Keywords
root canal therapy - microtomographic analysis - unprepared canal walls - rotary instruments
- Hyflex CM - XP-endo Shaper - curved canals
Introduction
During preparation of the root canal system, the mechanical action of instruments
combined with irrigation facilitates subsequent operative procedures; it promotes
canal debridement, creates space for intracanal medication, and optimizes the root
canal filling procedure.[1] With the introduction of nickel-titanium (NiTi) rotary systems, endodontic instrumentation
has become faster and safer, favoring the preservation of the original canal anatomy.[1] Despite the lower rates of procedural errors—such as zips and perforations—associated
with these instruments, a certain amount of dentinal wall area remains unprepared
after preparation, possibly causing failures in endodontic treatment.[2] In curved canals, incomplete dentin removal in one portion of the canal, and excessive
removal in the other, can increase the risk of apical transportation, as well as fracture
and weakening of the root structure.[3]
[4] Even though the improvements observed in the manufacturing process of NiTi rotary
instruments three-dimensional root canal preparation remains a major challenge to
endodontists, and instruments with different geometries and surfaces have been introduced
to render canal shaping more predictable.[1]
[2]
[3]
The first memory-controlled rotary instruments were launched in 2010, namely the Hyflex
CM system (Coltène/Whaledent, Allstätten, Switzerland). This controlled-memory effect
is achieved through a special thermomechanical treatment. The system has multiple
instruments with a triangular cross-section (sizes 25/.08 and 20/.06), and a quadrangular
cross-section (sizes 20/.04, 25/.04, 30/.04, and 40/.04). The flexural strength of
its instruments is up to 300% higher than that of instruments made with conventional
NiTi alloys,[5]
[6]
[7] enabled by the predominance of the martensitic phase in its alloy.[8]
[9]
The XP-endo Shaper instrument (FKG, La Chaux-de-Fonds, Switzerland) was introduced
in 2016 following the single-file preparation concept and is made from the MaxWire
alloy (Martensite-Austenite Electropolishing Flex, FKG). This alloy undergoes a martensitic-to-austenitic
phase transformation with an increase in temperature (35°C), thereby promoting expansion
of the instrument from its original 30/.01 size to a new 30/.04 size. This means that
expansion of the preparation takes place in a single step, without requiring the use
of successively larger files.[10]
[11]
[12]
[13]
Microtomographic (micro-CT) analysis has been widely used in endodontic research because
it is a non-destructive method with excellent accuracy and is considered the gold
standard for assessing the three-dimensional shaping ability of endodontic instrumentation
systems.[14]
[15]
[16]
It is important to study new rotary systems such as Hyflex CM and XP-endo Shaper,
as they present unique design and provide superior flexibility, thus allowing better
maintenance of the original canal curvature and greater efficiency and safety.[14]
[15]
Even after conducting an extensive literature review on the subject, no study was
found comparing a memory-controlled NiTi alloy multiple-file rotary system versus
a MaxWire NiTi alloy single-file rotary system with respect to root canal shaping
in teeth with curved roots. Thus, the aim of the present study was to assess the shaping
of curved mesial canals of mandibular molars produced with the Hyflex CM or XP-endo
Shaper systems by a micro-CT analysis of preparation centering (apical transportation),
canal volume, canal surface area, and unprepared dentin wall parameters. The null
hypothesis tested was that both systems would provide equivalent canal shaping with
respect to the parameters analyzed.
Materials and Methods
Sample Size Calculation
Sample size was calculated using the Cohen method (1988). Considering test power of
80%, a sampling error of 5%, and an effect size of 0.85, it was determined that a
minimum number of 17 specimens per study group were required.
Selection of Specimens
This study was approved by the local institutional research ethics committee (register
no. 2.270.631). The specimens used were teeth indicated for extraction for reasons
unrelated to this research and were donated by patients to one of the authors. After
extraction, they were kept in distilled water in the institutional bio-repository
for a maximum period of 3 months.
From pool of 102 mandibular molars, those that met the following criteria were selected:
intact roots, complete root formation, no previous endodontic treatment, and severe
curvature—between 20 and 30 degrees (Schneider 1971). In addition, the mesial roots
had to have two independent foramina (Vertucci type IV configuration), whose existence
was confirmed by introducing a n. 10 K-type file (Dentsply Maillefer, Ballaigues,
Switzerland) in either canal until it was seen exiting the respective apical foramen
under the lens of an operating microscope (Alliance 20x, Alliance Comercial de São
Carlos, São Carlos, SP, Brazil). Thus, 54 mandibular molars were selected and included
in the sample.
The roots were scanned using a SkyScan 1173 micro-CT apparatus (Bruker Micro-CT, Kontich,
Belgium) set to operate at 50 kV, 80 µA, 360 degrees rotation around the vertical
axis with a rotation step of 0.9 degrees and using a 1-mm thick aluminum filter and
a pixel size of 12.11 µm. Once obtained, the images were reconstructed from cross
sections using NRecon v1.6.9.0 software (Bruker Micro-CT). The values of the length,
volume, surface area, and structure model index parameters of the canals were obtained
using CTAn v.1.14.4 software (Bruker Micro-CT). Seventeen mesial roots of mandibular
molars with similar values (p> 0.05) for these parameters were finally included in the study. Similarly, the mesial
canals were matched based on their similarity in terms of micro-CT measurements, and
then randomly distributed (www.random.org) into the two experimental groups.
Seventeen mesial roots of mandibular molars with similar values (p> 0.05) for these parameters were eventually included in the study. Likewise, the
mesial canals were matched based on their similarity in micro-CT measurements and
randomly assigned (www.random.org) to the two experimental groups.
Root Canal Preparation
All the experimental procedures were performed by a single operator, who was experienced
in the tested systems. The instruments were used only once, as a deformity in the
instrument could occur without any visible warning sign. The working length (WL) was
established at 1 mm short of the apical foramen. The root apex was sealed with gingival
isolation material (Topdam; FGM, Joinville, Santa Catarina, Brazil) to create a closed
system. Preparation was performed with the Hyflex CM and XP-endo Shaper systems alternately
in each mesial canal, to minimize possible influence of anatomical variation between
specimens. Glide-path creation and maintenance of foraminal and canal patency throughout
the instrumentation procedure were performed in all the canals with a n. 15 K-type
file, up to 1 mm short of the actual tooth length, and with a n. 10 K-type file, up
to 1 mm beyond this length, respectively.
The procedures were performed inside a booth containing a heater (800-Heater; PlasLabs,
Lansing, Michigan, United States) that maintained an ambient temperature of 37°C to
simulate actual clinical conditions.
Hyflex CM (Group HFCM)
Hyflex CM instruments were driven by a VDW Silver electric motor (VDW, Munich, Germany),
operating at a speed of 500 rpm and a torque of 2.5 N.cm. The canals were instrumented
using the crown-down technique, starting with a 25/.08 instrument introduced up to
the first two-thirds of the canal, followed by the 20/.04, 25/.04, 20/.06, and 30/.04
instruments, taken up to the WL.
XP-endo Shaper (Group XPS)
The XP-endo Shaper instrument was driven by a VDW Silver electric motor (VDW), operating
at a speed of 800 rpm and a torque of 1.0 N.cm. Once the instrument was introduced
into the canal, two gentle in-and-out movements toward the apex and up to the WL were
performed. After five repetitions of this instrumentation sequence, canal shaping
was tested with a 30/.04 gutta-percha cone (VDW), at which time it was considered
complete.
Before beginning instrumentation, the pulp chamber of the specimens from both groups
was filled with 2 mL of a 2.5% sodium hypochlorite (NaOCl) solution.
During instrumentation, canal irrigation was performed with 2 mL of 2.5% NaOCl at
each instrument change, in Group HFCM, and after each in-and-out movement, in Group
XPS, totaling 10 mL of solution per canal. A disposable hypodermic syringe attached
to a NaviTip 30-G irrigation needle and positioned 2 mm short of the WL was used in
the procedure.
Final irrigation in both groups was performed with a 5 mL of 17% ethylenediaminetetraacetic
acid (EDTA), followed by 5 mL of 2.5% NaOCl; the canals were dried with absorbent
paper points (Dentsply-Maillefer).
Micro-CT Evaluation
After root canal preparation, a postoperative scan was performed, and the images were
reconstructed following the same protocol used in the initial scan. The model images
were color-coded to allow a qualitative comparison of the canal shaping produced by
the Hyflex CM and XP-endo Shaper systems; the initial scans were coded in green, and
the final scans—referring to the results obtained after using either system—in blue
or red, respectively ([Fig. 1]).
Fig. 1 (A–D) Representative of final microtomographic image after preparation of mesial canals
with the Hyflex CM (in blue) and XP-endo Shaper (in red) systems; the unprepared dentin walls are represented in green.
CTAn v.1.16.4.1 software (Bruker micro-CT) was used to measure the volume and surface
area of the canals. Pre- and postoperative images were superimposed using Image J
v. 1.50d software (National Institute of Health, Bethesda, Maryland, United States)
to determine the value of the parameter of the unprepared surface area of the canal.
The area of this surface was determined by computing the static voxels (those voxels
whose position on the canal surface was unchanged after preparation) and expressed
as a percentage.
Canal centering ability was evaluated based on data on the variation of the centers
of gravity for each canal segment, connected along the Z axis, and the mean transportation
observed in each root segment (in mm) was calculated by comparing the centers of gravity
before and after preparation in the respective segments. Similarly, the transportation
for the entire canal was determined by averaging the three segments in each specimen.
Statistical Analysis
Data distribution was analyzed using the Shapiro-Wilk normality test. The Wilcoxon
test was used to perform intragroup comparisons between the two evaluated canal segments,
0 to 10 mm and 0 to 4 mm from the apex, with respect to the canal volume and surface
area parameters. Friedman's test was used to perform intragroup comparisons among
the cervical, middle, and apical segments for the center of gravity parameter (canal
transportation). The Mann-Whitney test was used to perform intergroup comparisons
for pre-preparation values of canal volume, and post-preparation values of canal volume
increase, surface area increase (%), unprepared areas, and canal transportation in
the cervical and middle segments. The independent t-test was used to perform intergroup comparisons for pre-preparation surface area,
and post-preparation percentage of unprepared areas, in the 0 to 10 mm and 0 to 4 mm
segments, and for variation in center of gravity in the apical segment and in the
entire canal. All the statistical tests were performed using a level of significance
of 5% (p <0.05).
Results
Canal Volume, Surface Area, and Unprepared Surface Area
Root canal volume values are shown in [Table 1]. Surface area and unprepared surface area, before and after preparation, are shown
in [Table 2]. Initial canal volume and surface area were similar in both groups (p >0.05) and increased significantly after preparation with both the systems tested
(p <0.01). There was no significant difference between the groups with respect to the
percentages of volume increase in the entire canal, and with respect to unprepared
areas in the 0 to 10 mm segment (p >0.05; [Table 2]). However, a significant difference was observed with respect to unprepared areas
in the 0 to 4 mm segment, with HFCM specimens showing significantly higher percentages
of unprepared areas than XPS specimens (p <0.01).
Table 1
Root canal volume (mm3) along the entire length of the canal (from 0 10 mm from the apex), before and after
preparation performed with the systems tested in the study
|
Group
|
Segment
|
|
Mean (SD)
|
Median
|
Interval
|
|
|
Initial
|
114,915.29 (21,683.11)
|
105,147
|
85,411–166,519
|
|
0–10 mm
|
Final
|
17,627.14 (5,449.96)
|
17,012
|
8,555–27,369
|
|
|
% unprepared
|
15.81A (5.25)
|
16.37
|
5.67–24.26
|
|
HFCM
|
|
|
|
Initial
|
20,695 (8,132.84)
|
22,016.50
|
7,206–31,637
|
|
0–4 mm
|
Final
|
6,021.28 (2,874.96)
|
6,093.50
|
1,917–10,269
|
|
|
% unprepared
|
28.46B (4.48)
|
27.80
|
21.67–36.75
|
|
|
Initial
|
119,266.57 (46,491.43)
|
109,767.50
|
49,163–197,619
|
|
0–10 mm
|
Final
|
15,047.86 (9131.30)
|
15,807
|
3,721–31,138
|
|
XPS
|
|
% unprepared
|
12.89A (6.62)
|
13.36
|
3.32–23.44
|
|
|
Initial
|
25,588.43 (12,012.82)
|
23,176.50
|
9,554–51,912
|
|
0–4 mm
|
Final
|
3,170.64 (1671.38)
|
2,516.50
|
1,059–6,435
|
|
|
% unprepared
|
13.26A (4.82)
|
13.29
|
3.58–19.52
|
* Values followed by the same superscript letter within columns indicate a statistically
significant difference.
Table 2
Untouched root canal surface area (in number of static voxels) in the two segments
evaluated (0–4 mm and 0–10 mm from the apex), before and after preparation performed
with the systems tested in the study
|
Group
|
Segment
|
Mean (SD)
|
Median
|
Interval
|
|
Cervical
|
0.75 (0.38)A
|
0.92
|
0.05–1.15
|
|
Middle
|
0.52 (0.29)A
|
0.53
|
0.05–0.98
|
|
HFCM
|
|
|
|
|
|
Apical\
|
0.35 (0.21)B
|
0.31
|
0.03–0.69
|
|
0–10 mm
|
0.33 (0.15)
|
0.33
|
0.09–0.58
|
|
Cervical
|
0.56 (0.36)
|
0.50
|
0.02–1.07
|
|
Middle
|
0.46 (0.39)
|
0.52
|
0.02–0.98
|
|
XPS
|
|
|
|
|
|
Apical
|
0.36 (0.17)
|
0.35
|
0.08–0.60
|
|
0–10 mm
|
0.49 (0.26)
|
0.57
|
0.01–0.79
|
a Values followed by the same superscript letter within columns indicate a statistically
significant difference.
Canal Centering Ability
In general, there was no variation between the groups or among the segments within
the same group with respect to the center of gravity parameter (canal transportation;
p >0.05); The only exception was found in the HFCM intragroup analysis, which revealed
the occurrence of significantly less canal transportation in the apical segment than
in the cervical and middle segments (p <0.05).
Discussion
The aim of the study was to conduct a microtomographic comparison of two rotary instrumentation
systems—the single-file XP-endo Shaper and the multiple-file Hyflex CM—with respect
to their shaping ability in mesial root canals of mandibular molars. The volume and
surface area of the canal, the percentage of unprepared walls, and the centering of
the preparation were evaluated. The null hypothesis was partially accepted, since
there were statistically significant intergroup and intragroup differences with respect
to some of the parameters evaluated.
Root canal anatomy has a direct influence on the quality of the preparation produced
by endodontic instrumentation. Mandibular molars were used in the present study because
these teeth are most indicated for endodontic treatment.[17] Their roots have moderate to severe curvature, with a higher prevalence of curvature
in their mesial canals.[6]
[14]
This factor increases the risk of apical transportation during instrumentation.[18]
[19] In addition, mandibular molars often have independent root canals in the mesial
root,[20] this enabled specimen standardization between the groups in the present study, since
each of the two root canals of the same root was instrumented using either of the
systems tested, thus mitigating possible interference of anatomical variation in the
results. Furthermore, all the procedures were performed by the same operator in a
booth heated to 37°C, considering that instruments submitted to heat treatment during
their manufacturing process may present a structural phase change at higher temperatures.[10]
Micro-CT was selected as the method of analysis, because it is a widely used, non-destructive
method that provides a 3D evaluation of the instrumentation results, without requiring
any alteration of root anatomy. As a result, it is considered the gold standard for
this type of analysis 14 to 16.
The XP-endo Shaper system consists of a new generation of instruments that have the
ability to expand beyond their nominal size, thus touching a larger area of dentin
walls during preparation and removing a greater amount of debris.[21] In the present study, its shaping ability was compared with that of the Hyflex CM
system, which is composed of controlled-memory- NiTi instruments. Owing to their high
flexibility and heat treatment, Hyflex CM instruments are suitable for the instrumentation
of curved canals and have shown a canal-centering ability superior to that of conventional
NiTi files.[22] However, unlike the XP-endo Shaper, these instruments lack the ability to contract
or expand beyond their core to better adapt to root canal anatomy.[12]
In the present study, specimens from Group XPS were associated with a lower percentage
of unprepared walls in the apical segment; however, their walls were not completely
instrumented, corroborating the results of previous studies.[10]
[23] The XP-endo Shaper instrument is made from MaxWire alloy (Martensite-Austenite Electropolishing-Flex,
FKG), which imparts a more rectilinear shape to the instrument when cooled (martensitic
phase), and then a “snake-like” shape when submitted to body temperature (austenitic
phase). This expandability, combined to its small mass, seems to have contributed
to the three-dimensional shaping of the canal, especially in the apical third. In
addition, owing to this change in crystal structure and to its 6-edge “booster tip,”
the XP-endo Shaper instrument can begin instrumentation following a glide-path with
an n. 15 diameter, and then expand to an n. 30 diameter. Previous research has found
that 17,[10] 31,[24] and 9.42%[12] of the total root canal wall area remained unprepared by the XP-endo Shaper file,
whereas 13% of the canal walls were found to be unprepared in the present study. This
variation in results could be explained by differences among specimens with respect
to root canal anatomy.[25]
A significant difference was observed in relation to unprepared areas in the 0 to
4 mm segment (p <0.01), with specimens from the HFCM Group showing significantly higher percentages
of unprepared areas than those from the XPS group. Other authors reported that the
fewer the areas a file works in root canal system, the greater the remaining pulp
tissue and microorganisms that may persist, thus contributing to reinfection as well
as interfering with obturation.[15]
[26]
Inadvertent apical transportation during root canal preparation depends on the degree
and radius of canal curvature, and, even more so, on the choice of instruments.[22] In our study, no significant difference was found between the instruments regarding
this parameter, demonstrating that there was little variation between them with respect
to their ability to produce well-centered preparations. This result could be explained
by the high flexibility of both systems; furthermore, it suggests that the type of
alloy used in the instruments may be partially responsible for their mechanical behavior
in curved canals,[6]
[19]
[24]
[27]
[28]
Intragroup analysis revealed that the Hyflex CM system promoted a lower degree of
transportation in the apical third than in the cervical third. Although the instrument
used for apical preparation in this group was the 30/.04, the one used for cervical
preparation was the 25/.08, which has a triangular cross-section.[25] Although not specifically designed to act as a canal orifice enlargement file (or
“orifice shaper”), this 25/.08 instrument promoted an enlargement corresponding to
its size in the cervical region, which may have contributed to obtaining this lower
transportation result.
This aspect is noteworthy, because a substantial enlargement of the cervical third
can lead to root weakening due to widening of the walls facing the furcation region.[4]
[29] With respect to the apical third, any transportation above 0.3 mm may have a negative
impact on treatment success rates.[29]
[30] A previous study found low percentages of apical transportation for the Hyflex CM
system in the apical and cervical thirds[6]; however, it should be borne in mind that the preparation in their study was performed
only up to an n. 25 instrument.
The concept of employing a single NiTi instrument to prepare the entire root canal[31] has been proven to enable a shorter learning curve and provide effective preparation.[12]
[13]
[32] In the present study, the single-file XP-endo Shaper system displayed a shaping
ability similar to that of a multiple-file Hyflex CM system; in addition, it prepared
a larger area of dentinal walls in the apical segment. The XP-endo Shaper was used
following manufacturer's recommendations, according to which the preparation can be
considered complete after five movements toward the WL, followed by another five movements
if needed, totaling 10 instrument penetrations; however, other study demonstrated
that an increase in instrumentation time with this system led to an increase in the
percentage of prepared walls, canal volume, and dentin removal, suggesting a correlation
between preparation quality and instrument application time.[8]
[9]
[28]
Further research is warranted to confirm the indication for using a single-file system,
and to explore aspects still lacking investigation in the literature, including possible
adjustments to the protocols of use and the development of new treatments for NiTi
alloys.
