Comparison of the shaping ability of various nickel–titanium file systems in simulated curved canals

DENTAL NEWS, Volume XXIV, Number II, 2017

Comparison of the shaping ability of various nickel–titanium file systems in simulated curved canals

Dr. Mothanna Alrahabi  (mrahabi@taibahu.edu.sa) - Department of Restorative Dentistry, College of Dentistry, Taibah University, Al Madinah Al Munawwarah, Saudi Arabia
Dr. Ayman Alkady - Department of Restorative Dentistry, College of Dentistry, Taibah University, Al Madinah Al Munawwarah, Saudi Arabia

Abstract

Aim
The aim of this study was to compare the shaping time and shaping ability of the ProTaper Universal, ProTaper Next, WaveOne, and Twisted Files (TF) nickel–titanium (NiTi) systems.
Materials and Methods
This study was conducted using simulated root canals (n = 40). The specimens were divided into four experimental groups (n = 10 each).
Each group prepared by NiTi system as following: ProTaper Universal, ProTaper Next, and WaveOne, the canals were injected with black ink before instrumentation, a series of photographs of each canal was saved to a computer using a set protocol. The canals were reinjected using red ink postoperatively to define their outlines, and images were taken in the same standardized manner. Photoshop software was used to superimpose pre‑ and post‑instrumentation images in two different layers. Each 1 mm step from the apical end was measured using ImageJ Software and evaluated by measuring the amount of removed resin. The data were analyzed using SPSS version 20.0.
Significance was set at P < 0.05.
Results
There were significant differences (P < 0.05) in shaping time (in seconds) among the NiTi systems where the shortest preparation time was with TF system. There were significant differences (P < 0.05) in canal transportation values among NiTi systems. Canal transportation at D1, D2, D3, D5, and D7 was greater for the TF system than for the other systems (ProTaper Universal, ProTaper Next, and WaveOne).
The transportation with the TF System was toward the outside of the curvature, and it was greater than that with other systems at D1, D2, D5, and D7.
Conclusions
ProTaper Universal, ProTaper Next, and WaveOne preserved the original curvature of the canal better than the TF system.

Keywords

Nickel–titanium, ProTaper Next, ProTaper Universal, shaping, simulated canal, WaveOne

Introduction

Root canal shaping is considered an essential step in endodontic treatment. 1 According to Schilder,2 the preferred shape of the canal after mechanical shaping is a tapering funnel following the original shape and curvature of the canal, while keeping the original position of the foramen, and keeping it as small as practically possible. Traditionally, stainless steel instruments have been used to achieve these objectives.
The classic method of using stainless steel instruments to shape the canal from the apical end to the coronal part does not achieve Schilder’s mechanical objectives of root canal shaping in curved canals and can cause iatrogenic damage to the original shape of the canal 3 such as straightening a curved canal, transportation, zipping, ledging, and root perforations. 4,5
Stainless steel instruments’ stiffness is considered the main cause of these procedural errors. 6 The introduction of nickel–titanium (NiTi) instruments to the endodontic field has revolutionized endodontic treatment, reducing operator fatigue and treatment time and minimizing errors associated with the use of stainless steel instruments.1 ProTaper Universal rotary files (Dentsply Maillefer, Ballaigues, Switzerland), which constitute a common NiTi rotary system, have a convex triangular cross‑sectional design and progressive taper that allows efficient movement and cutting ability to flare the canal more coronally. 7 ProTaper Universal rotary files are made from a conventional superelastic NiTi wire. In previous studies, the ProTaper Universal system showed more cracks than other rotary NiTi instruments. 8,9
Recently, several brands of NiTi instruments have been marketed to simplify root canal shaping.
ProTaper Next (Dentsply Maillefer) is one of these brands with an off‑center rectangular design and progressive and regressive percentage tapers on a single file, which is made from M‑Wire technology. The off‑center rectangular design of ProTaper Next minimizes contact between the instrument and the dentin, which leads to decreased withdrawal of the instrument from the canal and reduces the risk of taper lock. 10 The novel WaveOne NiTi single‑file system (Dentsply Maillefer) is another example of new brands offered in 2011. This system is intended for use with a special reciprocating file motion. It is composed of three single‑use files: Small (ISO 21 tip and 0.06 taper) for fine canals, primary (ISO 25 tip and 0.08 taper) for most canals, and large (ISO 40 and 0.08 tapers) for large canals. 11
Files are manufactured by grinding M‑Wire NiTi alloy. Endodontic instruments made of M‑Wire are predicted to have higher strength, with wear resistance resembling conventional superelastic NiTi instruments because of M‑Wire’s unique nanocrystalline martensitic microstructure. 12
Recent improvements in NiTi systems have reduced the number of machining defects; Twisted Files (TF) (SybronEndo, Orange, CA, USA), whose use involves twisting a file blank, represent an example of these improvements. 13
TF are made by transforming basic austenite NiTi wire into the R‑phase through thermal processing through a series of heating and cooling cycles after achieving the required twisted shape. When R‑phase wire returns to the austenite crystalline structure, it becomes superelastic when stressed. 14
The aim of this study was to compare the shaping time and shaping ability of the ProTaper Universal, ProTaper Next, WaveOne, and TF NiTi systems.

Materials and methods

Simulated canals
This study was conducted using simulated root canals (n = 40) fabricated with clear resin blocks (Dentsply Maillefer, Ballaigues, Switzerland). The taper of the canals was 2, and they were 17 mm long (a straight coronal section of 12 mm and a curved apical section of 5 mm). The size of the canal equaled ISO file size 15#, and the curvature of the simulated root canals was 40°, based on the Schneider method of measuring curvature. 15
Photographic procedures
To provide standardized photographs of each canal, a special mold was used to position the camera (Nikon D3200, Nikon, Inc.) precisely. To improve the color contrast of photos, all canals were injected with black ink before instrumentation. In a standardized manner, a series of photographs of each canal was saved to a computer using a set protocol. The canals were reinjected using red ink postoperatively to define their outlines, and images were taken in the same standardized manner. To reduce the margin of error, all photographs were taken by the same operator.
Instrumentation
The specimens were divided into four experimental
groups (n = 10 each).
• Group 1 was prepared using the ProTaper Universal system according to manufacturer reference guide to F2 instrument (size 25, taper 8% over the first 3 mm from apical tip) file, the instrument replaced after preparing three canals
• Group 2 was prepared using the ProTaper Next system according to manufacturer reference guide to × 2 (size 25, taper 6% over the first 3 mm from apical tip) file, the instrument replaced after preparing three canals
• Group 3 was prepared using the WaveOne system according to manufacturer reference guide, the canals prepared by primary single file instrument (size 25, taper 8% over the first 3 mm from apical tip)
• Group 4 was prepared using the TF system according to manufacturer instructions, the canals prepared until TF instrument (size 25, taper 6% constant taper), the instrument replaced after prepare three canals
• All simulated canals were prepared by the same operator, who had more than 5 years’ experience in root canal therapy.
Assessment of root canal preparation
The time taken to prepare each canal was recorded including active instrumentation, instrument changes, and irrigation.
Shaping efficiency was evaluated by measuring the amount of material lost at various levels (1 mm [D1], 2 mm [D2], 3 mm [D3], 5 mm [D5], and 7 mm [D7]) from the apical foramen of the root canal. Photoshop (Adobe Systems, San Jose, CA, USA) was used to superimpose pre‑ and post‑instrumentation images in two different layers. Each 1 mm step was measured using ImageJ Software (National Institute of Mental Health, Bethesda, Maryland, USA) and evaluated as follows:
D (difference) = Do (outer resin removed) – Di (inner resin removed).
A positive value indicated the prevalence of outer resin removal, and a negative result indicated the prevalence of inner resin removal. The closer the value was to zero, the more balanced was the preparation.
Statistical analyses
Statistical analyses were carried out using SPSS software (version 20; SPSS, Inc., Chicago, IL, USA). To evaluate the results, one‑way ANOVA and Bonferroni post hoc tests were used. Statistical significance was set at P < 0.05.

Results

Shaping time evaluation
There were significant differences (P < 0.05) in shaping time (in seconds) among the NiTi systems. The order of NiTi systems according to preparation time, in descending order, was as follows: ProTaper Universal > TF > ProTaper Next > WaveOne.
Table 1 and Figure 1 show the preparation times of each NiTi system.


Figure 1: Average shaping time (in seconds) according to nickel–titanium system



Canal transportation evaluation
There were significant differences (P < 0.05) in canal transportation values among NiTi systems. Canal transportation at D1, D2, D3, D5, and D7 was greater for the TF system than for the other systems (ProTaper Universal, ProTaper Next, and WaveOne). At D1, there were no significant differences (P > 0.05) in canal transportation among ProTaper Universal, ProTaper Next, and WaveOne. At D2, there was no significant difference (P > 0.05) in canal transportation between ProTaper Universal and ProTaper Next.
In addition, canal transportation with ProTaper Next was lower than that with WaveOne at D2 and D3, and canal transportation with ProTaper Universal was less than that with WaveOne at D2 and D3. Furthermore, absolute values of canal transportation with ProTaper Universal and WaveOne were lower than transportation values for ProTaper Next at D5 and D7.
Table 2 shows means and standard deviation of transportation (in mm) at different apical levels.
Figure 2 shows the differences in canal transportation according to NiTi system.

Evaluation of canal transportation direction
There were significant differences (P < 0.05) in the direction of transportation such that transportation with the TF System was toward the outside of the curvature, and it was greater than that with other systems at D1, D2, D5, and D7.
ProTaper Next caused less transportation toward the outside of the canal curvature at D3 compared with the other systems.
WaveOne caused less transportation toward the inside of the canal curvature at D1 compared with ProTaper Universal and ProTaper Next. ProTaper Universal caused more transportation toward the inside of the canal curvature at D2 and D7 compared with ProTaper Next and WaveOne.

Figure 2: Average canal transportation values according to nickel–titanium system


Discussion

The aim of this study was to compare the shaping ability of several NiTi systems (ProTaper Universal, ProTaper Next, WaveOne, and TF) in simulated canals. The use of simulated canals in this study allowed standardization of canal shaping evaluation 16, 17 and root canal morphology. 18, 19 The canals also supplied the same hardness and abrasion characteristics every time. 20
Pre‑ and post‑instrumentation studies indicate that the analysis of root canal outlines provides a standardized study design and extremely reproducible conditions. 17, 21
In our study, there was a significant difference in preparation time among NiTi systems, where the longest time was required for the ProTaper Universal, and the shortest for the WaveOne system. This is logical because the procedure with the ProTaper Universal required four instruments, whereas the WaveOne system is a single‑file system, and the use of a single‑file NiTi system with reciprocating motion reduces the time of preparation in curved root canals. 17, 21
The results show that the TF rotary system caused more canal transportation than did the other NiTi systems in this study (ProTaper Universal, ProTaper Next, and WaveOne).
This is consistent with the results of one other study; 22
however, in other studies, TF produced less canal transportation than did ProTaper Universal.[23,20] A recent study revealed that ProTaper and TF rotary systems could shape curved canals safely and preserve the original canal in a satisfactory way without any significant difference in shaping ability and canal transportation between them. 24
TF instruments have a triangular cross section with a constant taper. While nonlanded, ground‑fluted instruments with aggressive cutting action have more difficulty negotiating a curvature and increase the risk of ledging or transportation, 25 the differences between this study and other studies may result from differences in assessment methods and the equipment used.
The reciprocating motion with WaveOne prevents instrument engagement, and that may enhance the safety of single‑file systems in curved canals.26
However, it is clear that instrumentation with single‑file systems is faster. 27 Furthermore, reciprocating motion facilitates centered instrumentation more than a continuous rotating motion does, as aggressive continuous rotating motion tends toward the external wall of the canal, especially in the apical third. 28
Comparisons of single‑file technique systems with the ProTaper NiTi system (continuous rotating motion) have yielded conflicting results. Berutti et al. 17
reported that the WaveOne system facilitated centering preparation better than the ProTaper system did, whereas Bürklein et al. 29  found no significant difference between the single‑file technique and a full NiTi file sequence technique.
The shaping ability of NiTi instruments is a multifactorial phenomenon that is related to the method of manufacture, 30 microstructure of the alloy, taper, cross‑sectional design, type of movement, and system composition.
The microstructure of NiTi wire has three phases: Austenite, martensite, and the R‑phase.
The strong, hard quality of NiTi alloy is present in the austenite phase, and the flexible and ductile quality are present in the martensite phase.[31] The progressive tapers and sharp cutting edges of ProTaper instruments are responsible for canal transportation toward the outside wall of the canal. 32
A microcomputed tomographic three‑dimensional [3D] analysis is a recent advance for evaluating root canal instrumentation. 33
It has recently been suggested that microcomputed tomographic 3D analysis provides clearer distinctions than photographic measurements do. However, evaluation of root canal instrumentation requires further studies on natural teeth to achieve accurate results. Within the limitations of the present study, the ProTaper Universal, ProTaper Next, and WaveOne systems preserved the original curvature of the canal better than the TF system did.

Conclusion

The results of this study revealed that ProTaper Universal, ProTaper Next, and WaveOne preserved the original curvature of the canal better than the TF system did, with transportation toward the outside wall of the canal. ProTaper Universal, ProTaper Next, and WaveOne produced satisfactory root canal instrumentation. Further studies on extracted teeth with 3D analysis are needed to obtain accurate results.

References

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