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Year : 2013  |  Volume : 3  |  Issue : 2  |  Page : 65-69

Stress distribution of new generation of Twisted Files in comparison with ProTaper: A finite element analysis

Department of Conservative Dentistry, Sri Dharmasthala Manjunatheshwara, SDM College of Dental Sciences and Hospital, Sattur, Dharwad, India

Date of Web Publication13-Sep-2013

Correspondence Address:
Harsha Pujari
E2 - 204 Emerald Chs Ltd, Highland Park, Amar Nagar, Darga [rd], Mulund [West], Mumbai - 400 080
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/1658-5984.118149

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Aim: 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, eight-noded, 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, 5-1) 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.

Keywords: Finite element analysis, stress distribution, Twisted File, white light scanning system

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;3:65-9

How to cite this URL:
Pujari H. Stress distribution of new generation of Twisted Files in comparison with ProTaper: A finite element analysis. Saudi Endod J [serial online] 2013 [cited 2022 Sep 27];3:65-9. Available from: https://www.saudiendodj.com/text.asp?2013/3/2/65/118149

  Introduction Top

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] Nickel-titanium (NiTi) was developed 40 years ago by Buehler et al., in the Naval Ordnance Laboratory (NOL) in Silver Springs, Maryland. [3] NiTi is 2-3 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 engine-files might not show any visible signs of permanent deformation during clinical uses, instrument fracture appears to occur suddenly. 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 well-respected 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 R-PHASE 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 R-phase 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 transverse-running machining marks (as a result of electro-polishing) 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 nickel-titanium rotary instruments, ProTaper and the latest Twisted Files using Finite Element Model.

  Materials and Methods Top

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 2]a and b. Finally a mesh of 4 noded-linear-tetrahedral 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 3]a and b. This numerical model of each instrument was entered into a 3D FE analysis package (ABAQUS V6, 5-1, Bangalore, INDIA) taking into consideration the non linear mechanical characteristic of NiTi material. Parameters were set in which the Young's modulus of the alloy was 36 GPA and the Poisson's ratio 0.3 for both the instruments as confirmed by the manufacturers.
Figure 1: Twisted File and ProTaper universal of similar tip diameter

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Figure 2: (a) 3D data of Twisted File (b) 3D data of ProTaper file

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Figure 3: (a) 24,663 nodes and 129,208 elements for Twisted File (b) 25,692 nodes and 103,778 elements for ProTaper universal

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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 4]a
    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)

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  • 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 4]b
  • 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 4]c.

  Results Top

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.

3-D von Mises Criterion

σvM = [½ {(σ12 ) 2+ (σ23 ) 2+ (σ31 ) 2}] 1/2

Here σ1 σ2 and σ3 are known as the principal stresses and σvM 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].
Figure 5: A color coding was given where red denoted a high stress bearing area and blue a low stress bearing area

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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 6]a and b.
Figure 6: (a) ProTaper files (b) Twisted files

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In case where torsional force was applied, maximum stress was seen at the core of the ProTaper Universal and at the cutting edge of Twisted Files and [Figure 7]a and b.
Figure 7: (a) ProTaper files (b) Twisted file

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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 [Table 2].
Table 1: Results when bending force was applied

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Table 2: Results when torsional forces are applied

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  Discussion Top

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]

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 (Young's modulus of elasticity) and v (Poisson's 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 be cut and the use of a lubricant. The cross-sectional 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 cross-sectional shape, helical angle, and radial lands. The ProTaper system (Dentsply Maillefer) has a cross-section of a triangle with convex sides. The vortex of the triangle in cross-section is claimed to reduce the contact area between the file and canal wall, hence a good cutting efficiency. The ProTaper Universal incorporates a shallow U-shaped groove at each of its convex triangle to improve flexibility. The modified design also reduces screw-in effect. [14]

Twisted Files (Sybron endo) has a triangular cross-section. They are non-landed 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 (R-phase), by a process of heating and cooling. In the R-phase, 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, R-phase 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 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 cross-sectional 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 engine-file to fracture. To increase safety, endodontic educators must emphasize the need for mastering the skill through supervised training. [17],[18]

  Conclusion Top

In this study, we saw that R-phase 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.

  Acknowledgment Top

We would like to thank Dr. Priya Horatti, Dr. Balaram D Naik, Dr. Nageshwar Rao, Dr. Deepti Rao and Mr. Nithin Kulur for their support.

  References Top

1.Mcspadden JT. Mastering Endodontic Instrumentation. 1 st ed. Ramsey, New Jersey: Arbor Books, Inc; 2006.  Back to cited text no. 1
2.Beruttii E, Chiandussi G, Gaviglio I, Ibba A. Comparative analysis of torsional and bending stresses in two mathematical models of nickel-titanium rotary instruments: ProTaper versus ProFile. J Endod 2003;29:15-9.  Back to cited text no. 2
3.Baumann MA. Nickel-titanium: Options and challenges. Dent Clin North Am 2004;48:55-67.  Back to cited text no. 3
4.Thompson SA. An overview of nickel-titanium alloys used in dentistry. Int Endod J 2000;33:297-310.  Back to cited text no. 4
5.Kim HC, Yum J, Hur B, Cheung GS. Cyclic fatigue and fracture characteristics of ground and twisted nickel-titanium rotary files. J Endod 2010;36:147-52.  Back to cited text no. 5
6.Kim TO, Cheung GS, Lee JM, Kim BM, Hur B, Kim HC. Stress distribution of three NiTi rotary files under bending and torsional conditions using a mathematic analysis. Int Endod J 2009;42:14-21.  Back to cited text no. 6
7.Clauder T, Baumann MA. ProTraper NT system. Dent Clin North Am 2004;48:87-111.  Back to cited text no. 7
8.Gambarini G, Grande NM, Plotino G, Somma F, Garala M, De Luca M, et al. Fatigue resistance of engine-driven rotary nickel-titanium instruments produced by new manufacturing methods. J Endod 2008;34:1003-5.  Back to cited text no. 8
9.Larsen CM, Watanabe I, Glickman GN, He J. Cyclic fatigue analysis of a new generation of nickel titanium rotary instruments. J Endod 2009;35:401-3.  Back to cited text no. 9
10.Peters OA, Paque F. Current developments in rotary root canal instrument technology and clinical use: A review. Quintessence Int 2010;41:479-88.  Back to cited text no. 10
11.Turnip YL, Chagneau F, Vulcain JM. Impact of two theoretical cross-sections on torsional and bending stresses of nickel-titanium root canal instrument models. J Endod 2000;26:414-7.  Back to cited text no. 11
12.Turnip YL, Chagneau F, Bartier, Cathelineau G, Vulcain JM. Impact of torsional and bending inertia on root canal instruments. J Endod 2001;27:333-6.  Back to cited text no. 12
13.Holmes DC, Diaz-Arnold AM, Leary JM. Influence of post dimension on stress distribution in dentin. The J Prosthet Dent 1996;75:140-7.  Back to cited text no. 13
14.Kim HC, Cheung GS, Lee CJ, Kim BM, Park JK, Kang SI. Comparison of forces generated during root canal shaping and residual stresses of three nickel-titanium rotary files by using a three-dimensional finite-element analysis. J Endod 2008;34:743-7.  Back to cited text no. 14
15.Mounce R. Rotary nickel titanium instrumentation revolutionized: The Twisted File. Oral Health J 2008;56:6-9.  Back to cited text no. 15
16.Kim HC, Kim HJ, Lee CJ, Kim BM, Park JK, Versluis A. Mechanical response of nickel-titanium instruments with different cross-sectional designs during shaping of simulated curved canals. Int Endod J 2009;42:593-602.  Back to cited text no. 16
17.Mandel E, Adib-Yazdi M, Benhamou LM, Lachkar T, Mesgouez C, Sobel M. Rotary Ni-Ti profile systems for preparing curved canals in resin blocks: Influence of operator on instrument breakage. Int Endod J 1999;32:436-43.  Back to cited text no. 17
18.Yared GM, Bou Dagher FE, Machtou P. Influence of rotational speed, torque and operator's proficiency on ProFile failures. Int Endod J 2001;34:47-53.  Back to cited text no. 18


  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]

  [Table 1], [Table 2]

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