Keywords Endodontics - instrumentation systems - micro-CT scan - root canal preparation - severely
curved roots
Introduction
The instrumentation of the curved root canals presents a challenge considering the
definite risks of canal transportation and perforation.[[1 ]],[[2 ]] In addition, in curved roots, the instrument cannot promote adequate three-dimensional
(3D) preparation, which leaves a significant percentage of the canal surface area
untouched.[[3 ]] The permanence of debris may predispose patients to persistent infection and consequently
compromise the root canal treatment.[[4 ]]
To minimize the limitations imposed by this anatomical complexity, instruments have
undergone modifications in the composition of their alloys to improve their flexibility.
Reciproc (RC) instruments (VDW, Munich, Germany) are made of an M-Wire nickel-titanium
(NiTi) alloy, which has greater flexibility and cyclic fatigue resistance than traditional
NiTi alloys.[[5 ]] However, the pressure exerted on the instrument during its use increases the possibility
of canal transportation.[[6 ]] The One-Shape (OS) (OS; Micro Mega, Besancon, France) is composed of a conventional
superelastic austenite 55-NiTi alloy.[[7 ]] The files have variable pitch length along the entire blade, which limits the risk
of minimal instrument fatigue, consequently eliminating the risk of instrument breakage.[[8 ]]
The twisted-file-adaptive (TFA) system (SybronEndo, Orange, CA, USA) consists of a
sequence of three NiTi instruments for use in a specific reciprocating motion.[[9 ]] These instruments are created by the transformation of a raw NiTi wire in the austenite
crystalline structure phase into a different phase of crystalline structure (R-phase).
Such a treatment confers instrument flexibility, allowing it to adjust the torque
values such that the elastic limit is not exceeded.[[10 ]]
The literature shows that TFA provides better root canal centralization than do systems
that work with continuous and alternate rotational kinematics.[[11 ]],[[12 ]] Another study suggested that rotary glide path preparation before biomechanical
performance improves the ability to shape the area while following the original root
canal curvature.[[13 ]] The ProGlider (PG) (Dentsply Sirona, Ballaigues, Switzerland) is a rotary glide
path instrument manufactured from the M-Wire alloy. The PG has a tip diameter of 0.16 mm
and a progressive taper ranging from 2% to 8%.[[14 ]]
Micro–computed tomography (micro-CT) has been used to evaluate the morphological changes
in the root canal generated by biomechanics since it offers more detailed images and
sample preservation.[[15 ]]
Morphological alterations occurring in dentin during excision are generally beneficial
for sanification, as long as they do not promote canal transportation. Therefore,
this study analyzed the two-dimensional (2D) and 3D parameters and canal transportation
of severely curved roots prepared by three mechanical systems with different kinematics
preceded or not by glide path using micro-CT. The null hypothesis was that there would
be no difference in the 2D and 3D parameters between systems with or without glide
path preparation.
Materials and Methods
Sample selection and preparation
The study was approved by the institute's research ethics committee (CEP/FORP-USP,
N. 2010.1.1478.58.5). A total of 170 human mandibular molars with complete root and
apical formation were selected from the human permanent tooth bank of the School of
Dentistry of Ribeirão Preto.
The teeth were radiographed with a digital radiograph sensor (IDA; Dabi Atlante, Ribeirão
Preto, SP, Brazil). The exclusion criteria were pulp nodules, root canal and/or pulp
chamber calcification, internal resorption, previous root canal treatment, and perforated
root. A total of 151 maxillary molars remained after the preselection. The same radiographic
images were used to determine the curvature angles and radius. The Dimension Angle
tool of CorelDraw Graphics Suite X6 software (Corel, Ottawa, Canada) was used for
this purpose. The angle severity pattern followed the recommendations of Schneider
(1971). Thus, a line (a) was drawn from the entrance of the root canal, following
the long axis of the root canal. Concomitantly, a second line (b) was drawn, extending
from the end of the apical foramen to the intersection with the first line. The point
of intersection of the lines indicated the beginning of the curvature [[Figure 1 ]]. Molars that exhibited mesial roots with an angle of curvature ≥25° were selected.
Subsequently, the radius of curvature was determined following the recommendations
of Pruett et al. (1997). Thus, we identified the point at which the root canal begins to deviate from
its long axis in each of the lines drawn (a, b), to determine the angle of curvature.
Finally, a circle encompassing the two previously defined points was drawn. The radius
of the circle was defined as the radius of curvature [[Figure 1 ]].
Figure 1: Determination of root canal curvature and radius of curvature
Among them, 105 molars with severe curvature (≥25°) and radius of curvature (≤5 mm)
were selected. The specimens were mounted in a sample holder and subjected to micro-CT
scanning (SkyScan 1174 v2; Bruker-microCT, Kontich, Belgium). The parameters used
were 50 kV, 800 mA, 360° of rotation with a step size of 1°, and an isotropic resolution
of 22.90 μm. Of the 105 scanned specimens, the first 30 teeth that had two independent
root canals with two separate apical foramina in the mesial root were selected.
Images of all specimens were reconstructed using NRecon v.1.6.3 software (Bruker-microCT,
Kontich, Belgium), which provided axial cross-sections of the inner structure of the
roots in bitmap format. Morphological 2D (area, perimeter, circularity, and major
and minor diameters) and in 3D parameters (volume and surface area) of the root canal
were investigated using CTAn v.1.14.4 software (Bruker-microCT, Kontich, Belgium).
Root canal preparation
The experimental groups consisting of 60 root canals of 30 molars were distributed
using a stratified sampling technique. Six groups (n = 10) were formed and paired by length, volume, and area of each root canal. The
sample data were normally distributed (Shapiro–Wilk, P > 0.05) with homogeneous variance (Levene's test, P > 0.05), confirming the uniformity of selection.
After endodontic access was achieved, the working length (WL) of each root canal was
determined by subtracting 1 mm from the length where size 10 was visible at the apical
foramen. Biomechanical preparation was performed according to the manufacturer's recommendation:
Group 1, TFA instruments (size 20, 0.04 taper; size 25, 0.06 taper). TFA size 20.04,
followed by a 25.06 instrument were taken into the root canal until WL was achieved
and powered by an electric Sybron Elements motor (SybronEndo, Orange, CA, USA); Group
2, RC (size 25, 0.08 taper). The instrument R25 was operated in a reciprocating motion
by an electric VDW Silver motor (VDW, Munich, Germany). The instrument was introduced
into the root canal until resistance was felt and then activated in an apical direction
using in-and-out pecking motion about 3 mm in amplitude with light apical pressure.
After the three pecking motions were made, the instrument was removed from the root
canal and cleaned; Group 3, OS instrument (size 25, 0.06 taper). The OS instrument
was used with a rotary motion by an electric X-Smart Plus motor (Dentsply Sirona,
Baillaigues, Switzerland) to reach 2/3 of the WL, WL – 3 mm, and the WL; Group 4,
glide path with a PG instrument followed by TFA instruments; Group 5, glide path with
a PG instrument followed by a RC; and Group 6, glide path with a PG instrument followed
by an OS instrument. Initially, in these groups, the PG instrument was used in one
or more passes until the WL was achieved, and then the different systems were used.
All of the instruments were operated using a torque-controlled endodontic motor according
to the manufacturers' recommendations. Thus, the TFA instrument was operated in the
TF Adaptive program Elements motor (SybronEndo, Orange, CA, USA), the RC was operated
in the “Reciproc ALL” program (VDW, Munich, Germany), the OS was operated at 400 rpm
and 2.5 Ncm torque, and the PG was operated at 300 rpm and 2 Ncm torque. All instruments
were scanned and discarded after each use. During preparation, 3 mL of a 1% sodium
hypochlorite solution was used between each file or after three pecking motions. In
all groups, the biomechanical procedure was performed by a specialist endodontist
(Souza-Flamini L.E.), who had clinical experience with the use of the three systems
assessed.
The prepared roots were subjected to micro-CT scanning (SkyScan 1174 v2; Bruker-microCT,
Kontich, Belgium). The pre- and post-preparation images [[Figure 2 ]] were analyzed using the CTAn v. 1.14.4.1+ (Bruker-microCT, Kontich, Belgium) to
calculate canal transportation.
Figure 2: Three-dimensional models of root canal systems of the mesial root of mandibular molars
before and after biomechanical preparation with the Twisted-File Adaptive (a), Reciproc
(b), and One-Shape (c) systems. The green color indicates uninstrumented areas of
the canals, whereas the red color indicates instrumented areas
Statistical analysis
Two-way analysis of variance was used to evaluate the influence of the glide path
(with and without glide) and file instruments (TFA, RC, and OS), followed by Tukey's
test (α = 0.05) using SigmaPlot v11.0 (Systat Software, Chicago, IL, USA). The confidence
level was 95%.
Results
Two-dimensional analysis
[[Table 1 ]] shows the 2D parameter analysis. Without the glide path, the increase in the root
canal area promoted by the RC in the final apical 5-mm section was significantly higher
than that promoted by the TFA (P < 0.001). No significant difference was observed between OS and the other techniques
(P = 0.744). With the glide path, the increase in the root canal area promoted by the
RC was significantly higher than that promoted by the OS and TFA instruments (P < 0.001). No significant difference was detected between the latter two instruments
(P = 0.274). The instrumentation with or without the glide path did not have any influence
on increasing the root canal area regardless of the system used (P = 0.744). The mean percentage of perimeter increase in the root canal was higher
for RC, followed by TFA and OS (P < 0.001). The use of PG did not influence the perimeter increase compared with that
of the system alone (P = 0.620). The higher mean percentage of diameter increase in the final 5-mm section
of the root canal was provided by RC, followed by TFA, and there were significant
statistical differences between them (P < 0.001). The increase in the minor canal diameter was not influenced by the use
of PG (P = 0.812).
Table 1:
Analyze the two.dimensional parameters (means of the final 5 mm)
Without PG
With PG
TFA
RC
OS
TFA
RC
OS
Area
Original
0.12±0.07
0.13±0.05
0.13±0.05
0.12±0.04
0.11±0.04
0.13±0.06
Δ
0.07±0.03B
0.13±0.04A
0.10±0.07AB
0.08±0.05b
0.13±0.06a
0.08±0.07b
Δ (%)
78.49±51.24
148.0±114.0
85.77±67.59
87.67±74.38
139.28±89.25
84.59±92.66
Perimeter
Original
1.26±0.23
1.31±0.29
1.30±0.16
1.20±0.31
1.28±0.19
1.27±0.18
Δ
0.40±0.40B
0.55±0.24A
0.25±0.21C
0.39±0.23b
0.50±0.22a
0.27±0.20c
Δ (%)
34.26±31.63
46.30±28.96
22.73±22.72
34.74±23.92
39.43±21.77
23.42±21.37
Roundness
Original
0.52±0.19
0.60±0.17
0.44±0.21
0.53±0.18
0.59±0.17
0.44±0.19
Δ
0.17±0.19A
0.24±0.19A
0.22±0.25A
0.17±0.22a
0.26±0.18a
0.24±0.23a
Δ (%)
47.08±60.89
56.16±62.17
91.34±126.99
49.10±72.26
51.94±62.63
76.14±82.04
Major diameter
Original
0.47±0.14
0.48±0.14
0.50±0.14
0.44±0.13
0.46±0.13
0.50±0.14
Δ
0.11±0.10A
0.12±0.11A
0.11±0.11A
0.12±0.15a
0.12±0.09a
0.09±0.08a
Δ (%)
25.95±22.77
32.70±34.80
26.15±28.98
27.52±30.89
28.89±2.72
21.38±23.57
Minor diameter
Original
0.28±0.07
0.32±0.08
0.30±0.10
0.31±0.08
0.30±0.07
0.32±0.12
Δ
0.13±0.07C
0.22±0.08A
0.16±0.09B
0.10±0.08c
0.20±0.10a
0.15±0.10b
Δ (%)
55.21±39.38
79.65±43.21
62.93±44.47
36.62±28.88
73.70±47.17
60.35±50.41
Different uppercase letters in rows are designed to compare instrumentation systems
without PG (P <0.05), Different lowercase letters in rows are designed to compare instrumentation
systems with PG (P<0.05), Δ – Mean±standard deviation, TFA – Twisted.file.adaptive,
OS – One.Shape, RC – Reciproc, PG – ProGlider
Three-dimensional analysis
The 3D parameter analysis is shown in [[Table 2 ]]. There was no significant difference in the root canal final volumes promoted by
RC and OS (P = 0.409). However, both instruments promoted a significant increase in root canal
volume compared with TFA (P < 0.001) regardless of the use of PG (P = 0544). The increased surface area obtained with RC was significantly higher than
that obtained with TFA (P = 0.024). There was no significant difference between OS and the other instruments.
The previous use of PG instrumentation did not affect the root canal surface area
(P = 0.123).
Table 2:
Analyze the three.dimensional parameters
Without PG
With PG
TFA
RC
OS
TFA
RC
OS
Volume
Original
1.63±0.64
1.76±0.78
1.75,±0.81
1.63±0.49
1.70±0.67
1.84±0.89
Δ
1.07±0.36B
2.03±0.45A
2.01±0.48A
1.22±0.23b
1.85±0.53a
1.83±0.50a
Δ (%)
72.1±25.7
136.2±67.0
149.0±89.1
88.2±32.1
121.2±48.3
119.6±75.8
Surface area
Original
17.84±5.10
20.62±5.62
22.84±7.70
20.20±4.26
20.08±4.77
23.75±7.12
Δ
3.40±1.31B
5.47±1.38A
4.94±1.84AB
3.49±1.14b
4.77±1.39a
3.91±1.11ab
Δ (%)
24.9±17.0
28.8±11.5
28.4±16.9
23.6±14.5
26.4±9.9
18.7±9.6
Different uppercase letters in rows are designed to compare instrumentation systems
without PG (P <0.05), Different lowercase letters in rows are designed to compare instrumentation
systems with PG (P <0.05). *In rows represent statistical significant difference between instrumentation
systems without PG and with PG (P <0.05). Δ – Mean±standard deviation, TFA – Twisted.file.adaptive, OS – One shape,
RC – Reciproc, PG – ProGlider
Root canal transportation
A significant statistical difference was observed in the values of canal transportation
between the instrumentation systems in only the cervical third [P = 0.045; [Table 3 ]]. For this third, OS yielded a higher rate of canal transportation than did RC (P < 0.001). There was no significant difference between OS and TFA. PG use had no effect
on canal transportation (P = 0.124).
Table 3:
Mean values±standard deviation of root canal transportation for each root canal third
Root third
Without PG
With PG
TFA
RC
OS
TFA
RC
OS
Cervical
0.13±0.05AB
0.13±0.03B
0.17±0.07A
0.12±0.05ab
0.18±0.05b
0.15±0.04a
Middle
0.11±0.03A
0.11±0.05A
0.15±0.07A
0.08±0.04a
0.12±0.07a
0.19±0.07a
Apical
0.15±0.08A
0.11±0.04A
0.14±0.06A
0.17±0.12a
0.10±0.04a
0.15±0.10a
Total
0.13±0.06AB
0.11±0.04B
0.15±0.07A
0.12±0.08ab
0.11±0.06b
0.14±0.08a
Different uppercase letters in rows are designed to compare instrumentation systems
without PG (P <0.05), Different lowercase letters in rows are designed to compare instrumentation
systems with PG (P <0.05). TFA – Twisted.file.adaptive, OS – One shape, RC – Reciproc, PG – ProGlider
Discussion
In the present study, 2D and 3D morphological changes, as well as canal transportation
caused by the continuous motion (OS), one reciprocating single-file system (RC), and
one combined continuous/reciprocation motion (TFA), were evaluated using micro-CT
scanning. The results revealed that the systems assessed showed significant differences
in the parameters studied. Thus, the null hypothesis was rejected.
Micro-CT scanning offers a simple and reproducible technique for a 3D noninvasive
assessment of the root canal system, and it can be applied quantitatively as well
as qualitatively.[[11 ]] This method has been used to assess the effects of instrument systems on root canal
geometry and transportation.[[12 ]] Mesial roots of extracted mandibular molars were used in this study. Even though
simulated root canals using prefabricated resin blocks allow better standardization
of the sample, the hardness and abrasion of acrylic resin and root dentin are not
identical, and consequently does not reflect the action of the instruments in root
canals of human teeth.[[16 ]],[[17 ]]
The RC instrument caused higher increases in the area, perimeter, and minor diameter
than did the other systems, regardless of the use of PG, confirming previous reports.[[12 ]],[[18 ]] The increase in these parameters suggests a greater cutting capacity of the instrument.[[12 ]] Their cutting capacity results from a complex relationship among different parameters,
such as the sectional design of the instrument, cutting angle of inclination, metallurgical
properties, metal surface of active parts, and motion kinematics.[[6 ]] The TFA and OS systems have triangular cross-sectional shapes, whereas the RC system
has variable tapers, a sharp double cutting edge, an S-shaped geometry, and a smaller
cross-sectional area, with greater cutting capacity.[[19 ]] It should also be considered that although the instruments of the three tested
systems had #25 diameter, the RC has the largest taper, consequently promotes greater
divergence of the root canal walls.
The increased area and perimeter imply a greater enlargement of the root canal diameter.
The removal of a greater amount of contaminated dentin is desirable in the case of
necrotic teeth, as bacteria can penetrate dentinal tubules to a depth of approximately
420 μm.[[20 ]]
The three systems assessed were similar in relation to the circularity parameter.
Considering that circularity values range from 0 to 1, where “0” corresponds to a
straight line and “1” to a perfect circle; the mean values of 0.17–0.26 in the present
study indicate that the root canal had a flattened shape at the end of root canal
treatment. Thus, the preparation of the complete circumference of the root canal is
probably not possible with any system. Instrumentation systems, using NiTi or stainless
steel instruments, are not able to prepare 100% of the circumference of curved and
flat root canals.[[18 ]]
Root canal volume is a variable used to analyze the effects of root canal instrumentation
on dentin removal. In the present study, the data showed that RC had a significantly
better volume and area increase than did TFA. The section design, taper angle, and
cutting capacity may explain this result. The more refined cut associated with greater
conicity resulted in a greater amount of excised dentin in more divergent root canal
walls and a consequently greater root canal volume.[[6 ]],[[13 ]] Notably, root canals that present an anatomical configuration with these parameters
favor the irrigation or aspiration of the irrigating solution, making the filling
phase easier. However, excessive root canal wear can promote root weakening or perforation.
A similar result was reported in the literature.[[13 ]] A study points out that although RC removes a greater amount of dentin than do
WaveOne and TFA, approximately 20%–35% of the surface of the root canal was found
to have no instrumentation after preparation in the three groups.[[13 ]] Untouched dentin provides a reservoir for biofilms and allows persistent infection.[[4 ]]
One study reported that the glide path minimizes the risk of canal transportation.[[16 ]] The results showed that in the middle and apical thirds, canal transportation was
similar to the RC, OS, and TFA systems, regardless of the use of PG. The lack of any
influence of the glide path on canal transportation corroborates the findings of previous
studies.[[21 ]],[[22 ]],[[23 ]] Flexibility may influence the instrument's ability to properly shape curved root
canals.[[24 ]] The RC and TFA instruments are composed of NiTi produced with M-Wire thermomechanical
processing, which in the literature shows better properties in terms of flexibility
and resistance to mechanical stress.[[25 ]] In contrast, the OS instrument is made of traditional NiTi alloys.[[26 ]] Based on the results of the present study, this thermal treatment does not appear
to increase the instrument's flexibility under our simulated clinical conditions.
However, canal transportation is also related with instrument cross-section.[[27 ]] Different cross-sectional shapes can promote higher or lower flexibility of the
instrument.[[27 ]] Therefore, a less flexible instrument may yield similar canal transportation performance
results as reported by Brasil et al. [[28 ]] Another study[[16 ]] reported that the glide path affects the transportation of the root canal. These
contrasting results can be explained by the type of instrument used. In this study,
the glide path was created with three types of instruments: 1 made of stainless steel
and 2 of NiTi. Canal transportation was significantly more frequent with the use of
the stainless steel instrument, with no difference between the other two. The rigidity
and lower flexibility of the stainless steel instrument compared to the NiTi instruments
favored canal transportation. Although the results of the present study indicate that
the creation of the glide path does not interfere with transportation of the root
canal, previous work emphasizes that the glide path decreases the working time.[[16 ]],[[23 ]],[[29 ]] as well as extrusion of material beyond the apical foramen.[[16 ]] The extrusion of debris may cause postoperative complications and pain.[[26 ]] Thus, to obtain safe and efficient outcomes, the establishment of a glide path
before root canal treatment is still recommended. In this study, the morphological
changes of the root canal with the use of the three systems were assessed in the apical
5 mm of the root canal. The apical third is the critical area of the root canal, and
remaining pulpal and inorganic debris have been detected in this area.[[30 ]] Thus, based on the results obtained and considering the limitation of an “ex vivo ” study, CR was more efficient in promoting enlargement of the apical third without
leading to significant canal transportation compared with the other systems. This
observation may have clinical relevance mainly in cases of pulp necrosis where the
removal of contaminated dentin is essential for the success of the treatment.
Conclusions
Use of the RC instrument produced greater modifications in most of the 2D and 3D parameters,
which can be a positive factor mainly in the treatment of teeth with root canal infections.
The RC, OS, and TFA systems were similar in terms of canal transportation, and although
the glide path did not influence the parameters assessed, its clinical use should
be taken into consideration.
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