EVALUATION OF DEFORMATION AND FRACTURE OF THREE SINGLE-FILE NITI ROTARY INSTRUMENTS: PROTAPER F2, WAVEONE PRIMARY AND ONESHAPE IN SIMULATED CURVED CANALS

Purpose. The aim of the current study was to compare the incidence of deformation and fracture in three single-file NiTi instruments: ProTaper F2, WaveOne Primary and OneShape. Methods. Fifteen instruments were equally divided into three groups: ProTaper F2 in reciprocation, WaveOne Primary in reciprocation and OneShape in continuous rotation. Each instrument was used to prepare standardized simulated curved canals in resin blocks until fracture had occurred. Following each canal preparation, the instruments were examined for deformation both by naked eye inspection and stereomicroscopic examination. The average number of canals prepared until the first incidence of cracks and the average lifespan of the instruments were calculated. Data were analysed using one-way ANOVA and two-sample t-test. Results. There was no statistically significant difference between ProTaper and WaveOne instruments in both the incidence of cracks and the average lifespan (P > 0.05). OneShape instruments had a significantly delayed incidence of cracks and a longer lifespan than both ProTaper and WaveOne instruments (P < 0.05); however, OneShape instruments showed a noticeable early plastic deformation. Conclusion: PT F2 instrument was comparable to WO Primary instrument in terms of fracture resistance, while OS instrument had more fracture resistance than both PT F2 and WO Primary instruments.


INTRODUCTION
Single-file technique is a revolutionary concept, which has been lately introduced in the field of endodontics.This concept was first proposed by Yared by using ProTaper (PT) F2 file as a single reciprocating instrument [1].The rationale behind this concept was to simplify the canal shaping procedure by using a NiTi rotary instrument to achieve the whole canal preparation.Using only a single instrument is more cost effective than the multi-file sys-tems.This, in turn, can encourage practitioners to use such instruments for a single case, which reduces the risk of cross-contamination and instrument separation.
On the basis of Yared's report, several single-file reciprocating systems were developed, such as WaveOne (WO; Dentsply Maillefer, Ballaigues, Switzerland) and Reciproc (VDW, Munich, Germany).WO system consists of three files: Small (ISO 21 with a constant taper of 6%), Primary (ISO 25 with an apical taper of 8%) and Large (ISO 40 with an apical taper of 8%).It should be noted that WO Primary instrument shares many design features with PT F2 instrument, as both instruments have a tip size of 0.25 mm, apical 3-mm taper of 8% and a progressively decreasing taper from D4 to D16.Additionally, both the instruments have a convex triangular cross-section, with the exception of the apical 8 mm of WO Primary instrument, where the cross-section is a modified convex triangle to enhance the file flexibility.The two major dissimilarities between these instruments are the manufacturing alloy and the direction of flutes.WO Primary instrument is manufactured from a special NiTi alloy called M-wire and has reverse (left handed) flutes.However, PT F2 instrument is manufactured from conventional NiTi alloy and has regular (forward) flutes [2].
OneShape (OS; Micro Méga, Besançon, France) is another single-file system that is used in a continuous rotation motion.OS system was first introduced in the market as a single-file instrument with a tip size of 0.25 mm and a 6% constant taper.Later, another version of OS system was launched in the market, namely 'OS new generation'.In this version, the 25/0.06original file was modified by lengthening the pitch.In addition, two finishing files were added to the system to further enlarge the apical portion of the root canal following the original OS file.OS instruments are manufactured from conventional NiTi alloy.The instruments are modified by electropolishing surface treatment to aid in eliminating the surface manufacturing defects.According to the manufacturer, OS instruments have an additional feature called anti-breakage control (ABC), which allows instrument unwinding to provide an alarm before instrument separation.
Few studies have compared PT F2 and WO Primary instruments by using the same kinematics.Furthermore, little data are available concerning the deformation and fracture in OS instrument.Therefore, the purpose of the current study was to directly compare the incidence of deformation and fracture in PT F2, WO Primary and OS instruments by using standardised simulated curved canals.

Sample selection and grouping:
Fifteen new instruments with 0.25-mm tip size and 25-mm length were used in this study: five PT F2 (Dentsply Maillefer, Ballaigues, Switzerland), five WO Primary (Dentsply Maillefer, Ballaigues, Switzerland), and five OS new generation (size 25/0.06;Micro-Mega, Besancon, France).All the instruments were examined preoperatively by using stereomicroscope at 40x magnification to exclude defective instruments.Instruments were categorised into three equal groups according to the instrument used: PT F2 file in reciprocating motion (PT group), WO primary file in reciprocating motion (WO group) and OS new generation file in continuous rotation (OS group).

Simulated canal preparation:
The selected instruments were repeatedly used to prepare standardised simulated curved canals embedded in resin blocks (Endo Training-Bloc, Dentsply Maillefer, Ballaigues, Switzerland).Canals had a 45º angle of curvature, 6-mm radius and 2% taper.All the instruments were driven by an electric motor (iEndo Dual; Acteon, Mérignac, France) with a 16:1 contra angle (EB-75; W&H, Bürmoos, Austria).According to the instrument used, the motor was adjusted as follows.
PT group: PT F2 instruments were used in a reciprocating motion with reciprocation angles of 150º clockwise and 30º counter clockwise and a speed of 350 rpm (the same settings as that for WaveOne file but in the opposite direction).
WO group: WO Primary instruments were used according to the manufacturer's instructions in a reciprocating motion with reciprocation angles of 150º counter clockwise and 30º clockwise [2] and a speed of 350 rpm.
OS group: OS new generation instruments were used according to the manufacturer's instructions [3] in a continuous rotation motion with a speed of 400 rpm and a torque of 2.5 N.cm.
Canal patency was confirmed using a size 8 K-file, and a glide path to the full working length was established using a size 15 K-file.The rotary instruments were coated with EDTA cream for lubrication.Each rotary instrument was used in a slow pecking motion with light apical pressure.After three consecutive pecking motions [4], the instrument was removed, and the canal was irrigated with 2 ml of water by using a 25-gauge needle.Subsequently, patency and glide path were rechecked, and the file was cleaned using a wet gauze.The previously mentioned procedure was repeated until the instrument reached the full working length.Canal preparation was performed by the same operator; in addition, all the resin blocks were maintained at a fixed position throughout the procedure by us-ing a bench vice.
Instrument examination for deformation: Following each individual canal preparation, the instruments were examined for the incidence of plastic deformation and cracks as shown in Table 1.The working portion of the instrument was first inspected by naked eye and then using stereomicroscope (Leica M26, Heerbrugg, Switzerland) at 40x magnification.The number of canals prepared until the first incidence of any deformation and the type of incident deformation were recorded.In addition, the progress of deformation upon further usage was traced, and sample images were obtained at a magnification ranging from 12x to 80x.From the recorded data, in each group, the mean number of canals prepared until the first incidence of cracks was calculated and compared.

Lifespan determination:
Each instrument was reused to prepare additional canals, following the same procedure as mentioned earlier, until the incidence of file fracture.The number of simulated canals prepared until fracture was recorded and the average lifespan of each of the three tested instruments was calculated and compared.

Statistical analysis:
Data were presented as mean and standard deviation values.One-way ANOVA was used for comparison between groups.The two-sample t-test was used for comparison between the means of each two groups when the ANOVA test was significant.The significance level was set at P ≤0.05.Statistical analysis was performed using IBM® SPSS® Statistics Version 20 for Windows.

Instrument examination for deformation:
No visible signs of plastic deformation could be detected by naked eye inspection throughout multiple usages of PT F2 instruments.Stereomicroscopic examination revealed the incidence of small cracks after an average of 3 ± 0.71 canals were prepared (Fig. 1a).Upon further usage, crack propagation occurred in all PT F2 instruments with the formation of large cracks prior to fracture (Fig. 1b).
WO Primary instruments exhibited a pattern of deformation similar to that of PT F2 instruments.No visible signs of deformation could be detected in WO Primary instruments by naked eye inspection upon multiple usages.Small cracks were detected by stereomicroscopic examination after an average of 2.60 ± 0.55 canals (Fig. 1c).These cracks later propagated into large cracks ahead of fracture (Fig. 1d).Only one WO Primary instrument showed unwinding defect, which was detected only by stereomicroscopic examination.
Unlike PT and WO Primary instruments, all the tested OS instruments showed an obvious plastic deformation following the preparation of the first simulated canal, which was detected upon naked eye inspection.These defects included unwinding of the helical structure, reverse winding, bending, or a combination of these defects (Fig. 2).Following multiple usages, stereomicroscopic examination revealed the incidence of small cracks in the areas with plastic deformation, which occurred after an average of 7.40 ± 0.55 canals (Fig. 1e).The incidence of small cracks was followed by crack propagation and fracture in four OS in-struments (Fig. 1f).Only one OS instrument was fractured directly, with no crack propagation, following the first incidence of small cracks.
A comparison of the number of canals prepared until the first incidence of cracks showed that there was no significant difference between the PT and WO groups (P > 0.05).The OS group demonstrated a significantly larger number of uses until the first incidence of cracks than both the PT and WO groups (P < 0.05).

Lifespan of tested instruments:
The lifespan of the instruments was represented as the number of simulated canals prepared until fracture.There was no statistically significant difference in the average lifespan between the PT (6.8 ± 0.45 canals) and WO groups (6.2 ± 1.1 canals) (P > 0.05).However, the OS group exhibited a significantly longer lifespan (10.4 ± 1.34 canals) than both the PT and WO groups (P < 0.05).

DISCUSSION
The aim of the current study was to compare the deformation and fracture resistance of three Ni-Ti rotary instruments, used in a single file technique, which vary in the type of manufacturing alloy and surface characteristics.To facilitate the comparison between PT F2 and WO Primary instruments, both the instruments were activated in a reciprocating motion using the same values of reciprocation angles.However, the reciprocation direction was reversed, because PT instruments cut in a forward direction while WO instruments cut in a reverse direction.Standardised simulated canals in resin blocks were used in the current study to ensure the reproducibility of examination conditions while allowing the instruments to perform an active cutting action during rotation.
Stereomicroscopic examination of the instruments was performed at considerably low magnification (40x).Several studies have recommended the usage of more meticulous methods for instrument examination [5,6].Nevertheless, our findings revealed that using this relatively low magnification was effective in tracking formation and propagation of cracks prior to instrument fracture in all the tested instruments.Hence, it could give a close prediction of the actual time of instrument fracture.
None of the tested PT F2 or WO Primary instruments showed visible signs of deformation prior to separation.This finding concur with that of previous reports, which demonstrated that it is impractical to predict instrument separation upon naked eye inspection [7,8].However, all the tested OS instruments exhibited a noticeable early visible deformation.This observation was previously reported by Saber et al. [9], where all the used OS files were deformed following the preparation of four curved root canals in extracted mandibular molars.Such an early plastic deformation can be attributed to the ABC feature in OS instruments.The manufacturer claims that the ABC feature allows file unwinding before separation; therefore, it acts as an alarm to discard the instrument [3].More details on this feature are not clearly provided by the manufacturer, which need further investigations.
Despite the early observed plastic deformation in the OS group, this deformation failed to indicate the time of instrument fracture.Results showed that the occurrence of fracture in all the tested instruments of all three groups was much related to the incidence of cracks rather than the incidence of plastic deformation.In addition, the dimensions of the incident cracks were found to be a crucial factor for the occurrence of fracture in the tested instruments.In 14 of the 15 instruments tested, the incidence of large cracks was evident directly prior to separation, as cracks had to achieve a critical size before the fracture occurs.These findings reveal the role of crack formation in the process of instrument fracture [7,10] and indicate the significance of crack formation as a powerful predictor of the instrument failure.Another important consideration regarding crack formation is the risk of cross-contamination.Previous studies have found that cracks harbour tooth structure and organic debris regardless of meticulous ultrasonic cleaning and decontamination [11].Consequently, this emphasises the importance of the singleuse concept when using NiTi rotary instruments.
Our results showed no significant difference between the PT and WO groups in both the average lifespan and the average number of canals prepared until the first incidence of cracks.These results are consistent with those of Gambarini et al. [12], who found no significant difference in the cyclic fatigue resistance between the instruments manufactured from conventional NiTi and those manufactured from M-wire alloy.In addition, our results are in agreement with those of Bürklein et al. [13] and Saleh et al. [14], who suggested that M-wire alloy is not a prerequisite for single-file systems in terms of avoiding instrument fracture.In contrast to these findings, Pirani et al. [15] found that WO Primary instrument was significantly more resistant to cyclic fatigue than PT F2 when both instruments were activated in a reciprocating motion using the same "WaveOne All" setting for both instruments.Further, several studies have demonstrated the superior cyclic fatigue resistance of M-wire alloy than that of traditional NiTi alloy [16,17].This diversity in the findings can be attributed to the fact that instruments in the current study were tested while they were actively cutting rather than being rotating freely in a curved tube, which is the commonly used method in cyclic fatigue tests [18].
The OS group had a significantly longer lifespan and a significantly delayed incidence of cracks than both the PT and WO groups.This can be related to several factors.The first important factor is the electropolishing surface treatment of OS instruments.Electropolishing was reported to prolong the fatigue life of rotary NiTi endodontic instruments, by reducing surface irregularities that provide points for stress concentration and crack initiation [19].The second factor is the design of OS instruments.OS instruments have a 6% taper and a variable cross-section; this is in contrast to the apical 8% taper and the larger convex triangular cross-section in PT F2 and WO Primary instruments.This could have improved the cyclic fatigue resistance of OS instruments because instruments with smaller metal core were reported to pose a higher cyclic fatigue resistance [20].

CONCLUSION
Within the limitations of the current study, PT F2 instrument was comparable to WO Primary instrument in terms of fracture resistance.OS instrument had more fracture resistance than both PT F2 and WO Primary instruments, although it showed an early plastic deformation.

Sample selection and grouping:
A total of 117 resin blocks with embedded severely curved simulated canals were used in the current study.However, sample selection and grouping were performed depending on the instrument used rather than the canals, because the instruments were the target objects to be tested.

Sample size calculation:
The sample size was calculated from the lifespan (represented as the number of canals prepared) of single-file NiTi rotary instruments in simulated curved canals.As reported in a previous publication [21], the average lifespan was 5 canals.We considered that the SD equals one canal and the least clinically important effect size equals three canals.Accordingly, we calculated that the minimum proper sample size was five instruments in each group, which can reject the null hypothesis with 80% power at a = 0.05 level by using Student's t test for independent samples.The sample size was calculated using Stats Direct statistical software version 2.7.2 for MS Windows, StatsDirect Ltd., Cheshire, UK.

Instrument examination for deformation:
For file stabilisation during the examination, an endodontic ruler (Mini-Endo-Bloc, Ballaigues, Switzerland) was used.The ruler was modified (Fig. 3a) by adding a rest and lock (Fig. 3b) to fix the file in place.The modified ruler allowed 360º rotation of the file during the examination.Throughout the file examination procedure, only 3-4 mm of the file could be viewed under the 40x magnification; therefore, a permanent pen was used to set marks on the ruler, such that, the file tip and the marks were used as a guide to determine the position of deformation in relation to the file tip under stereomicroscope.

Table 1 :
Types of examined deformations and their definitions.A. Plastic deformation [8]: Loss of regular geometry of the instrument, which included the following defects: • unwinding of the helical structure • reverse winding • bending of the instrument over its long axis • or any combination of the above B. Cracks: Formation of fracture lines without complete separation of the instrument