A finite element study on the mechanical behavior of reciprocating endodontic files

Mohamed I. El-Anwar Ayman O. Mandorah Salah A. Yousief Tarek A. Soliman Tarek M. Abd El-Wahab About the authors

Abstract

AIM:

To evaluate the mechanical behavior of reciprocating endodontic files, comparing nickel-titanium NiTi and stainless steel 316L St.St. 316L as manufacturing material for such instruments.

METHODS:

A three-dimensional finite element model was designed for this study. The simplified instrument model geometry was created on commercial CAD/CAM software. Real strain stress curves of St.St. 316L and NiTi were used in the analysis. Non-linear static analysis was performed to simulate the instrument inside root canal at an angle of 45° in the apical portion, and subjected to torsion of 0.3 N.cm.

RESULTS:

Non-linear NiTi material showed super elasticity and high functionality in such applications. Very high levels of stress appeared in the file at 3 mm from the tip close to yield point.

CONCLUSIONS:

St. St. 316L is not suitable for manufacturing reciprocating instruments. Modeling of the instrument with equivalent circular cross-sectional area did not affect results quality. Reciprocating instruments have short lifespan, thus manufacturers recommend using one file per tooth. Reciprocating instruments are recommended for less experienced dentist.

endodontics; stainless steel; nickel; titanium


Introduction

The goals of endodontic instrumentation are to shape without deviating from the original canal position, to enlarge until the walls are smooth and free of soft tissues, to completely remove microorganisms and debris, and to create a canal form that converges toward the foramen11. Del Bello P, Wang N, Roane JB. Crown-Down Tip Design and Shaping. J Endod. 2003; 29: 513-8.. Stainless steel endodontic instruments, whose characteristics include stiffness that increases with size, may set limitations to successful shaping. During enlargement of the apical third, this characteristic may be responsible for curvature defects such as apical transportation, ledging or zipping, which might compromise the outcome of treatment22. Schäfer E, Tepel J, Hoppe W. Properties of endodontic hand instruments used in rotary motion, Part 2, Instrumentation of Curved Canals. J Endod. 1995; 21: 493-7..

In the early 1960s, a nickel-titanium NiTi alloy was developed by Buehler, during the investigation of nonmagnetic, salt resisting, and waterproof alloys for the space program at the Naval Ordnance Laboratory in Silver Springs, Maryland, USA33. Saunders EM. Hand instrumentation in root canal preparation. Endod Top. 2005; 10: 163-7.. Nitinol is the name given to a family of inter-metallic alloys of nickel and titanium, which has unique properties of shape memory and super-elasticity. Nickel titanium instruments are more flexible than stainless steel instruments and have the ability to revert to their original shape after flexure. It has been reported that NiTi instruments are 2 to 3 times more flexible than stainless steel instruments and more resistant to fracture44. Walia H, Brantley WA, Gerstein H. An initial investigation of the bending and torsional properties of Nitinol root canal files. J Endod. 1988; 14: 346-51..

However, despite these advantages, the main problem with rotary NiTi instruments is a probable failure of the instruments. Instrument fracture is a serious problem and can jeopardize the outcome of the root canal treatment. Separation of rotary NiTi instruments can occur due to two reasons: torsional fracture or cyclic fatigue55. Fishelberg G, Pawluk JW. Nickel titanium rotary file canal preparation and intracanal file separation. Compend Contin Educ Dent. 2004; 25: 17-24.. Torsional failure occurs when the tip of the instrument is locked in the canal while the shaft continues to rotate. If the elastic limit of the metal is exceeded, the instrument undergoes plastic deformation, which can be followed by fracture if the load is high enough. Failure by torsional overload was reported as the most common cause of separation of rotary NiTi instruments66. Sattapan B, Nervo GJ, Palamara JE, Messer HH. Defects in rotary nickel titanium files after clinical use. J Endod. 2000; 26: 161-5..

Endodontic files are subjected to stress when they are bent around a curve. Every bent segment of the file experiences a cycle of both compressive and tensile stresses; this may lead to cyclic fatigue. Due to initiation, propagation, coalescence of micro-fractures it may ultimately cause an overt fracture of the file. Because the micro-fractures cannot be seen even with the aid of a surgical operating microscope, there is no warning preceding fracture77. Pruett JP, Clement DJ, Carnes DL. Cyclic fatigue testing of nickel titanium endodontic instruments. J Endod. 1997; 23: 77-85..

Possible strategies to increase efficiency and safety of NiTi rotary instrument include improving the manufacturing process or using new alloys that provide superior mechanical properties. The stiffness and flexibility of endodontic files are greatly dependent on their geometric design, including taper, helix angle, cross section shape, tip size and length88. He R, Ni J. Design improvement and failure reduction of endodontic files through finite element analysis: application to V-Taper file designs. J Endod. 2010; 36: 1552-7..

In an attempt to increase the resistance to cyclic fatigue of the rotary instruments, M-Wire was introduced by applying a series of heat treatments to NiTi wire blanks99. Shen Y, Zhou HM, Zheng YF, Peng B, Haapasalo M. Current challenges and concepts of the thermomechanical treatment of nickel-titanium instruments. J Endod. 2013; 39: 163-72.. Before the grinding process, the alloy was thermally treated to improve its properties. The final goal was to produce instruments with superelastic behavior reduced generation and accumulation of lattice defects during each load-unload cycle and increased resistance to cyclic fatigue, compared to those constructed from traditional NiTi alloy1010. Pereira ES, Peixoto IF, Viana AC, Oliveira II, Gonzalez BM, Buono VT, et al. Physical and mechanical properties of a thermomechanically treated NiTi wire used in the manufacture of rotary endodontic instruments. Int Endod J. 2012; 45: 469-74.. The first commercially available endodontic rotary system using the new M-Wire NiTi material was GTX Dentsply Tulsa Dental Specialties, Tulsa, OK, USA.

Recently, costly experimental studies1111. Plotino G, Rubini AG, Grande NM, Testarelli L, Gambarini G. Cutting efficiency of Reciproc and waveOne reciprocating instruments. J Endod. 2014; 40: 1228-30.

12. Braga LC, Faria Silva AC, Buono VT, de Azevedo Bahia MG. Impact of heat treatments on the fatigue resistance of different rotary nickel-titanium instruments. J Endod. 2014; 40: 1494-7.
- 1313. Pedullà E, Lo Savio F, Boninelli S, Plotino G, Grande NM, Rapisarda E, et al. Influence of cyclic torsional preloading on cyclic fatigue resistance of nickel - titanium instruments. Int Endod J. 2014 Oct 29. doi: 10.1111/iej.12400. were implemented on different brands and types of instruments like Reciproc, WaveOne, HyFlex, Mtwo, ProTaper, EndoWave, etc., according to ISO 3630-1. Tens of instruments were tested in order to statistically prove that heat treated instruments have better cutting efficiency and higher fatigue resistance than conventional NiTi files.

In 2008 Yared1414. Yared G. Canal preparation using only one Ni-Ti rotary instrument: preliminary observations. Int Endod J. 2008; 41: 339-44. proposed a new preparation motion reciprocating, using only F2 ProTaper rotary file to prepare the root canal. It has been claimed that rotary NiTi endodontic files show more resistance to cyclic fatigue, when used in reciprocating motion. Various cyclic fatigue tests have been conducted to compare file systems which allow the files to rotate till fracture. Flexural-fatigue failure occurs when the instrument rotates inside a curved canal while subjected to an excessive number of tensile-compressive strain cycles in the region of maximum canal curvature.

The stresses generated during flexural loading are directly associated to the fatigue life of the material1515. Câmara AS, de Castro Martins R, Viana AC, de Toledo Leonardo R, Buono VT, de Azevedo Bahia MG. Flexibility and torsional strength of ProTaper and ProTaper Universal rotary instruments assessed by mechanical tests. J Endod. 2009; 35: 113-6.. The stress conditions within instruments cannot be revealed by inspection of broken segments but require a mathematical/numerical simulation. Finite element analysis FEA has been applied as alternative to study the mechanical behavior of endodontic instruments for detailed assessment of stress distributions in instruments1515. Câmara AS, de Castro Martins R, Viana AC, de Toledo Leonardo R, Buono VT, de Azevedo Bahia MG. Flexibility and torsional strength of ProTaper and ProTaper Universal rotary instruments assessed by mechanical tests. J Endod. 2009; 35: 113-6. - 1616. Min-H o Lee, Anth eunis Versluis, Byung-Min Kim, Chan-Joo Lee, Bock Hur, Hyeon-Cheol Kim. Correlation between experimental cyclic fatigue resistance and numerical stress analysis for nickel-titanium rotary files. J Endod. 2011; 37: 1152-7..

Arruda Santos et al.1717. de Arruda Santos L, López JB, de Las Casas EB, de Azevedo Bahia MG, Buono VT. Mechanical behavior of three nickel-titanium rotary files: A comparison of numerical simulation with bending and torsion tests. Mater Sci Eng C. 2014; 37: 258-63. confirmed the potential of FEA as a numerical method to assess the mechanical behavior of endodontic instruments comparing the behavior of three different types of instruments: Mtwo VDW, Munich, Germany, RaCe FKG Dentaire, La-Chaux-de-Fonds, Switzerland size 25, .06 taper 0.25 mm tip diameter, 0.06% conicity and PTU F1 Dentsply Maillefer, Ballaigues, Switzerland experimentally and numerically by FEA1717. de Arruda Santos L, López JB, de Las Casas EB, de Azevedo Bahia MG, Buono VT. Mechanical behavior of three nickel-titanium rotary files: A comparison of numerical simulation with bending and torsion tests. Mater Sci Eng C. 2014; 37: 258-63..

The aim of this study was to evaluate the mechanical behavior of reciprocating endodontic files, comparing nickel-titanium NiTi and stainless steel 316L St.St. 316L as manufacturing material for such instruments.

Material and methods

In this study the WaveOne Dentsply Maillefer, Ballaigues, Switzerland primary file equivalent to ProTaper F2 file was modeled in 3D Figure 1. The file tip size was ISO-25 with an apical taper of 8% that reduces towards the coronal end. The 3D geometric model was prepared on a commercial general purpose CAD/CAM software AutoDesk Inventor version 8.0; Autodesk Inc., San Rafael, CA, USA. Although the cross section of the Wave One primary file is a convex triangle, it was simplified in this study to be circular with equivalent cross-sectional area, while the change in cross section in the apical part was neglected. The NiTi instruments and wires with different cross sections showed similar behavior in previous studies1717. de Arruda Santos L, López JB, de Las Casas EB, de Azevedo Bahia MG, Buono VT. Mechanical behavior of three nickel-titanium rotary files: A comparison of numerical simulation with bending and torsion tests. Mater Sci Eng C. 2014; 37: 258-63. - 1818. Cheung GS, Zhang EW, Zheng YF. A numerical method for predicting the bending fatigue life of NiTi and stainless steel root canal instruments. Int Endod J. 2011; 44: 357-61. which inspired the simplification of the modeled file cross section.

Figure 1.
Modeled File geometry on Inventor screen

The geometric model was transferred as IGES file to the meshing and finite element analysis package ANSYS version 14.5; ANSYS Inc., Canonsburg, PA, USA. The meshing element was 20 nodes "Solid 186" which has three degrees of freedom translations in the global directions. Mesh density is a parameter that improves the result accuracy and reduces artificial peak stresses by improving the representation of the actual geometry. The mesh density effect was evaluated before extracting results with 20,935 nodes and 13,057 elements.

Multi-linear material was defined as presented in Figure 2. NiTi alloy and AISI 316L stainless steel, widely used in biomedical applications and described by an elasto-plastic constitutive model with kinematic hardening1919. Necchi S, Petrini L, Taschieri S, Migliavacca F. A comparative computational analysis of the mechanical behavior of two nickel-titanium rotary endodontic instruments. J Endod. 2010; 36: 1380-4.. In both material models, the values of the characteristic parameters were derived from the literature. Considering the NiTi alloy, the transformation starting stress σs and the transformation finishing stress σf, are the stress values when, at the working temperature, the transformation between austenite and single-variant martensite starts and finishes, respectively. The limit transformation strain εL is the amplitude of the transformation strain interval. The capacity to recover all the deformation i.e. pseudo-elastic behavior ends when the martensitic yielding stress 'σy ' and the martensitic yielding strain 'εy ' indicated as pseudo-elastic limits are assessed1919. Necchi S, Petrini L, Taschieri S, Migliavacca F. A comparative computational analysis of the mechanical behavior of two nickel-titanium rotary endodontic instruments. J Endod. 2010; 36: 1380-4..

Figure 2.
NiTi and St.St. 316L material properties uniaxial stress-strain curves19

The boundary conditions were imposed to simulate the behavior of the files under bending and torsional conditions in compliance with the ISO 3630-1 specification2020. International Organization for Standardization. 3630-1: Dentistry. Root-canal instruments - Part 1: General requirements and test methods. 2nd ed. Geneva: ISO; 2008.. To test the bending resistance, the bending moment was calculated while the file was clamped 3 mm from the tip and the shaft was deflected until 45° inclination2121. Santos LA, Bahia MG, Casas EB, Buono VT. Comparison of the mechanical behavior between controlled memory and superelastic nickel-titanium files via finite element analysis. J Endod. 2013; 39: 1444-7.. To evaluate the torsional resistance, the file was held at 3 mm from the tip, and a clockwise torsional moment of 0.3 N.cm was applied. The boundary conditions used in the bending and torsional simulations are presented in Figure 3.

Figure 3.
a file outlines and regions, b boundary conditions for the bending and torsion simulations21

The model used in this study was confronted with previously published researches1818. Cheung GS, Zhang EW, Zheng YF. A numerical method for predicting the bending fatigue life of NiTi and stainless steel root canal instruments. Int Endod J. 2011; 44: 357-61. - 1919. Necchi S, Petrini L, Taschieri S, Migliavacca F. A comparative computational analysis of the mechanical behavior of two nickel-titanium rotary endodontic instruments. J Endod. 2010; 36: 1380-4., and showed close and comparable results.

In this study was planned to perform non-linear static analysis under the worst loading conditions. This analysis was followed by fatigue failure check on St.St. 316L and NiTi, stress vs number-of-cycles curves of both materials to estimate the instrument lifespan, as evaluation procedure steps in previous study by Cheung et al.1818. Cheung GS, Zhang EW, Zheng YF. A numerical method for predicting the bending fatigue life of NiTi and stainless steel root canal instruments. Int Endod J. 2011; 44: 357-61.. As presented in Figure 4a Cheung et al.1818. Cheung GS, Zhang EW, Zheng YF. A numerical method for predicting the bending fatigue life of NiTi and stainless steel root canal instruments. Int Endod J. 2011; 44: 357-61. used Bannantine et al.2222. Bannantine JA, Comer JJ, Handrock JL. Fundamentals of metal fatigue analysis. Upper Saddle River: Prentice Hall; 1989. p.38. equations and tabulated parameter values Table 1 to estimate the instrument lifespan. In Figure 4b Norwich et al.2323. Norwich DW, Fasching A. A study of the effect of diameter on the fatigue properties of NiTi wire. JMEPEG. 2009; 18: 558-62. showed the endurance strain amplitude % limit of 0.6 for 0.23 mm diameter NiTi wire.

Figure 4.
a Comparison between triangle and square cross sections of StSt and NiTi wires Life-time vs. tip displacement18 and b NiTi Ti-55.8/55.9 wt%Ni 0.23 & 0.61 mm heat treated wire diameters; strain % vs. Life-time curves23. LCF is Low Cycle Fatigue, HCF is Low Cycle Fatigue, SQ is square cross section, TR is triangular cross section.

Table 1.
used in evaluating fatigue behavior of St.St. and NiTi wires18

Where E is Young's modulus, εap is plastic strain amplitude, εa and σa are total strain and total stress amplitudes respectively.

The solid modeling and finite element non-Linear static analysis were performed on a Server HP ProLaint ML150, with Intel Xeon 3.2 GHz processors with 12 MB L2 cache, 10 GB RAM.

Results

The linear static analysis showed unrealistic stress level, thus a multi-linear stress strain curve is essential in non-linear analysis. Therefore, two runs on the constructed model were performed, simulating the use of the files under bending and torsional conditions in compliance with the ISO 3630-1 specification2020. International Organization for Standardization. 3630-1: Dentistry. Root-canal instruments - Part 1: General requirements and test methods. 2nd ed. Geneva: ISO; 2008..

First simulation was performed for St.St. 316L as the file material and its stress strain curve was imported to ANSYS as multi-linear material. As presented in Figures 2, 4, and 5, the generated stress and strain levels on the instrument exceeded the fracture point of St.St. 316L, which indicated instrument failure. Thus St.St. 316L as a material is not suitable for manufacturing rotary/reciprocating files.

Figure 5.
St.St. 316L file behavior a Von Mises stress distribution and b Plastic Von Mises strain distribution, with deformed/un-deformed shape

Second simulation was performed for NiTi as the file material, and its stress strain curve was imported to ANSYS as Multi-linear material, as illustrated in Figure 2. The meshed model, deformed shape and total deformation of the studied model are presented in Figure 6.

Figure 6.
NiTi file a Mesh & deformed shape and b total deformation distribution

Figures 7a and 8a illustrated the distributions of Von Mises strain ca 0.61 and stress ca 480 MPa respectively, showing the critical points at file tip and outer layer in the bending region. The total plastic strain in Figure 7b represents the majority of total strain that indicated fatigue failure expectation. Maximum tensile stress in Figure 8b dominated the total stress and showed the critical point maximum total stress at the outer layer at the bending region 3 mm from tip.

Figure 7.
NiTi file a Von Mises strain and b Total plastic strain distributions

Figure 8.
NiTi file a Von Mises Stress and b Maximum tensile stress distributions

Discussion

Using basics of mechanics from literature55. Fishelberg G, Pawluk JW. Nickel titanium rotary file canal preparation and intracanal file separation. Compend Contin Educ Dent. 2004; 25: 17-24. - 66. Sattapan B, Nervo GJ, Palamara JE, Messer HH. Defects in rotary nickel titanium files after clinical use. J Endod. 2000; 26: 161-5. to compare between the two systems rotary and reciprocating with the same file will give good indications. The reciprocating system will have three advantages and one drawback. The first advantage is reducing the torsional stress, since during reciprocation movement the instrument engages dentin at its tip during the counterclockwise movement, whereas the clockwise movement disengages the instrument immediately afterwards2424. Pirani C, Ruggeri O, Cirulli PP, Pelliccioni GA, Gandolfi MG, Prati C. Metallurgical analysis and fatigue resistance of WaveOne and ProTaper nickel-titanium instruments. Odontology. 2014; 102: 211-6..

Second, extended fatigue life of the NiTi file when used in reciprocation movement compared to continuous rotation, which may be explained by smaller angular movement that induces less stress on the file in case of locking2525. Lopes HP, Vieira MVB, Elias CN, Siqueira Jr JF, Mangelli M, Lopes WSP, et al. Fatigue life of WaveOne and ProTaper instruments operated in reciprocating or continuous rotation movements and subjected to dynamic and static tests. Endod Pract Today. 2013; 7: 217-22.. Finally, reciprocating action working as hammer at the file's cutting edge, increased the applied load on the root canal to double its value. The drawback is an expected shorter lifespan in comparison to rotating one, due to increased load applied at the blade tip.

Finite element analysis has been applied for detailed assessment of stress distributions in instruments. Few researches have studied the influence of reciprocating motion on cyclic fatigue resistance of rotary NiTi files using finite element analysis1616. Min-H o Lee, Anth eunis Versluis, Byung-Min Kim, Chan-Joo Lee, Bock Hur, Hyeon-Cheol Kim. Correlation between experimental cyclic fatigue resistance and numerical stress analysis for nickel-titanium rotary files. J Endod. 2011; 37: 1152-7..

In this research, St.St. 316L file analysis showed that the generated stress levels on the instrument exceed the fracture stress ca 568 MPa2626. Neeharika VB, Narayana KS, Krishna V, Kumar MP. Tensile and creep data of 316L N stainless steel analysis. J Eng Appl Sci. 2014; 9: 699-705., which indicated instrument failure. This may be attributed to the rigidity of stainless steel alloy, which is not suitable for instrumentation of curved canals under high torsional load.

On the other hand, the NiTi file analysis showed maximum values of total Von Mises stress ca 480 MPa and strain ca 0.61 as indicated in Figures 7 and 8 which are matching similar studies2222. Bannantine JA, Comer JJ, Handrock JL. Fundamentals of metal fatigue analysis. Upper Saddle River: Prentice Hall; 1989. p.38. , 2727. Arbab-Chirani R, Chevalier V, Arbab-Chirani S, Calloch S. Comparative analysis of torsional and bending behavior through finite-element models of 5 Ni-Ti endodontic instruments. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2011; 111: 115-21., although with less instruments' cross sectional details in current study. Plastic stress and strain components are dominating the total Von Mises stress and strain that indicates fatigue failure expectation after certain number of such type of load45° bend without locking. The expected number of cycles under such level of stress may be obtained from Figure 4, is approximately 500 cycles file lifespan before failure.

Pessoa et al.2828. Pessoa OF, da Silva JM, Gavini G. Cyclic fatigue resistance of rotary NiTi instruments after simulated clinical use in curved root canals. Braz Dent J. 2013; 24: 117-20. reported a comparable average number of 310 cycles to failure for RaCe rotary NiTi files FKG Dentaire, La-Chaux-de-Fonds, Switzerland, size #5, taper 0.04 inside buccal canals with an angle of curvature of 40°, curvature of 5 mm radius and 21 mm long, and the beginning of the curve was positioned 14 mm from the canal orifice.

Three roots per tooth require about 2 min to be treated by reciprocating instrument, if the average rotating speed of the instruments is 250 rpm. That was experimentally proven by You et al.2929. You SY, Bae KS, Baek SH, Kum KY, Shon WJ, Lee W. Lifespan of one nickel-titanium rotary file with reciprocating motion in a curved root canals. J Endod. 2010; 36: 1991-4. who estimated the time for one root preparation by reciprocating instrument at 21.15 ± 6.70 second2929. You SY, Bae KS, Baek SH, Kum KY, Shon WJ, Lee W. Lifespan of one nickel-titanium rotary file with reciprocating motion in a curved root canals. J Endod. 2010; 36: 1991-4. i.e. less than 2 min. In addition, applying equations 1 and 2 on the obtained results with NiTi instrument with locking at 3 mm from tip and 45° bend with 3 N.cm torque as the worst possible loading condition, it will resist up to three locking cases during its life, which is too difficult to occur during root preparation of one tooth.

In addition, the improvement in cyclic fatigue resistance shown by the reciprocating movement is related to two main factors. First, the rotation cycles are slightly reduced and consequently the overall number of rotations is reduced. Second, during reciprocation there is a different distribution of the same tensile stress values in time and this may reduce the overall accumulation of fatigue2929. You SY, Bae KS, Baek SH, Kum KY, Shon WJ, Lee W. Lifespan of one nickel-titanium rotary file with reciprocating motion in a curved root canals. J Endod. 2010; 36: 1991-4.. Reciprocating preparation with only one file was much faster than root canal instrumentation with continuous rotation. However, one file can be safely used a limited number of times in reciprocating motion2929. You SY, Bae KS, Baek SH, Kum KY, Shon WJ, Lee W. Lifespan of one nickel-titanium rotary file with reciprocating motion in a curved root canals. J Endod. 2010; 36: 1991-4.. Inexperienced operators achieved better canal preparations with reciprocating and/or rotary Ni-Ti instruments than with manual stainless steel files. However, rotary preparation was associated with significantly more fractures3030. Sonntag D, Guntermann A, Kim SK, Stachniss V. Root canal shaping with manual stainless steel files and rotary Ni-Ti files performed by students. Int Endod J. 2003; 36: 246-55..

Within the limitations of this study, the following conclusions can be drawn:

  1. - Finite element analysis indicated that modeling the instrument with fewer details using equivalent circular cross sectional area did not affect results quality.

  2. - St.St. 316L, the traditional material used in manufacturing manual files, is not suitable for rotary or reciprocating instruments.

  3. - Reciprocating instruments have short lifespan, and it is usually recommended to use one file per tooth.

  4. - Reciprocating system has great advantages over other root therapy instruments. Therefore, it is recommended to be used by less experienced dentist.

References

  • 1
    Del Bello P, Wang N, Roane JB. Crown-Down Tip Design and Shaping. J Endod. 2003; 29: 513-8.
  • 2
    Schäfer E, Tepel J, Hoppe W. Properties of endodontic hand instruments used in rotary motion, Part 2, Instrumentation of Curved Canals. J Endod. 1995; 21: 493-7.
  • 3
    Saunders EM. Hand instrumentation in root canal preparation. Endod Top. 2005; 10: 163-7.
  • 4
    Walia H, Brantley WA, Gerstein H. An initial investigation of the bending and torsional properties of Nitinol root canal files. J Endod. 1988; 14: 346-51.
  • 5
    Fishelberg G, Pawluk JW. Nickel titanium rotary file canal preparation and intracanal file separation. Compend Contin Educ Dent. 2004; 25: 17-24.
  • 6
    Sattapan B, Nervo GJ, Palamara JE, Messer HH. Defects in rotary nickel titanium files after clinical use. J Endod. 2000; 26: 161-5.
  • 7
    Pruett JP, Clement DJ, Carnes DL. Cyclic fatigue testing of nickel titanium endodontic instruments. J Endod. 1997; 23: 77-85.
  • 8
    He R, Ni J. Design improvement and failure reduction of endodontic files through finite element analysis: application to V-Taper file designs. J Endod. 2010; 36: 1552-7.
  • 9
    Shen Y, Zhou HM, Zheng YF, Peng B, Haapasalo M. Current challenges and concepts of the thermomechanical treatment of nickel-titanium instruments. J Endod. 2013; 39: 163-72.
  • 10
    Pereira ES, Peixoto IF, Viana AC, Oliveira II, Gonzalez BM, Buono VT, et al. Physical and mechanical properties of a thermomechanically treated NiTi wire used in the manufacture of rotary endodontic instruments. Int Endod J. 2012; 45: 469-74.
  • 11
    Plotino G, Rubini AG, Grande NM, Testarelli L, Gambarini G. Cutting efficiency of Reciproc and waveOne reciprocating instruments. J Endod. 2014; 40: 1228-30.
  • 12
    Braga LC, Faria Silva AC, Buono VT, de Azevedo Bahia MG. Impact of heat treatments on the fatigue resistance of different rotary nickel-titanium instruments. J Endod. 2014; 40: 1494-7.
  • 13
    Pedullà E, Lo Savio F, Boninelli S, Plotino G, Grande NM, Rapisarda E, et al. Influence of cyclic torsional preloading on cyclic fatigue resistance of nickel - titanium instruments. Int Endod J. 2014 Oct 29. doi: 10.1111/iej.12400.
  • 14
    Yared G. Canal preparation using only one Ni-Ti rotary instrument: preliminary observations. Int Endod J. 2008; 41: 339-44.
  • 15
    Câmara AS, de Castro Martins R, Viana AC, de Toledo Leonardo R, Buono VT, de Azevedo Bahia MG. Flexibility and torsional strength of ProTaper and ProTaper Universal rotary instruments assessed by mechanical tests. J Endod. 2009; 35: 113-6.
  • 16
    Min-H o Lee, Anth eunis Versluis, Byung-Min Kim, Chan-Joo Lee, Bock Hur, Hyeon-Cheol Kim. Correlation between experimental cyclic fatigue resistance and numerical stress analysis for nickel-titanium rotary files. J Endod. 2011; 37: 1152-7.
  • 17
    de Arruda Santos L, López JB, de Las Casas EB, de Azevedo Bahia MG, Buono VT. Mechanical behavior of three nickel-titanium rotary files: A comparison of numerical simulation with bending and torsion tests. Mater Sci Eng C. 2014; 37: 258-63.
  • 18
    Cheung GS, Zhang EW, Zheng YF. A numerical method for predicting the bending fatigue life of NiTi and stainless steel root canal instruments. Int Endod J. 2011; 44: 357-61.
  • 19
    Necchi S, Petrini L, Taschieri S, Migliavacca F. A comparative computational analysis of the mechanical behavior of two nickel-titanium rotary endodontic instruments. J Endod. 2010; 36: 1380-4.
  • 20
    International Organization for Standardization. 3630-1: Dentistry. Root-canal instruments - Part 1: General requirements and test methods. 2nd ed. Geneva: ISO; 2008.
  • 21
    Santos LA, Bahia MG, Casas EB, Buono VT. Comparison of the mechanical behavior between controlled memory and superelastic nickel-titanium files via finite element analysis. J Endod. 2013; 39: 1444-7.
  • 22
    Bannantine JA, Comer JJ, Handrock JL. Fundamentals of metal fatigue analysis. Upper Saddle River: Prentice Hall; 1989. p.38.
  • 23
    Norwich DW, Fasching A. A study of the effect of diameter on the fatigue properties of NiTi wire. JMEPEG. 2009; 18: 558-62.
  • 24
    Pirani C, Ruggeri O, Cirulli PP, Pelliccioni GA, Gandolfi MG, Prati C. Metallurgical analysis and fatigue resistance of WaveOne and ProTaper nickel-titanium instruments. Odontology. 2014; 102: 211-6.
  • 25
    Lopes HP, Vieira MVB, Elias CN, Siqueira Jr JF, Mangelli M, Lopes WSP, et al. Fatigue life of WaveOne and ProTaper instruments operated in reciprocating or continuous rotation movements and subjected to dynamic and static tests. Endod Pract Today. 2013; 7: 217-22.
  • 26
    Neeharika VB, Narayana KS, Krishna V, Kumar MP. Tensile and creep data of 316L N stainless steel analysis. J Eng Appl Sci. 2014; 9: 699-705.
  • 27
    Arbab-Chirani R, Chevalier V, Arbab-Chirani S, Calloch S. Comparative analysis of torsional and bending behavior through finite-element models of 5 Ni-Ti endodontic instruments. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2011; 111: 115-21.
  • 28
    Pessoa OF, da Silva JM, Gavini G. Cyclic fatigue resistance of rotary NiTi instruments after simulated clinical use in curved root canals. Braz Dent J. 2013; 24: 117-20.
  • 29
    You SY, Bae KS, Baek SH, Kum KY, Shon WJ, Lee W. Lifespan of one nickel-titanium rotary file with reciprocating motion in a curved root canals. J Endod. 2010; 36: 1991-4.
  • 30
    Sonntag D, Guntermann A, Kim SK, Stachniss V. Root canal shaping with manual stainless steel files and rotary Ni-Ti files performed by students. Int Endod J. 2003; 36: 246-55.

Publication Dates

  • Publication in this collection
    Jan-Mar 2015

History

  • Received
    11 Nov 2014
  • Accepted
    17 Mar 2015
Faculdade de Odontologia de Piracicaba - UNICAMP Avenida Limeira, 901, 13414-903 - Piracicaba - SP, Tel.: 55 19 2106 5706, Fax: 55 19 2106 5218 - Piracicaba - SP - Brazil
E-mail: brjorals@fop.unicamp.br