INTRODUCTION

Endodontic treatment seeks to restore the function of the dental organ through chemical-mechanical procedures, the success or failure of the procedure depends directly or indirectly on the response of adjacent structures1,2, to pulpal alterations and their repercussions on periapical tissues³.

The file systems were initially made of carbon steel, such as the K file; however, in curved root canals, there were drawbacks such as the rigidity of the alloy and the working time^4,5, this promoted the evolution of the instruments to achieve a better adaptation to the original anatomy of the root canal⁶.

With instruments the best quality, iatrogenic events, shelves, racks, perforations, transportation and deviations were reduced thanks to the elasticity, flexibility and cutting efficiency⁷, reducing the cyclic fatigue or files fracture caused when the instrument axis is compressed against the internal wall of the bend, while the opposite side is tensioned, the movement of the file inside the canal would cause a maximum compression point in the bend causing the unexpected fracture^7,8.

The NiTi alloy allowed greater resistance to the cyclic fatigue of the files, it was developed by WF Buehler, a metallurgical engineer (Naval Ordenance Laboratory), who gave it the name of Nitinol (Ni nickel, Ti titanium and "nol" for short) from the laboratory). The thermodynamic properties of the alloy and the concepts related to the shape memory effect and super-elasticity at high temperatures were known, as well as high resistance to corrosion and biocompatibility with the dental organ. NiTi is currently widely used due to the characteristics and specific properties of resistance, damping, elasticity and hysteresis to stress^9-11; In addition, patented heating-cooling treatments have given the instruments a layer of titanium oxide that can be visualized on its surface, thus allowing control of transition temperatures thanks to an alloy in which the files have the ability to return to their natural state¹².

WO is a system with an S-shaped cross section, with three zones along its working length, triangular convex in the apical zone and two trapezoidal convex zones in the medial and coronal zone^13,14, allows to eliminate the Due to its 170° counterclockwise, 50° hour and 350 oscillations per minute¹⁵ movement, the dentin of the canal is available in three presentations: Small, Primary and Large^13,16, it is manufactured with M-Wire alloy¹⁷.

WOG comes in four presentations: Small, Primary, Medium and Large^18-19, unlike its predecessor, it has a parallelogram-shaped cross section²⁰, it is manufactured under a heating-cooling treatment with a surface layer of titanium oxide that gives it the golden coloration¹⁹.

The R and RB systems have an S-shaped cross section^21,20, they come in three presentations: R25, R40 and R50 at 21, 25 and 31 mm^7,19,20,22, are manufactured with a “M-Wire” alloy, improved in the case of the R17 system and a heating-cooling treatment with a titanium oxide surface layer that gives it the bluish coloration in the RB19,²⁰ systems, work with a movement of 150 ° anti-clockwise, 30 ° hourly and 300 oscillations per minute, which allows correct removal of debris from inside the root canal¹⁵.

When root canals have pronounced curvatures, reciprocating systems have become the first choice due to the help they provide, a better instrumentation technique and less working time than the conventional technique²³.

Studies on the fracture resistance of various rotary systems are important, especially when there is root curvature; The objective of the study was to evaluate the resistance to cyclical fatigue of the WO, WOG, R and RB systems, evaluating the useful life time, degrees of curvature and area of the fracture file in curved root canals.

 

Materials and methods

In vitro study, in simulated ducts, the fatigue resistance of files from different reciprocating systems was evaluated. For the experiment, a gutter was made in an 8 x 14 cm metal box in which three canals were designed to simulate natural tooth curvatures: 45°, 60° and 90°20. Following the recommendations of Mohammad and colls.7, and Keskin and colls.²⁰, three conical shaped artificial ducts were created, according to the dimensions of the instruments to be used: 1.5mm in diameter, a bend angle of 10mm from the tip of the instruments and a radius of bend of 5mm (figure N ° 1); an open channel at the top for the entrance of the files and a circular deposit at the end of the channel for the portion of the fractured file; the front part was covered with a 4mm thick glass to prevent the file from slipping or jumping^7,20.

Figure 1. Dynamic gutter of simulated roots

Fuente: Base de datos de la Investigación
Elaborado: Los autores

The sample was made up of 48 files (25 / 0.25), divided into four groups of 12 files: WO and WOG (Denstply Maillefer, Ballaigues, Switzerland) Ø = 25 mm, with taper of 0.08 and 0.07 respectively, as well as, R and RB (VDW, Munich, Germany) Ø = 25 mm, taper 0.08. Additionally, 3 subgroups of 4 files were used in each of the three curvatures: 45°, 60° and 90°.

All files were rotated until fracture and the useful life time and area of the file that fractured in each conduit were determined20. The files of the reciprocating systems were new and were used in an X-Smart Plus® (Dentsply Sirona) motor, files that did not save the speed values in the motor system were excluded, as well as the shanks of the files that did not enter correctly in the contra-angle of the motor handpiece or that at the moment of actuation they fractured outside the artificial canal.

Glycerin was placed inside the artificial ducts to prevent friction and heating of the metal, then the file was installed in the contra-angle and selected in the engine file library using the preset program "Reciproc ALL" designed specifically for the R R25 and RB R25 instruments and the "WaveOne ALL" program designed specifically for WO and WOG instruments, and were automatically driven at 300 - 500 oscillations per minute as directed by each manufacturer; all files were activated 23 mm into the canal; The time to fracture was determined by means of a chronometer, and the fracture area (apical area: 1-8 mm, mean: 8-16 mm or coronal: 16-23 mm) was also measured through an endodontic ruler (figure N ° 2).

Figura 2. Sistema reciprocante accionado. A. Toma del tiempo B. Medición de la zona fracturada

Fuente: Base de datos de la Investigación
Elaborado: Los autores

The data was analyzed and processed using the BioEstat® Version 5.3 statistical program. As there was heterocesticity, it was compared between groups using the Kruskal Wallis test with Student-Newman-Keuls comparisons and a 95% reliability, to determine the statistical difference between groups. Results less than 0.05 were considered statistically significant.

 

Results

The descriptive statistics of the reciprocating systems at the different angulations showed that at a 45° bend there was greater resistance to cyclical fatigue in the RB system with an average of 1461.0 seconds (± 89.58), all files were fractured in the apical portion between 3 and 4 mm (table N° 1).

Table 1. Means and standard deviation at 45°

Fuente: Base de datos de la Investigación
Elaborado: Los autores

When the curvature was 60°, the longest half-life was reached by the RB system, with an average of 1146.0 seconds (± 28.23), higher than the other systems; all fractures occurred in the apical portion of the file between 2 and 6 mm (table No. 2).

Table 2. Means and standard deviation at 60°

Fuente: Base de datos de la Investigación
Elaborado: Los autores

At the largest bend, at 90°, the time was drastically reduced in all the systems, there was a greater resistance to cyclic fatigue of the RB system, where the time for fracture averaged 195.0 seconds (± 14.76); the fracture zone was kept in the apical portion for all groups between 4 and 7 mm (table No. 3).

Table 3. Mean and standard deviation 90°

Fuente: Base de datos de la Investigación
Elaborado: Los autores

Descriptive statistics showed that as angulation increases, fatigue resistance decreases. To determine if the differences were statistically significant, the Kruskal Wallis test was performed with 95% reliability, comparing with each of the three different angulations, a significant difference was determined between groups (p = <0.01). At 90 ° WO and WOG with 60 ° and R and RB with 45 °, which were the lowest values in each group (table 4).

Table 4. Kruskal Wallis test for comparison between angulations of each group

Fuente: Base de datos de la Investigación
Elaborado: Los autores

For the comparison between groups, the Kruskal Wallis test was used with a reliability of 95%. From the results it was determined that there was a significant difference (p = <0.05). The Student-Newman-Keuls method was used to make comparisons between groups and determine which values were statistically significant. RB instruments were associated with the highest cyclic fatigue resistance values among all instruments (P <.05) at 90°.

At 45° and 60°, RB presented a statistically significant longer half-life (p = <0.05) than WO and WOG, but not than R. On the other hand, at 90° RB it had statistically greater resistance to cyclical fatigue (p = <0.05 ) than WO, WOG and R (table 5).

Table 5. Test de Kruskal Wallis; Comparison between systems at different angulations (45°, 60° and 90°). Kruskal Wallis test

Fuente: Base de datos de la Investigación
Elaborado: Los autores

Statistically, at 45° and 60° R it was higher than WOG, but not that WO; however, only RB was greater than 90°. There were no statistically significant differences between R and WO in any of the angulations (table 5).

Regarding the fracture area, there was no difference between the study groups, 100% of the instruments fractured in the apical portion, with an average of 3 to 6.5 mm.

 

Discussion

Based on the methodology proposed in this study, it was found that the resistance to cyclic fatigue of the files decreases when the curvature is greater, especially at 90°; the half-life of the files would be equivalent to a few minutes of continuous activation, increasing the risk of fracture with the degree of angulation and working time. RB proved to be more resistant to cyclic fatigue than WO and WOG at all angulations and R at 90° of curvature. All files were fractured in the apical portion.

Keskin and colls.20, presented a simulation of the anatomy of natural roots in their size, diameter, shape, length, angle and radius of curvature through resin blocks, however, the hardness of the material compared to dentin was different but equals contact softening during the movement of the file within the canal. For this reason, a metallic dynamic gutter was used according to the methodology proposed by Mohammad et al.7 and Keskin et al.^20.24.

The gutter is a dynamic model to determine cyclical fatigue, it makes the files increase their resistance, however, in the clinical part these variables are subjective, the main limitation is that the files can fit loosely and need more flexibility than in a real duct, however, the proposed method is useful since it allowed adjusting the variables that would affect the resistance to cyclical fatigue such as the diameter of the canal, standardized to 1.5 mm to achieve a minimum contact between the file, the canal wall and the length of entry of it^7.8.

Keskin, and colls.33, compared the resistance to cyclic fatigue of WOG, R and RB, at 60° angulation, in agreement with our study; they observed that RB is superior to WO and R. However, in their study WOG had greater resistance to the cyclic fatigue that R, differs it from our results where WO had less resistance at 60°. Regarding the fracture zone, they found no difference between the mean length of the fractured fragments, as observed in our data.

The manufacturers of WOG and RB have explained that the heat treatments they underwent increased the flexibility and resistance to cyclic fatigue of the instruments^7,13,20, however, we did not find a significant difference at 45° and 60° between R and RB or between WO and WOG; on the other hand, at 90° if a longer half-life of WOG and RB was demonstrated than WO and R respectively.

Previous studies usually have considered 60° as a single curvature reference to determine resistance to cyclical fatigue ^7,13,20, in the present study, the comparison of three curvatures, 45°, 60° and 90° was made with a radius of 5mm, a comparison that has not been made previously, these curvatures are considered as severe according to the method of Schneider in 1971. The resistance to cyclic fatigue was determined to decrease if the degree of curvature increases.

Through this study, it was found that the RB system has the highest resistance to cyclical fatigue compared to R, WO and WOG at 45°, 60° and 90°, a result that is confirmed in a recent study that reported superiority in terms of the resistance presented by RB at 60 °, the effect of the heat treatment of the alloy is evident in the results obtained¹⁹.

Resistance values of WOG have been significantly higher than those of R at 60° (20), data that differs with those of the present study. Nieto and Mendoza21, also compare the WOG system with R., stated that WOG presented greater resistance to fracture at 90°, since they had a higher incidence of deformed tips, this is attributed to the fact that the gold alloy is less resistant than the traditional M-Wire alloy, on the contrary, we found no difference between these systems at 90°.

In the study by Pedullá and colls.13, the WO systems are compared with R, the authors did not find significant differences, results concordant with our study since we also observed no significant difference at 45°, 60° and 90° degrees of curvature.
In the same way as the results obtained in the articles taken as reference7,20 with those obtained in the present study, there was no significant difference regarding the fracture area (mm) since the four systems suffered a rupture at the area level. apical, on an average of 3 to 6.5 mm.

Mohammad and colls.⁷, affirm that the recommended speed for the movement of the reciprocating systems is 300 to 500 rpm, values that were used as ranges used when activating the files to assess the resistance to cyclical fatigue.

Although the fracture occurred in the apical portion of the files, Yared and colls.²⁵, recommend that NiTi rotary systems can be used safely up to the working length; however, when the ducts are very narrow or have curvatures in the middle or coronal third, they will not be as successful, since the heat treatment they have been subjected to makes them safe when only the apical portion is curved. This consideration is important, if expansion is required, it will be necessary to increase the taper of the instrument or perform a manual termination.

For reciprocating systems it is recommended that the instruments have a single use; but it must be considered that the single use may include the preparation of at least 3 root canals in most molar teeth, which can be complex, depending on the anatomy of the root canal system¹⁴. Our results at 90° are equivalent to a few minutes of work, suggest the possibility of fracture in the roots of molars with greater curvature where the single use by canal should be considered.

Similar to the presented results, several studies have shown greater resistance to cyclic fracture of RB, which could determine that it is indicated to work more safely in a curved canal; however, new investigations should be carried out trying to include other clinical variables, in addition to the sample size being a limitation in this experimental study, other experimental investigations or randomized clinical trials with an influential number of samples should allow finding more conclusive findings.

 

Conclusion

The cyclic fatigue strength of WaveOne, WaveOne Gold, Reciproc and Reciproc Blue is reduced when the degree of curvature increases to 90°. The null hypothesis was rejected because the resistance to cyclic fatigue of Reciproc Blue files was significantly higher than WaveOne and WaveOne Gold at 45°, 60° and 90° and that Reciproc at 90°. The fracture zone occurs in the apical portion of the files in simulated curved canals.

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	Daicy Elizabeth Correa Abad; https://orcid.org/0000-0001-9149-7902
	Paola Daniela Hidalgo Araujo; https://orcid.org/0000-0002-8751-0753