Comparative Analysis of Canal Centering and Transportation: Edgefile X7 vs HyFlex EDM in Moderately Curved Mesiobuccal Root Canals of Mandibular Molars

Introduction

Root canal cleaning and shaping, which aims to remove infected tissues and debris, is the most critical phase of root canal treatment [1]. The instruments used during the procedure should preserve the original canal shape, without the transportation of the canal [2–4]. Nonetheless, these iatrogenic mistakes, which manifest as canal transportation, frequently happen when the curved canals are being instrumented. According to the American Association of Endodontists, Glossary of Endodontic Terms, canal transportation is defined as the removal of canal wall structure on the outside curve in the apical half of the canal due to the tendency of files to restore themselves to their original linear shape during canal preparation, which can lead to ledge formation and possible perforation [5]. Canal-centering ability can be described as the ability of the file to be in the center during root canal instrumentation, to provide a correct enlargement, without excessive weakening of the root structure [6]. Nickel-titanium (NiTi) rotary files are known to preserve the natural canal morphology more effectively than stainless-steel hand files; however, instrumenting curved canals remains a challenge [7–9]. To improve the mechanical characteristics, functionality, and safety of NiTi instruments, manufacturers have developed several innovative technologies, including M-wire, R-phase, controlled memory, and unique heat technology [10]. The newest generation of NiTi rotary instruments includes the Edgefile X7 (Edge Endo, NM, USA) and HyFlex EDM (Colténe/Whaledent, Switzerland) systems.

The HyFlex EDM was introduced with an innovative manufacturing process using electric discharge machining (EDM) followed by controlled memory treatment [11]. It has a variable cross-section design, which is almost triangular near the shaft, trapezoidal in the middle, and quadratic at the tip. The HyFlex EDM sequence for curved canals is as follows: Orifice Opener (25/.12), Glidepath File (10/.05), Preparation File (20/.05), and OneFile (25/~) [12].

The EdgeFile X7 was recently launched by EdgeEndo. It is manufactured by a patented Fire Wire heat-treating process that gives the file “Canal Contouring Technology” extreme flexibility and reduced shape memory. According to the manufacturer, there is a decreased chance of ledge formation, transportation, and apical perforation because the flexible EdgeFile X7 files can precisely follow the anatomy of the canal without straightening out [13]. EdgeFile X7 instruments have a triangular cross-section and variable helix angles that give sufficient strength, high flexibility, and increased resistance to flexural fatigue [14]. The instruments are available as a constant 04 taper for all types of canals and 06 taper for straight to mildly curved canals [15].

The present study evaluates and compares the Edgefile X7, a reduced shape memory file, with HyFlex EDM, a type of controlled memory file, for the transportation and the centering ability of the moderately curved canal. The null hypothesis of the present study was that there would be no difference between the tested rotary file systems in terms of canal transportation and centering abilities. Therefore, this cone beam computed tomography (CBCT)-based endodontics study aimed to compare the canal-centering ability and canal transportation of the newly introduced Edgefile X7 and HyFlex EDM NiTi file in 30 permanent mandibular first molar teeth with 10° to 24° root curvature in the mesial root.

Material and Methods

SAMPLE SELECTION:

The study protocol was reviewed and approved by the Ethics Committee for Scientific Research, Jazan University (reference number REC-44/05/396). The hopeless mandibular first permanent molar teeth indicated for extraction were collected from the Oral Surgery Clinics, College of Dentistry, Jazan University. The reason for extraction was extreme tooth mobility due to periodontal diseases. Patient informed consent was obtained to use the extracted teeth for research purposes. Teeth with no previous endodontic treatment, root caries, external or internal resorption, calcifications, or open apices were included. Teeth were disinfected with 5.25% sodium hypochlorite for 5 min, cleaned of residual tissue tags, rinsed under running water, and stored in saline solution until the experiment. Teeth were radiographed in a buccolingual direction, and the mesiobuccal (MB) canal curvature was measured according to Schneider’s criteria [16]. Thirty mandibular first molar teeth with moderate root canal curvature (10° to 20°) in the mesial root were chosen [17].

SAMPLE PREPARATION:

Endodontic access cavities were prepared by round diamond and tapered fissure burs with a high-speed handpiece under air-water spray. For standardization of the samples, the crowns were cut with the diamond disc, and a final dimension of 16 mm in length was achieved for each tooth. To determine the working length, a #10 K-file was placed into the MB canal until it was visible at the apical foramen, and the working length was established as 0.5 mm short of this length. The glide path was created with a #15 K-file, up to the working length. To identify the MB canal on CBCT, a vertical groove (approximately 1 mm in length and 0.5 mm in depth) was made on the occlusal surface at the MB line angle of the tooth. The light cure acrylic sheets in rubber base former were used to mount the teeth such that their roots were embedded in the acrylic material without covering the crowns. Rubber base formers were selected to arrange teeth because of the simulation of the mandibular jaw for acquiring CBCT images.

PRE-INSTRUMENTATION CBCT SCAN:

All teeth were scanned with the CBCT device (3D Accuitomo 170, MORITA, Japan) pre-operatively at 90 KV and 7mA, 0.2 mm³ voxel size, 15.8 s exposure time, and 140×100 field of view to determine the root canal shape before instrumentation. The high-resolution images were processed and reconstructed using i-Dixel 3D imaging software (MORITA, Japan).

ROOT CANAL PREPARATION:

The 30 MB canals were split into 2 experimental groups of 15 each at random: group 1: the HyFlex EDM, and group 2: the Edgefile X7.

The root canal preparation was performed by an experienced endodontist, using rotary instrumentation. The full clockwise rotation motion using an endo motor with a handpiece (X Smart, Dentsply, Maillefer) was used for both groups. Each file was inserted into the canal with minimum pressure in an inward-outward motion. The file was retrieved from the canal after 3 inward-outward motions to clean file flutes from debris and again inserted until the file reached the working length. The canals were frequently irrigated with 5 mL solution of 5.25% NaOCl in a syringe with a 27-gauge needle for each insertion of the file. After complete instrumentation, the root canals were irrigated with 1 mL solution of 17% EDTA for 1 min, followed by 5 mL solution of 2.5% NaOCl to remove the smear layer. Finally, the root canals were flushed with 5 mL sterilized saline solution. Each file of both systems was used for 3 canals and then discarded to avoid accidental instrument separation during root canal instrumentation.

POST-INSTRUMENTATION CBCT SCAN:

CBCT scans were acquired for the experimental groups with the same exposure parameters as the pre-instrumentation scan, to determine the root canal shape after instrumentation. The images were reconstructed using i-Dixel 3D imaging software (MORITA, Japan) in a similar way to the pre-instrumentation scan.

MEASUREMENT OF CANAL TRANSPORTATION AND CANAL-CENTERING ABILITY:

From pre-instrumentation and post-instrumentation CBCT scans, the axial sections of the MB canals were measured at 3 mm, 6 mm, and 9 mm from the apical end of the root, and the data were recorded in an Excel sheet. The pre-instrumentation measurements were recorded as a1 and b1, where a1 is the distance from the mesial wall of the uninstrumented root canal to the outer wall of the root, and b1 is the distance from the distal wall of the uninstrumented root canal to the outer wall of the root. The post-instrumentation measurements were recorded as a2 and b2, where a2 is the distance from the mesial wall of the instrumented root canal to the outer wall of the root, and b2 is the distance from the distal wall of the instrumented root canal to the outer wall of the root (Figure 1).

CANAL TRANSPORTATION CALCULATION: The amount of canal transportation was measured by using the formula (a1–a2) - (b1–b2), given by Gambill et al [7]. With the formula, the following interpretations of canal transportation were made: “0” indicates no canal transportation, a value other than “0” means canal transportation occurred, a negative value denotes canal transportation toward distal, and a positive value signifies canal transportation toward mesial.

CANAL-CENTERING ABILITY CALCULATION: The canal-centering ability was measured by using the formula (a1–a2) / (b1–b2) or (b1–b2) / (a1–a2) given by Gambill et al [7]. The fraction with the lesser value was selected for statistical analysis in these formulas. With the formula, the following canal-centering ability interpretations were made: “1” indicated perfect canal-centering ability; the closer the value to “0”, the worse the instrument’s ability to keep itself in the canal’s central axis.

STATISTICAL ANALYSIS:

Data were presented as mean, standard deviation, and median. Intragroup and intergroup analyses were performed using the unpaired t test. The paired t test was used to compare the mean difference between 3 mm vs 6 mm, 3 mm vs 9 mm, and 6 mm vs 9 mm in each group. The Pearson correlation coefficient was used to measure the linear correlation between canal transportation and canal-centering ability. The threshold of significance for all statistical tests was established at 5% (P<0.05).

Results

CANAL TRANSPORTATION RESULTS:

At 3 mm, 6 mm, and 9 mm, the average canal transportation for the HyFlex EDM was 0.01±0.17 mm, −0.10±0.21 mm, and 0.03±0.33 mm, respectively; for the Edgefile X7, the average canal transportation was −0.02±0.21 mm, 0.07±0.22 mm, and −0.12±0.23 mm, respectively (Table 1). The Edgefile X7 demonstrated a negative mean at 3 and 9 mm, which was shown to encourage canal transportation on the distal surface of the canal curvature. In contrast, the HyFlex EDM mean exhibited positive values, suggestive of canal transportation on the mesial surface of the canal curvature. The HyFlex EDM, on the other hand, displayed a negative mean at 6 mm, which was shown to facilitate canal transportation on the distal surface of the canal curvature, whereas Edgefile X7 was on the mesial surface of the curve. The HyFlex EDM demonstrated considerable canal transportation at 6 mm (P<0.05), but Edgefile X7 and HyFlex EDM did not differ significantly at 3 mm or 9 mm (P>0.05).

CANAL-CENTERING ABILITY RESULTS:

At 3 mm, 6 mm, and 9 mm, the HyFlex EDM average centering ability was 0.52±0.30 mm, 0.38±0.23 mm, and 0.48±0.33 mm, respectively; for the Edgefile X7, it was 0.57±0.40 mm, 0.47±0.39 mm, and 0.53±0.29 mm, respectively (Table 2). Edgefile X7 demonstrated a superior ability to maintain within the central axis of the root canal (the highest average value that is closest to 1), compared with HyFlex EDM, even though the differences between the 2 files at all tested levels were not statistically significant (P>0.05). The correlation between canal transportation and canal-centering ability in the 2 experimental groups showed a significant correlation at 9 mm (P<0.05) but not at 3 mm or 6 mm (P>0.05) (Table 3).

Discussion

LIMITATIONS OF THE STUDY:

Although the results of this in vitro study could be applied to clinical practice, because natural teeth were used, the complete simulation of clinical scenarios is difficult and considered a limitation. Therefore, more in vivo studies should be conducted to evaluate the canal transportation and centering ability of rotary file systems.

Conclusions

Within the constraints of this study, it can be concluded that in moderately curved canals at 3 mm, 6 mm, and 9 mm from the apex, both the Edgefile X7 and the HyFlex EDM files are shown to promote canal transportation either on the mesial or distal surface of the canal curvature. Compared with HyFlex EDM, the Edgefile X7 file system demonstrated lower canal transportation and a better ability to maintain within the central axis of the root canal. From a clinical perspective, these findings are significant, because files with better centering ability and less canal transportation are desirable during root canal instrumentation.