Keywords HyFlex EDM file - OneShape - ProTaper NEXT - vertical force
Introduction
Superior chemomechanical debridement of the intricate root canal system is essential
for the attainment of optimal endodontic treatment outcomes.[1 ] Canal shaping procedures aim to develop a continuously tapering root canal that
facilitates the delivery of chemical irrigating solutions and intracanal medicaments
as well as three-dimensional obturation of the root canal system.[2 ] Nowadays, engine-driven nickel–titanium (NiTi) files are predominantly used to shape
root canals due to superior flexibility, improved cutting efficiency, proven canal
centering ability, reduced instrumentation errors, superior cyclic and torsional fatigue
resistance, reduced treatment time, and favorable treatment outcome compared with
hand instrumentation with stainless steel files.[3 ]
[4 ]
[5 ] Despite such advantages, fracture of NiTi instruments may occur during clinical
use as a result of either cyclic or torsional fatigue, or a combination of both mechanisms.[3 ]
[6 ] Such an incident can impede the cleaning and shaping of the apical portion of the
root canal system which, in turn, may affect the outcome of the case.[7 ] Therefore, various changes have been incorporated into the manufacturing process
of NiTi instruments to enhance the clinical performance and safety of root canal shaping
procedures. Such enhancements include alterations in file design, thermomechanical
treatment, motion kinematics, and electropolishing.[5 ]
[8 ]
Single file NiTi systems have gained widespread acceptance among dental practitioners
owing to reduced cost, instrumentation time, and cross-contamination risks.[9 ] The use of single file instruments was found to be equivalent to traditional multifile
systems in terms of canal cleanliness, although it took less time to complete the
shaping procedure.[10 ]
[11 ] OneShape system (OS; MicroMega, Besançon, France) is a single file NiTi rotary system
(25/.06) that is made of conventional NiTi alloys. OS is characterized by having an
asymmetric horizontal cross-sectional geometry with three sharp cutting edges and
a nearly triangular cross-section in the tip region. Near the shaft, the instrument
design changes into an S-shaped cross-section with two cutting edges.[10 ] HyFlex EDM file (HEDM; Coltene/Whaledent AG, Altstatten, Switzerland) is a new-generation
single file instrument that works in continuous rotation. Like HyFlex CM files (Coltene/Whaledent
AG), HEDM instruments are manufactured from a controlled memory alloy using electrodischarge
machining technology. HEDM OneFile (25/∼) has a constant taper of 0.08 in the apical
4 mm of the file but that decreases to 0.04 in the coronal part of the instrument.
This novel instrument has three different cross-sectional designs over the length
of its working part: a quadratic cross-section near the tip, a trapezoidal cross-section
in the middle area, and an almost triangular cross-section near the shaft. The hardened
surface created by the EDM process and the variable cross-sectional design of the
instrument increase the cutting efficiency and fatigue life of the instrument.[12 ]
It is known that during root canal shaping, NiTi files are subjected to repeated cycles
of multidirectional loadings which negatively affect the fatigue resistance of the
instrument resulting in file fracture and other intraradicular defects such as root
canal aberrations and dentinal cracks.[11 ]
[13 ]
[14 ] The shaping forces generated during root canal preparation can be influenced by
different factors such as the contact area between the file and the canal walls, instrument
design and geometry, preoperative canal size, file's motion kinematics, and forces
exerted by the clinician.[5 ]
[15 ]
[16 ]
[17 ]
[18 ]
[19 ] Hence, assessing the magnitude of vertical forces generated during root canal shaping
procedures is essential. The shaping force generated with single file systems during
root canal preparation has not been adequately studied. Therefore, this study was
conducted to assess the vertical forces produced with OS and HEDM single file systems
and compare them with ProTaper Next (PTN) during canal shaping. The null hypothesis
was there would be no difference between the three tested systems in the vertical
forces generated during canal shaping.
Materials and Methods
Selection of Teeth and Preparation of Samples
This study was performed in accordance with the ethical guidelines of the Institutional
Review Board (RC17/008/R) and the principles of Declaration of Helsinki. Before the
commencement of the study, estimation of required sample size was established based
on initial piloting. The sample size was determined using a 5% significance level
and 80% power to detect a 2.1 N force difference between the three tested systems.
The standard deviation was set at 2 N. According to this, 15 root canals were included
for each group.
In this experiment, sound and fully formed two-rooted maxillary premolar teeth with
two distinct canals starting at pulp chamber floor level were chosen from a pool of
extracted human teeth that were maintained in distilled water. Facial and proximal
digital periapical radiographs were obtained using Planmeca ProX™ (Planmeca®, Helsinki,
Finland) to confirm the presence of mature roots, constricted canals, and relatively
straight root canals (less than 10-degree curvature).[20 ]
Access cavity preparation was accomplished and the canals were checked for patency
and size. The working length (WL) was determined by placing a size 10 K-file into
the canal until the tip of the instrument became flush with the apical foramen. The
distance from the reference point in the occlusal surface to the file tip minus 0.5 mm
was considered the WL of the canal. The root canal was considered sufficiently constricted
by placing a size 15 stainless steel file that would bind 3 mm or more away from the
canal length. If these criteria were not met, the tooth was excluded from the study
sample. Hence, 45 root canals were selected for this study. Thereafter, the apical
root tip was covered with utility wax, and the whole tooth was embedded in a mixed
autopolymerizing resin (DuraLay; Reliance Dental Manufacturing Co, Worth, IL) that
enclosed the root surface along with the surrounding wax and left to set in room temperature.
After the complete resin setting, a manual glide path was established for each canal
using sizes 10 and 15 K-file. Throughout the glide path preparation phase, canals
were irrigated with 1% NaOCl followed by a final rinse with saline. Afterward, each
sample was wrapped in moist gauze and stored in a test tube.
Vertical Force Measurements
A force gauge (M5-20 Advanced Digital Force Gauge; Mark-10 Corporation, Long Island,
NY) with a force capacity of 100 N and a capability to measure data every 0.1 second
was used to assess canal shaping forces in real time ([Fig. 1 ]). This force gauge is designed for tension and compression testing across various
industries. The device features a sampling rate of 7,000 Hz making it capable to produce
accurate readings even for quick-action test. Accuracy is ±0.1% of full scale while
resolution is 1/5,000.
Fig. 1 Schematic drawing of the experimental setup. The force gauge was used for recording
vertical forces in inward and outward directions and the received data are presented
on a personal computer using MESUR Lite software (Mark-10 Corporation).
The generated forces during canal shaping procedures were displayed using the MESUR™
Lite software (Mark-10 Corporation, NY). Before the commencement of canal shaping
for each sample, each tooth was set on a fixed stage and the force gauge was zeroed.
The vertical shaping forces generated during canal shaping were obtained in both apical
(inward) and coronal (outward) directions. The vertical force data were expressed
in newtons.
Root Canal Shaping
Following the preparation of a manual glide path to WL with a size 15 K-file, the
root canals were randomly assigned based on their WL to three similar experimental
groups; OS, HEDM, and PTN systems (n = 15 each). The average WLs of the OS, HEDM, and PTN groups were 20.57 ± 1.43, 21.20 ± 1.81,
and 21.07 ± 2.05 mm, respectively. There were no statistically significant differences
in WLs among the three groups (p = 0.594).
All canal shaping procedures were performed using the Element Motor (SybronEndo, Orange,
CA) under continuous rotary motion. Canals in the OS group were shaped with the file
size 25/.06 at a speed of 400 rpm. In the HEDM group, the OneFile instrument was used
to complete canal shaping at a speed of 500 rpm. In the two groups, each canal was
shaped with one file inserted three times until reaching the WL. Each insertion cycle
incorporated a maximum of three pecking motions. In the PTN group, canal shaping was
completed using an X1 file inserted once into WL, followed by an X2 file inserted
into WL through two insertions at a speed of 300 rpm. In agreement with the manufacturer's
recommendations, the torque setting was standardized for all experimental groups at
a value of 2.5 Ncm. Shaping procedures were completed using gentle in-and-out movements
under copious irrigation with 1% NaOCl.
After each instrument insertion, the canal was irrigated with 1 mL of 1% NaOCl, apical
patency was verified using a size 10 K-file, and the file's flutes were cleaned with
alcohol swab. Each rotary file was used to shape a maximum of four root canals unless
evidence of deformation or fracture was noted earlier. The root canals were shaped
by a single experienced operator. The mean force values of the three shaping insertions
were used for comparison. The shaping time, defined as the active instrumentation
time, was recorded.
Data Analysis
Since the vertical force data were not normally distributed (Shapiro–Wilk test; p < 0.05), differences between the mean coronal and apical maximum force values were
analyzed using the Kruskal–Wallis and Mann–Whitney U tests.
The WL of selected teeth and canal shaping time were tested for statistical significance
using a one-way ANOVA test. All statistical analyses were performed using SPSS software
version 22 (SPSS Inc, Chicago, IL). The level of significance (α) was set as 0.05.
Results
In this study, each canal was shaped through three file insertions. The magnitude
of the vertical forces increased with successive advancements of the files within
the canal ([Fig. 2 ]). During canal shaping procedures in all groups, the apical and coronal maximum
force values ranged from 2.2 to 7.2 N and 1.3 to 2.9 N, respectively. There were significant
differences in maximum force values among the tested groups at each insertion.
Fig. 2 The means and standard deviations of apical (positive values) and coronal (negative
values) forces for all insertions in the tested systems. The asterisk (*) indicates
statistical significance. HEDM, HyFlex EDM; OS, OneShape; PTN, ProTaper Next.
For forces generated during apical file advancement, PTN generated the lowest peak
loads during the second and third insertions (p < 0.05). For vertical forces induced during coronal withdrawal of the instrument,
HEDM generated significantly less maximum forces than both OS and PTN during the first
insertion while the use of OS was associated with the highest amount of force values
in the second and third insertions (p < 0.05).
The average shaping times of the OS, HEDM, and PTN groups were 21.95, 23.61, and 21.91 seconds,
respectively, with no significant differences between the groups (p = 0.606). No file fracture or plastic deformation was encountered throughout the
experiment.
Discussion
Safe and effective chemomechanical preparation of the root canals relies on the endodontic
file performance.[21 ] Single file systems produce predictable canal cleanliness with reduced instrumentation
time and cost.[11 ]
[22 ]
[23 ] Based on this, the current study was conducted to provide data on the vertical forces
generated with OS and HEDM during the shaping of narrow canals, in comparison with
PTN.
OS and HEDM systems successfully shaped the root canals with only one file with no
prior shaping with smaller files. This results in a large contact surface with the
canal walls that would generate great stresses and strains on the dentin and file.[24 ] This is evident in the current findings wherein the magnitude of the vertical forces
increased with successive use of the files within the canal. Consistently, previous
studies showed that more forces are needed to reach the WL in single file systems
or to shape the canal with a larger file size in multifile systems.[15 ]
[16 ]
[24 ]
[25 ]
[26 ]
It should be mentioned that this study results should be interpreted with caution.
Considering that the roots were fixed to acrylic resin and the forces were transmitted
to the gauge through the acrylic resin, the observed values are probably greater than
the forces generated in a clinical setting. In a clinical scenario, the periodontal
ligament and surrounding bone would disperse the forces. Nonetheless, the testing
conditions were standardized throughout the experiment and differences in vertical
forces were shown between the tested systems in shaping narrow canals. In terms of
the apical forces, OS and HEDM generated apical forces that exceeded 7 N in the last
insertion. The results were found to be higher than PTN in the second and third insertions.
Similarly, a previous study showed that apical forces generated with WaveOne and WaveOne
Gold single file systems were high.[17 ] Another study showed that multifile systems generated apical forces of no more than
6.4 N.[26 ] This supports the idea that shaping the canal successively with a smaller file size
led to lower forces generated by the last file and that canal shaping with single
file systems has been mostly based on simplicity but not proven effectiveness.[11 ] Nevertheless, a recent study revealed that preshaping canal with a smaller file
could reduce the generated forces induced during the shaping under reciprocating motion.[27 ]
The “screwing-in” effect of the examined files was tested by measuring how much force
was needed to pull the file coronally.[28 ] It may cause file penetration beyond the apex, leading to overshaping and cracks
on the apical root surface.[29 ] The results showed significant differences between OS and HEDM in the three insertions.
Although the HEDM file had a larger taper in the apical part, its rotational speed
is lower than the OS file. It has been reported that the rotational speed affects
the contact time of the file with the canal walls.[30 ] The high speed releases the close binding of cutting edges to the canal wall which
reduces the screwing-in force.[30 ] This could explain the favorably low coronal forces required to shape the canal
with HEDM.
Clinically, the shaping procedure creates unavoidable stresses mainly at or near the
file tip.[31 ] This stress induces strain on the canal walls that would lead to the development
of dentinal defects.[24 ]
[29 ]
[32 ] Moreover, it has been shown that the shaping force generates operational torque
that creates higher stresses on the file, resulting in file fracture.[6 ]
[13 ]
[25 ] Therefore, gentle shaping is needed.[33 ] Although higher vertical forces were observed with the tested single file systems
(OS and HEDM) compared with PTN, no plastic deformation or fracture took place. This
result may indicate that the OS and HEDM are resistant to fracture and can shape up
to four narrow canals using the described shaping technique. This is in agreement
with previous findings.[17 ]
[23 ]
In this study, there are certain limitations that must be acknowledged. First, the
study investigated only one aspect of file behavior, which is the vertical force induced
during canal shaping procedure. Hence, future experiments should examine the effect
of generated force on other attributes such as associated torque, file deformation,
and fracture, as well as on the root canal wall. Second, despite the variations in
root canal morphology of natural teeth, the research team attempted to maintain comparability
of the experimental groups. For instance, only defect-free two-rooted maxillary premolar
teeth with two constricted and relatively straight canals starting at pulp chamber
floor level were considered for inclusion in the study, and the three groups were
balanced with respect to the WL. Another limitation of this study relates to the possible
differences in pressure exerted by the operator during canal shaping. Future studies
should consider using automated devices to allow for consistent vertical pressure
and pecking motion to eliminate the potential for operator bias. Nevertheless, canal
shaping was performed by one experienced operator who maintained gentle “in-and-out”
movements of 2 to 3 mm amplitude each to ensure consistency across the experimental
groups.
Conclusion
Within the current study parameters, the amount of vertical forces increased with
the progressive advancements of the instruments within the canal. PTN generated the
lowest maximum apical forces during the second and third insertions. HEDM generated
significantly less maximum coronal forces than both OS and PTN during the first insertion
while the use of OS was associated with the highest amount of force values in the
second and third insertions. Thus, the null hypothesis was rejected.
Further investigations should be conducted to simultaneously examine the effect of
developed vertical forces on instrument performance such as associated torque and
fatigue behavior as well as on the canal structure in other groups of teeth with wider
variety of pulp space features.
