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65PB Saudi Endodontic Journal MayAug 2013 Vol 3 Issue 2 Saudi
Endodontic Journal MayAug 2013 Vol 3 Issue 2
Address for correspondence: Dr. Harsha Pujari, E2 204 Emerald
Chs Ltd, Highland Park, Amar Nagar, Darga [rd], Mulund [West],
Mumbai 400 080. Email: [email protected]
Stress distribution of new generation of Twisted Files in
comparison with ProTaper: A finite element analysis
Harsha PujariDepartment of Conservative Dentistry, Sri
Dharmasthala Manjunatheshwara, SDM College of Dental Sciences and
Hospital, Sattur, Dharwad, India
INTRODUCTION
The two primary goals for root canal instrumentation are to
provide a biological environment that is conducive to healing and
to provide a canal shape that is comfortable to sealing.[1]
Introduction of nickel titanium in rotary system has greatly
simplified shaping of root canal systems.[2] Nickeltitanium (NiTi)
was developed 40 years ago by Buehler et al., in the Naval Ordnance
Laboratory (NOL) in Silver Springs, Maryland.[3] NiTi is 23 times
more flexible than stainless steel.[2] They have a low
modulus of elasticity (about one fourth to one fifth that of
stainless steel) but are tougher and more resilient.[3] Nitinol is
the name given to a family of intermetallic alloys of nickel and
titanium which have been found to have unique properties of shape
memory and super elasticity.[4]
The super elasticity of the material allows the NiTi rotary
instruments to be used in continuous rotation, even in curved root
canals, to produce a desirable tapered root canal form, with a low
risk of transporting the original canal lumen. However, there is a
general perception that NiTi instruments have a high risk of
fracture in use. Clinically, there is a real potential for rotary
NiTi instruments to separate in the canal; even new instruments
might demonstrate unexpected breakage on first use. Because NiTi
enginefiles might not show any visible signs of permanent
deformation during clinical uses, instrument fracture appears to
occur suddenly.
ABSTRACTAim: To compare and evaluate the stress distribution of
new generation of Twisted File in comparison with ProTaper under
bending or torsional conditions using a finite element analysis
model. Materials and Methods: Two NiTi files, a ProTaper file and
the latest generation nickel titanium file which is the Twisted
File of similar tip diameter were scanned using White light
scanning system. Through this a real size digitized models of the
two brands of NiTi instruments were obtained. Then, the outline of
the instrument was extracted from the stacks of 3D data in
software. Finally a mesh of linear, eightnoded, hexahedral elements
was overlaid onto the rendered 3D image. The behavior of the
instrument under bending or torsional loads was then analyzed
mathematically in the software (ABAQUS V6, 51) taking into
consideration the non linear mechanical characteristic of NiTi
material. The results were expressed as von Mises stresses and were
calculated by the von Mises criteria. Results: Higher stress values
were seen in Twisted Files than the ProTaper universal, however,
the angular deflection was seen to be more in Twisted Files.
Conclusion: As more angular deflection was seen in Twisted File it
was more flexible than ProTaper Universal but did not have the
uniform stress distribution like the ProTaper universal.
Key words:
Finite element analysis, stress distribution, Twisted File,
white light scanning system
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DOI:
10.4103/16585984.118149
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Pujari: Finite element analysis of rotary NiTi instruments
66 Saudi Endodontic Journal May-Aug 2013 Vol 3 Issue 2
Increasing the resistance to fracture has been a focus in the
design of new NiTi rotary systems. Although excess torsion and
cyclic fatigue have both been implicated as a reason for file
fracture, the latter is probably the more prevalent cause of
unexpected breakages.[5]
To date, many NiTi rotary systems have been introduced to the
market.[6] The ProTaper system (DENTSPLY/Maillefer, Ballaigues,
Switzerland) represents a new generation of NiTi instruments
currently available, which was introduced in 2001. The system was
developed by a group of wellrespected endodontists (Prof. Pierre
Machtou, Dr. Clifford Ruddle, and Prof. John West).[7] Twisting, as
it is done with stainless steel K files and K reamers, is
impossible due to the superelastic properties and the memory
effect. Therefore, machining and grinding is the only way for
NiTi.[3] Recently, new manufacturing processes for NiTi endodontic
instruments have been developed by manufacturers attempting to
overcome these limitations.[8] The Twisted File with its RPHASE
TECHNOLOGY is the second advancement in the manufacturing process
of NiTi.[9]
The manufacturer claimed that Twisted File (TF) has a different
surface texture (natural grain structure) that runs in the
longitudinal direction and that the instrument is made of the
Rphase of NiTi alloy (although no transition temperature data are
presented). It was further claimed that these features serve to
raise the flexibility and the fracture resistance of the
instrument. There is also an absence of transverserunning machining
marks (as a result of electropolishing) that would result in slower
crack initiation and propagation. To date, only very few reports of
the fatigue behavior of this new Twisted NiTi File are available.
[5] This study was aimed to compare torsional and bending stresses
in two simulated models of nickeltitanium rotary instruments,
ProTaper and the latest Twisted Files using Finite Element
Model.
MATERIALS AND METHODS
Two brands of NiTi instrument Twisted File and ProTaper
Universal (F2) of identical instrument sizes (0.25 tip diameter)
were tested [Figure 1]. Real digitized models were obtained by
using white light scanning system (APM technologies, Delhi, INDIA).
Then the outline of the instrument was extracted from the 3D data
in a software (IDEAS: UGS, Delhi, INDIA) [Figure 2a and b]. Finally
a mesh of 4 nodedlineartetrahedral elements was overlaid onto
the rendered 3D image. Such a 3D model consisted of 24,663 nodes
and 129,208 elements for Twisted File and 25,692 nodes and 103,778
elements for ProTaper universal [Figure 3a and b]. This numerical
model of each instrument was entered into a 3D FE analysis package
(ABAQUS V6, 51, Bangalore, INDIA) taking into consideration the non
linear mechanical characteristic of NiTi material. Parameters were
set in which the Youngs modulus of the alloy was 36 GPA and the
Poissons ratio 0.3 for both the instruments as confirmed by the
manufacturers.
Then the behavior of instruments were analyzed numerically under
following simulated conditions in the Finite element model (FEM)
analysis. Cantilever bending with a constant load Applying
a concentrated load of 1 N at the tip of the file with its shaft
rigidly held in place [Figure 4a]
Stress distribution under cantilever bending at fixed
displacement The tip of the file was deflected for a distance of 2
mm and held there [Figure 4b]
Application of a shear moment (torsion) A 2.5 Nmm moment of
force was applied to the shaft in clockwise direction whilst 4 mm
was kept constrained [Figure 4c].
RESULTS
Principal stresses were used in the assessment from which a
value of the von Mises stresses was calculated according to the
formula 3 dimensional von Mises criterion.
3D von Mises Criterion
vM = [ {(12) 2+ (23)
2+ (31) 2}]1/2
Here 1 2 and 3 are known as the principal stresses and vM is the
von Mises stress. Principal stresses are found from the existing
tensile, compressive and shear stresses upon loading. Results are
shown in terms of colored contoured pattern.
A color coding was given where red denoted a high stress bearing
area, and blue a low stress bearing area [Figure 5].
In case where bending force was applied the stress was mainly
concentrated at the cutting edges of both the instruments, however,
a more uniform stress distribution was seen with ProTaper and
increased angular deflection with Twisted File [Figure 6a and
b].
In case where torsional force was applied, maximum stress was
seen at the core of the ProTaper Universal
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Pujari: Finite element analysis of rotary NiTi instruments
67Saudi Endodontic Journal May-Aug 2013 Vol 3 Issue 2
In Finite element model (FEM), structures of various shapes are
modeled and subdivided with a digital computer into simpler
geometric shapes or elements whose apexes meet to form nodes. The
elastic constants E (Youngs modulus of elasticity) and v (Poissons
ratio) of the modeled material are specified for each element. A
system of simultaneous equations is generated and solved to yield
stress distributions in each element. The major advantage of Finite
element model (FEM) is the ability to solve complex biomechanical
problems for which other methods of study are too complex.[13]
In the last decade, the use of NiTi rotary instruments has grown
in popularity and there has been increasing number of proprietary
systems introduced commercially. NiTi engine files operate by way
of continuous rotation in the root canal and as such, are subjected
to unidirectional torque. The value of the shear (torsional) stress
varies depending on the canal size, hardness of the dentine to
Figure 2: (a) 3D data of Twisted File (b) 3D data of ProTaper
file
ba
Figure 4: (a) Case 1-Cantilever bending with a constant load (b)
Case 2-Stress distributions under cantilever bending at fixed
displacement (c) Case 3-Application of a shear moment (torsion)
c
b
a
and at the cutting edge of Twisted Files and [Figure 7a and
b].
The results showed that ProTaper universal had lesser amount of
stress when calculated by the von Mises criterion as compared to
twisted files, however the values also showed that Twisted Files
had a greater amount of deflection than ProTaper universal
indicating more flexibility [Table 1 and 2).
DISCUSSION
File fracture is a major concern during endodontic treatment.
Although multiple factors are responsible for instrument separation
in use, cyclic fatigue has been shown as one of the leading causes.
Cyclic fatigue occurs when a metal is subjected to repeated cycles
of tension and compression that cause its structure to break down
(as a result of concentration of stress at the propagating crack
front) and ultimately leading to fracture.[9] Torsional overload
and fracture typically happens when an instrument tip is forced
into a canal that is smaller than the tip diameter.[10] Several
studies of the stresses generated in NiTi instrument have been
completed using Finite Element (FE) analysis.[2,6,11,12]
Figure 3: (a) 24,663 nodes and 129,208 elements for Twisted File
(b) 25,692 nodes and 103,778 elements for ProTaper universal
b
a
Figure 1: Twisted File and ProTaper universal of similar tip
diameter
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Pujari: Finite element analysis of rotary NiTi instruments
68 Saudi Endodontic Journal May-Aug 2013 Vol 3 Issue 2
Twisted Files (Sybron endo) has a triangular crosssection. They
are nonlanded with positive rake angles.[9,15] TF instruments are
created by taking a raw NiTi wire in the austenite crystalline
structure and transforming it into a different crystalline
structure rhombohedral (Rphase), by a process of heating and
cooling. In the Rphase, NiTi cannot be ground, but it can be
twisted. Once twisted, file is heated and cooled again to maintain
its new shape and convert it back into austenite crystalline
structure, which is superelastic once stressed.[8] The
manufacturers claim the three new design methods of this process,
namely, Rphase heat treatment, twisting of the metal and special
surface conditioning, significantly increase instruments resistance
to cyclic fatigue and flexibility.[9]
Flexure of the instrument tip was measured and the von Mises
stress distribution was evaluated when it was deformed by applying
a static load of 1N on the tip of each NiTi instrument with its
shaft rigidly held in place. A von Mises stress is a so called
equivalent stress, which represents the three dimensional stress
conditions with a single value according to von Mises criterion. In
a similar experiment, bending displacement (2 mm) of the instrument
tip was simulated to obtain the resulting von Mises stresses and
the load. The instrument was rotated using a torsional moment of
2.5 mm at the shaft and von Mises stress distribution were
calculated whilst the instrument were clamped rigidly 4 mm from
their tip.[16]
Based on a mathematical comparison Twisted Files had a greater
deflection indicating that it possesses a higher flexibility but
ProTaper had lower and more evenly distributed stresses as compared
to Twisted File. The
be cut and the use of a lubricant. The crosssectional
configuration is also an important determinant of the distribution
of stresses on the instrument.[6]
Various brands of NiTi rotary system have been introduced to the
market, each having slightly different design for its
crosssectional shape, helical angle, and radial lands. The ProTaper
system (Dentsply Maillefer) has a crosssection of a triangle with
convex sides. The vortex of the triangle in crosssection is claimed
to reduce the contact area between the file and canal wall, hence a
good cutting efficiency. The ProTaper Universal incorporates a
shallow Ushaped groove at each of its convex triangle to improve
flexibility. The modified design also reduces screwin
effect.[14]
Table 1: Results when bending force was appliedCondition applied
ProTaper universal Twisted file
Case 1 (Figure 4a) von mises stress577.7 MPMax deflection
4.852 mm
von mises stress1057 MPaMax deflection
7.3 mmCase 2 (Figure 4b) von mises
stress263.4 MPaMax deflection
2.2 mm
von mises stress512 MPaMax deflection
3.54 mmMPa=Mega pascal
Table 2: Results when torsional forces are appliedCondition
applied ProTaper universal Twisted file
Case 3 (Figure 4c) Max von mises stress350.236 MPa
Max von mises stress582.276 MPa
MPa=Mega pascal, Max=Maximum
Figure 5: A color coding was given where red denoted a high
stress bearing area and blue a low stress bearing area
Figure 6: (a) ProTaper files (b) Twisted filesb
a
Figure 7: (a) ProTaper files (b) Twisted fileb
a
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Pujari: Finite element analysis of rotary NiTi instruments
69Saudi Endodontic Journal May-Aug 2013 Vol 3 Issue 2
highest stress concentration was found at the cutting edge of
both the rotary files when bending forces were applied. This is
expected from the mechanics of bending a beam of triangular cross
section.[6]
There are various other factors that could affect the stress
distribution, it could be crosssectional configuration, depth of
the flute or the area of the inner core.[6] It is therefore
important to have uniform distribution of stresses to avoid
creating stress accumulation zones and thus areas of least
resistance.[2] Neither of the systems studied was both highly
flexible and yet able to withstand and distribute the stress evenly
in bending and torsion. Despite a truer representation of the
actual geometry of the instrument in this study, the actual
stresses may differ when the instrument is actively filing against
the dentine wall during clinical use. Further studies through other
methods to verify the relationship between instrument design,
stress distribution, fatigue fracture and the influence of
microscopic notches are required.[6]
Clinicians should understand not only the general guidelines for
NiTi rotary instrumentation but also the structural characteristics
which might influence the durability or the risk of an enginefile
to fracture. To increase safety, endodontic educators must
emphasize the need for mastering the skill through supervised
training.[17,18]
CONCLUSION
In this study, we saw that Rphase technology definitely led to
increased flexibility in Twisted Files because of the increased
deflection seen. However, there was more uniform stress
distribution seen in ProTaper universal indicating that it avoids
stress accumulation zones.
We conclude that:
Twisted Files had greater flexibility than ProTaper files, but
also greater stress concentration that could predispose to fatigue
fracture.
ProTaper Universal was less flexible but had a more uniform
stress distribution under load hence are more strong.
The quest for the best file with both increased flexibility and
better strength is still on. However, new technologies must be
tested against a benchmark and verified in independent studies to
give confidence for clinicians to make their choice.
ACKNOWLEDGMENTWe would like to thank Dr. Priya Horatti, Dr.
Balaram D Naik, Dr. Nageshwar Rao, Dr. Deepti Rao and Mr. Nithin
Kulur for their support.
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How to cite this article: Pujari H. Stress distribution of new
generation of Twisted Files in comparison with ProTaper: A finite
element analysis. Saudi Endod J 2013;3:659.
Source of Support: Nil. Conflict of Interest: None declared.
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