[19]
[20]
[21]
This study aimed to evaluate if the difference in the design of the endodontic files
and the motion kinematics may have an implication on canal centering, canal transportation,
and the change in canal volume after preparation.
Microcomputed tomography has been used in this study as it represents an innovative
technique for 3D imaging. It produces scanned images with very high resolution and
fine details, overcoming many of the drawbacks of the other imaging techniques. Micro-CT
is considered a highly acceptable, standardized, and reliable method to be used in
the evaluation of many endodontic studies.[12]
[22]
[23]
To study the performance of the instruments examined through the whole canal length,
the analysis of three different canal levels—apical, middle, and coronal levels, represented
by 3 mm, 5 mm, and 7 mm from the apex, respectively, was performed.[21]
[24]
[25]
The results of this study showed that at the middle and the coronal levels, there
was no significant difference between the four groups in canal centering ratio or
the amount of canal transportation. However, at the apical level, WaveOne Gold files
showed the best centering ratio and the least amount of canal transportation. WaveOne
Gold files are manufactured from the Gold wire by using advanced metallurgy and proprietary
thermal treatment.[26] The cross-section of WaveOne Gold is a parallelogram with two 85°cutting edges that
contact the canal wall, alternating with off-center cross-section, only one cutting
edge is in contact with the canal wall, it is utilized in a reciprocating motion and
has a regressive taper,[27] the special heat treatment the file received and these characteristic design features
improved the file performance inside the root canals.[28] RECIPROC Blue is another file used in reciprocating motion, the files are produced
with Nickel-Titanium that goes through an innovative heat treatment, modifying its
molecular structure to give the files an enhanced characteristics,[29] the cross-section of RECIPROC Blue files is an S-shaped with regressive taper.[30] Although WaveOne Gold and RECIPROC blue are both used in reciprocating motion, however,
the difference between them in the centering ratio and the amount of canal transportation
suggest that the file characteristics such as the surface treatment, cross-sectional
design or the taper have more influence on the file performance rather than the motion
kinematics. This finding is consistent with previous studies that showed that the
instrument's features had significant influences on the mechanical properties of the
NiTi instruments.[27] Although previous study reported the superiority of WaveOne Gold over RECIPROC Blue
in canal centering and transportation,[26] another study showed that no difference was found in canal transportation between
HyFlex EDM and WaveOne Gold; these discrepancies in the results may be attributed
to the study design where they used simulated resin canals which is not the case in
this study where natural canals were used and analyzed with the highly accurate micro-CT.[31] It has been reported that WaveOne Gold showed a better centering ability than One
Shape which is evident in this study.[32] Previous studies showed similar performance for WaveOne Gold.[25]
[33]
[34] It was found that all the four groups showed a centering ratio more than one at
the apical level indicating the tendency of all the instruments used to prepare more
of the outer surface of the canal curvature rather than the inner surface.[17]
The results of the change in canal volume showed that the four file groups behaved
similarly at the apical level, middle level, and for the total volumetric change;
however, at the coronal level, the HyFlex EDM group produced the highest percentage
change in canal volume. HyFlex EDM group and One Shape group showed a difference in
canal volumetric changes between different levels of the canal when prepared with
the same type of file. Studies on canal volumetric changes using these file types
have not been extensively studied before, and hence it was difficult to correlate
our current results with previous data. HyFlex EDM has a cross-section that is variable
along its entire length. The file has an almost triangular cross-section design at
the coronal part, followed by a trapezoidal cross-section design and ends with a quadratic
cross-section design at the apical third.[35] There are always four points of contact for the file with the canal walls. The file
used in a rotation motion, has a continuous taper and made of a controlled memory
alloy using electric discharge machining technology, which enhanced its cutting efficacy.[36] Although WaveOne gold and RECIPROC Blue received a thermal treatment that improved
their cutting efficiencies, the difference in the taper and hence in the amount of
the metal core may influence the difference in the canal volumetric change, where
it is continuous for HyFlex EDM and regressive for WaveOne Gold and RECIPROC Blue.[37] One shape file is designed to have three different cross-section designs along the
length of the files starting with three symmetrical cutting edges at the apical part,
it ends with two S-shaped cutting edges in the coronal part.[38] Although HyFlex EDM and One Shape both have a continuous taper, however, the lack
of surface treatment in One Shape may explain the higher canal volume change in the
HyFlex EDM group at the coronal level.[36]
The difference in canal volumetric change between the different levels of the canals
prepared with HyFlex EDM group or One Shape group may be correlated to the transition
from different designs in the cutting edges and the continuous increase in metal core
of these two groups.[39]
As the differences exist among the four single-file systems in the centering ratio
and canal transportation at the apical level and in the volumetric canal change at
the coronal level, the null hypothesis has to be rejected. It is important to evaluate
the behavioral nature of different files to ensure that the proper selection of the
files that will respect the original canal anatomical features is applied and the
subsequent achievement of long-term successful endodontic treatment.
Conclusion
Within the limitations of this study, it was concluded that WaveOne Gold showed the
best performance regarding canal centering ratio and the amount of canal transportation
at the apical level, all the files performed similarly at the middle and the coronal
levels of the canal.
Preparation with HyFlex EDM produced a greater change in canal volume than the other
groups at the coronal level. No difference in canal volumetric change between the
tested files was found at the other two levels of the canal.
The performance of the endodontic files during canal instrumentation is dependent
mainly on the interim cross-sectional design, the taper, and the thermal or surface
treatment.
