Comparison of canal transportation and centering ability of Twisted Files, HyFlex controlled memory, and Wave One using computed tomography scan: An in vitro study.

AIM: The aim of the present study was to compare the canal transportation and centering ability of three rotary nickel-titanium (NiTi) systems (Twisted Files [TF], HyFlex controlled memory [CM], and Wave One [WO]) in curved root canals using computed tomography (CT).
MATERIALS AND METHODS: Sixty freshly extracted single-rooted teeth having curved root canals with at least 25-35 degrees of curvature were selected. The teeth were randomly divided into three experimental groups of twenty each. After preparation with TF, HyFlex CM, and WO, all teeth were scanned using CT to determine the root canal shape. Pre- and post-instrumentation images were obtained at three levels, 3 mm apical, 9 mm middle, and 15 mm coronal above the apical foramen were compared using CT software. Amount of transportation and centering ability were assessed. The three groups were statistically compared with analysis of variance and post hoc Tukey's honestly significant difference test.
RESULTS: Least apical transportation and higher centering ability were seen in HyFlex CM file system in all the three sections followed by TF. WO file system showed maximum transportation.
CONCLUSIONS: The canal preparation with HyFlex CM file system showed lesser transportation and better centering ability than TF, WO file system.

INTRODUCTION

Cleaning and shaping of the root canal have been recognized as an important phase in endodontic therapy while maintaining the original canal configuration and canal form.[1] However, in curved canals, these goals are not easily attainable, and root canal instrumentation becomes more difficult because there is a tendency for all preparation techniques and instruments to divert the prepared canal away from its original axis.[23] To remove this error, the introduction of nickel-titanium (NiTi) instruments has represented a major breakthrough in root canal preparation by permitting faster instrumentation while maintaining the original canal shape.[45] In rotary endodontics, two types of motions are there, one is continuous rotation and other is reciprocating motion.

Apart from motion, other variants among rotary instruments are their design and alloy characteristics. It has been shown that the design features and methods of manufacturing might significantly affect the clinical performances of NiTi rotary instruments.[67] Following this concept of NiTi technology, various rotary file systems have been introduced, claiming better root canal preparation.

HyFlex controlled memory (CM) rotary instruments are made of CM wire and exhibit the property of CM making the files extremely flexible. Twisted Files (TF) are manufactured by R-phase.[8] Wave One (WO) single file system is manufactured by M-wire alloys, which works on the principle of reciprocating motion.[9] As limited literature was available on canal transportation and centering ability of WO, HyFlex CM, and TF, this study was conducted to compare canal transportation and centering ability of WO, HyFlex CM, and TF using computed tomography (CT) scan.

MATERIALS AND METHODS

The present study consisted of 60 single-rooted human teeth with curvatures within 25–35 degrees extracted from patients because of orthodontic and periodontal reasons. Radiographic evaluation was done for all extracted teeth to maintain standardization.

All teeth were cleaned and stored in saline. Canal curvature was measured using Schneider method.[10] The crowns of all teeth were sectioned with a diamond disk, and the canal length was standardized to 18 mm. Access preparation was made on each tooth using a high-speed number two round bur (SS White) with continuous water spray.

A size 10 K-file (Dentsply, Maillefer) was placed into the canal until it was visible at the apical foramen, and the working length was established 0.5 mm short of this length. Roots were embedded into transparent acrylic (DPI, India). The teeth were randomly divided into three experimental groups.

Group 1: Teeth were prepared using TF according to the manufacturer's recommendations up to 25/0.6

Group 2: Teeth were prepared using HyFlex CM files up to 25/0.4

Group 3: Teeth were prepared using WO files up to 25/0.8.

All teeth were scanned by spiral CT (GE Brightspeed Elite) to determine the root canal shape before instrumentation. The sections were 0.6 mm thick from apical to the canal orifice. Three sections from each tooth, the number of the tooth, and its level were archived onto a magnetic optical disk (EDM 650B; Sony Corp, Tokyo, Japan). The first two sections were 3 mm from the apical end of the root (apical level) and 3 mm below the orifice from the coronal level (15 mm from the apex). A further section at the mid-root level (9 mm from the apex) was recorded, dividing the distance between the sections into 2 equal lengths. After initial scans, root canals were instrumented by the same operator using a standardized technique.

In Group 1 (n = 20), the samples were prepared using TF (SybronEndo, Orange, CA, the USA) according to manufacturer's instruction. A size 25/0.08 TF was used to resistance and then withdrawn immediately. If the working length was reached a size 25/0.06, TF was used to the working length.

In Group 2 (n = 20), the samples were prepared using HyFlex CM (ColteneWhaledent, Allstetten, Switzerland) according to manufacturer's instructions. A size 25/0.08 HyFlex CM was used as an orifice shaper followed by size 20/0.04 to working length, and the final apical preparation was done by size 25/0.04.

In Group 3 (n = 20), the samples were prepared using WO reciprocating files (Dentsply Maillefer, Ballaigues, Switzerland). WO primary file with size 25/0.08 was used for the preparation of canals with the preprogrammed motor (X-Smart Plus-DentsplyMaillefer, Ballaigues, Switzerland), according to manufacturer's instruction.

Consequently, the final apical preparation resulting was standardized to size 25 for all the three groups. For all groups after the use of each file, canals were irrigated with 1 ml of 17% EDTA was used for 1 min followed by 3 ml of a 5.25% NaOCl solution using a 27 gauge needle.

Teeth were scanned before and after instrumentation with their roots being perpendicular to the beam of the CT device (GE Brightspeed Elite). Calculation and comparison of all scans were made at 3, 9, and 15 mm from the apical foramen using a CT scan software (GE AW Volume Share 5).

Evaluation of canal transportation

The amount of canal transportation was determined by measuring the shortest distance from the edge of uninstrumented canal to the periphery of the root (mesial and distal) and then comparing this with the same measurements obtained from the instrumented images. All values were measured by two evaluators, and a mean value was taken.

The following formula was used for the calculation of transportation:

|(a1–a2)−(b1–b2)|

Where a1 is the shortest distance from the mesial edge of the root to the mesial edge of the uninstrumented canal, b1 is the shortest distance from distal edge of the root to the distal edge of the un-instrumented canal, a2 is the shortest distance from the mesial edge of the root to the mesial edge of the instrumented canal, and b2 is the shortest distance from distal edge of the root to the distal edge of the instrumented canal [Figure 1]. According to this formula, a result other than 0 indicates that transportation has occurred in the canal.

Figure 1

Representation drawing of tooth sections showing how transportation and centering ratios were derived. Uninstrumented image (left): Original canal space represented by dark shaded area. Instrumented image (right): Light shaded area represents canal's shape after instrumentation

Evaluation of centering ability

The mean centering ratio indicates the ability of the instrument to stay centered in the canal. It was calculated for each section using the following ratio:

(a1–a2)/(b1–b2) or (b1–b2)/(a1–a2).

If these numbers are not equal, the lower figure is considered as the numerator of the ratio. According to this formula, a result of 1 indicates perfect centering.

Statistical analysis

One-way analysis of variance followed by post hoc Tukey's honest significant difference multiple comparisons were conducted to explore a significant difference in mean degree of canal transportation and centering ability between the three shaping procedures. SPSS version 22.0 (IBM, USA) software was used for statistical analysis.

RESULTS

On comparing, three shaping techniques at various sections for canal transportation and centering ability using one-way ANOVA, there were no statistical significant difference between the three at the apical region (P > 0.05) whereas the difference was statistically significant in middle (P < 0.05) and coronal section (P < 0.05) [Tables 1 and 2]. When comparing the average mean degree of canal transportation, it was seen that the canal transportation was smallest in Group B (0.133 ± 0.112) followed by Group A (0.210 ± 0.119) and Group C (0.226 ± 0.135). Group A and C were statistically not significant (P > 0.05, post hoc Tukey test). When comparing the average mean centering ratio, it was seen that the centering ability was highest in Group B (0.604 ± 0.200) followed by Group A (0.472 ± 0.207) and Group C (0.42 ± 0.222). Group A and C were statistically not significant (P > 0.05, post hoc Tukey test) [Graphs 1 and 2].

Table 1

Mean canal transportation

Table 2

Mean centering ratio

Graph 1

Mean transportation in HyFlex controlled memory, Wave One, and Twisted Files at all sections

Graph 2

Mean centering ratio of HyFlex controlled memory, Wave One, and Twisted Files at all sections

DISCUSSION

Curvatures may pose difficulty in root canal instrumentation. Deviations and apical transportation are the most common undesirable accidents that can occur during preparation of curved root canals.[11] By preserving the original canal as far as possible, iatrogenic complications arising from cleaning and shaping can be avoided.[12] Various file systems are available in the market which are based on different NiTi wire technology and claim to preserve the canal anatomy.

The files used in our study are TF, HyFlex CM files, and WO files. The choice of the instruments used for this study took into account several factors that make them different from other systems, such as manufacturing processes and motion TF, a continuous rotation full-sequence NiTi system, having a triangular cross section has been developed by Sybron Endo (Orange, CA) with three unique design features, namely, the R-phase heat treatment, twisting of the metal, and special surface conditioning. These features proved to increase the instrument's resistance to fracture, provide greater flexibility, maintain the original canal center, and minimize canal transportation.[9]

HyFlex CM is continuous rotation rotary instruments, (Coltene Whaledent, Allstetten, Switzerland) with double fluted cross section are made from a new type of NiTi wire, CM wire. It 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 loss of shape memory typical of other NiTi files.[13]

WO reciprocating files (Dentsply-Maillefer, Ballaigues, Switzerland) are made of a special NiTi alloy called M-wire that is created by an innovative thermal treatment process. This procedure has been developed using super elastic NiTi wire blanks that contain substantial stable martensite under clinical conditions. The benefits of M-wire are increased flexibility of the instruments and resistance to cyclic fatigue.[14] These files have modified convex triangular cross section at the tip end and a convex triangular cross section at coronal end.

To detect the shaping characteristics of these NiTi rotary instruments, canal transportation and centering ability should be evaluated.[15] To investigate the efficiency of instruments and techniques developed for root canal preparation, a number of methods such as radiography, serial sectioning method, CT, cone beam CT, and micro CT, can be used to compare canal shape before and after preparation. Radiography only provides a two-dimensional image, and a cross section of the root canal is impossible to observe. The serial sectioning technique is a commonly used method, but it is more invasive procedure.[1617]

CT imaging techniques have been evaluated as an accurate noninvasive methods for the analysis of canal geometry and efficiency of shaping techniques.[1819] This study utilized CT to evaluate canal transportation and centering ability which was in accordance with the protocol employed by Hartmann et al. and Nagaraja and Sreenivasa Murthy.[2021]

The main parameters that were used to evaluate the shaping ability are protecting the curvature of the canal and maintaining a good centering ability. To describe the canal curvature, Schneider method which is the first and still most common method was used in this study.[10]

The results of instrumentation of curved root canals are influenced by several factors, such as flexibility and diameter of the endodontic instruments, instrumentation techniques, location of the foramen, and hardness of the dentin.[11] The flexibility of root canal instruments depends on complex interrelationships among different parameters, such as the cross-sectional design, core diameter, pitch, metallurgical properties, and surface treatment of the instruments.[22] Pongione et al.[23] evaluated flexibility and resistance to cyclic fatigue of endodontic instruments made with different NiTi alloys and concluded that the instruments made from CM wire are more flexible as compared to the other super-elastic NiTi wire technology. According to McSpadden,[24] less canal transportation occurs when the file has greater flexibility, an asymmetrical cross-section design and a radial land. Schäfer et al.[22] maintained that the size of a taper is one of the main factors involved in apical root transportation because an increase in the taper reduces instrument flexibility; therefore, recommended that NiTi files with tapers greater than 4% should not be used to shape the apical area of curved canals.

In this study, the HyFlex CM used was 4% taper which was least among the other file system used and therefore shows least canal transportation and highest centering ability.

CONCLUSIONS

Under the limitations of the present study, it could be concluded that HyFlex CM showed least transportation and remained better centered in the canal compared with TF and WO and it can be safely used in curved canals at full working length with satisfactory preservation of the original canal shape. However, research should continue to further improve instrument design, preparation techniques, and methodologies that are used to evaluate the action of endodontic instruments inside the root canal, aiming at solving the problems inherent to shaping of canals an important and difficult phase of the endodontic therapy.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.