Shaping ability of thermomechanically treated files in simulated S-shaped root canals

doi:10.4236/ojst.2013.37065

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Open Journal of Stomatology, 2013, 3, 386-391 OJST

http://dx.doi.org/10.4236/ojst.2013.37065 Published Online October 2013 (http://www.scirp.org/journal/ojst/)

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: *[email protected]

Received 2 August 2013; revised 21 October 2013; accepted 28 October 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

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

Aberration

1. INTRODUCTION

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.

OPEN ACCESS

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. MATERIALS AND METHODS

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

(www.tfwithrphase.com-TF-brochure). 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

curve.

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.

N. A. Taha et al. / Open Journal of Stomatology 3 (2013) 386-391

388

OPEN ACCESS

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 ˂

0.05.

3. RESULTS

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 =

0.003).

4. DISCUSSION

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

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

design.

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-

N. A. Taha et al. / Open Journal of Stomatology 3 (2013) 386-391

390

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.

5. CONCLUSION

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

25/0.06.

6. CLINICAL SIGNIFICANCE

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.

7. ACKNOWLEDGEMENTS

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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