Alteration in surface roughness of reciprocating endodontic instruments [version 1; peer review: 1 not approved]

Background: Surface roughness is one of the most important characteristics of endodontic instruments, correlating to instrument fracture. The purpose of this study was to measure the surface roughness values of these instruments before and after resin root canal preparation without previous glide path preparation, with the new method. Data was obtained from field emission scanning electron microscopes (FE-SEM) combined with independent ImageJ software (NIH, Bethesda, MD, USA). Methods: A total of 20 simulated J-shape resin blocks with a radius of 4.5 mm, length of 16 mm, and angle of inflection of 600 were chosen and distributed into two equal groups. Each group was prepared by the WaveOne Gold Primary (Dentsply Sirona, Maillefer, Ballaigues, Switzerland) or the Reciproc Blue R25 (VDW, Munich, Germany) instruments, without glide path preparation. Special molds were used to confirm the same areas on the cutting blade at 3 mm and the instruments’ tips were scanned by FE-SEM, at different observed times. The parameters of Ra, Rq, and Rz in each sample were collected using the ImageJ software for analyses. The data was processed using the paired t-test with a significance level of 0.05. Results: Right after the first resin canal instrumentation, the surface roughness parameters of the two reciprocating investigated instruments were decreased. Conclusions: The FE-SEM images processed using the ImageJ software offered a trustworthy and suitable method for assessment of the NiTi endodontic file surface roughness.


Introduction
Root canal preparation plays an important role in endodontic treatment. This step is critical and facilitates obturation. 1 Advancements in science and technics have led to the production of the rotary nickel-titanium (NiTi) root canal instruments using two manners of movement: continuous or reciprocating motion. To reduce time required, complicatedness of the process, and exhaustion of clinicians in root canal preparation, the single-file instruments are introduced using continuous or reciprocating rotation. 2 Since the first introduction of the single-file system with proprietary integrated technique, the system has been continuously developed in material improvement, taper modification, and cross-sectional geometry, especially in distinctive heat treatment processes.
Recently, the Reciproc Blue (RB) (VDW, Munich, Germany) and WaveOne Gold (WOG) (Dentsply Sirona, Maillefer, Ballaigues, Switzerland) instruments have been developed with many distinctive features in designs and thermal treatments. 3 The instruments are subjected to the proprietary post-machining heat processes after being ground by machine. This treatment produces the oxide layer with the specific blue or gold. 4 The risk of fracture is inherent in an endodontic instrument, especially for rotary nickel-titanium files. Both brittleness or ductility are a possible cause of breakage. 5 Typically, a beginning crack on the material facet could lead to the fracture of the file. 5 Defect on the surface of the material could be the crack beginning. 6 There could be a correlation between surface characteristics and the breakage mechanism of the root canal rotary file. 7 So far, there are limited studies of the root canal instrument surface roughness, [8][9][10][11] especially using the data obtained from the field emission scanning electronic microscope (FE-SEM). 12 There is modest data available for the surface roughness parameters of the WOG Primary (WOGP) and RB R25 (RBR25) instruments for root canal preparation with the previous glide path preparation using the FE-SEM. 12 The purpose of this study was to measure the surface roughness values of these instruments before and after resin root canal preparation without previous glide path preparation, with the new method, using the data obtained from FE-SEM combined with independent ImageJ software (NIH, Bethesda, MD, USA).

Methods
The present study was performed in 2019, at the University of Medicine and Pharmacy at Ho Chi Minh City, Viet Nam. The method used for the present study was introduced in a previous study. 12 An acrylonitrile butadiene styrene (ABS) former was produced using automatically computerized controlled apparatus for ensuring the root canal file at the exact location before and after resin root canal preparation with dimethicone (Sylgard 184, Dow Corning Corp., USA) impression. 12 The combination of the ABS mold and the polydimethylsiloxane (PDMS) impression with instrument inside was then introduced and fastened into the gear of the vacuum room of the FE-SEM (NOVA NanoSEM 450, FEI, UNSW, Sydney, Australia) after complete removal of bubbles and drying at 60°C in 3 hours using the vacuum extractor (Diener Electronic, Germany). 12 Sample size was calculated using the data of the previous study 12 with a total of 20 J-shape limpid plastic blocks for endodontic training (Dentsply Sirona, Maillefer, Ballaigues, Switzerland). Ten blocks were used for each group. Clear resin simulated root canal was a radius of 4.5 mm, length of 16 mm, and 60°angle of inflection, using the Pruett's manner. 13 All endodontic preparations were conducted by an endodontist. The resin root canal was first explored using the ISO #10 K-file (Dentsply Sirona, Maillefer, Ballaigues, Switzerland) to confirm the patency of the entire plastic canal. X-Smart IQ cordless handpiece (Dentsply Sirona, Maillefer, Ballaigues, Switzerland) combined with an iPad Mini were operated using proper modules for root canal instrumentation.
In total, ten WOGP (lot no. 1493642) and ten RB25 (lot no. 258192) files were chosen for the study. The marks on the shanks of all files were produced with the tiny round bur #014 ISO (Dentsply Sirona, Maillefer, Ballaigues, Switzerland). These marks assisted in reintroducing the files into the mold after preparation and in inspecting the identical sectors on the file before and after preparation. 12 Group 1 was prepared (without the glide path preparation) by the WOGP (Dentsply Sirona, Maillefer, Ballaigues, Switzerland) and group 2 was prepared (without the glide path preparation) by the RB25 (VDW, Munich, Germany) until the working length was reached with the gentle in-and-out pecking movement to approach clinical settings. Plastic root canal was thoroughly irrigated using 3% NaOCl solution (Canal Pro, Coltene Whaledent, Altstätten, Switzerland). One reciprocating file was only used for each plastic canal. After complete instrumentation, the instrument's surface was wiped off using moisture gauze, then the instrument was placed into an ultrasonic cleaner to be vibrated with alcohol solution for further cleansing and dried using gauze afterward.
The files were captured before and after endodontic preparation using the FE-SEM (Â1000, 10kV). The surface of the instrument pinpointed at 3 mm from the peak of the file was assessed using the method described the previous study. 12 The marks on the shank of the file were used as reference points. The evaluated sections were located, first by displacing the gear containing specimen from the mark on the instrument's shaft, toward the peak until it could be observed. Then, the backward movement of the gear was performed with the distance of 3 mm, and at this position, the cutting blade was evaluated and captured with the dimensions of 150 μm Â 150 μm. These figures were analyzed using the ImageJ 1.52 (NIH, Bethesda, MD, USA). The mean roughness parameters (R a ), root-mean-square roughness (R q ), the average length in the range of the highest tip and lowest groove in each length of specimen (R z ), and the three-dimension graphs were calculated and obtained from the ImageJ. 12 Shapiro-Wilk statistic was performed initially to check the distribution of the data and the paired t-test was executed to interpret the data using the SPSS 22.0 package (IBM-SPSS Inc, Armonk, NY, USA).

Results
The statistical values of the R a , R q, and R z were showed in the Table 1. 22 All data were normally distribution using the Shapiro-Wilk test. R a and R q parameters of the group 1 decreased after root canal instrumentation (P > 0.05). For the group 2, all these values decreased after endodontic, and these values were significantly different (P < 0.05).

Discussion
The result of the present study showed that almost all of the surface roughness parameters of both experimental instruments were decreased after root canal instrumentation. In the WOGP group, two of three valueswere reduced. In the RBR25 group, all values were decreased significantly. This result showed that surface changes, for such small sample of ten instruments, in the very beginning stage of root canal instrumentation, even the tiniest ones, could be discovered by this alternative modality with the FE-SEM at superior preciseness.
The atomic force microscope (AFM) and the conventional SEM were used for assessment of the surface of the root canal file before and after endodontic instrumentation. Although the three-dimension reconstruction feature of the AFM was been highly evaluated, this modality could just be used for scanning on tiny and absolute square, flat and rigid surfaces, whilst the conventional SEM could only capture the two-dimensional figures. 10 In the touch mode of the AFM, the tiny end of the stylus might be damaged itself or it could destruct the scanned surface, affecting the outcome, and increasing the considerable time required for image building as well. 10 Recently, a touchless profilometer has been reported by Ferreira et al. for investigation of the root canal instrument topography. 10 However, this technique has been found to give inaccurate outcomes when scanning in extreme slope inclines or intermissions. 14 One important shortcoming of this modality is that it does not properly work on the breakage areas. 15 The contemporary profilometer does not work properly without integrated software supplied by the manufacturer.
The extreme tiny resolution of 1.4 mm was one of the most advantage of this modern FE-SEM and the specimen treatment was unnecessary. One of the utmost significant advantage of the FE-SEM is the integration of the data obtained from the FE-SEM and the calculation using the ImageJ program in reconstruction of the three-dimension figures and automatic production of the surface roughness values. 12 Some other studies have affirmed the correctness and reliability of the integration of the data obtained from FE-SEM and the ImageJ program in calculation of the material surface roughness. [16][17][18] In the present study, the above viewpoint was securely confirmed with the results of precise and reliable surface roughness and reconstructed three-dimension figures using the data obtained from FE-SEM and the independent ImageJ program. All surface roughness parameters of the RBR25 group were significantly reduced, these reductions might be emerged from the efficacy of the instruments in root canal preparation.
Almost all surface roughness values in the WOGP group were receded after endodontic instrumentation, however, the differences were not statistically significant. This showed that the RBR25 files were subjected to the friction more than the WOGP instruments did. This could be led from the discrepancies in the construction, proportions, and substance of the two experimental NiTi instruments.
Although there were still certain disadvantages of resin blocks used for root canal experiments, 8,19 the plastic blocks might still be used appropriately for endodontic investigation. 12 The results of the present study were similar to those of the two previous studies. 11,20 The surface roughness parameters of the WaveOne Primary (WOP) and Reciproc R25 (RR25) were increased after resin root canal preparation in the study using the AFM and those of the ProTaper F2 (Dentsply Sirona, Mailefer, Ballaigues, Switzerland) instruments were reduced. 11 The ProTaper F2 instruments were performed following the F1 instruments, therefore, the F2 instruments were subjected to less load than the WaveOne Primary and Reciproc R25. In the present study, the two instruments were subjected to a heavy load when using the resin root canal at the beginning of the preparation without any previous glide path preparation. However, they had the reduced surface roughness after first utilization, therefore; the difference came from the construction, dimension, and substance of these instruments. In the previous study using a non-contact optical profilometer for investigation of the surface roughness of the WOP and RR25, these roughness values also reduced from the second use to the first use and from the second use to the unused instruments after resin root canal preparation. 20 This result agreed with the outcome of the present study, came from the wear of cutting blades of the files after root canal preparation.
Opposed to the result of the present study, other studies have reported that the surface roughness of the endodontic instruments increased after endodontic instrumentation. 8,9,21 All these previous studies used the extracted human teeth, one investigated the files used in the clinical setting, 9 and two others used the extracted human teeth for the in vitro evaluations. 8,21 The instruments were used five times and sterilized six times in the other study, 9 and the instruments were used in the severely curved canals for three times before examining under the AFM 8 or four-time utilizations and subjected to one sterilization cycle before investigation under the non-contact optical profilometer. 21 All the above different conditions affected the surface roughness of the instruments after instrumentation. Further research on the WOGP and RBR25 using many times both in-vitro or in-vivo situations needed to be performed to explore more characteristics of these instruments' surface.

Conclusion
The FE-SEM images processed using the ImageJ software offered a trustworthy and suitable method for assessment of the NiTi endodontic file surface roughness. In total, two of the three surface roughness parameters of the WOGP and RBR25 were significantly decreased after the first manipulation in plastic root canal preparation.

Partly
Competing Interests: No competing interests were disclosed.

Reviewer Expertise: Endodontology
I confirm that I have read this submission and believe that I have an appropriate level of expertise to state that I do not consider it to be of an acceptable scientific standard, for reasons outlined above.
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