Shaping ability of reciprocating motion of WaveOne and HyFlex in moderate to severe curved canals: A comparative study with cone beam computed tomography.

INTRODUCTION: M-Wire and reciprocating motion of WaveOne and controlled memory (CM) wire) of HyFlex were the recent innovations using thermal treatment. Therefore, a study was planned to evaluate the shaping ability of reciprocating motion of WaveOne and HyFlex using cone beam computed tomography (CBCT).
METHODOLOGY: Forty-five freshly extracted mandibular teeth were selected and stored in saline until use. All teeth were scanned pre- and post-operatively using CBCT (Kodak 9000). All teeth were accessed and divided into three groups. (1) Group 1 (control n = 15): Instrumented with ProTaper. (2) Group 2 (n = 15): Instrumented with primary file (8%/25) WaveOne. (3) Group 3 (n = 15): Instrumented with (4%/25) HyFlex CM. Sections at 1, 3, and 5 mm were obtained from the pre- and post-operative scans. Measurement was done using CS3D software and Adobe Photoshop software. Apical transportation and degree of straightening were measured and statistically analyzed.
RESULTS: HyFlex showed lesser apical transportation when compared to other groups at 1 and 3 mm. WaveOne showed lesser degree of straightening when compared to other groups.
CONCLUSION: This present study concluded that all systems could be employed in routine endodontics whereas HyFlex and WaveOne could be employed in severely curved canals.

INTRODUCTION

Biomechanical preparation is one of the most important procedures in endodontic treatment which influences root canal irrigation and obturation.[1] Nickel-titanium (NiTi) instruments offer many advantages to overcome the procedural errors of stainless steel.[23] They are flexible, maintain the original canal shape during preparation, and have a reduced tendency to transport the apical foramen.[23] Despite these advantages, the biggest problems of NiTi files are their high cost and unexpected instrument fracture.[2]

Traditional continuous rotation of files in curved root canals requires the instrument to flex during every rotation, resulting in cyclic compression and elongation, which produces metal fatigue and fracture.[4]

Recently, root canal shaping with reciprocating motion has been postulated to reduce the possibility of unexpected file fractures.[1] WaveOne utilizes reciprocating motion and is claimed to be able to completely prepare and clean root canals with only one single use instrument. These files are made of a special NiTi-alloy called M-Wire which is created by an innovative thermal treatment process. The benefits of this M-Wire NiTi are increased flexibility of the instruments and improved resistance to cyclic fatigue.[567]

However, the single use of endodontic instruments is more expensive and the high cost of NiTi files has forced many clinicians to reuse them, which in turn, lead to a higher incidence of instrument fracture.[28]

To overcome this, recently, thermal treatment of NiTi alloy has been used. Hyflex controlled memory (CM) rotary instruments are made from a new type of NiTi wire, namely, CM wire which has been subjected to proprietary thermomechanical processing. It has been manufactured by a unique process that controls the material's memory, making the files extremely flexible but without the shape memory of other NiTi files. With about 300% resistant to separation, they attain strength by sterilization.[910]

Very few literature is available comparing the shaping ability of WaveOne and HyFlex. Therefore, this study was planned. Null hypothesis tested as new manufacturing methods and type of rotary motion will not have any effect on their shaping ability of root canals.

METHODOLOGY

Forty-five freshly extracted human mandibular molars having moderate to severely curved canals (20–45°) were collected and cleaned for any debris and stored in saline until use. All teeth were subjected to radiovisiography to rule out any calcification. These teeth were divided into three groups of 15 each after randomization of the sample.

All teeth were accessed with Endo-Z bur (#2, Dentsply Maillefer, Ballaigues, Switzerland) and working length was determined by passing #10 K file until it was visible beyond apex using ×3 magnifying loops (Seilar Zolar) at which point 1 mm was deducted from that length.

Group 1 (control n = 15): Instrumented with ProTaper (Dentsply Maillefer, Ballaigues, Switzerland) in the sequence S1>SX>S1>S2>F1>F2 (8%/#25) as it can be considered as standard control preparation in continuous rotation in pecking motion according to manufacturer's recommendations

Group 2 (n = 15): Instrumented with primary file (8%/#25) WaveOne (Dentsply Maillefer, Ballaigues, Switzerland) in reciprocating motion.(clockwise140°, counterclockwise 45°)

Group 3 (n = 15): Instrumented with (4%/#25) HyFlex CM (Coltene-Whaledent, Allstetten, Switzerland) file in reciprocating motion (clock wise 140°, counterclockwise 45°).

All canals were prepared by one experienced operator. WaveOne Endomotor was used to prepare all canals. During instrumentation, 3 ml 3% NaOCl, 17% ethylenediaminetetraacetic acid, and saline were used in all groups. New files were used to prepare three canals.

Cone beam computed tomography analysis

All teeth were mounted in modeling wax template and subjected to cone beam computed tomography (CT) (KODAK 9000 3D) at 60 kV and 2.5 mA with isotropic resolution of 90 μm for pre- and post-instrumentation. Only five teeth were scanned per time for better resolution. Cross-sectional images were obtained using CS3D software (KODAK 9000, KODAK Carestream Health, Trophy, France) at 1, 3, and 5 mm from the root tip.

The amount of transportation was measured from the cross-sectional images before and after the canal preparation according to the shortest distance from the edge of the uninstrumented canal to the periphery of the root and was compared with the value obtained from the postinstrumentation image. A 1 mm reference scale is also drawn adjacent to the slice. Image is captured at 200% magnification using the software [Figure 1a and b].[11]

Figure 1

(a) Preoperative cone beam computed tomography sections using CS3D software, (b) postoperative cone beam computed tomography sections using CS3D software, (c) preoperative angle measurement using Schneider's technique, (d) postoperative angle measurement using Schneider's technique, (e) measurement using Adobe Photoshop software

Measurement using Adobe Photoshop

Scanned images were exported to Adobe Photoshop software (Version 7.0; Adobe systems Inc., San Jose, CA, USA). The area of interest was cropped along with the 1 mm reference bar. At magnification, 200% magic wand tool was used with tolerance set to 10 to outline the canal and the root outer surface. Then, the image was zoomed to 1200% and observed as pixels. Based on the number of pixels present in the 1 mm reference bar, the size of each pixel is noted in millimeters. The number of pixels was counted to improve the accuracy of the measured values by CS3D software. This method was employed to eliminate the operator difficulty in identifying the margin of the canal accurately. Error was minimized by calculating in the pixel level [Figure 1e].[11]

The amount of apical transportation can be measured using the formula (a1 − a2) − (b1 − b2).[12]

Where,

a1 was the shortest distance from the lateral edge of the uninstrumented canal to the lateral edge of the root,

b1 was the shortest distance from the medial edge of the uninstrumented canal to the medial edge of the root,

a2 was the shortest distance from the lateral edge of the instrumented canal to the lateral edge of the root,

b2 was the shortest distance from the medial edge of the instrumented canal to the medial edge of the root.

The transportation values were analyzed in 2 ways. The degree of transportation was measured by calculating the absolute values that resulted from the above formula, and the tendency of the transportation direction was observed using the total values obtained. The positive value obtained from the formula represents that transportation has occurred lateral to the curvature, whereas the negative value represents that transportation has occurred in the direction facing the furcation. Transportation in the 1, 3, and 5 mm points from the apical foramen was measured from each of its corresponding cross-section image.[113]

Degree of straightening was measured based on the formula.

Degrees of straightening = Preoperative angle – postoperative angle.

The canal curvature was measured according to Schneider's technique[14] using CS3D software in pre- and post-instrumentation scans. A line drawn from the center of canal orifice is extended apically and the point where it goes out of the canal is noted. This point is intersected by the line drawn from root apex and angle between them is measured as Schneider's angle.[14] The change in the curvature was measured and recorded as canal straightening [Figure 1c and d].

All the observed values were noted and statistical analysis was performed using the software, version 20.0 (SPSS Inc., Chicago, III, USA). Group differences were analyzed by ANOVA and intergroup comparison was done using post hoc Tukey test. P value was adjusted to (<0.05).

RESULTS

Table 1 represents the results of the statistical analysis of apical transportation at 1, 3, and 5 mm. Tukey test revealed the degree of transportation is in the following order.

Table 1

Post hoc analysis of apical transportation between different techniques

At 1 mm - HyFlex < WaveOne < ProTaper

At 3 mm - HyFlex < ProTaper < WaveOne

At 5 mm - HyFlex = Waveone = ProTaper.

Table 2 represents the Tukey analysis of results of degree of straightening which was recorded by measuring the difference between pre- and post-operative angles according to Schneider's.[15]

Table 2

Post hoc analysis of degree of straightening between different techniques

To summarize:

WaveOne < HyFlex < ProTaper.

DISCUSSION

Based on the results obtained in this study for apical transportation at 1 mm, reciprocating motion has shown lesser transportation (HyFlex = 0.0163 mm, WaveOne = 0.0293 mm) than continuous motion (ProTaper = 0.0513 mm) with statistical significant difference (P < 0.05) which is in accordance to a study where the values are <0.1 mm.[16] Greater apical transportation in ProTaper is due to the screw in effect and aggressive cutting design.[1718] Instrument in reciprocation is claimed to continuously progresses toward the apex of the root canal[6] which explains the lesser apical transportation seen at 1 mm in reciprocating group. Flexibility could be another adjunct for lesser transportation in these groups along with reciprocating motion as M-Wire of WaveOne and CM wire of HyFlex were found to be more flexible than traditional alloy.[19]

At 3 mm, the reason for greater apical transportation of reciprocating group (WaveOne = 0.0541 mm) when compared to continuous motion group (ProTaper = 0.0035 mm) could be explained based on the modified cross-section of WaveOne at 3 mm. Sufficient evidence is not available to eliminate the confounding factors associated along with the reciprocating motion. Hence, it is difficult to arrive at a conclusion.

Comparing the reciprocating groups (HyFlex and WaveOne), HyFlex showed better at 3 mm with statistical significance (P < 0.05). The probable reasons could be discussed based on the material aspect and design of the file. HyFlex is manufactured with low percentage of nickel (52%wt) contrary to the traditional NiTi (54.5%–57%wt). The potential role of this still remains uncertain. In a study, it was found that CM wire was more flexible than M-Wire and lesser taper instruments causes less transportation.[20212223] These instruments consist of mixture of martensite and austenite structure in the body temperature which is a unique property in HyFlex, whereas in ProTaper and WaveOne only austenitic phase exists.[23]

The martensitic phase transformation has excellent damping characteristics because of the energy absorption characteristics of its twinned phase structure. When the material is in its martensite form, it is soft, ductile and can be easily deformed. For instance, the martensite phase is less stiff and more pliable, possessing an elastic modulus of 31–35 GPa compared with 84–98 GPa for austenite.[232425] This probably explains the lesser apical transportation of HyFlex.

Regarding the direction of the transportation, ProTaper showed tendency toward the outer aspect of the curve at 1 and 3 mm which is in accordance with Schäfer and Vlassis and Stern et al.[2627] At 5 mm, it showed a tendency toward the inner aspect of the curve toward the furcation which is in accordance with a study conducted.[28] WaveOne showed tendency toward inner aspect of the curvature at 1 and 5 mm which is in accordance to a study.[29] Whereas HyFlex showed tendency toward the outer aspect of the curve at all levels which is in accordance to a study.[12] Based on the results obtained at all levels, HyFlex in reciprocating motion showed lesser transportation with the direction toward the outer aspect of the curve away from the furcation.

Degree of straightening is also referred to as change in canal curvature. Ideally, there should not be any change. However, no instrumentation technique so far has come up maintaining the normal curvature. Reciprocating group showed lesser degree of straightening (WaveOne-2.1333, HyFlex-3.2667) when compared to continuous group (ProTaper-6.8) with statistical significance (P < 0.05). The probable reason could be explained based on the instrument taper.

S1 and S2 have an increasing taper over the working part from 2% at D1 to 11% at D14 for S1 and 4% on D1 to 11.5% at D14 for S2. Whereas WaveOne has continuously decreasing taper when compared to ProTaper which explains lesser coronal diameter. Schneider's angle mainly depends on the curvature in the coronal region. The results obtained probably could be because of the greater coronal enlargement by ProTaper influencing Schneider's angle. The result of this study is in contrary to a study wherein, they have found no significant difference between continuous and reciprocating motion.[1]

Among the reciprocation groups, WaveOne (2.13°) showed significantly lesser degree of straightening compared to HyFlex (3.26°). The probable reason could be the decreasing taper of WaveOne resulting in lesser coronal diameter when compared to HyFlex which is having continuously increasing taper.

Considering the results of the study, the null hypothesis was rejected. Based on the results, all systems could be indicated for routine clinical use, whereas HyFlex and WaveOne in reciprocating motion could be employed in severe canal curvatures. No strict adherence to the severe curvatures and not using micro-CT for more accurate results could be some possible limitations in this study. Future research possibilities could be on the stress distribution of these files using finite element and fracturographic analysis to determine the mode of fractures.

CONCLUSION

The following conclusions have been drawn from the study.

Apical transportation was significantly more with ProTaper at 1 mm when compared to other groups with direction toward the inner aspect of the curvature (furcal side) at 5 mm

WaveOne produced greater apical transportation at 3 mm with direction toward inner aspect of the curvature (furcal side) at 1 and 5 mm

HyFlex showed lesser transportation at 1 and 3 mm with statistical significance

WaveOne showed significant lesser degree of straightening when compared to other groups.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.