Open Journal of Stomatology, 2013, 3, 386-391 OJST Published Online October 2013 (
Shaping ability of thermomechanically treated files in
simulated S-shaped root canals
Nessrin A. Taha1*, Ghada A. Maghaireh1, Dyana El Sadek1, Rafat Bagheri2, Mohammad Al-Omari1
1Department of Conservative Dentistry, Jordan University of Science and Technology, Irbid, Jordan
2Biomaterial Research Center, School of Dentistry, Shiraz University of Medical Sciences, Shiraz, Iran
Email: *
Received 2 August 2013; revised 21 October 2013; accepted 28 October 2013
Copyright © 2013 Nessrin A. Taha et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Aim: the aim of this study was to investigate the
shaping ability of thermomechanically treated files
manufactured by twisting (Twisted files) and com-
pare it to conventional rotary system (K3, Sybron
Endo, Orange, CA) in S-shaped canals, including
formation of ledges, zipping, elbow, outer widening,
danger zone, perforation and file deformation. Mate-
rials & Methods: Forty S-Shaped canals in resin
blocks were randomly divided into 2 groups of 20
each. Pre-instrumentation images of the canals were
taken via a digital camera and superimposed on im-
ages taken after preparation with TF and K3 systems
to apical size of 25/06 and 30/06. Canal aberrations
were measured from the superimposed image at five
levels using AutoCAD system. Fisher exact test and
Mann Whitney test were used for analysis of the data.
Results: the incidence of zipping, elbow and apical
transportation was significantly lower in the TF
group (P = 0.04). Generally the incidence of aberra-
tion increased when the apical size increased to
30/0.06 regardless of the file system. Significant file
deformation was evident in the TF after single use (P
˂ 0.001). Conclusion: Under the conditions of this
study, TF manufactured by new technique performed
better than K3 systems when used up to size 25/06 in
simulated S-shaped canals. Clinical significance: The
flexibility of thermomechanically treated files is bene-
ficial in canals with multiple curvatures; however,
attention should be paid to the instrument taper and
final apical size of the preparation.
Keywords: S-Shaped Canal; Twisted Files; K3; Canal
The ultimate biological aim of root canal treatment is to
treat or prevent apical periodontitis. The biological ob-
jectives of chemomechanical preparation are to remove
all infected tissues from the root canal space and confine
instrumentation within the root canal without forcing
necrotic debris beyond the apical foramen [1]. The me-
chanical objectives of cleaning and shaping are to facili-
tate delivery of antimicrobial irrigants and create resis-
tance form against which a root filling can be compacted,
maintain the path of original canal and consider the mul-
tiple geometric planes and curves more than the roots
that house them. Moreover, the apical foramen should
remain in its original position, hence canal transportation
may damage the apical foramen and create foraminal zip
or tear which was found to have a negative effect on the
apical seal especially when curved root canals are obtu-
rated with cold lateral compaction technique [2].
Numerous studies have shown the ability of nickel ti-
tanium (NiTi) rotary instruments to effectively produce a
well tapered root canal with minimal risk of transporting
the original canal [3-6]. Most NiTi systems are manu-
factured by grinding method which may result in micro-
cracks that can become the focus of fracture if the file is
exposed to excess torsion and cyclic fatigue. Therefore, a
new manufacturing method by twisting has been intro-
duced to overcome limitations of the grinding method by
increasing the hardness, reducing torsional failure and
cyclic fatigue [7] which subsequently maximizes file
flexibility and improves its shaping ability [8,9].
Canal anatomy including S-shaped curvature can pose
serious challenges to the operator. The S-shape canal is
common in maxillary premolars, lateral incisors and ca-
nines and mandibular molars. These canals have two
curves, with the apical curve being the most difficult to
negotiate. The chances of creating a strip perforation in
this area are high and are dependent on the degree of
*Corresponding author.
N. A. Taha et al. / Open Journal of Stomatology 3 (2013) 386-391 387
apical curvature. Preflaring of the coronal third has been
suggested to reduce the angle of curvature and to facili-
tate negotiation of the apical curve. [10] The use of
S-shaped resin blocks is relatively common in studies
evaluating shaping abilities including apical zipping and
ledging; [11-13] hence they offer the advantages of canal
shape standardization and the ability to visualize intra-
canal accidents. The aim of this study was to evaluate the
shaping ability of Twisted files (TF; SybronEndo, Or-
ange, CA) manufactured from R-phase alloy using a
twisting method and compare it to the K3 rotary file sys-
tem (SybronEndo, Orange, CA) manufactured by the
conventional grinding method in simulated S-shaped root
canals after two apical preparation sizes.
2.1. Simulated Canal Preparation
Fourty simulated S-shaped root canals in clear resin
blocks (Endo training Bloc-S; Dentsply Mallifer, bal-
ligues, Switzerland), with a 30˚ coronal curvature, 20˚
apical curvature and 16 mm canal length were randomly
assigned into 2 groups, the TF group and the K3 group.
Two orientation grooves were drilled in each block to
facilitate superimposition of the images.
A preoperative image of each simulated resin block
was recorded by a digital camera (Nikon digital camera
D70s, Japan) and macro lens (Sigma 105 mm macro,
Japan). In order to take standardized and reproducible
pictures, a custom made camera stand was used and
camera was placed on a fixed distance (14.5 mm) from
the block which was placed in a custom made template.
Each canal was injected with red ink before taking the
image, and the image was saved as JPEG file on a linked
computer [14].
A glyde path was then established in the simulated
canals to the full working length (16 mm) with sizes 8,
10, 15 stainless steel K-files, canals were irrigated with
1ml distilled water using 27 gauge needles. Subsequently
in the TF group canals were prepared according to the
manufacturer’s recommendations using crown down
technique with rotary handpiece (DentaPort ZX J. Morita
MFG Corp., Tokyo, Japan) at 500 rpm without torque
( The shaping pro-
cedure was started with size 25/0.08 in the coronal one
third of the canal, then 25/0.06 inserted to resistance.
Finally the following 3 files 25/0.04, 25/0.06, 30/0.06
were used for apical preparation at full working length.
In the K3 group canals were prepared with K3 rotary
system with constant speed of 250 rpm at torque 2 using
a crown down approach. The shaping procedure was
started with size 25/0.12 in the orifice then size 25/0.10
and 25/0.08 in the coronal one third, followed by size
25/0.06 taken to 2 mm short of the length and finally the
following 3 files were used for apical preparation at full
working length 25/0.04, 25/0.02, 25/0.04, 25/0.06 and
30/0.06. Each canal was shaped with new instruments
lubricated with Glyde-Prep (Dentsply Maillefer), patency
was confirmed with size 10K-file, and canals were irri-
gated with 1ml distilled water after each instrument. All
used files were evaluated for deformation.
2.2. Assessment of Canal Preparation
Two postoperative images were taken; one after prepara-
tion with size 25/0.06 and the second after preparation
with size 30/0.06. The preoperative and post operative
images were superimposed using image Adobe Photo-
shop (Adobe Systems Inc, San Jose, CA).
The final composed images were imported into Auto-
cad 2007 and were blindly assessed by two experienced
clinicians at five levels [15]. (Figure 1):
Position 1: half way of the straight part of canals (7
mm from orifice).
Position 2: beginning of the first curve. This was de-
termined as the point where the canal starts to deviate
from the long axis of the straight part of the canal at first
Position 3: apex of the first curve. This was deter
Figure 1. Superimposed image
show the five assessment levels
in the S shape canal: (A) Posi-
tion 1: Half way of straight part
of canals, (B) Position 2: Be-
ginning of the first curve. (C)
Position 3: Apex of first curve.
(D) Position 4: Apex of second
curve. (E) Position 5: Apical
end, this represent the end
point of the preparation.
Copyright © 2013 SciRes. OPEN ACCESS
N. A. Taha et al. / Open Journal of Stomatology 3 (2013) 386-391
Copyright © 2013 SciRes.
mined by intersection of two lines one drawn along the
outer border of straight part of canal and the second
drawn along the outer border of a line extending between
the two curves.
Position 4: apex of second curve. This was determined
by the intersection of two lines one drawn along the out-
er border of a line extending between the two curves and
the second drawn along the outer border of the apical
aspect of the canal.
Position 5: apical end which represents the end point
of the preparation.
Each canal was assessed for the presence of canal ab-
errations including, zip, elbow, ledges, perforation, dan-
ger zone and outer widening according to al-Omari et al.
[16], in addition to recording instrument failure or de-
formation and changes in working length (WL). Statisti-
cal analysis was done using Fisher exact test for instru-
ment deformation and aberrations and Mann Whitney
test for width measurements. Significance was set at P ˂
None of the used rotary files separated inside the canal
however TF showed significant deformation after single
use compared to K3 (20/20 files of size 25/0.08 and
16/20 of size 25/0.06, P ˂ 0.001). K3 files resulted in
significant changes in the WL implying straightening of
the canals in 100% of the canals, compared to 30% in the
TF group.
Generally more aberrations were found in canals pre-
pared with K3 than TF including ledges, perforation and
creating danger zones, however statistical significance
was reached only in the incidence of zipping and elbow
formation P = 0.04. Table 1 summarizes the incidence of
aberrations in the two files systems, when used up to an
apical size of 25/06 and 30/06. Width measurements for
the two systems are presented in Table 2; after prepara-
tion with size 30/06 the incidence of danger zone in-
creased in both systems. However it was significantly
greater in K3 system as it tends to remove more material
from the inner aspect of the curve at many levels (1, 2, 3)
after preparation with size 30/06 (P = 0.001, P = 0.003, P
= 0.007) implying creating a danger zone, in addition to
outer widening (transportation) at the apical end after
size 30/06 (P < 0.001).
Whereas TF created a danger zone at the apex of sec-
ond curve after preparation with 25/06 and 30/06 (P =
Understanding material properties and its impact on in-
strument performance is crucial for the clinician. Re-
cently thermomechanical treatment and alteration in the
manufacturing process has been attempted to improve
Table 1. Incidence of aberrations after final apical preparation with size 25/0.06 and 30/0.06 using TF and K3 systems.
K3 (n = 20)% TF (n = 20)%
Aberration after preparation with file size 25/0.06
10 0 Ledges
40* 0 Zipping
40* 0 Elbow
0 0 Outer widening in first curve
0 0 Outer widening in 2nd curve
90 50 Danger zone in first curve
100 80 Danger zone in second curve
10 0 Perforation
Aberration after preparation with file size 30/0.06
50 20 Ledges
50 20 Zipping
40 20 Elbow
0 10 Outer widening in first curve
0 0 Outer widening in 2nd curve
100 70 Danger zone in first curve
100 90 Danger zone in second curve
0 0 Perforation
N. A. Taha et al. / Open Journal of Stomatology 3 (2013) 386-391 389
Table 2. Mean width measurements (mm) after preparation with TF and K3 systems.
Width measurement Total Inner Outer
M ± SD
M ± SD
M ± SD
M ± SD
M ± SD
M ± SD
Preparation ended with size 25/0.06
Half way from orifice 0.79 ± 0.05 0.72 ± 0.03* 0.17 ± 0.05 0.20 ± 0.02 0.24 ± 0.03 0.22 ± 0.03
Beginning of 1st curve 0.62 ± 0.04 0.60 ± 0.03 0.21 ± 0.05 0.23 ± 0.04 0.14 ± 0.04 0.12 ± 0.02
Apex of 1st curve 0.58 ± 0.03 0.57 ± 0.02 0.25 ± 0.12 0.24 ± 0.03 0.08 ± 0.01 0.08 ± 0.03
Apex of 2nd curve 0.33 ± 0.03 0.33 ± 0.26 0.11 ± 0.26 0.08 ± 0.02* 0.07 ± 0.06 0.06 ± 0.02
Apical end 0.26 ± 0.05 0.30 ± 0.30 0.02 ± 0.02 0.02 ± 0.02 0.07 ± 0.04 0.11 ± 0.07
Preparation ended with size 30/0.06
Half way from orifice 0.84 ± 0.03 0.81 ± 0.03 0.19 ± 0.02 0.25 ± 0.02* 0.26 ± 0.03 0.32 ± 0.21
Beginning of 1st curve 0.69 ± 0.02 0.71 ± 0.04 0.27 ± 0.04 0.34 ± 0.04* 0.14 ± 0.02 0.13 ± 0.02
Apex of 1st curve 0.62 ± 0.02 0.67 ± 0.05* 0.28 ± 0.04 0.35 ± 0.04* 0.07 ± 0.02 0.08 ± 0.02
Apex of 2nd curve 0.35 ± 0.04 0.38 ± 0.04 0.13 ± 0.03 0.10 ± 0.02* 0.06 ± 0.02 0.10 ± 0.19
Apical end 0.31 ± 0.05 0.31 ± 0.05 0.03 ± 0.00 0.02 ± 0.02 0.12 ± 0.07 0.13 ± 0.06*
flexibility and fatigue resistance [17]. Studies evaluating
the impact of this property on the shaping ability of files
manufactured by this procedure are scarce, with variation
in assessment criteria.
This study evaluated the shaping ability of TF, manu-
factured with R-phase, which has been shown to be su-
perelastic with a different path of crack propagation
compared to non electropolished ground files. [18] The
results of this study are in accordance with previous
findings regarding better shaping ability of TF [8,9].
However this is the first study to investigate shaping
ability in S-shaped canals. El Batouty et al. have shown
that TF resulted in the least change in the overall canal
curvature compared to K3 system in distal root canals of
mandibular teeth with 25 and 35 degrees curve [9].
The K3 system resulted in excessive resin removal
from the inner aspect at multiple points which is a seri-
ous problem clinically that can result in strip perforation,
in addition to apical transportation at the end of second
curve which is in accordance with the findings of Gergi
et al. [8], this could be attributed to the interaction of two
aspects of the file systems; the cross sectional design and
the taper of instrument used for apical preparation. On
the other hand one study found K3 to be the best choice
among 5 systems for preparation of S-shaped canals; this
could be explained by the small taper and apical size
used in that study (25/0.04) and the variation in prepara-
tion sequence that included 8 files for every canal [19]. It
is worth noting however that the design of K3 system has
been updated by SybronEndo via the introduction of the
thermomechanically treated K3XF, which is claimed to
have better flexibility and fatigue resistance.
Although in previous studies TF were not found supe-
rior to Flexmaster files when used in 0.04 taper in terms
of apical transportation [20], they demonstrated signifi-
cantly lower incidence of transportation compared to the
Protaper 0.08 taper when evaluated by computed tomo-
graphy [8]. Considering the above findings and the fact
that both FlexMaster and K3 files are manufactured by
grinding process the authors concluded that the superior-
ity of TF my not be attributed solely to the manufactur-
ing process but rather to the taper and cross sectional
In this study potential contributing factors were stan-
dardized by matching the size and taper of the final api-
cal file. Advantage of TF include creating less aberration
than K3 implying more flexibility. However it is worth
noting that it resulted in danger zone at the apex of sec-
ond curve even after preparation with 25/0.06, which
may be related to the complexity of the S-shaped curva-
ture rather than suboptimum characteristics of the file
system. Therefore to optimize shaping results it may be
advantageous to use small taper in curved canals [21]. It
is worth noting that when the apical size increased to 30,
both systems resulted in high incidence of danger zone
(70% - 100%). Considering the fact that debridement is
optimized by larger apical preparations [22], it may be
wise to balance between the size and taper of instruments
used in S-shaped canals.
Second advantage is that instruments made from the
R-phase alloy including Twisted files are flexible, which
allows a greater amount of deformation at a similar tor-
que level than austenitic phase [17]. The significant de-
formation occurred in TF in this study may be a positive
warning sign before fracture following multiple uses.
However combined with the reported negative effects of
multiple autoclave cycles it highlights the manufacturer
recommendations of single use. [23] This may be advan-
Copyright © 2013 SciRes. OPEN ACCESS
N. A. Taha et al. / Open Journal of Stomatology 3 (2013) 386-391
tageous in terms of infection control while dearer on the
other hand.
However considering variation in material properties
between dentine and resin, and the two dimensional as-
sessment method employed, complete extrapolation of
the results to the clinical practice may not be wise and
validation of these results using three dimensional tech-
nologies in a clinical setup is required.
Within the limitation of this study, it can be concluded
that twisted files manufactured by new technique can be
used in preparation of S-Shaped root canals up to size
The flexibility of thermomechanically treated files is
beneficial in preparing canals with multiple curvatures.
However attention should be paid to the instrument taper
and the final apical size of the preparation.
The authors deny any conflicts of interest. We affirm that we have no
financial affiliation (e.g., employment, direct payment, stock holdings,
retainers, consultantships, patent licensing arrangements or honoraria),
or involvement with any commercial organization with direct financial
interest in the subject or materials discussed in this manuscript, nor
have any such arrangements existed in the past three years.
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