| Abstract|| |
Objective: The aim of this study was to evaluate defects caused by torsional fatigue in used hand and rotary nickel-titanium (Ni-Ti) instruments by stereomicroscopic examination.
Materials and Methods: One hundred five greater taper Ni-Ti instruments were used including Protaper universal hand (Dentsply Maillefer, Ballaigues, Switzerland), Protaper universal rotary (Dentsply Maillefer, Ballaigues, Switzerland), and Revo-S rotary (MicroMega, Besanηon, France) files. Files were used on lower anterior teeth. After every use, the files were observed with both naked eyes and stereomicroscope at 20Χ magnification (Olympus, Shinjuku, Tokyo, Japan) to evaluate defects caused by torsional fatigue. Scoring was assigned to each file according to the degree of damage.
Statistics: The results were statistically analyzed using the Mann-Whitney U test and the Kruskal-Wallis test.
Results: A greater number of defects were seen under the stereomicroscope than on examining with naked eyes. However, the difference in methods of evaluation was not statistically significant. Revo-S files showed minimum defects, while Protaper universal hand showed maximum defects. The intergroup comparison of defects showed that the bend in Protaper universal hand instruments was statistically significant.
Conclusion: Visible defects in Ni-Ti files due to torsional fatigue were seen by naked eyes as well as by stereomicroscope. This study emphasizes that all the files should be observed before and after every instrument cycle to minimize the risk of separation.
Keywords: Protaper universal for hand use; Protaper universal for rotary use; Revo-S; torsional fatigue
|How to cite this article:|
Asthana G, Kapadwala MI, Parmar GJ. Stereomicroscopic evaluation of defects caused by torsional fatigue in used hand and rotary nickel-titanium instruments. J Conserv Dent 2016;19:120-4
|How to cite this URL:|
Asthana G, Kapadwala MI, Parmar GJ. Stereomicroscopic evaluation of defects caused by torsional fatigue in used hand and rotary nickel-titanium instruments. J Conserv Dent [serial online] 2016 [cited 2019 Dec 11];19:120-4. Available from: http://www.jcd.org.in/text.asp?2016/19/2/120/178684
| Introduction|| |
Nickel-titanium alloy (Ni-Ti) was developed by W.F Buehler (1960) at the Naval Ordinance Laboratory (White Oak, Maryland, United States). In 1988, Walia et al. introduced Ni-Ti files with special properties of super elasticity and shape memory.  Ni-Ti files are more flexible, biocompatible, corrosion-resistant, and resistant to torsional stress than stainless steel files. ,
Despite the many advantages, fracture of Ni-Ti files during clinical use is the most common procedural error.  Fracture of Ni-Ti files can occur with or without any visible defects of previous permanent deformation.
Stress buildup within the files causes fatigue during its use in the root canals. Fatigue of Ni-Ti instruments occurs in two ways:
- Flexural/cyclic fatigue.
- Torsional fatigue.
Flexural fatigue is caused by work hardening and metal fatigue, because of repeated compressive and tensile stresses accumulated at the point of maximum flexure when the file is used within the curved canals (Sotokawa 1988). Such a fatigue mechanism initiates microscopically within the elastic limit of metal, therefore it is not visible by visual inspection with the naked eyes. ,,,
Torsional fatigue is another form of stress buildup in the file during its rotation in the canal.
Torsional fatigue generally occurs in three situations:
- If a large surface of the instrument rubs excessively against the canal walls.
- If instrument tip is larger than the canal section to be shaped.
- If the operator exerts excessive pressure on the handpiece during filing. 
It is further classified into two types:
- Dynamic torsional fatigue: Occurs due to frictional forces caused by the resistance of dentin to cutting by the file. 
- Static torsional fatigue: Occurs due to continuous rotation of the file at one end while the other end stops spinning, i.e., when the tip or a part of file binds in the root canal and handpiece that holds the shank of the file and continues to rotate, it leads to torsional failure. ,,,
In torsional fatigue, the instrument exceeds the elastic limit of the metal and shows plastic deformation, followed by fracturing. 
Unlike cyclic fatigue, which is largely dependent on the original anatomy of the canal and is difficult to modify, we can intervene in cases of torsional fatigue through the correct use of the instruments and discarding them before they get fractured. 
Thus, the aim of this study was to evaluate the defects that occur during clinical uses by torsional fatigue, which will help clinicians to identify such defects and discard the instrument at the right time to prevent its separation within the root canal.
| Materials and methods|| |
One hundred five greater taper new Ni-Ti files for hand and rotary use were taken, used, and observed under naked eyes and stereomicroscope at 20× magnifications (Olympus, Shinjuku, Tokyo, Japan) before and after every use.
Group 1: Protaper universal hand (Dentsply Maillefer, Ballaigues, Switzerland) - S1, S2, F1, F2 (9, 9, 9, 8).
Group 2: Protaper universal rotary (Dentsply Maillefer, Ballaigues, Switzerland) - S1, S2, F1, F2 (9, 9, 9, 8).
Group 3: Revo-S (MicroMega, Besançon, France) - SC1, SC2, SU (12, 12, 11).
* () number of individual files used, respectively.
Teeth with straight canals confirmed by the Schneider method (less than 5 degree).
- Teeth with open apex.
- Teeth with developmental anomalies in root canal shape.
- Teeth having internal resorption.
Thirty-five new files from each group were taken and evaluated under stereomicroscope (20× magnification) for manufacturing defects. Standard access cavities were prepared on extracted lower anterior teeth stabilized with wax model, straight line access was achieved, and working length was determined. To standardize the canal size, apical gauging was done, and the canals with size 20 K file bound at the working length were selected for the study. Biomechanical preparation was done using the crown-down technique using ethylenediaminetetraacetic acid (EDTA) as lubricant, and the access cavity was filled with 3% sodium hypochlorite at every stage. Files were cleaned with alcohol sponge by twisting the file against it after each use. Instrumentation was done in sequential manner with torque and speed setting in Protaper universal rotary as 2-3 N.cm and 250/min and in Revo-S 2 N.cm and 400/min, respectively as advised by the manufacturer. Files were used with NSK endomate DT endomotor (Nakanishi, Chiyoda-ku, Tokyo, Japan). All files were operated by the same operator under pecking motion. Files were removed, cleaned, and inspected. 
Files were used for up to (maximum) 15 uses. If any distortion was observed, the files were not used further.
Evaluation of files
After every use, files were ultrasonically cleaned, sterilized, and examined by a single observer with the naked eyes and stereomicroscope at 20× magnification (Olympus, Shinjuku, Tokyo, Japan) to see the defects in the files.
A record was maintained about the number of uses and consequent changes in files. The following score  was given according to the degree of deformation or defects:
- No changes.
- Bent instrument.
- Tip deformation.
- Cutting edge changes.
- Reverse twisting.
- Change in length.
- Fracture of file.
Statistical analysis was done between three groups using the Kruskal-Wallis test and multiple groups using the Mann-Whitney test with the help of Statistical Packages for Social Sciences version 18.0 (SPSS Inc., Chicago, USA).
| Results|| |
The overall frequency distribution of defects showed that out of 105 files, using two different types of examination methods, i.e., by naked eyes and stereomicroscope, defects were seen in 17 files (16.2%) and 24 files (22.9%), respectively.
The percentage distribution of various types of defects as observed by different methods of examination, i.e., by naked eyes and stereomicroscope showed that bend was seen in 3 and 4 files, tip changes were seen in 5 and 7, and stretching was seen in 9 and 11 files, respectively. Other types of defects such as cutting edge change and partial reverse twisting were seen only by stereomicroscope. None of the files included in this study fractured after 15 uses [Figure 1] and [Figure 2].
|Figure 1: Stereomicroscopic (20×) photographs of defects seen in files. (A) Protaper hand instrument - stretching seen after 14 uses (B) Protaper hand instrument - bend seen after 11 uses (C) Protaper hand instrument - cutting edge change seen after 8 uses (D) Protaper Universal - tip changes seen after 11 uses (E) Protaper Universal - stretching seen after 13 uses (F) Protaper Universal - partial reverse twist seen after 14 uses|
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|Figure 2: Stereomicroscopic (20×) photographs of defects seen in fi les (contd). (G) Revo-S file-stretching seen after 15 uses (H) Revo-S file-tip changes seen after 10 uses. Cross sections of Protaper and Revo-S files. (I) Cross section of Protaper Uniersal file (convex triangular). (J) Cross section of Revo-S file (asymmetrical triangular)|
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Comparison of the methods of examination of the defects showed no statistically significant difference (P value 0.292).
The percentage distributions of various types of defects in the different groups (intragroup) are shown in [Table 1].
|Table 1: Defects in Ni-Ti instruments seen by stereomicroscopic examination|
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Intergroup comparisons of various defects were done and statistical analysis was carried out. The null hypothesis for the analysis was that there is no significant difference in defects between the three file groups.
Statistical analysis of the defects in different groups is shown in [Table 2].
Intergroup comparison of the frequency of different types of file distortion showed that bend in Protaper hand was statistically significant by the Kruskal-Wallis test (P value 0.016) and the Mann-Whitney U test (P value 0.041).
| Discussion|| |
Ni-Ti rotary instruments of varying cross-sectional designs and tapers have flooded the market. Using Ni-Ti rotary files for root canal instrumentation has enabled clinicians to predictably and efficiently create consistently tapered preparations while minimizing procedural mishaps, especially in curved canals. 
Despite their higher strength and flexibility, separation can still occur with Ni-Ti instruments, especially after extended use.  Unfortunately, many of these fractures happen unexpectedly, without any visible signs of permanent deformation. Cyclic, static torsional, and dynamic torsional fatigue are the most common causes of rotary Ni-Ti instrument fracture. ,
Classification of the different types of defects was given by Sotokawa (1988),  Roane and Sabala (1984), and Bonetti et al. (1998).  Though many classifications exist, a newer classification  was designed for the ease of clinical usage, to visually and microscopically gauge the torsional fatigue. This classification also indicates the severity of damage.
In this classification, defects caused by torsional fatigue were divided into two groups depending on the severity of defects, as follows:
1. Defects indicating mild damage:
- Bent instrument/tip deformation.
- Stretching/straightening of twist contour.
- Cutting edge changes.
2. Defects indicating severe damage.
- Partial reverse twisting.
- Change in length.
- Fracture of instruments.
In a study by Sattapan et al. in 2000, the authors stated that files that broke due to torsional fatigue exhibited signs of deterioration before fracture, while files that broke due to flexural fatigue did not show any defects related to their successive fracture. 
Dynamic torsional fatigue is seen in narrow canals as it results from frictional forces caused by the resistance of dentin cutting by the file.  Unlike in curved canals where both torsional and cyclic fatigue are seen, in straight canals only torsional fatigue can be seen. 
Since we have included only straight canals, cyclic fatigue was not seen and only torsional failure that demonstrates variable deformation, such as instrument unwinding, straightening, reverse winding, and twisting, was observed. ,,
Torsional failure depends on torque, which in turn depends on the tip size and taper of the instrument and on canal size. 
For standardization of torsional fatigue, tip size 25 and 6 degree taper was used. To standardize the apical diameter of the canal, apical gauging was done, and canals with size 20 K file bound at the working length were selected for the study.
Files were selected without any manufacturing defects by examination under stereomicroscope (20×), as manufacturing defects can cause fracture of new instruments even during first use. 
Files were cleaned after each use because dentin chips wedged in a file may magnify the effect of torsional fatigue. This may play a role in the clinical failure of these instruments. 
Sattapan et al. suggested that all the files should be analyzed after each use to reduce risk of separation in root canal systems.  This contradicts Pruett et al. (1997), who in their study suggested that visual examination is not a reliable method for evaluating used Ni-Ti instruments.  Sattapan et al. also suggested that minor defects, both manufacturing imperfection and plastic deformation, may not be detectable by naked eyes, and they recommended to use a magnification of at least 10×.
In this study, files were observed under both the naked eyes and stereomicroscope (20×), but the results showed no statistically significant difference. The overall percentage of defects observed in group I (Protaper universal hand) was 34.3%, in Group II (Protaper universal rotary) was 20%, and in Group III (Revo-S) was 14.3%. This shows that among the study groups, Group I (Protaper universal hand) had the highest percentage of defects, followed by Group II (Protaper universal rotary), and the lowest percentage of defects was seen in group III Revo-S files. Among all these groups, bend in Protaper hand instruments showed a statistically significant difference from the other groups.
The results showed the highest percentage of defects in Protaper hand files, which may be attributed to the difference in manner of use between hand and rotary files, wherein the amount of pressure applied was not uniform and was perhaps higher in case of hand instruments, leading to severe damage. ,
Protaper universal rotary instruments have a progressively larger, variable tapered shaft and symmetrical cross section with deep cutting flutes. Thus, when a Protaper universal rotary file rotates within the root canals, it simultaneously touches the surrounding wall of the canal.  The forces experienced by the files are correlated with the contact areas of friction between the instrument, the root canal wall, and the load required to cut the dentin.  Thus Protaper Universal rotary files require higher torque, making them more prone to metal fatigue  compared to fifth-generation Revo-S files, which have asymmetrical cross sections and a swaggering (snakelike) motion that allows easy escape of debris and prevents clogging , [Figure 2]. Reduction of the contact length of the blade to the dentin and reduction of stress in Revo-S may have contributed to fewer defects compared to Protaper Universal. In addition, the Auto reversal motor (Nakanishi, Chiyoda-ku, Tokyo, Japan) helps in withdrawing a file before it locks and fractures in the root canals, as it controls both rotation speed and torque during instrumentation. If the torque of instruments that rotate at a constant speed reaches the preset level, the rotation direction is reversed automatically. 
This emphasizes that all the files should be observed for any visible defects, before and after every instrumentation cycle, and any files showing defects should be discarded immediately, to minimize the risk of separation and to prevent iatrogenic mishaps.
| Conclusion|| |
Within the limitations of this study, the following conclusions were drawn:
- Defects identified with stereomicroscope numbered more as compared to those identified with the naked eyes, although the difference was not statistically significant. If these files had been used further, their potential for fracturing would have greatly increased.
- Hand instruments showed a greater number of defects than did rotary instruments.
- Revo-S files showed fewer defects compared to Protaper universal rotary files.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2]