Gentlefile Versus Protaper in the Shaping Ability in Simulated J-shaped Root Canals


 Background: The aim of this study was to compare the shaping ability of Gentlefile (GF), a unique stainless-steel (SS) rotary system, and ProTaper (PT), a popular mechanical nickel-titanium (NiTi) system, in J-shaped simulated canals. Methods: Thirty-three resin blocks with a J-shaped canal were selected and randomly divided into 3 groups according to tip diameters: GF red (#23); PT F2(#25); GF blue (#26). They were prepared in one training model which imitated human dentition. Specimens were photographed by a dental operating microscope before and after preparation. The images were processed and measured using professional software. Centering ability and canal transportation were calculated, canal aberration and instrument separation were also recorded. Results: The GF system was significantly less transported than PT F2 in the apical and middle section. What’s more, better centering ability of GF was shown in all sections, including the coronal level. However, there was no difference between GF red and GF blue based on above criteria. Two cases of instrument separation of GF were noted.Conclusion: Under the conditions of this study, the two GF files showed better shaping ability, but were more easily separated than PT F2. As a novel SS rotary system, Gentlefile is more flexible to prepare narrow and curved root canals by scraping dentin with high-speed rotation, which might help remove smear layer much thoroughly and cause less dentin microcracks.


Background
Root canal preparation is one of the most important steps in root canal therapy. The ultimate goal of root canal preparation is to clean and shape the root canal system while maintaining the original con guration [1]. NiTi instruments with good exibility, super-elasticity, and shape-memory, have gradually become the mainstream in endodontic treatment [2]. However, it is often reported that the preparation of NiTi instruments may produce microcracks in dentin [3][4][5] and the fracture of instruments caused by torsion and cyclic fatigue seems to be inevitable during preparation.
ProTaper (PT, Dentsply Maillefer, Ballaigues, Switzerland), as a traditional NiTi rotary system, is still widely applied in clinical practice due to its great cyclic fatigue resistance and shaping ability [6][7][8]. It is generally accepted that NiTi mechanical instruments are more suitable for root canal preparation than SS ones, which caused the rare number of SS mechanical instruments.
Gentle le (GF, MedicNRG, Kibbutz A kim, Israel), a unique SS rotary system with limited studies, has been proved to be more exible than NiTi instruments [9]. The system consists of six les: one 18 mm coronal le (GF gray) and ve 25 mm nal les (GF yellow, GF red, GF blue, GF green, GF black). Each le is composed of three SS wires, which divide the le into a bi-layer-structure apical section and a three-layerstructure upper section, the taper of which varies between 3% and 4% [10], and its apical 0.5 mm edge sharpened at a 45° angle results in a non-cutting passive tip. These instruments are connected by friction to a dedicated motor (Gentle le Drive, Medic NRG) that automatically adjusts to load, operating at a maximum speed of 6500 rpm. High-speed rotation in uences the intracanal uid dynamics and increases effective irrigation [11]. GF instruments have been reported [12] to have great shaping and cleaning ability in oval canals, but that in the severely curved simulated canals is still lacking.
This study aimed to compare the shaping ability of PT F2, GF red, and GF blue in simulated curved canals. The null hypothesis was that there is no difference in the shaping ability of these instruments.

Selection of Samples
A total of 33 J-shaped simulated root canals in clear resin blocks were included. Each one had an initial 0.02 taper, 0.10 mm diameter at the apex,17 mm working length, and 30° curvature angle. Given the fact that special apical sizes of Gentle le, these root canals were divided equally into three different experimental groups to get comprehensive results: a basic preparation was performed up to GF red (#23) PT F2(#25) GF blue (#26), respectively.

Canal Instrumentation
All the root canals were prepared in one training model (Endo Training Model Castillo, VDW, Munich, Germany) which imitated human dentition to simulate the clinical environment. The curved part of each root canal was towards the mesial of the arti cial dentition.

Group 1: GF red
According to the latest manufacturer's instructions [10], after instrumentation with a manual #10 K le(Dentsply Maillefer, Ballaigues, Switzerland), GF grey was inserted into the canals and activated using the Gentle le Drive, applying inward action(pecking) [13] 5 times with very slight apical pressure for 5 seconds. Repeat the motions until the instrument reached 1 mm less than the working length. Then, GF red was chosen to proceed towards the working length (WL). Once GF red had prepared to the WL and rotated freely, it was removed from the canal.

Group 2: PT F2
Files were used with a 16:1 gear reduction handpiece powered by an electric and torque-controlled endodontic motor (X-Smart, Dentsply Maillefer, Ballaigues, Switzerland). According to the manufacturer's recommendations, S1, S2 were sequentially used at 300 rpm/3.0 N.cm and 300 rpm/1.0 N.cm until the instrument reached 1 mm less than the working length. Then, F1, F2 were rotated at 300 rpm/1.6 N.cm, 300 rpm/2.0 N.cm to the full WL, respectively. Outward action(brushing) [13] was performed 5 times for 5 seconds throughout the whole preparation. Once the instrument had prepared to the WL and rotated freely, it was removed from the canal.

Group 3:GF blue
We repeated the same conduction as in the Group 1. Then, GF red and GF blue were chosen to proceed towards the working length. Once GF blue had prepared to the WL and rotated freely, it was removed from the canal.
During canal preparation, irrigation was performed using 3 ml 0.9% NaCl solution before and after using each instrument. EDTA (GLYDE™ FILE PREP, Dentsply Maillefer, Ballaigues, Switzerland) was used as a lubricating agent.

Image Processing and Data Collection
All the root canals were injected by black ink before and after preparation. The dental operating microscope (Leica m525 F20, Leica Camera AG, Solms, Germany) was used to capture the pre-and postinstrumentation images under the same parameters and positions. The two images of each root canal were superimposed into a composite image using a computer software program (Adobe Photoshop CC2018, Adobe Systems, San Jose, CA, USA). Layers of post-instrumentation were treated by using the " nd edges" lter.
After calibration with the scale in the pictures, ten circles were drawn on the composite images by Auto CAD2010(Autodesk, San Rafael, CA). Twenty points were chosen for analysis. The rst two points were 1 mm away from the apical ending, and following points were located at 1-mm intervals. The last two were 10 mm from the apical stop, then connected the two points in each level and generated ten lines ( Fig. 1). 0-3 mm from the apical foramen consisted the apical part, 4-6 mm represented the middle part, and 7-10 mm were the coronal part of the canal. Image J (National Institutes of Health, Bethesda, MD; public domain) was used to measure the amount of resin removed in the mesial and distal direction.
Transportation and Centering ratio, according to the following formulas, were considered as two important criteria of shaping ability. X1 and X2 were the resin removed from mesial and distal section during preparation, respectively. In the rst formula, "0" indicated no transportation. whereas positive and negative values showed mesial and distal transportation, respectively. In the second formula, "1" indicated complete centering. The closer to the value 1, the stronger the centering ability. Canal aberration and instrument separation were also recorded.

Data Analysis
Variables were expressed as the mean ± standard deviation. Statistical analysis was performed with SPSS software 22.0 (SPSS Inc, Chicago, IL). Data were analyzed using Shapiro-Wilk test, Levene test, and one-way multivariate analysis of variance (ANOVA). The level of signi cance was set at 0.05.

Aberration and instrument separation
The three groups were able to prepare continuous taper without aberration, but two cases of instrument separation occurred in GF red and GF blue groups (Table 1). Instrument separation 0 0 0 0 0 1 0 0 1 Aberration 0 0 0 Transportation The GF system caused signi cantly less transportation than PT F2 in the apical (P < 0.001) and middle section (P < 0.001). However, in the coronal preparation, there was no signi cant difference in transportation between the two systems(P = 0.057). As for GF red and GF blue, no difference was found in root canal transportation at all three levels above(P>0.05) ( Table 2).

Centering ratio
Better centering ability of GF system was shown in all three sections of the root canal, but there was no difference between GF red and GF blue (P>0.05) ( Table 3).

Discussion
The preparation of curved root canals has always been a challenge. Many studies [14][15][16] have showed that canal curvature results in an increased risk of canal transportation and instrument fracture, which leads to the failure of root canal therapy. Unfortunately, around 84% human teeth have clinically noticeable root canal curvatures [17]. It is well-known that NiTi instruments are superior to SS ones in shaping the narrow and curved canals.
However, several of the recent studies [9,12] indicated that, as a unique rotary system made from SS, GF was more exible and prepared better for oval root canals than some of the NiTi instruments, and our study showed the GF system was also more centered and less transported than PT in the middle and apical sections of the narrow and curved root canals. This nding is consistent with that of Moreinos [9] who reported that GF les created less vertical force on bent canals.
There is a large volume of published studies [18,19] describing the threat of root canal transportation. Once canal transportation scores exceed 0.15 mm, the possibility of preparation failure will increase. Excessive transportation results in unreliable canal walls-one side of canals could not be completely cleaned, While the other side remains thin dentin, which will lead to a series of complications such as perforation, root fracture, periapical periodontitis, and so on [20,21]. The instrumentation of PT F2 in 5-7 mm in our study seemed to be riskier than GF on account of their excessive results at these levels (average transportation value is 0.174,0.305,0.224, respectively), obvious transportation even can be observed in the middle section (Fig. 2).
The results seemed that PT is not suitable for narrow and severely curved canals. In addition to the curvature mentioned above, two other possible reasons are the big taper and brushing movement of PT system. PT F2 is a multi-taper instrument, the maximum taper can reach 0.08, which means that, compared with GF system, more dentin needs to be removed to achieve the same shaping effect and several reports [22,23] have shown that 0.06 or smaller taper is enough for mechanical debridement.
Furthermore, brushing motion [24] has been proved to be more aggressive but safer than pecking. Multiplying the number of unidirectional brushing strokes will lead to signi cant cutting of the canal wall and result in unnecessary root canal enlargement in one direction [24].
However, in the present study, two cases of instrument separation occurred in GF groups. This outcome was contrary to that of Moreinos [9] who found GF les have greater resistance to cyclic fatigue than NiTi les. It is worth mentioning that resin materials have some limitations in simulating human teeth preparation, especially under high rotation speed. Although plastic blocks make it possible to standardize and visualize samples and the credibility of resin blocks as an experimental model for the analysis of preparation effect has been proved [25], due to the difference of hardness, preparation in simulated root canals would generate much heat, which might soften the resin material and increase the risk of fracture, especially at a speed of nearly 6500 rpm in GF groups. Thus, it is necessary to take further studies in extracted teeth with curved root canals.
Interestingly, there occurred a new mode of instrument separation in the present study. The separated le was just like a ring in the canal, thus it was hollow so that other instruments can insert into the canal and take it out easily (Fig. 3). The reason for this is not clear but it may have something to do with its coiling method of the three SS wires.

Conclusions
Under the limitations of this study, GF les, with the new concept of preparation, showed better shaping ability than PT F2, except the centering ability in the coronal section of simulated curved root canals. However, because of the different hardness of resin and dentin, further studies should be undertaken in extracted teeth.

Declarations
Ethics approval and consent to participate Not applicable Consent for publication Not applicable Availability of data and materials The data supporting the results reported in this manuscript can be provided upon request to the journal by the authors.

Competing interests
The authors declare that they have no competing interests. Authors' contributions XX and JY participated in the design of this study, and they both collected important background information. XX performed the study and statistical analysis, he also drafted the manuscript. And JY revised the manuscript. They both approved the nal manuscript.