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Table of Contents   
ORIGINAL ARTICLE  
Year : 2013  |  Volume : 16  |  Issue : 4  |  Page : 323-326
Comparative evaluation of the effects of multiple autoclaving on cyclic fatigue resistance of three different rotary Ni-Ti instruments: An in vitro study


1 Department of Conservative Dentistry & Endodontics, SRM Dental College & Hospital, SRM University, Chennai, Tamil Nadu, India
2 Department of Conservative Dentistry and Endodontics, Mahatma Gandhi Institute of Dental Science, Pondicherry, India

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Date of Web Publication2-Jul-2013
 

   Abstract 

Aims: The aim of this in vitro study was to evaluate comparatively the effects of multiple autoclaving on surface topography and cyclic fatigue of three different rotary nickel-titanium (Ni-Ti) instruments.
Materials and Methods: Twenty-one files each of Twisted Files (TF), Revo-S, and RaCe were used in this study. They were grouped as group 1-TF, group 2-Revo-S, and group 3-RaCe. All the files were autoclaved and subjected for scanning electron microscopic analysis and cyclic fatigue analysis.
Results: Data were subjected to statistical analysis using one way analysis of variance (ANOVA) and post hoc Tukey's Kramer multiple comparison test. TF exhibited the highest cyclic fatigue resistance followed by Revo-S and RaCe. Scanning electron photomicrographs showed no obvious surface changes in the files after three autoclaving cycles.
Conclusions: Within the limitation of this study, it can be concluded that Twisted Files was the most fatigue resistant compared to Revo-S and RaCe, and multiple autoclaving cycles did not have any significant alterations in the cyclic fatigue resistance of rotary Ni-Ti files.

Keywords: Autoclaving; cyclic fatigue; rotary nickel titanium files

How to cite this article:
Sundaram KM, Srinivasan N, Ebenezar RA, Narayanan L A, Rajkumar K, Mahalaxami S. Comparative evaluation of the effects of multiple autoclaving on cyclic fatigue resistance of three different rotary Ni-Ti instruments: An in vitro study. J Conserv Dent 2013;16:323-6

How to cite this URL:
Sundaram KM, Srinivasan N, Ebenezar RA, Narayanan L A, Rajkumar K, Mahalaxami S. Comparative evaluation of the effects of multiple autoclaving on cyclic fatigue resistance of three different rotary Ni-Ti instruments: An in vitro study. J Conserv Dent [serial online] 2013 [cited 2019 Jul 24];16:323-6. Available from: http://www.jcd.org.in/text.asp?2013/16/4/323/114359

   Introduction Top


Nickel Titanium (Ni-Ti) rotary endodontic files are known for its shape memory and super-elastic properties. [1] Super-elastic property of Ni-Ti enables greater file flexibility and resiliency. The major drawback of rotary NiTi instruments is torsional or flexural fracture. Torsional failure occurs when an instrument tip or another part of the instrument is locked in a canal while the shank continues to rotate. When torque exerted by the handpiece exceeds the elastic limit of the metal, fracture of the instrument tip occurs. [2] Instrument fractures caused by torsional fatigue often carry specific warning signs such as plastic deformation. [3]

On the contrary, flexural fracture occurs because of metal fatigue. Material fatigue occurs due to the microcracks produced during manufacturing process. These microcracks aids in crack propagation, thereby, leading to fracture of the instrument during work. The instrument actually does not bind in the canal, but it rotates freely in a curvature, thereby, generating alternative tension/compression cycles at the point of maximum flexure until the fracture occurs. [4] This is known as cyclic fatigue and initiation of fatigue crack usually occurs at the surface of a working part. It is especially vulnerable if the area with the highest stress coincides with the machining marks or miniature grooves. [5] After multiple usage surface defects like pitting, strip formation, micro-fractures/micro-crack formation, and disruption of cutting edges are evident on the instrument surface and if the instrument is used further, it may result in cyclic fracture. [6]

Revo-S (Micro-Mega, France) has a different geometrical design and asymmetrical cross section which allows more flexibility and less stress and it offers better ability to negotiate curved canals. A proprietary process of heating, cooling and twisting constitutes the manufacturing of Twisted File (TF- Sybron Endo, Orange, CA). TF is not ground against the material's natural grain structure to create the cutting edges.

Rotary files are supposed to be single use instruments as claimed by the manufacturers. [7] But owing to the high cost of rotary instruments, these Ni-Ti files are frequently reused after sterilization by autoclaving. Autoclaving helps in minimizing the risk of cross-infection during endodontic treatment.

Hayashi et al., have shown that additional heat treatment of Ni-Ti instruments during autoclave conditions may increase the flexibility of Ni-Ti rotary instruments. [8] There are varied reports on autoclaving conditions as to either improving [9] or degrading [10] both the performance and physical properties of different rotary Ni-Ti systems.

There are very few studies regarding the autoclaving effects of ground and twisted manufacturing methods on cyclic fatigue resistance. Hence, the aim of this study was to evaluate the effects of multiple autoclaving on surface topography and cyclic fatigue of Twisted Files and Revo-S compared with RaCe (FKG Dentaire, La Chaux-de-Fonds, Switzerland) rotary Ni-Ti files as control.


   Materials and Methods Top


Grouping

Group 1 (n = 21)-Twisted Files, 0.06 taper, 23 mm length, and tip diameter 25, (Batch number 822-6253, Sybron Endo, Orange, CA),

Group 2 (n = 21)-Revo-S, 0.06 taper, 25 mm length, and tip diameter 25, (Batch number 091410, Micro-Mega, France)

Group 3 (n = 21)-RaCe, 0.06 taper, 25 mm length, and tip diameter 25, (Batch number 2388, FKG Dentaire, La Chaux-de-Fonds, Switzerland).

Selection and preparation of natural teeth for instrumentation

Sixty-three single rooted and single canaled mandibular central incisors extracted for periodontal problems were selected for this study. Teeth with calcified canals, caries, and root fractures were not included.

The teeth were decoronated approximately at 1 mm above the level of cemento-enamel junction so as to standardize the length of the remaining root canal to 13 mm. Coronal portion of the canal was prepared using Gates Glidden drills of size 2, 3 and 4. The working length was obtained by introducing a #15 size K file until its tip was just seen at the apical foramen. From this length one mm was reduced to obtain the final working length.

The canals were then instrumented to a size of # 25 K files (Mani, Matsutani Seisakusho Co., Takanezawa-Machi Tochigi-Ken, Japan) and the canals were copiously irrigated with 5 ml of 17% Ethylenediaminetetraacetic acid (EDTA) (Glyde, Dentsply-Maillefer, Ballaigues, Switzerland) and 5 ml of 3% sodium hypochlorite (Prime Dental Products Pvt. Ltd., Mumbai, India) followed by 10 ml of 0.9% w/v of saline (Freseni us Kabi India Pvt. Ltd., Pune, India). The teeth were then stored in 0.9% saline until further instrumentation with rotary Ni-Ti endodontic files.

Custom-made jig preparation

The experimental set up for evaluating the cyclic fatigue of the Ni-Ti instruments comprised of an endodontic micro motor (X-Smart, Dentsply, Maillefer, Switzerland) which operated with a 16:1 reduction gear hand piece attached to the descending cross head of Universal Testing Machine (Lloyds Instruments, UK) and a stainless steel shaping block for endodontic files. The shaping block was fixed and consisted of a concave tempered steel radius and a convex tempered steel cylinder. This concave radius incorporated a notch for guiding the instrument into a canal which had a radius of curvature of 5 mm and angle of curvature of 45°. The three different file systems were subjected for evaluation of surface topography and cyclic fatigue after three cycles of autoclaving.

Procedure for autoclaving and scanning electron microscopic (SEM) analysis

Autoclaving of all rotary Ni-Ti instruments was performed in steam autoclave (T and S 14N, Cixi City Tongshuo medical appliances Co. Ltd., Japan) at 121°C at 15 lb pressure for 15 minutes. The stored mandibular central incisors were instrumented with the respective files of each group after 2 nd autoclaving following which they were cleaned with a brush and rinsed with water. The same procedure was followed after 3 rd autoclaving. The files were then mounted on SEM for surface topography analysis under scanning electron microscope (Hitachi S3400N, Tokyo, Japan).

Cyclic fatigue testing

The instrument was mounted on the reduction gear hand piece which was attached to universal testing machine (Lloyds Instruments, UK) with a cross head speed of 2 mm/s and a load of 500 kgf. The files were then guided into the notch of the radius and were allowed to rotate until fracture occurred. As the instrument is rotating in metal canal, synthetic oil was used to reduce the friction between the instrument and the canal wall. The instruments were rotated at a speed of 300 rotations per minute with a torque set at 5.0 Ncm. All instruments were rotated until fracture, which was synchronized with time using a chronometer (stopwatch). The time to fracture was multiplied by the number of rotations per minute to obtain the number of cycles to fracture (NCF) for each instrument. Mean values were then calculated for each group.

NCF = Time to fracture x Number of rotations per minute.

The above mentioned protocols of three cycles of autoclaving, SEM analysis, instrumentation, and cyclic fatigue testing were followed for the other two groups of rotary Ni-Ti instruments and number of cycles to fracture was calculated. The results were tabulated and statistically analyzed using one way ANOVA and post hoc Tukey's Kramer multiple comparison test by using Graph Pad Instat 3 software (San Diego, USA).


   Results Top


One way analysis of variance (ANOVA) of cyclic fatigue shows that there was statistical significance among the groups during all the autoclaving cycles (P < 0.001).

Intra group comparison reveals that there is an increase in cyclic fatigue resistance for TF from first to second autoclaving cycle followed by a slight decrease in cyclic fatigue resistance in the third autoclaving cycle. Revo-S showed a statistically significant difference from baseline to second autoclaving cycle and marginal increase in third autoclaving cycle which was not significant. RaCe also showed similar results as Revo-S but when compared with the other two the overall cyclic fatigue resistance was lower.


   Discussion Top


Ni-Ti endodontic instruments always possess a risk of separation caused by cyclic fatigue or torsional stress. [3],[4] It has been suggested that cyclic fatigue has accounted for 50% to 90% of mechanical failures. Hence, to reduce the incidence of instrument fracture, various methods have been employed by manufacturers to increase the fatigue resistance which include electro-polishing the surface of Ni-Ti instruments (RaCe and TF), nitrogen ion implantation, twisting instead of machining (TF), and alteration of geometric characteristics such as asymmetric design (Revo-S). The geometrical design of the files also aides in minimizing cyclic fractures. [11]

In this study, canals were simulated with a stainless steel device that guaranteed a fixed radius of curvature along with a fixed angle of curvature. The method used in our study for cyclic fatigue is based on a method described by Pruett et al.[12] This method accurately describes the root canal curvature based on the angle of curvature (α) and radius of curvature (r). The radius of curvature represents how abruptly or severely a specific angle of curvature occurs as the canal deviates from a straight line. The smaller the radius of curvature, the more abrupt is the canal deviation. [12] Stress on the instrument is inversely proportional to the radius of curvature. In our study, we have chosen a 45° angle of curvature and 5 mm radius of curvature as this was deemed to be within the clinically relevant values to simulate an abruptly curved canal as suggested by Pruett et al., [12] in their study.

Autoclaving have a slight influence on the physical properties of stainless steel instruments. [13] There are varied reports on the sterilization of Ni-Ti rotary instruments. Silvaggio and Hicks et al., [14] concluded that heat sterilization of Ni-Ti rotary files did not increase the likelihood of instrument fracture. Similarly Mize et al., [15] and Hilt et al., [16] showed that heat treatment as a result of autoclave sterilization did not extend the life of Ni-Ti instruments. On the contrary, de Melo et al., [17] showed that sterilization increased the fatigue life of rotary Ni-Ti instruments through the increase in hardness and torsional resistance of the material.

The results of this study [Table 1] reveals that the machined rotary files Group 2 (Revo-S, Mean value-342.50) and Group 3 (RaCe, Mean value-180.50) took fewer numbers of rotations for failure when compared with Group 1 (TF, Mean value-716.08) after first autoclaving cycle. Out of these two machined rotary files, RaCe showed the least fatigue resistance. This might be attributed to the fact that RaCe files have alternating cutting edge, which tends to increase the torsional resistance and not the flexural resistance. [18] The better results of Revo-S over RaCe may be attributed to its asymmetrical cross section, which tends to reduce the core diameter, flute depth, and forces generated during rotations in the curved canal.
Table 1: Mean and standard deviation of cyclic fatigue of three NiTi rotary instruments after first, second and third cycles of autoclaving

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The number of rotations for failure of TF was significantly greater than for the other two files tested, which is concurrent with the results of Gambarini [19] and Larsen. [20] This may be attributed to the fact that TF are manufactured by twisting the file blank in combination with an R-phase heat treatment and deoxidation surface treatment, which increases the surface hardness and sharpness of the cutting flutes. [21] The twisting process of TF optimizes the grain structure and eliminates the formation of micro fractures, making the file more durable. [22]

Ni-Ti endodontic instruments are machined from wires, which are previously cold drawn and then annealed in the temperature range that favors partial recovery of dislocations and precipitation of Ti 3 Ni 4 . After the machining process, no thermal treatment was applied to the Ni-Ti endodontic instruments. [1] From the metallurgical point of view, the temperature used in sterilization may not be high enough to cause significant changes in the alloy structure. The required changes to increase hardness, cutting efficiency, and fatigue life would be related to precipitation of Ti 3 Ni 4 . [23],[24] However, it is possible that consecutive cycles of sterilization give rise to cumulative effects, leading to increase in hardness in rotary Ni-Ti endodontic instruments after sterilization, as observed by de Melo et al. [17] Hayashi et al., [8] also reported that the additional heat treatment of hybrid Ni-Ti instruments can effectively increase the flexibility and fatigue resistance. On the other hand, Hilt et al., [16] did not observe this effect on similar instruments.

Even though our results show a small increase in fatigue resistance after three different autoclaving cycles, it was not statistically significant. So it is plausible to say that even if sterilization does not render Ni-Ti endodontic instruments stronger, it does not result in deleterious effects that could reduce their mechanical resistance. This is an important result, which implies that sterilization does not compromise the mechanical behavior of rotary Ni-Ti endodontic instruments, assuring the possibility of their re-use after sterilization, thereby, preventing cross infection between patients and increasing the success rate of endodontic therapy. [7]

SEM photomicrographs showed no obvious surface changes in the files after three autoclaving cycles [Figure 1]. Similarly, there were also no obvious surface morphological changes of the autoclaved files after instrumentation. This may be due to the fact that the number of autoclaving cycles were restricted to three, whereas other studies have employed up to 10 autoclaving cycles, which in turn, may have produced an increase in the depth of surface irregularities due to surface corrosion and the cumulative effects of autoclaving. [10],[25]
Figure 1: SEM photomicrographs of RaCe, Revo-S, and twisted files

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   Conclusion Top


Within the limitation of this study, it can be concluded that TF was the most fatigue resistant compared to Revo-S and RaCe, and multiple autoclaving cycles did not have any significant alterations in the cyclic fatigue resistance and surface morphology of rotary Ni-Ti files.

 
   References Top

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Thompson SH. An overview of nickel-titanium alloys used in dentistry. Int Endod J 2000;33:297-310.  Back to cited text no. 1
    
2.
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Kim HC, Cheung GS, Lee CJ, Kim BM, Park JK, Kang S. 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. 5
    
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Subramaniam V, Indira R, Srinivasan MR, Shankar P. Stress distribution in rotary nickel titanium instruments: A finite element analysis. J Conserv Dent 2007;10:112-8.  Back to cited text no. 11
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Haikel Y, Serfaty R, Bleicher P, Lwin TT, Allemann C. Effects of cleaning, chemical disinfection, and sterilization procedures on the mechanical properties of endodontic instruments. J Endod 1997;23:15-8.  Back to cited text no. 13
    
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Silvaggio J, Hicks L. Effect of heat sterilization on the torsional properties of rotary nickel-titanium endodontic files. J Endod 1997;23:731-4.  Back to cited text no. 14
    
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Mize SB, Clement DJ, Pruett JP, Carnes DL. Effect of sterilization on cyclic fatigue of rotary nickel-titanium endodontic instruments. J Endod 1998;24:843-7.  Back to cited text no. 15
    
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Correspondence Address:
Kanthimathinathan Meenakshi Sundaram
27, 23rd Cross Street, Maharaja Nagar, Tirunelveli . 627 011, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0707.114359

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[Pubmed] | [DOI]



 

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