|Year : 2018 | Volume
| Issue : 2 | Page : 82-86
Cyclic fatigue resistance of new and used ProTaper universal and ProTaper next nickel-titanium rotary instruments
Neslihan Büşra Keskin1, Taha Özyürek2, Gülşah Uslu2, Uğur İnan2
1 Balgat Oral and Health Care Center, Ankara, Turkey
2 Department of Endodontics, Faculty of Dentistry, Ondokuz Mayis University, Samsun, Turkey
|Date of Web Publication||5-Apr-2018|
Dr. Taha Özyürek
Department of Endodontics, Faculty of Dentistry, Ondokuz Mayis University, Samsun
Source of Support: None, Conflict of Interest: None
Aim: This study aims to compare the cyclic fatigue resistance of new and used ProTaper Universal (PTU) and ProTaper Next (PTN) rotary nickel-titanium systems using artificial canals.
Materials and Methods: Twenty sets of PTU and PTN were included in the present study. The groups were set as follows: Group A: 10 new sets of PTN instruments; Group B: 10 clinically used sets of PTN instruments; Group C: 10 new sets of PTU instruments; and Group D: 10 clinically used sets of PTU instruments. The cyclic fatigue tests were performed using a specially manufactured dynamic cyclic fatigue-testing device, which has an artificial stainless steel canal with 60° angle of curvature and a 2 mm radius of curvature. New and used files were rotated until fracture, and the cyclic fatigue device stopped automatically by the fracture and showed the fracture time on the screen. The number of cycles to fracture for each group was calculated and compared using paired and unpaired t-tests using SPSS 21.0 software. Statistical significance was set at 5%.
Results: For both rotary systems, the new instruments showed statistically higher cyclic fatigue resistance than the used instruments (P < 0.05). The mean number of cycles to fracture of PTN instruments was significantly higher than equivalent file sizes of PTU instruments (P < 0.05).
Conclusions: Within the limitation of the present study, reduction in the cyclic fatigue resistance for PTN and PTU instruments was observed after clinical use when compared to new groups.
Keywords: Cyclic fatigue, dynamic test, endodontics, nickel-titanium, rotary files
|How to cite this article:|
Keskin NB, Özyürek T, Uslu G, İnan U. Cyclic fatigue resistance of new and used ProTaper universal and ProTaper next nickel-titanium rotary instruments. Saudi Endod J 2018;8:82-6
|How to cite this URL:|
Keskin NB, Özyürek T, Uslu G, İnan U. Cyclic fatigue resistance of new and used ProTaper universal and ProTaper next nickel-titanium rotary instruments. Saudi Endod J [serial online] 2018 [cited 2018 Apr 22];8:82-6. Available from: http://www.saudiendodj.com/text.asp?2018/8/2/82/229348
| Introduction|| |
Nickel-titanium (NiTi) instruments have the advantages of being more flexible and more resistant to cyclic fatigue than stainless steel instruments; however, they have a tendency to fracture without any visible signs.,
Many factors influence instrument fractures, such as rotational speed,, preparation techniques, manufacturing technology, the radius and angle of canal curvature, and clinical use. Decrease in resistance of cyclic fatigue for different brands of rotary NiTi instruments following clinical use has been reported in a number of studies.,,,,
Manufacturers have developed various manufacturing methods, such as electropolishing, ion implantation, heat treatment, and recently, thermomechanical methods to enhance the fracture resistance of rotary NiTi instruments. In 2007, manufacturers introduced a new thermomechanical NiTi wire (M-Wire) that had greater superelasticity to stress and improved fatigue resistance compared with conventional NiTi wire.
ProTaper Universal (PTU; Dentsply Maillefer, Ballaigues, Switzerland) is produced with conventional NiTi, has variable tapers, and has a triangular cross-section design, and it has widely been used for root canal shaping in the past years. ProTaper Next (PTN; Dentsply Maillefer, Ballaigues, Switzerland) rotary NiTi system, produced with the M-Wire technology, was introduced and the manufacturer claims that PTN is more flexible than traditional NiTi files and more resistant to cyclic fatigue. PTN files exhibit a unique asymmetric rotary motion for enhancing the efficiency of root canal preparation and the rectangular cross-section for superior mechanical characteristics.
Studies showed that the clinical usage of NiTi rotary files caused surface deformations on the files. The deformations that occur during clinical usage of NiTi files may affect the cyclic fatigue resistance of the files. Thus, the aim of the study was to compare the cyclic fatigue resistances of new and used PTN and PTU instruments. The null hypothesis was that there would be no difference between the cyclic fatigue resistances of the new and used instruments.
| Materials and Methods|| |
Twenty sets of PTU (S1, S2, F1, F2, F3) and 20 sets of PTN (X1, X2, X3) were included in the present study. The groups were set as follows:
Group A: 10 new sets of PTN instruments.
Group B: 10 clinically used sets of PTN instruments.
Group C: 10 new sets of PTU instruments.
Group D: 10 clinically used sets of PTU instruments.
The new instruments in Groups A and C were subjected to cyclic fatigue test. The instruments in Groups B and D were used for the preparation of 1 mandibular first molar tooth that had straight root canals in routine clinical use.
PTU and PTU files were used according to the manufacturer's recommendations. In Group B, the pulp chamber was filled with 2.5% sodium hypochlorite (NaOCl) solution; then, the working length was determined with a size 10 K-file (Dentsply Maillefer) using an electronic apex locator (Morita Root ZX Mini; J Morita Corp., Tokyo, Japan). Preparation was performed with X1, X2, and X3 files to working length. The preparation was completed with X3 file. The instruments were ultrasonically cleaned and sterilized in an autoclave for 18 min at 134°C before use on another patient and before cyclic fatigue test after use for 3 times.
In Group D, the pulp chamber was filled with 2.5% NaOCl solution; then, the canal was irrigated, and the auxiliary SX file was used to move the coronal aspect of the canal to improve radicular access. The canal was irrigated, and a size 10 K-file (Dentsply Maillefer) was used for the recapitulation. Preparation continued with S1, S2, F1, F2, and F3 files to working length. The preparation was completed with F3 file. The instruments were ultrasonically cleaned and sterilized in an autoclave for 18 min at 134°C before use on another patient and before cyclic fatigue test after use for 3 times.
Cyclic fatigue test
To measure the cyclic fatigue resistance of the files, an artificial canal made of stainless steel with an inner diameter of 1.5 mm, a 60° angle of curvature, and a curvature radius of 2 mm was used. The working length of the artificial canal was 14 mm. To mimic clinical conditions, the front-rear axial motion length of the cyclic fatigue device was set at 3 mm/s. The VDW Reciproc Gold (VDW, Munich, Germany) endodontic motor was connected to the test device and run at the speed and torque (300 rpm and 3 Ncm) recommended by the manufacturer until the files fracture [Figure 1]. To minimize excessive heating during rotation and to decrease the friction of the files, the artificial canals were lubricated with a synthetic lubricant (WD-40; Milton Keynes, Buckinghamshire, UK).
The number of cycles to failure (NCF) was calculated using the following formula: NCF = the duration to failure (seconds) rotating speed/60.
Eight files, 2 from each group, were evaluated under a scanning electron microscope (SEM; JSM-7001F; JEOL, Tokyo, Japan) to determine the type of failure.
The data were normally distributed according to Kolmogorov–Smirnov test. The differences between new and used instruments of the same file size were compared using paired t-test. Unpaired t-test was used to compare the files of different brands. As the manufacturer stated the equivalent file sizes for PTN and PTU were X1-S1 and S2; X2-F1 and F2; and X3-F3, these file sizes were compared statistically. Statistical significance was set at 5%, and SPSS 21.0 software (IBM-SPSS Inc, Chicago, USA) was used for statistical analysis.
| Results|| |
Cyclic fatigue resistances of new instruments (Groups A and C) were higher than used ones for all instrument sizes tested. There was a significant reduction in the NCF between new and used PTN X2 and X3 instruments (P< 0.05) [Table 1]. The reduction in lifespan observed for X2 and X3 instruments was 33.54% and 19.93%, respectively. Although there was a decrease in NCF between new and used PTU instruments for all sizes, the difference was significant only between F2 and F3 instruments (P< 0.05) [Table 2].
|Table 1: Means and standard deviations of number of cycles to fracture of new and used ProTaper Next instruments|
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|Table 2: Means and standard deviations of number of cycles to fracture of new and used ProTaper Universal instruments|
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When the means NCFs of new and used instruments were compared between the two brands, the mean NCF of PTN instruments was significantly higher than PTU instruments. The differences among PTN X1-PTU S1 and S2, PTN X2-PTU F1 and F2, and PTN X3-PTU F3 were statistically significant in both new and used groups (P< 0.05).
When the samples were evaluated under SEM, fatigue striations were observed under high magnifications confirming the failure reason as flexural [Figure 2].
|Figure 2: Scanning electron microscope images of the fracture surfaces of ProTaper Next X3 (a) and ProTaper Universal F3 (b) files showing fatigue striations (arrows)|
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| Discussion|| |
In the present study, reduction in cyclic fatigue resistance was exhibited after clinical use for both PTU and PTN instruments. Similar results were reported in the previous studies.,,,,, These results were expected because continuous tensile and compressive stresses occur on NiTi rotary instruments at the maximum curvature of the canal while shaping curved root canals, and this leads to flexural fatigue and fracture. However, instrument fracture has various reasons, including root canal anatomy, physician's ability, instrument design, and usage. Many studies examining the NiTi files cyclic fatigue resistance were carried out on the artificial canals., The aim of using artificial canals is to minimize the anatomic variation, which might arise from the natural teeth, and to ensure certain level of standardization. After the cyclic fatigue testing, the fractured NiTi files were subjected to the SEM imaging to ensure the cyclic fatigue fracture.
According to the previous studies, clinical use has a negative effect on the cyclic fatigue resistance of rotary NiTi instruments. Gambarini  reported a significant reduction in fracture resistance for ProFile (Dentsply Maillefer) instruments after using 10 clinical cases. Furthermore, Plotino et al. stated a decrease in cyclic fatigue resistance but suggested to use Mtwo (VDW) rotary instruments 10 times in molar teeth. Aydin et al. reported a reduction in lifespan ranging from 18% to 51% after clinical use of RaCe (FKG, La Chaux-de-Fonds, Switzerland) instruments. Wolcott et al. suggested that PTU instruments might be reused more than four times. However, Fife et al. demonstrated that prolonged reuse of PTU instruments affects the fracture resistance of the instruments. Ounsi et al. reported that fatigue resistance of PTU instruments significantly decreased after two clinical applications. The differences among the studies that investigate the number of usage of NiTi rotary files might be due to the different brand of NiTi files tested in each study. In our study, owing to PTN, which was recommended for single use by the manufacturer, instrument groups were used once clinically. Our results showed that a single clinical use reduced the NCF of each brand of rotary instrument compared with new ones. A significant difference between new and used PTN instruments was observed, with the exception of X1 instruments, as well as a reduction in the life spans of X2 (−33.54%) and X3 (−19.93%).
Some researchers reported that instruments with larger diameters would fracture earlier under dynamic loads.,, Our results supported those of previous studies as the NCF of the instruments with larger tapers decreased in both new and used instruments. Furthermore, in new and used groups, NCF decreased respectively for sizes X1, X2, and X3.
In our study, cyclic fatigue resistance of PTN instruments was significantly higher than the PTU instruments. The reason of this might be that PTN has M-Wire technology and an offset design. The M-Wire technology is based on a series of heat treatments to the NiTi wire, and according to the manufacturers, M-wire is more flexible and resistant to cyclic fatigue than superelastic wire. Conventional NiTi alloys are in the austenite phase at mouth and room temperatures. At room temperature, M-Wire alloys that manufactured with heat treatment exist in both austenite and martensite phases because martensite is softer than austenite, and the higher martensite content of files made of M-Wire alloy has a positive effect on their cyclic fatigue resistance. Johnson et al. reported that M-Wire instruments were stronger than conventional instruments in cyclic fatigue tests. Gao et al. compared the fatigue life and flexibility of instruments made from different materials and found that M-Wire exhibited more functional advantages than conventional NiTi. The manufacturing technology may influence NiTi instruments regarding cyclic fatigue resistance. Besides, the cross-section design may affect the fatigue resistance of instruments. PTN instruments have variable tapers and reduced contact points between the instrument and canal wall to enhance fracture resistance. The PTU instrument has a triangular cross-section, and it is produced by a traditional grinding technique in the manufacturing of conventional instruments, which leads to microcracks and defects along the surface of the file. Cracks lead to stress concentration points that weaken the instruments and can cause unexpected instrument fractures.
| Conclusion|| |
Within the limitation of the present study, reduction in the cyclic fatigue resistance for PTN and PTU instruments was observed after clinical use when compared to new groups. PTN produced by M-Wire technology enhanced cyclic fatigue resistance compared with PTU produced by traditional NiTi alloy.
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]