Saudi Endodontic Journal

ORIGINAL ARTICLE
Year
: 2020  |  Volume : 10  |  Issue : 2  |  Page : 111--115

ProTaper Next files edge wear after conforming vestibular canals of maxillary molars: An in vitro study


Daniel Aracena1, Sergio Marín2, Eduardo Borie3, Angella Aracena4, Luis Bustos5, Mario Guzmán6,  
1 Dental School, Faculty of Sciences, Universidad Mayor, Temuco, Chile
2 Postgraduate Endodontic, Universidad Mayor, Temuco, Chile
3 Research Centre in Dental Sciences, Dental School, Universidad de La Frontera, Temuco, Chile
4 Undergraduate Research Group in Odontology, Faculty of Health Sciences, Universidad Autoínoma de Chile, Chile
5 Department of Public Health, CIGES, Faculty of Medicine, Universidad de La Frontera, Temuco, Chile
6 Clinical Investigation and Dental Innovation Center, Dental School, Universidad de La Frontera, Temuco, Chile

Correspondence Address:
Prof. Daniel Aracena
Avenida Alemania 0281, Temuco
Chile

Abstract

Introduction: The aim of this study was to evaluate the wear of ProTaper Next rotary files, X1, X2, and X3 after shaping 6, 10, and 14 maxillary molar vestibular canals. Materials and Methods: One hundred and sixty-two rotary files were divided into three groups according to the number of prepared canals, namely Group 1 (6 canals), Group 2 (10 canals), and Group 3 (14 canals). After canal preparation, files were embedded in resin molds, sectioned at 6 mm from the tip, and the wearied surface was microscopically evaluated. AutoCAD® software was used to measure the wear of edges in the different sections and multilevel mixed linear regression model to statistically analyze the data. Results: The files exhibited progressive wear as the number of uses increased. X2 and X3 files of Group 2 (10 canals) showed the highest wear, which decrease at 14 canals. Conclusions: X1, X2, and X3 files can shape until 10 canals without loss of their edge effectiveness.



How to cite this article:
Aracena D, Marín S, Borie E, Aracena A, Bustos L, Guzmán M. ProTaper Next files edge wear after conforming vestibular canals of maxillary molars: An in vitro study.Saudi Endod J 2020;10:111-115


How to cite this URL:
Aracena D, Marín S, Borie E, Aracena A, Bustos L, Guzmán M. ProTaper Next files edge wear after conforming vestibular canals of maxillary molars: An in vitro study. Saudi Endod J [serial online] 2020 [cited 2020 Jul 12 ];10:111-115
Available from: http://www.saudiendodj.com/text.asp?2020/10/2/111/283146


Full Text

 Introduction



Technological advances in the development of new alloys have increased the number of different mechanized file systems available. Such advances improve the biomechanical preparation stage of the root canal system while also making it more efficient.[1]

The nickel–titanium (Ni-Ti) alloy was introduced to the field of endodontics by Walia et al. in 1988 and has become the most used memory alloy, possessing a wide range of biomedical applications.[2] This alloy presents better characteristics compared to stainless steel in terms of its ductility, flexibility, biocompatibility, and corrosion resistance.[3],[4]

ProTaper Next® system is fabricated with an improved Ni-Ti alloy, denominated M-Wire, which has physical and mechanical properties that allows more flexibility and more fatigue resistance than those fabricated with a conventional Ni-Ti alloy.[5],[6]

Its design has a noticeable and variable taper along the instrument and with a rectangular cross-section decentered. These features minimize the contact between the surface of the instrument and dentin, avoiding the debris extrusion outside the canal and improving the flexibility of the active section.[7]

The mechanical performance of an endodontic file is determined mainly by its geometrical configuration, which not only affects the possible risk of fracture but also generates stress in the walls of the root canal during the shaping procedure. An overused instrument could lead to fracture due to deformed surface or worn edges.[8] In addition, the removal of fractured instruments in the root canals is very difficult, and in some cases, fractured files that cannot be removed could have a negative effect on the endodontic treatment outcome. These facts showed that overcoming or reducing this risk is of a high clinical significance.[9]

Previous studies have investigated different aspects of the ProTaper Next™ and ProTaper Universal® files, including canal shaping capacity,[10] resistance to cyclic fatigue,[11] or the amount of extruded waste.[12] However, there are few studies that have investigated how the number of uses influences the instrument effectiveness.[13],[14]

The aim of this study was to evaluate the wear of ProTaper Next Files after shaping different numbers of maxillary molar vestibular canals.

 Materials and Methods



The present study was approved by the Ethics Committee of Universidad Mayor, Temuco, Chile (F: 0062). Edge wear of 162 ProTaper Next X1, X2, and X3 (Dentsply-Maillefer®, Ballaigues, Switzerland) files, divided into three groups (n = 54 each group), were evaluated microscopically. Group 1 was tested in six canals (3 uses), Group 2 in ten canals (5 uses), and Group 3 in 14 canals (7 uses).

A total of 162 root canals of first maxillary human molar extracted teeth due to caries or periodontal disease were randomly distributed among the three groups. The root canals were selected according to the following criteria: mature apices, root curvature equal to or <34°, canals with independent foramens, no calcifications, and no resorption.

Molar teeth were subjected to the following protocol once extracted: they were submerged into a sodium hypochlorite solution of 5.25% for 20 min to ensure the disinfection and elimination of organic residues. After rinsing with water, any caries and restorations were removed from samples and then maintained in a 0.9% saline till use. The access opening was prepared with a high-speed round diamond bur 801 L (Jota®, Rüthi, Switzerland) and subsequently finished by removing all of the chamber roofs with an Endo-Z Bur (Dentsply-Maillefer®, Ballaigues, Switzerland) both cooled with air–water spray. The canals were located, and the work length was established using K-10 file where a 1 mm was subtracted after the tip of the file was emerged from the apical foramen.

Subsequently, a radiograph was taken of each sample using a 70 KV device (Soredex, Tuusula, Finland) using periapical radiographic Kodak films (Ultra Speed, DF58, Kodak®, Rochester, NY, USA). A working table was used that allowed standardization of the technique by positioning teeth at a distance of 47.2 cm.

Radiographic films were processed in an automatic developer (Perio Mat Plus, Dürr Dental AG, Bietigheim-Bissingen, Germany), and the obtained images were photographed and processed according to the Schneider technique.[15] Then, AutoCAD 2015 software (Autodesk®, California, USA) was used to identify and determine the angulation of the root canals.

Each sample was placed on a plaster block to continue with thein vitro instrumentation. Root canals were shaped using the protocol defined by the manufacturer. The canals were instrumented with #10 and 15-K-Flexo Dentsply-Maillefer® manual files (Ballaigues, Switzerland) using the balanced forces technique, and then, a sweeping motion with X1, X2, and X3 ProTaper Next files until reaching the previously determined work length.

Canals were permeabilized with a K#10 file between each file used and irrigated with 5 cc of 5% sodium hypochlorite solution. ProTaper Next files were cleaned with gauze soaked in a 70% ethanol to eliminate organic and inorganic residues. An X-Smart Plus motor (Dentsply/Maillefer®, Ballaigues, Switzerland) was used with the software indicated for ProTaper Next system files.

Endodontic file encapsulation and cutting

After instrumentation, files were encapsulated in cylindrical molds with Vinylester A-430 resin filling (BASF®, Concón, Chile). A transversal cut was performed using a diamonded EZ lock disc (Dremel® México DF, México). The cut was performed at 6 mm from the tip of the file. Samples were subjected to unidirectional fine polishing to prepare them for microscopic analysis.

Measurement of the file's detachment angles

A specific platform was built to obtain standardized images of the files. It was used as a mobile device in which the file was always located in a vertical position and at the same distance from the microscope [Figure 1]. These images were obtained with a Moticam 5 camera (Motic®, Hong-Kong, China) attached to a Motic® stereoscopic microscope, model SMZ168 (Motic Xiamen, Fujiam, China) with a direct USB connection to a computer. Then, images were analyzed using AutoCAD® 2015 software (Autodesk®, California, CA, USA).{Figure 1}

Once the image was uploaded into the AutoCAD® program, the cross-section of each sample was studied according to the technique employed by Ha et al.[8] This technique involves performing a peripheral trace of the file's cross-section wear out and locating its rotation point (O), which determines the geometric shape, detachment of reference angles and detachment following the cutting. In addition, the lines needed to obtain the detachment angles were delineated [Figure 2].{Figure 2}

The first reference is the measurement of the preprocedure angle, which is the angle between the EG face of the file and the OE radius. Later, as the file wears down, the new tip of the tool is moved from point E to point F. A higher detachment angle than the last one is generated between the FG face and the new radius OF. The wear in tip E corresponds to the mass losses along the file's edge and is represented bidimensionally by the DEFD area.

The second reference is the measurement of the preprocedure angle between the BE face of the tool and the OB radius. After the file is worn down, the new tip of the file is moved from point B to point C. This generates a higher detachment angle between the CE face and the new OC radius. The wear of the B tip corresponds to the mass loss along the file's edge and is represented bidimensionally by the ABCA area.

Statistical analysis

Statistical analysis was performed using STATA 12.0 software (StataCorp LLC, Texas, USA), with a significance level of 5%. For data description, average and standard deviations were used. The differences between average wear were estimated using a multilevel fixed linear regression model.

 Results



When the X1 files were analyzed, it was observed that with more uses, the degree of wear increases compared to the initial measurement of the unused file. Regarding files X2 and X3, the most wear was shown after five uses (10 canals) which decreased slightly after seven uses (14 canals) [Table 1]. When comparing the average degree of wear between X2 and X1 files, no statistically significant differences were observed (P > 0.05). Significant differences were identified between X3 and X1 files (P = 0.03), with a higher wear in X1 related to the number of uses [Table 2].{Table 1}{Table 2}

 Discussion



There is no consensus regarding the number of times that endodontic instruments can be used. The frequency with which they are changed should be based on the number of uses, instrument size, canal complexity, and the protocols and skills of the operator among other factors.[16]

Few researchers have evaluated the changes made on the surfaces of Ni-Ti instruments after their use; mainly, file fractures, deformation, and resistance to cyclic fatigue.[17]

Subha and Sikri studied the wear of the ProFile, rotary ProTaper, Manual ProTaper, and K3 Endo after being used in 300 mesiobuccal canals of mandibular molars. The instruments were analyzed through a scanning electron microscope. The observing signs of deterioration after its first use in all instruments showed cracks, microfractures, and disruption of the cutting angles.[18] These results were different than the findings of the current study, where the wear in the edges of instruments was analyzed in the buccal canals of maxillary molars. After 6, 10, and 14 times of shaping the canals, a higher wear at 6 and 10 canals was observed, and this wear decreased when conforming 14 canals. This decrease reveals that when a file exceeds its uses, it only slides in the dentinal wall and does not have an effective cut. Furthermore, these results show that the edges of the ProTaper Next files are effective only until the conformation of 10 canals given that once 14 canals are reached, they have already lost their edges, and therefore, their effectiveness is lost. In this sense, after the shaping of ten canals, these instruments should not be used to minimize the risk of fracture.

A previous study that examined the wear of 40 reciprocating primary files of the WaveOne system after being used in 2, 4, and 6 buccal canals of the maxillary molars concluded that the files presented a slight wear of their edges after performing 2 and 4 canals and that the degree of wear significantly increased by the sixth use.[19] In contrast, You et al. studied the behavior of the F2 file of the ProTaper Universal system with reciprocal movement in the buccal canals of maxillary and mandibular molars and established their safe use until six canals.[20] The conflict in results could be attributed to the differences between the conventional Ni–Ti alloys of the ProTaper Universal system with the M-Wire alloy of WaveOne. M-Wire presents molecules with great cohesion force, which allows the material to be denser and the edge more compact, and as a result, it increases the resistance to cyclic fatigue and has less wear in its use. On the other hand, in conventional Ni–Ti alloys, the molecules have less cohesion, and the edge is less compact. Several investigations support these differences between these alloys.[21],[22],[23],[24]

The ProTaper Next system has improved the clinical efficiency through its incorporation of its off-centered rectangular cross-section, which is based on the concept that the asymmetric spinning movement, where the canal's wall is only conformed in the contact points, reduces the torsional strain and allows an easy way to follow the canal's shape, thereby diminishing vibrations generated during shaping.[22],[25] This winding movement likely allows the files to wear down more slowly since they do not permanently cut in their circular spin and its rectangular cross-section provides more space for the debris layer removal. The results of the present study confirmed the cutting efficiency of these files as they were only worn down on two of their vertexes, leaving a larger space for the evacuation of dentin debris. This can prevent the file from getting stuck or fracture in the canal. Despite that, the wear was progressive between the different groups; there was no fractured instrument which was probably due to the strict root canal shaping protocol and the facilities granted by thein vitro procedure. On the contrary, a study which examined 593 conventional Ni-Ti Mtwo files showed that 16% of them fractured,[26] whereas the other research that evaluated the fracture and deformation of 571 ProTaper Next files after clinical use observed a high percentage of fracture in X1 files (19.87%) and deformation (11.8%).[17] The likelihood of these accidents is being reduced by the optimization of the alloys' microstructure through new processing technologies. In this sense, the thermomechanical treatment of conventional Ni–Ti alloys has accentuated their properties such as flexibility and elasticity. Among these instruments, those fabricated with M-Wire and more recently with CM-Wire technology have shown good mechanical behaviors.[21],[27]

The thermocycling process has guaranteed higher tool flexibility, and therefore, higher resistance to cyclic and torsional fatigue compared to instruments manufactured with conventional Ni–Ti.[21],[22],[28]

The irrigation environment may affect the resistance to cyclic fatigue of ProTaper Next files. Abuhaimed observed that chlorhexidine 2% exhibited a significantly higher resistance to cyclic fatigue compared with 17% ethylenediaminetetraacetic acid solution and 6% sodium hypochlorite.[29] In the present study, the irrigants' action was not considered, but it would be important to assess in a further study.

Besides the above-mentioned characteristics of cinematic and geometry, and due to the requirement of using 2 or 3 files to complete the root canal procedure, it is important to mention the fact that M-Wire files of the ProTaper Next system suffer less stress than reciprocating system files, where only one file conforms the root canal completely. However, the reciprocal instrument moves with a shorter angular distance than the rotary instrument, justifying its higher cyclic fatigue resistance.[20],[30],[31] This could explain why the ProTaper Next system, using a similar methodology to primary WaveOne single files, could be used in six more canals than reciprocating files.[18]

 Conclusions



According to thisin vitro study, the use of ProTaper Next system files is recommended for shaping until ten root canals before the edges lose cut efficacy.

Acknowledgment

The authors want to acknowledge the FDP project 2017 (Universidad Mayor).

Financial support and sponsorship

The study was financially supported by FDP, 2017, Universidad Mayor, Temuco, Chile.

Conflicts of interest

There are no conflicts of interest.

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