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ORIGINAL ARTICLE
Year : 2021 | Volume
: 24
| Issue : 3 | Page : 418-424
The effect of heat-treated single-file systems on dentinal crack formation
E Ozlek, H Gunduz
Department of Endodontics, Faculty of Dentistry, The University of Van Yuzuncu Yıl, Van, Turkey
Correspondence Address:
Dr. E Ozlek
Department of Endodontics, Faculty of Dentistry, The University of Van Yuzuncu Yil, Van
Turkey
Source of Support: None, Conflict of Interest: None
CheckDOI: 10.4103/njcp.njcp_250_20
Abstract
Objectives: The aim of this study was to compare the formation of dentinal crack after root canal preparation performed with different heat treated single-file systems. Materials and Methods: Forty mandibular premolar teeth were randomly divided into four groups according to the file system to be used during the preparation (n = 10): Group 1: Reciproc Blue; group 2: One Curve; group 3: HyFlex EDM; group 4: (control group) hand files. During the cleaning and shaping, irrigation was performed with a total of 10 ml of 5.25% sodium hypochlorite and 2 mL of distilled water. Then sections were taken from the roots (3, 6 and 9 mm). Sections were examined on a stereomicroscope to determine the presence of cracks. Chi-square test was used for data analysis. Results: There was a significant difference between the groups in dentinal crack frequency (P <.05). HyFlex EDM (13.3%) caused less dentinal crack than One Curve (26.7%) and Reciproc Blue (30%). No cracks were observed in the control group. When 3, 6, and 9 mm were evaluated within themselves, dentinal crack formation did not differ according to the groups (P > 0.05). A statistically higher mean value was obtained in the control group compared to the other groups in terms of working time (P < 0.001). Conclusions: All the files used during root canal preparation formed dentin crack. Hy-Flex EDM caused less dentinal cracks than other file systems. The flexibility of nickel-titanium instruments because of heat treatment seems to have a significant influence on dentinal crack formation.
Keywords: Dentinal crack, Hyflex EDM, one curve, Reciproc Blue, single-file systems
Introduction 
A successful root canal treatment is possible by expanding and shaping the root canals in a suitable form, disinfecting, and filling them tightly. Complications such as disruption of the original shape of the canal, perforation, root canal transportation, and step formation may occur during the shaping of the root canals.[1] In addition, stress is applied to the root dentin with the files used, and microcracks and craze lines may occur as a result of this stress.[2] These microcracks and craze lines can result in vertical root fractures in the long term.[3]
Many studies report that the use of NiTi rotary files causes cracks in the root dentin, there is a direct relationship between the amount of dentin removed and crack formation, and file design affects apical stress and strain concentration.[1],[4],[5] On the other hand, however, it is also known that bacteria and infected debris must be completely removed from the root canal for a successful endodontic treatment.[1] Companies have manufactured many NiTi rotary file systems with different kinematics to improve clinical performance and safety during the shaping of the root canals.[6] In recent years, technological advances in NiTi rotary file systems have allowed the opportunity to shape the root canals with single-file systems activated in rotary or reciprocating motions.[7] The use of single-file techniques in preparing root canals is considered advantageous due to the ease of learning the technique, decreased number of instruments, reduced potential for file separation and dramatic reduction in preparation time.[8]
Reciproc (VDW, Munich, Germany) is a single-file system made of M-Wire NiTi alloy and producing reciprocating motion. Thanks to M-Wire NiTi alloy, the flexibility of the file has been increased while maintaining its cutting efficiency.[9] The Reciproc file has an S-shaped cross section, a non-cutting tip, and a sharp edge. It is used by moving back and forth with a speed of 300 rpm.[10],[11] Studies have reported that the reciprocating motion distributes the stress forces applied to the root canal walls more evenly compared to conventional rotary file systems.[5] Thermomechanical processes have recently been applied to NiTi alloys. The structure and transformation performance of the files are tried to be increased with this heat treatment.[12] Reciproc Blue is the heat-treated version of Reciproc, which was recently released. Studies show that Reciproc Blue shows much higher flexibility and resistance to metal fatigue compared to Reciproc thanks to heat treatment.[8],[9]
HyFlex EDM (Coltene/WhaledentAG, Altstatten, Switzerland) is another single-file system introduced to the market in recent years. HyFlex EDM files are controlled memory instruments, manufactured using a unique process called electrical discharge machining (EDM).[13] Electrical Discharge Machining uses spark erosion to harden the surface of NiTi files in the manufacturing process, which gives the files excellent flexibility and fracture resistance.[14] In addition, thanks to the controlled memory, the risk of root transportation and perforation is reduced by the files more easily following the root canal anatomy.[9],[14]
One Curve (Micro-Mega, Besancon, France) is a heat-treated single-file system that was released in 2018. The heat-treated NiTi alloy is called C. Wire. C. Wire technology is reported to provide a controlled memory and a pre-bending feature to the file for easier access to the root canal.[15] Although One Curve files have a single tip size (size 25) and constant taper (6%), different shape designs are available. The variable cross-sections with a triangular-shaped at the tip of the instrument and S-shaped near the shaft are claimed to allow effective cutting and centered trajectory.[16]
To the best of our knowledge, there are a limited number of studies evaluating the effect of single-file systems on dentinal crack to date. Besides, no study in the literature has evaluated the effectiveness of One Curve files, which is a fairly new system, on dentinal crack formation after root canal shaping. Thus, the aim of this study was to test the null hypothesis that single-file systems (Reciproc Blue, Hyflex EDM One, One Curve) had no effect on dentinal cracks.
Methodology Specimen selection
The study protocol was approved by The Research Ethics Committee, presenting with statement number 2019/17-10. Freshly extracted, single-rooted mandibular human premolars (n = 40) with completely formed roots and closed apices, with no cracks or structural anomalies were used for this study. After cleaning the soft tissue debris and calculus from the surface of the teeth, the teeth were decoronated with a low-speed diamond saw (Isomet 1000, Buehler, Lake Bluff, IL, USA) under water cooling to obtain the roots of standardized lengths of 12 mm. As previously described, the roots were wrapped in aluminum foil and embedded in auto-polymerized acrylic resin. The roots were then removed from the acrylic resin, and the aluminum foil around the roots was peeled off. To mimic the periodontal ligament, light-body polyvinyl siloxane impression material was inserted into the space formed by the aluminum foil, and the roots were repositioned immediately.[17]
Root canal preparation
A #15 K-file (VDW GmbH, Munich, Germany) was inserted into the root canal until the tip of the file was visible at the apical foramen and the working length was determined after subtracting 1 mm from this length. The specimens were randomly divided into four groups (n = 10) based on the instrumentation technique used:
Group 1, Hyflex EDM (HEDM)
The root canals were prepared with the One File (Size 25) using gentle apical strokes and pecking movements in accordance with the manufacturer's recommendations. The VDW endodontic motor (VDW Silver Reciproc, VDW, Munich, Germany) was used to operate the file in continuous rotation at 500 rpm speed and 2.5 Ncm torque.
Group 2, Reciproc Blue (R25)
The Reciproc Blue file (Size 25) was used in the “Reciproc ALL” program using the VDW endodontic motor (VDW) in accordance with the manufacturer's recommendations.
Group 3, One Curve File
The One Curve file (Size 25) was used in the VDW endodontic motor (VDW) in continuous rotation at 300 rpm and 2.5 Ncm torque in accordance with the manufacturer's recommendations.
Group 4, Control group
The root canals were prepared with stainless steel hand files. The root canals were prepared to apical size #25 and preparation up to # 50 using a step-back technique with 1-mm increments.
Each instrument was used to prepare only 1 canal. If resistance was felt that would have required more apical pressure to advance the instrument, the instrument was removed, and the root canals were irrigated before using the instrument again. A total volume of 10 mL 5.25% NaOCI was used with a NaviTip irrigation needle (30-G; Ultradent Products Inc, South Jordan, UT) placed 1 mm short of the working length. After root canal preparation, all canals were irrigated with 2 mL of distilled water, and the roots were kept moist in distilled water throughout all experimental procedures to prevent the effect of dehydration on the roots.[2]
Sectioning and microscopic examination
All roots were cut 3, 6, and 9 mm horizontally from the apical region with a low-speed diamond saw under water cooling. The sections were then viewed under a stereomicroscope. The specimens were examined and photographed under the stereomicroscope at 2.5X and 5X magnifications (Nikon SMZ25; Nikon Tokyo, Japan) to determine the presence of microcracks [Figure 1]. A total of 120 digital images were examined by 2 operators and classified as “crack” and “no crack”. All lines extending from the root surface or root canal lumen into the dentin were defined as “crack”. Notch lines or microcracks from the inner surface of the root canal wall or the outer surface of the root were defined as “no crack”.
Figure 1: Representative microscopic cross sections from each experimental group at the 6-mm level. White arrows indicate dentinal defects. (A, B, C and D) 2,5 and (a, b, c and d) 5,0 magnifications. (D and d) No microcracks were observed in the control group. Reciproc Blue (A and a), One Curve (B and b), HyFlex EDM (C and c), Control (D and d)Calculation of working time
A digital stopwatch was used to determine the working time of the files. The stopwatch was started when the file was placed in the canal and stopped when it was taken out of the canal in all groups. The stopwatch was not operated in the processes such as irrigation of the root canals, file replacement and file cleaning.
Data presentation and statistical analysis
The data were analyzed with IBM SPSS V23 (IBM Corp. Armonk, NY, USA). The Chi-square test was used to carry out inter-group and intra-group examination of the presence of dentinal crack. The Shapiro Wilk test was used to examine the suitability of the procedure times to normal distribution. The Kruskal-Wallis test was used to compare the data without normal distribution by the groups.
Results Dentinal crack was observed in all specimens of all groups except for the control group in this study evaluating the effects of heat-treated single-file systems on dentinal crack formation during root canal shaping. When dentinal crack was evaluated only according to the groups regardless of the region where it was formed (3 mm, 6 mm, 9 mm), a statistically significant difference was found between the groups (P = 0.008) [Table 1]. While 30% of the cases in the Reciproc Blue group, 26.7% of the cases in the One Curve group and 13.3% of the cases in the Hyflex group had dentinal cracks, there was no dentinal crack formation in the control group.
Table 1: The number and percentage of roots slices exhibiting dentinal cracks after the preparation of each group (n=10)As a result of the evaluation made according to the region where dentinal crack was formed, there was no statistically significant difference between the groups in the apical, middle, and coronal regions (P > 0.05) [Table 2]. Dentinal crack was observed at the rate of 40% in the Reciproc Blue group, 40% in the One Curve group and 20% in the Hyflex EDM group in the apical region; it was observed at the rate of 30% in the Reciproc Blue group, 20% in the One Curve group and 10% in the Hyflex EDM group in the middle region; it was observed at the rate of 20% in the Reciproc Blue group, 20% in the One Curve group and 10% in the Hyflex EDM group in the coronal region [Figure 2]. When 3, 6, and 9 mm were evaluated within themselves, dentinal crack formation did not differ according to the groups (P > 0.05).
Table 2: The Number and Percentage of Dentinal Cracks in the Dentinal Cross Sections at 3, 6 and 9 mm from the Apex (n=10)
Figure 2: The percentage of dentinal cracks formation Cross Sections at 3, 6, and 9 mm from the Apex (n = 10)A statistically significant difference was found between the Hyflex EDM, Reciproc Blue, One Curve and control groups in terms of working time (P < 0.001) [Table 3]. A statistically higher mean value was obtained in the control group compared to the other groups. Although there was no statistically significant difference between Hyflex EDM, Reciproc Blue, and One Curve, One Curve had the shortest working time, followed by Hyflex EDM and Reciproc Blue, respectively. As a result, Reciproc Blue yielded the most negative result between the groups in terms of both dentinal crack formation and working time efficiency.
Table 3: The Mean and Standard Deviation of preparation time in each group Discussion This study compares 3 different single-file systems in terms of the effect on dentinal crack formation and root canal shaping time. The null hypothesis tested was that these 3 file systems had no effect on the formation of dentinal cracks and working time. The null hypothesis was rejected according to the results obtained. After shaping the root canals, dentinal crack was observed in all groups except for the manual file group, which was considered as the control group, and a statistically significant difference was found between the groups (P < 0.05). The fewest dentinal cracks were observed with Hyflex EDM, followed by One Curve and Reciproc Blue, respectively. When evaluated in terms of root canal shaping time, a statistically significant difference was found between the groups (P < 0.05). One Curve was the fastest to shape the root canals, followed by Hyflex EDM and Reciproc Blue, respectively.
Studies have reported that all rotary file systems form microcracks in the dentine and the degree of dentinal damage can vary depending on the tip design, cross-section geometry, constant or progressive taper type and flute form of the instrument.[4],[5],[17] In this study, rotary file systems with different geometrical properties were used, and microcracks were observed in all specimens in accordance with the literature. However, there are also studies in the literature reporting that dentinal crack formation does not differ significantly depending on the geometrical properties. These studies report that microcrack formation may be affected by NiTi alloy and kinematic properties.[7],[18],[19]
Studies have shown that the files manufactured from M-Wire NiTi alloy have higher flexibility and higher cyclic fatigue resistance compared to conventional NiTi files.[20],[21] In addition, controlled memory files have been reported to be more flexible than both M-Wire NiTi alloy and conventional NiTi wire.[7],[21],[22] In this study, the fewest microcracks were observed in the HyFlex EDM group, while the most cracks were observed in the Reciproc Blue group. Reciproc Blue is manufactured from M-Wire NiTi alloy. On the other hand, HyFlex EDM and One Curve are two controlled memory files, one of which is manufactured by electrical discharge machining (EDM) method and the other is manufactured from C. Wire alloy. The results of this study are consistent with the results of the studies conducted by Capar et al.[18], Yoldas et al.[4], Liu et al.[5], and Bier et al.[1], and we can state that microcrack formation is affected by NiTi alloy from which the files are manufactured.
Taper angles of the files used in root canal preparation are effective on dentinal crack formation.[1] Studies have reported that increasing taper angles cause higher stress in the canal wall, and therefore more dentinal cracks are formed.[23] Of the files used in this study, Reciproc Blue and Hyflex EDM (size #25) have variable tapers from 0.08 at the tip to 0.04 in the coronal part, while One Curve has a 6% constant taper. As a result of the study, high-tapered Reciproc Blue produced statistically significantly more dentinal cracks in accordance with the literature. On the other hand, Hyflex EDM, which has the same taper as RB, was observed to produce the fewest dentinal cracks. The study conducted by Pedulla et al.[7], which is the only study in the literature comparing Hyflex EDM and RB, also provides results that support our study. We think that different effects of these two files on the formation of dentinal crack despite having the same tapers may be due to the fact that they have different kinematic properties.
There are a limited number of studies in the literature on the effectiveness of different kinematic motions on dentinal crack formation. In addition, these studies have reported conflicting results.[2],[4],[17],[18] Liu et al.[5] stated that the reciprocating motion caused less dentinal crack formation since it minimizes torsional and bending stresses. However, Bürklein et al.[24] reported that the risk of dentinal crack formation increased due to the increased torsional force of reciprocating systems combined with cutting edges. In this study, Reciproc Blue, which produces reciprocating motion, caused statistically significantly more dentinal cracks compared to One Curve and Hyflex EDM file systems producing rotary motion. We think that these conflicting results among the studies conducted may be caused by different methodologies such as periodontal ligament simulation, the use of instruments of different types and sizes and the use of gates-glidden drills. Liu et al.[5] did not cover the apical 3 mm of the roots with auto-polymerized acrylic resin and polyvinyl siloxane to simulate the periodontal ligament in their study reporting that the reciprocating motion produced less dentinal crack formation. We think that the difference from our study is due to periodontal ligament simulation.
Besides, Hyflex EDM and One Curve files evaluated in this study produced rotary motion and caused dentinal crack formation at the rate of 13.3% and 26.7%, respectively. Although they have the same kinematics, the reason for crack formation at different rates with these file systems is primarily because the recommended speed of Hyflex EDM is higher than that of One Curve. Therefore, we think that the reason why Hyflex EDM caused less dentinal crack formation compared to One Curve is that it has high cutting efficiency due to high speed.[20] In addition, studies have reported that the files with less taper reduces dentinal crack formation.[3],[25] We are of the opinion that One Curve files (constant taper from the apical to the coronal region) produce more dentinal cracks because of being more tapered compared to Hyflex EDM.
The 3 mm apical part of the root is considered to be the most critical region in terms of the risk of dentinal crack formation during root canal shaping.[18] In this study, more dentinal cracks were observed in the apical region compared to the middle and coronal regions with all tested files, and no statistically significant difference was found in accordance with previous studies.[26],[27] Observing more cracks in the apical region may be due to the features of the files such as taper angle, flexibility, and cross section.[18] Additionally, the use of a single-file system for root canal shaping causes more stress in the apical region, the most fragile part of the root. We speculate that more cracks were formed due to this situation. However, there are not enough studies in the literature comparing single- and multiple-file systems. Further studies are needed to be conducted.
Working time is not one of the most important factors while evaluating the effectiveness of the files used in root canal shaping. However, since it may be advantageous for clinical use, working time of all groups tested was recorded in this study. According to the results obtained in this study, a statistically significant difference was observed between the groups in terms of working time. The control group using manual files had the longest working time among all groups. In addition, when single-file systems were evaluated within themselves, One Curve performed root canal shaping in the shortest time, while Reciproc Blue performed root canal shaping in the longest time. However, no comparison can be made since there are no studies in the literature evaluating these file systems. Thus, single-file systems can be said to reduce working time compared to multiple-file systems.[28] This finding is consistent with previous studies.
One of the most important limitation of this study was that the sectioning method. It is possible that the defects existing in the root can spread to different levels of the root during cross-sectioning and can be counted as defects. In addition, although the steoromicroscopic observations used in such studies are reported to provide accuracy between 39% and 58%, when the results obtained in the studies are taken into consideration, it turns out that endodontic treatment poses a serious risk in patients. Therefore, it should be noted that the correlation of such studies with clinical reality is questionable.
Conclusion Within the limits of this study, all single-file systems tested caused dentinal cracks in the apical, middle and coronal regions of the root. In addition, fewer dentinal cracks were observed in the Hyflex EDM group compared to the One Curve and Reciproc Blue groups. Thus, it can be stated that Hyflex EDM single-file systems reduce trauma in the dentinal wall more than other systems. As a result, we think that the clinical use of Hyflex EDM single-file systems is an appropriate alternative.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
References 
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