Comparison of resistance to cyclic fatigue of one novel reciprocating endodontic file system with two novel rotary endodontic file systems

Drs. Simon M. Ghattas and Michael M. Hoen test files for cyclic fatigue

Abstract

Introduction: The purpose of this study was to compare the cyclic fatigue resistance of one reciprocating and two rotary endodontic file designs.
Materials and methods: All tested files had a similar size .25-mm tip diameter and a taper of .08 mm/mm.

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The reciprocating file included a WaveOne® NiTi (Dentsply Tulsa Dental Specialties). The rotary files included the latest generations of NiTi metallurgy from Coltène Whaledent Gruppe (Langenau, Germany) (HyFlex® CM™) and Sybron Dental Specialties (K3™XF). The tempered stainless steel artificial canal-testing device had a milled 60 degree curve with a 5-mm curvature radius shape. The canal was lubricated between each individual instrument test. All files, handpieces, and motors were used according to the manufacturer’s specific directions. A jig was utilized to ensure the precise repeatable placement of all tested files. Each instrument was either rotated or reciprocated until a fracture was visually and audibly detected. Fracture times were recorded using a 1/100 second chrono-meter. A laser phototachometer (Nekio) was utilized to standardize and ensure that the rpm matched the individual manufacturer’s recommendations.
Results: All files functioned as expected. The mean times to failure of the instrument groups follow:

• WaveOne — 61 seconds
• HyFlex — CM 59 seconds
• K3XF — 89 seconds

Statistical analysis was completed using a one-way ANOVA and Tukey Post-hoc test.
Conclusions: The K3XF files had a statistically significantly longer mean time to failure (P<0.05) when compared to the WaveOne and HyFlex CM.

Introduction

The biologic objectives of cleaning and shaping procedures are to remove all pulp tissue, bacteria, and endotoxins from the root canal system.39 Mechanical enlargement of the root canal system is therefore one important part of endodontic treatment. Properly shaped canals are essential for two key reasons. One reason is to allow for adequate chemical irrigation to disinfect the canal system. Another is to be able to achieve proficient hydraulics that is required for a total 3D obturation.

In the field of endodontics, there have been many different instruments of various shapes and designs used to clean and shape the canal. These include files, reamers, and broaches made of various types of metals. However, originally, endodontic files were only made of stainless steel. The shaping of curved canals presents a considerable problem when stainless steel instruments are used. Due to their stiffness, there is a tendency to transport the prepared canal from its original axis. Deviation from the original curvature can lead to procedural errors, such as ledge formation, zipping, stripping, and perforations.

As a consequence, new endodontic instruments and techniques have been introduced that help minimize these risks. One of the most noteworthy advances was the development of nickel-titanium rotary instruments by Walia in 1988.
These alloys can exist in various crystallographic forms. The unique properties of nickel-titanium can be explained by specific crystal structures of the austenite and martensite phases of the alloy. These changes occur as a function of temperature and stress, and are due to its inherent ability to alter their type of atomic bonding. This phase transformation is the basis for the unique properties of these alloys, in particular shape memory effect and superelasticity.

NiTi alloys have greater strength and a lower modulus of elasticity than stainless steel. These properties allow NiTi instruments to flex far more than stainless steel instruments, allowing easier instrumentation of curved canals while minimizing canal transportation.4 Even with an increase in flexibility, file separation is still a concern with NiTi instruments. Even though NiTi files are perceived as being more prone to fracture than stainless steel files, a recent review of the literature by Parashos found that fracture rates were actually very similar, with stainless steel files having a fracture frequency of 1.6% and NiTi files only 1%.When the force of separation of the atoms exceeds the force of attraction, file breakage occurs. Molecules within the metal are set in patterns denoting its crystalline structure or grain.

One cause is from slippage between the planes of the crystalline boundaries, due to the excessive forces of torsion. Torsional fracture occurs when one part of a file rotates at a different rate than another part. As the file rotates in the canal, a portion of the file may bind in the canal or anatomy preventing the portion of the file apical to this binding from continuing rotation, and the portion coronal to this continues to rotate. This allows the file to unwind and creates stress in the file, which leads to separation of the instrument at this point. An example of this is when an instrument tip is locked in a canal while the shank continues to rotate.

Another fracture may occur across the grain of the metal with little or no apparent deformation. This type of fracture can be seen as a result of fatigue most often caused from the excessive stresses of the repetitive compression and tension that occurs during rotation of a file around a curvature. On the inside of the curvature of a canal, a rotating file is compressed. On the outside of the curvature, the file undergoes tension. During continuous rotation around a curvature, each surface of the file undergoes compression and tension until faults in the file begin to spread, and the file fatigues.

There are many factors that can be implicated in the failure of rotary endodontic instruments, including the properties of the alloys, the design of the instrument, rotational speed, torque, number of uses, sterilization method, method of use, operator efficiency, and root canal anatomy.There are several factors that can affect cyclic fatigue resistance of a file. These are the radius of curvature and angle of curvature of the canal, the length of the arc, the diameter of the instrument, and the design of the instrument. The angle of curvature describes the amount of curvature, and as the angle increases, cycles to failure decrease. The radius of curvature describes the abruptness of curvature, and as the radius decreases, cycles to failure also decrease. Canals can have the same angle of curvature while having different radii of curvature, resulting in more abrupt curves and, thus, decreasing cyclic fatigue resistance more. The diameter of the instrument also affects cyclic fatigue. As the diameter of the instrument increases, it becomes less flexible compared to narrower diameter instruments, which increases its binding in the canal anatomy and increasing stress on the instrument as it rotates. Generally, the greater the distance between the stress of tension and the stress of compression, the greater the total stress on the instrument. The file’s resistance to fatigue has a close inverse relationship with the square of the file radius. Thus, larger file diameters are more susceptible to cyclic fatigue than their smaller counterparts. It should be noted that the diameter of the instrument is directly related to the tip size as well as the taper of the instrument. The design of the file also can impact the outcome. Features such as core diameter, cross-sectional shape, and flute depth may account for incongruent fatigue properties seen among different file brands. One study compared similar size files with different file designs and determined file design to be the most important determinant of cyclic fatigue resistance.
The superelasticity of nickel-titanium also dictates its production in which files must be milled or ground, rather than twisted, as in the Twisted rotary NiTi files (TF®, Sybron Dental Specialties). Consequently, NiTi instruments may have characteristic imperfections such as milling marks, metal flashes, microfractures, or rollover. Some researchers even speculate that fractures in nickel-titanium instruments originate at such surface imperfections.
New methods of manufacturing NiTi alloys have been developed recently in an effort to enhance the file’s performance, including its resistance to cyclic fatigue. Previously, all NiTi files were manufactured in the austenite phase. Austenite is a metallic, nonmagnetic allotrope of iron or a solid solution of iron with an alloying element. This required an application of stress to transform it into the more malleable martensitic phase, which most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by diffusionless transformation. This is undesirable as the austenitic phase is stiffer and can lead to unwanted outcomes. By utilizing various methods of heat treatment, files can be produced where a portion of the alloy remains in the martensitic phase or R-phase. Dentsply uses this method of processing to make an alloy called M-Wire to make its WaveOne files. Sybron also uses a similar method they call R-phase heat treatment technology to manufacture their new K3XF files. Having more of the alloy in the martensitic phase is supposed to increase flexibility and thus resistance to cyclic fatigue.

Dentsply Tulsa Dental Specialties recently introduced its own reciprocating file, the WaveOne reciprocating file and WaveOne handpiece and motor. The manufacturer claims that “only one WaveOne NiTi instrument is required to shape a canal in most cases” and is “manufactured using M-Wire™ thermal treatment process for greater flexibility and increased strength.”

Coltène Whaledent recently introduced the HyFlex CM rotary endodontic file. The manufacturer claims that with Controlled Memory, these “files are up to 300% more resistant to cyclical fatigue compared to other NiTi files, which substantially helps reduce the incidence of file separation” and that “the shape and strength of files with straightened spirals can be restored during autoclaving and reused.”

Sybron Dental Specialties recently introduced the K3XF rotary endodontic file. The manufacturer claims that with “the third radial land, the file has better centering capability and more stability.” The K3XF file utilizes their proprietary heat treatment called R-Phase™ Technology. Fractured instruments are a definite hindrance to the goals of cleaning, shaping, and filling root canals, and they may adversely affect the outcome of endodontic treatment. Overall, it has been shown that a fractured file should not affect the successful healing after endodontic therapy.However, if the broken file impedes adequate cleaning of the canal beyond the obstruction, the prognosis may be affected.

With the preceding information in mind, it is important to remember that fractured instruments are a definite hindrance to the goals of cleaning, shaping, and filling root canals, and they may adversely affect the outcome of endodontic treatment. It is therefore important to avoid file separation. One way to do this is through new manufacturing processes that enhance the instruments’ ability to resist cyclic fatigue. Therefore, the purpose of this study was to compare the cyclic fatigue resistance of two reciprocating and two rotary endodontic file designs.

Materials and methods

Three different brands of endodontic files were tested, including one reciprocating file system, the WaveOne NiTi 25/.08 25 mm Medium (Dentsply Tulsa Dental Specialties), and two rotary endodontic file systems, the HyFlex CM NiTi 25/.08 19-mm rotary files (Coltène Whaledent Gruppe, Langenau, Germany) and the K3XF NiTi 25/.08 25 mm (Sybron Dental Specialties). Cyclic fatigue testing of the endodontic file instruments was performed with an apparatus used to immobilize the micromotor/contra-angle and artificial canal, allowing the instruments to rotate freely inside an artificial stainless steel canal and at the same time allow visualization of a fracture taking place.
Thirty of each file type were tested, for a total of 90 files. The artificial canal consisted of a tempered-steel block with a milled canal shape having a 60° canal curvature with a 5-mm radius of curvature. The width of the canal was 1.5 mm wide and 2 mm deep. A thin and translucent acrylic plate (2-mm thickness) was attached to the canals, allowing visualization of the instrument in the working canal. An aluminum prism was specially machined to support an electric micromotor and contra-angle. All of these parts were mounted on an aluminum base designed to hold the files in a precisely repeatable and reproducible position, which ensured a three-dimensional alignment and positioning of the instruments to the same depth each time. The canals were lubricated with one drop of synthetic oil (Mobil 1™ Synthetic oil, 5W-30) between each file to reduce friction. The instruments were used according to the manufacturer’s recommendations. For the reciprocating file, a 6:1 reduction reciprocating handpiece powered by the Dentsply e3 WaveOne motor (Dentsply Tulsa Dental Specialties) was used for the WaveOne files. The HyFlex CM and the K3XF files were tested for cyclic fatigue fracture at a rotational speed of 300 rpm. The rpm value was verified for accuracy using a digital laser phototachometer (Nekio Tools, USA, Wenzhou, China). The fracture time and fracture occurrences were recorded with a video camera (Sony, Tokyo, Japan) mounted on a camera stand. Each instrument was reciprocated until a fracture occurred by visual and audible detection. The fracture time was recorded using a 1/100 second chronometer. The number of cycles to fracture was calculated by multiplying time to fracture and revolutions per minute. All fractured instruments were investigated under scanning electron microscope (SEM) to observe the entire fracture surface detail and identify any detrimental points, possible cracks, and signs of fatigue and corrosion. Fractured instruments were cleaned in an ultrasonic bath containing acetone for 7 minutes. Following ultrasonic cleaning, instruments were rinsed with running tap water for 5 minutes and then patted dry with paper towels. Instruments were held in a custom-made jig and were studied under SEM (Hitachi S-3200N, Japan). All test groups were analyzed by using one-way analysis of variance (ANOVA). Tukey Post-hoc analysis was applied to identify the specific groups that were significantly different. Statistical significance was set at P<0.05.

Results

All other files functioned as expected. The mean times to failure of the instrument groups follow (Figure 1):

• WaveOne (25/.08) — 61 seconds
• HyFlex (25/.08) — CM 59 seconds
• K3XF (25/.08) — 89 seconds

Statistical analysis was completed using a one-way ANOVA, Tukey Post-hoc test, and a statistical significance level of a = 0.05. The K3XF files had a statistically significantly longer mean time to failure (P<0.05) when compared to the WaveOne and HyFlex CM. Statistics will not be applied to the SEM analysis due to the observational nature of this data.

Discussion

Today, most of the file systems that are available are used to shape canals by rotating via a continuous rotational movement with a mechanical handpiece. Even though reciprocation may seem like a relatively new concept, it has been around for decades. Some of the available examples of reciprocating motors and handpieces as of late include the WaveOne e3 Motor (Dentsply Tulsa Dental Specialties), the M4 (SybronEndo), and the Endo-Eze® AET™ (Ultradent). Today, with the exception of the WaveOne e3 Motor, these motors work in a manner wherein the clockwise degree of rotation equals that of the counterclockwise.

One of the first articles discussing reciprocating movement was by Frank, et al. (1967), wherein the Giromatic reciprocating handpiece had been advocated for use as a safe and effective alternative to hand instrumentation. There are advantages and disadvantages associated with utilizing either a continuous rotational movement versus a reciprocating movement. Continuously rotating active files have an advantage in that they have a greater efficiency in smaller diameter and more curved canals, but this must be balanced with the potential risks associated with torque and cyclic fatigue failures. New file designs and research and development of nickel titanium have greatly reduced these risks. In comparison to reciprocation, continuous rotational movement utilizing well-designed active NiTi files requires less inward pressure and improves hauling capacity augering debris out of a canal. On the other hand, a mechanical reciprocating movement has merit because it somewhat mimics manual movement and reduces the various risks associated with continuously rotating a file through canal curvatures. However, current motors that drive reciprocating shaping files have recognized limitations, including decreased cutting efficiency, more required inward pressure, and a limited capacity to auger debris out of a canal.

All of the factors involved in this study were standardized such as the operator and method of use to investigate the incidence of cyclic fatigue failure with the curvature of the apparatus. In the present study, an apparatus was used to hold the electric micromotor and consequently eliminate the potential interference of operator-induced tensions on endodontic instruments during the fatigue test. The fracture and overall failure of the files in this study occurred in the curved angle of 60 degrees.

The WaveOne files, introduced in 2011, are made of M-Wire NiTi technology. It was illustrated that files manufactured from M-Wire NiTi significantly improved the resistance to cyclic fatigue by almost 400%. The manufacturer claims that WaveOne should be considered if the clinician has concerns with any of the following:

1. Using stainless steel files for shaping canals
2. Breaking mechanically driven files
3. Ledging curved canals
4. Transporting the prepared foramen
5. Using too many shaping files
6. Mastering hybrid techniques
7. Spending too much time preparing canals

The three WaveOne instruments available are termed small (yellow 21/.06), primary (red 25/.08), and large (black 40/.08). The file used in this study, the primary 25/.08, has a fixed taper of 8% from D1-D3, whereas from D4-D16, it has a unique progressively decreasing percentage tapered design that the manufacturer claims serves to improve flexibility and conserve remaining dentin in the coronal two-thirds of the finished preparation. The WaveOne files have non-cutting modified guiding tips, which enable these files to safely progress through virtually any secured canal. The engaging angle of the file is 5 times the disengaging angle; and after three engaging/disengaging cutting cycles, the file will have rotated 360 degrees, or one full counterclockwise circle that enables the file to physically advance toward the desired working length compared to other reciprocating files, such as the Endo-Eze AET (Ultradent) files, which are driven with equal bidirectional movements. Because of its unequal bidirectional movement, the WaveOne hauls debris out of the canal.

Numerous apparatuses and methods have been used to study in vitro the cyclic fatigue fracture resistance of NiTi rotary endodontic instruments.21-24 All of these studies attempted to simulate the rotation and reciprocation of the instrument within a curvature to determine the length of time before a fatigue fracture occurred. The rotating instrument is either confined in an apparatus, or jig, which is used to impose a curvature on the rotating instrument and is considered an appropriate means for examining the fatigue behavior of NiTi files. The present study is the first study to compare one reciprocating NiTi instruments with two rotary NiTi instruments all in the using the same file size.
No other study to date has shown a direct comparison of the WaveOne files with the HyFlex CM and K3XF files. This is because up until recently the K3XF files were not yet available. There have been numerous studies conducted regarding cyclic fatigue. The effect size has on cyclic fatigue has already been determined. It was not the point of the study to determine whether or not size had an effect. The files chosen in this study were selected in one size and one taper only: 25/.08. This was done to match that of the WaveOne Primary 25/.08 file, which the manufacturer claims “is called the primary file because it will be used virtually in all cases.” This allowed as close a comparison as possible to see if there is a difference between the files.

The mechanical properties of NiTi alloy can be influenced by changes in composition, impurities, and heat treatment conditions.31 In recent times, a proprietary thermomechanical processing procedure has been developed with the objective of producing superelastic NiTi wire blanks and is termed M-Wire. The M-Wire technology allows the NiTi instruments more flexibility and resistance to cyclic fatigue compared with non-M-Wire NiTi instruments. Gao, et al.,30 reported that instruments made of M-Wire exhibited superior cyclic fatigue resistance compared with those made of regular superelastic wire. The results of that study indicated that NiTi instruments made from CM Wire were nearly 300% to 800% more resistant to fatigue failure than instruments made from conventional NiTi wire. Another finding by Shen, et al., is that the fatigue of NiTi alloys is sensitive to temperature, both locally and environmentally. Upon repeated loading, the latent heat of the stress-induced martensitic transformation released can elevate the local temperature, leading to a shortened fatigue life.In this study, the cyclic fatigue resistance of files produced with new manufacturing processes were studied. Based on the results, it appears that the manufacturer’s claims appear to be true. They are indeed more flexible and resistant to cyclic fatigue. The R-Phase Technology was superior to all other alloys in this study. The K3XF file was superior to both the WaveOne and the HyFlex CM files. It would seem that the difference in flexibility of these second-generation NiTi alloys is more pronounced as the sizes increase. This is important since it suggests that these files would allow for safer shaping of curved canals with larger size instruments. More studies would be necessary to validate this with greater differences in sizes and angle or radiuses of curvature as these were controlled in this experiment.

There have only been a handful of studies to date that have tested the cyclic fatigue resistance of the WaveOne. Prichard, et al. (2012), stated that the use of NiTi instruments in a reciprocating motion reduced the fracture incidence and negated the need for a glide path. Plotino, et al. (2012), showed that the Reciproc 25/.08 instruments was associated with a statistically significant increase in the mean time to fracture compared to the WaveOne 25/.08. Kim, at al. (2012), showed that both reciprocating files (Reciproc and WaveOne) demonstrated significantly higher cyclic fatigue and torsional resistances than ProTaper F2 (P<.05). Gavini, et al. (2012), showed that the reciprocation motion improves flexural fatigue resistance in nickel-titanium instrument Reciproc R25 when compared with continuous rotation movement. The results of our study matched both the Kim, et al. and Gavini, et al. studies.
In another comparable study, Shen, et al.,32 studied the fatigue behavior of NiTi instruments from a novel controlled memory NiTi wire (CM Wire). In Shen’s study, all the files were a size 25/.04 and were subjected to rotational bending at the curvature of 35° and 45°. Not surprisingly, they found that instruments made from CM Wire had a significantly higher number of revolutions to fracture and thus a substantial impact on fatigue lifetime.There have been many more studies conducted regarding cyclic fatigue resistance throughout the years and almost as many different models used to test the cyclic fatigue resistance. To date, there is no ISO standard for testing cyclic fatigue of rotary nickel-titanium instruments as there is for torsional fatigue testing. Given that the results in regards to time until fracture are only accurate in the exact same canal configurations, these conditions must be precisely duplicated in order to make fair comparisons. As there are constantly new files entering the market, it would be easier to compare their properties if there was a universal testing standard.As an additional portion of this study, SEM images were taken of the separated instruments. No study, to date, has described the SEM cyclic fatigue file appearance of the WaveOne 25/.08. In comparing the plastic deformation of a rotary endodontic file with a reciprocating endodontic file, there is no difference in plastic deformation
(Figure 2). The SEM investigation of the fractured surfaces of both the WaveOne 25/.08 and the HyFlex CM 25/.08 instruments showed mainly the characteristic patterns of ductile fracture with the predominance of dimples. The surface textures were similar for all fractured instruments regardless of the type of the instrument. No cracks were detected that would verify the presence of metal fatigue on the fractured instruments.

Conclusions

The mean times to failure of the instrument groups follow:

• WaveOne — 61 seconds
• HyFlex — CM 59 seconds
• K3XF — 89 seconds

Statistical analysis was completed using a one-way ANOVA and a Tukey Post-hoc test. The K3XF files had a statistically significantly longer mean time to failure (P<0.05) when compared to the WaveOne and HyFlex CM. The new manufacturing processes appeared to offer greater resistance to cyclic fatigue in this in vitro model. However, more research will be needed to further investigate if these new methods make a difference in clinical outcomes.

Acknowledgments

A special thanks to the AAE Foundation for funding this research project and also to Dentsply Tulsa Dental Specialties, Sybron Dental Specialties, and Coltène Whaledent for donating files that were used in this study.

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