Keywords
micro–computed tomography - nickel–titanium alloy - apical transportation - ProTaper
Next - HyFlex EDM
Introduction
Understanding the anatomy of tooth canals is an essential step for effective endodontic
therapy. Studies report changes to root canal morphology during canal preparation,
which may vary according to applied instrumentation technique.[1] In addition, root curvature introduces a complexity that influences the ability
of instruments to prepare and clean all root canal walls, regardless of the applied
instrumentation system.[2]
[3] These difficulties in preparation may predispose the root canal to persistent infection,
as bacteria may remain in unprepared areas, adhered to canal walls or in areas inaccessible
to the chemical–mechanical preparation, which can lead to failures in endodontic treatment.[4]
Automated instruments made from a nickel–titanium (NiTi) alloy have become widely
applied in clinical practice. The NiTi alloy presents a lower elasticity modulus than
stainless steel, allowing for easy and efficient preparation of curved root canals.[5]
[6] However, errors in iatrogenic procedures, such as deviations, perforations, or root
canal transportation, may occur due to the applied instrumentation technique, particularly
in curved canals, culminating in deviations of the original root canal pathway.[7]
[8]
As a way of controlling these factors, HyFlex EDM (HEDM; Coltene/Whaledent AG, Alstätten,
Switzerland) instruments have been proposed, manufactured with a controlled memory
alloy using the electric discharge machining technology. This manufacturing process
improves the fracture strength and efficiency of the cutting blade.[9]
[10]
[11] Another system comprises the Protaper Next (PTN; Dentsply Maillefer, Ballaigues,
Switzerland), manufactured with lower mass in its M-Wire alloy (containing martensite
portions in its microstructure) which, through a heat treatment process, provides
greater flexibility while maintaining cutting efficiency and higher resistance to
cyclic fatigue when compared with the conventional NiTi alloy.[3]
[12]
The vast majority of the preparation evaluation methods described earlier only assess
two-dimensional changes. However, root canal anatomy is altered in three dimensions
(3D) during chemical–mechanical preparation.[1] Studies have applied micro–computed tomography (CT) to evaluate the chemical–mechanical
preparation ability of different endodontic instruments. This type of methodology
displays advantages over other methodologies, mainly because it is nondestructive.[13]
[14] In addition, the risk of root fracture is greater when the canal diameter is widened
by more than 40% of the root width.[15]
In this context, the aim of the present study was to compare the modeling ability
of the PTN (Dentsply Sirona, Tulsa, Oklahoma, United States) and HEDM (Coltene/Whaledent
AG) instrument systems in mandibular molar mesial canals using micro-CT as an evaluation
method.
Materials and Methods
Sample Preparation and Selection
This study was approved by the local ethics committee under CAAE number 79232617.7.0000.5374.
Twenty-one teeth (based on a sample calculus) were selected from an initial sample
of 122 mandibular human molar teeth, displaying complete rhizogenesis, patent canals
presenting mesial roots, individualized canals and foramina, Vertucci's type IV classification,
and root curvature between 20 and 40 degrees.[16] Tooth crowns were removed by a diamond disc to standardize canal length at 16 mm.
The 16 mm was divided into three thirds. The apical third considered at 1 to 5 mm,
the middle at 6 to 10 mm, and the cervical at 11 to 16 mm from the apex. After specimen
selection and standardization, the specimens were submitted to an initial scanning
using a SkyScan 1173 device (Bruker-microCT, Kontich, Belgium) at 17.09 μm pixel size,
114 mA, 70 kV, 360 degrees, 1.0 for ∼18 minutes per specimen.
The acquired images were reconstructed in transverse slices using the NRecon 1.7.1.0
software (Bruker-microCT). The 3D images of the mesial roots were obtained and evaluated
by the CTVol v.2.2.1 software (Bruker-microCT). The internal morphology of the Vertucci's
type IV root canal was confirmed by the micro-CT images. Canal morphological parameters
(volume and surface area) were acquired using the CTAn v.1.14.4 software (Bruker-microCT)
and served as basis for sample matching.
Canal Preparation
A single experienced operator performed the setup for both systems. The instrumentation
was applied to a same root, in alternating mesial canals. The working length was determined
as 1 mm below the foraminal constriction. The instruments used in the PTN system were
as follows: SX-cervical, X1 - 17.04 middle and apical thirds, and X2 - 26.06 in the
apical third, driven by the VDW Gold engine at 300 rpm and 2 NCm, according to the
manufacturer's instructions. The HEDM system followed the sequence: 25.12, cervical
third, 10.05 and 25.08 files, in the middle and apical thirds, at 500 rpm and 2.5
NCm (25.12 and 25.08) and 300 rpm and 1.8NCm (10.05) respecting the manufacturer's
instructions. Each file made three in-and-out movements in each canal and each instrument
was used once per canal and then discarded, in both groups. At each instrument change,
each canal was irrigated with 10 mL of a 2.5% NaOCl solution using a 30G needle. For
smear layer removal, 3 mL of a 17% EDTA solution was used for 1 minute and a final
irrigation was applied using 2.5 mL of a 2.5% NaOCl solution, totaling 25 mL of NaOCl
2.5% per root.[17]
[18]
[19]
Analysis of Postinstrumentation Images—Micro-CT Evaluation
After preparation, the roots were subjected to a new micro-CT scan using the same
parameters described previously. The following morphometric parameters were analyzed:
canal centralization after chemical–mechanical preparation, apical transportation,
and canal and root widths. The 3D Slicer 4.4.0 software (available at http://www.slicer.org) was used to coregister the 3D models of the pre- and postoperative phases.
Canal Transportation and Centralization
Using the centralization data extracted from the CTAn program (Bruker-microCT), the
XLSTAT-3DPLOT for Windows plugin for Excel (Addinsoft, New York, United States) was
used to elaborate a center of gravity variation diagram connected along the Z-axis. Root canal transportation was evaluated from the center of gravity variation
(in mm), comparing the centers of gravity before and after preparation for all radicular,
cervical, middle, and apical canal segments. Representative measurements were also
presented graphically in the form of diagrams ([Fig. 1]).
Fig 1 (A) Three-dimensional images and diagram displaying the combination of the central
axis of the HyFlex EDM (HEDM) preinstrumentation (green line) and postinstrumentation
(red line) root canals and ProTaper Next (PTN) preinstrumentation (green line) and
postinstrumentation (blue line) root canals. (B) Cross-sections representative of
overlapping root canals before (green) and after (blue) HEDM and PTN preparation,
in the cervical (c), middle (m), and apical (a) thirds.
Canal/Root Width and Potential Risk for Root Fracture
The CTAn software v.1.14.4 (Bruker-microCT) was used to measure the diameter of the
pre- and postoperative canals in relation to root diameter at the cervical, middle,
and apical levels, as well as the variations in canal diameter in relation to the
width of the external dentin, as per the method reported by Gambill et al, with adaptations,[20] in which the apical third was considered at 1 to 5 mm, the middle at 6 to 10 mm,
and the cervical at 11 to 16 mm from the apex. Extensions were determined by measuring
the shortest distance from the edge of the unprepared canal to the tooth border, both
in the mesial and distal directions, and then comparing with the same measurements
obtained from the treated canal images. The following formula was used: ([X1 − X2] − [Y1 − Y2]) × X1.
The distance represented by X1 was measured before instrumentation, from the edge
of the canal to the edge of the root in the mesial region, and Y1 represents the distance
from the canal to the root border in the distal region before instrumentation. The
distance represented by Y1 was measured from the edge of the canal to the edge of
the root in the middle region, after instrumentation. Y2 represents the distance from
the canal to the root border in the distal region after instrumentation. The canal
diameter variation in relation to external dentin was measured by tracing a straight
line from the middle border to the distal border, followed by initial canal diameter
and diameter after canal enlargement assessments. A potential fracture risk was considered
if the diameter of the postoperative canal corresponded to more than 40% of the root
width[15]
[20]
[21] ([Fig. 2]).
Fig 2 Representative mandibular molar root shape showing the unprepared (left) canal diameter,
with X1 and X2 representing dentin thicknesses in the inner wall of the root and Y1
and Y2 representing dentin thicknesses in the outer wall. D represents the dentin
diameter, and C1 and C2 represent the canal diameters before and after preparation.
Statistical Analyses
Data distribution was analyzed by the Shapiro–Wilk's normality test. Intragroup and
intergroup analyses were performed by applying the unpaired t-test. The analysis of variance test was used for intragroup comparisons. The level
of significance was set at 5% for all statistical tests (p < 0.05).
Results
Canal Transportation and Centralization
Variations in the center of gravity were not statistically different between the groups.
A statistically significant difference in the HEDM group for the middle and apical
levels was observed. The results of root canal transportation and centralization are
summarized in [Table 1] and [Fig. 1].
Table 1
Transport and centralization of the mandibular molar mesial canals after preparation
by the assessed systems
|
HyFlex EDM
|
ProTaper Next
|
|
Level
|
Mean ± SD
|
Median
|
Range
|
Mean ± SD
|
Median
|
Range
|
|
Cervical
|
0.65 ± 0.38
aA
|
0.72
|
0.05–1.15
|
0.54 ± 0.37
aA
|
0.51
|
0.02–1.07
|
|
Middle
|
0.47 ± 0.26
abA
|
0.47
|
0.05–0.98
|
0.47 ± 0.34
aA
|
0.48
|
0.02–0.98
|
|
Apical
|
0.32 ± 0.18
bA
|
0.32
|
0.03–0.69
|
0.32 ± 0.16
aA
|
0.34
|
0.08–0.6
|
|
Total
|
0.40 ± 0.22
A
|
0.36
|
0.04–0.88
|
0.52 ± 0.22
A
|
0.58
|
0.01–0.79
|
Abbreviation: SD, standard deviation.
Note: Different lowercase letters represent significantly intragroup differences.
Different uppercase letters represent significant differences between the groups.
Canal/Root Width and Potential Risk of Root Fracture
The width of the root canal in relation to root width was significantly increased
by both systems (p < 0.05) ([Table 2]). No statistically significant differences between the groups was observed regarding
the root canal and root width relationship, comparing cervical, middle, and apical
levels before and after preparation (p > 0.05). The cervical and middle thirds of the canal diameter were increased by more
than 40% of the root width in both groups. Root diameter enlargement was not more
than 40% in the apical third ([Figs. 2] and [3]; [Table 2]).
Fig 3 Canal/root width and potential risk of root fracture after preparation by the assessed
systems.
Table 2
Canal/root diameter ratio and potential risk of root fracture after preparation by
the assessed systems
|
Section
|
HyFlex EDM
|
ProTaper Next
|
|
Mean ± SD
|
Median
|
Range
|
Mean ± SD
|
Median
|
Range
|
|
Cervical canal (mm)
|
1.40 ± 0.94
aA
|
1.13
|
0.32–3.83
|
0.94 ± 0.69
aA
|
0.85
|
0.22–3.34
|
|
Middle canal (mm)
|
1.25 ± 0.91
aA
|
1.00
|
0.14–4.00
|
0.97 ± 0.76
aA
|
0.92
|
0.00–2.84
|
|
Apical canal (mm)
|
1.16 ± 0.81
aA
|
1.12
|
0.08–2.86
|
0.76 ± 0.48
aA
|
0.58
|
0.00–1.80
|
|
Cervical dentin (%)
|
49.66 ± 8.65
aA
|
50.00
|
32.37–65.03
|
51.02 ± 11.81
aA
|
47.34
|
30.14–74.86
|
|
Middle dentin (%)
|
46.48 ± 14.29aA
|
43.28
|
24.69–80.89
|
45.48 ± 10.79
aA
|
40.16
|
29.84–74.40
|
|
Apical dentin (%)
|
36.85 ± 15.64
bA
|
33.26
|
17.86–86.30
|
32.29 ± 9.33
bA
|
27.24
|
15.29–51.8
|
Abbreviation: SD, standard deviation.
Note: Different lowercase letters represent significantly intragroup differences.
Different uppercase letters represent significant differences between the groups.
Discussion
Root canal preparation includes both root canal system enlargement and modeling, along
with disinfection. A variety of instruments and techniques have been developed and
described for this critical root canal treatment phase. Although many root canal preparation
reports are found in the literature, definitive scientific evidence on the quality
and clinical suitability of different instruments and techniques remains undefined.[22] Micro-CT images have been used to assess the chemical–mechanical preparation capacity
of different endodontic instruments. This methodology presents advantages over other
methodologies, mainly because it is nondestructive.[13]
[14]
The present study compared the centralization capacity of preparations using two rotary
systems. The HEDM system is a memory-based file system, manufactured by electrical
discharge machining that allows for high fatigue strength and greater flexibility.
It comprises varied triangular conicity in its cervical portion, is trapezoidal in
its middle portion, and quadrangular in the apical portion. The PTN file system presents
variable conicity (increasing and decreasing in the apical to cervical direction,
with fixed X1 and X2, an eccentric rectangular cross-section and is composed of the
M-Wire alloy).
Canal transportation was evaluated based on center of gravity variations and, although
the HEDM and PTN systems do not share similarities in alloy metallurgy, file and tip
geometry, and diameter in D0, the results obtained herein indicated no significant
differences in relation to the morphological parameters assessed after canal preparation.
Both groups maintained preparation centralization. The similar results observed for
both systems may be due to the alloy metallurgy of each system, as both undergo alloy
heat treatment, allowing for greater flexibility and preparation centralization.[12]
[13] These results are in agreement with other studies assessing canal centralization.[12]
[23]
Root canal diameter and root in relation to dentin wear in mandibular molar mesial
canals were evaluated. Dentin thickness evaluations are essential, considering that
excess dentin removal can predispose tooth root fractures.[15]
[21]
[24]
[25] The HEDM and PTN systems increased the cervical and middle thirds of the canals
by more than 40%. This may have occurred because both files used in the cervical and
middle thirds, HEDM 25.12 and SX (19/.035), share the same cervical portion geometry,
a triangular cross-section.[26]
Although no statistically significant difference was observed between the groups,
the PTN system led to an increased cervical third in comparison to the HEDM system.
This difference may have been caused because the file used in the PTN group to prepare
the cervical third was the SX (19/.035) file, prepared with a conventional NiTi alloy,
as part of the ProTaper Universal file system. Conventional NiTi alloy as compared
with heat-treated or memory-controlled alloys tends to further increase the canal
diameter and further decentralize the preparation.[6]
[23]
[24]
[25]
[26]
[27] This study indicates contrasting results when compared with other studies that both
shared[21] and did not share[15]
[20]
[28]
[29]
[30] the same methodology.
Conclusion
No differences in the assessed morphological modeling parameters were observed for
both root preparation systems. Both groups maintained the preparation centralization.
The assessed systems increased the cervical and middle thirds of the root canals by
more than 40%, increasing potential fracture risks due to excessive dentin removal.
