Root canal treatment aims to eradicate tissue debris as well as microorganisms from the root canal system. Chemomechanical preparation (CMP) achieved through instrumentation provides a canal with a larger diameter, smoother walls, and optimal apical size to allow copious irrigation along with 3-dimensional obturation. CMP is critical for successful endodontic treatment outcome. ¹ The evolution of file systems over the years from stainless steel hand files system to NiTi rotary file system has made CMP effortless and predictable. Normally while preparing the root canal, the multiple NiTi files of different diameters and taper are used to gradually enlarge the root canal. In the preparation of the root canal, the advent of the single file system (SFS) is an exciting all-new concept. In SFS, only one file is required to prepare the canal to an adequate size and taper, even in narrow and curved canals. In addition to shaping the canal, SFS reduces the working time, lowers cross-contamination, and reduces the instrument fatigue without compromising the cutting efficiency when compared to multiple file systems. SFS is developed for shaping the vast majority of canals, regardless of their length, diameter, or curvature. ² , ³ Neolix Neoniti (NEOLIX, Chatres-la-Foret, France), OneShape files (Micro-Mega, Besanc¸on Cedex, France), OneCurve (Micro-Mega Company) are some of the recently introduced SFSs with continuous rotation used for CMP. ²

CMP of root canal induces stresses on dentin and causes microcracks and craze lines into the root dentin. Propagation of microcracks and craze lines over a period of time may lead to vertical root fracture (VRF) which generates serious endodontic complication rendering the tooth for extraction. ⁴ , ⁵ Introduction of scanning electron microscope (SEM) has proved to be a valuable method for the assessment of the ability of endodontic procedures to remove debris from root canals, thus enabling comparison of instruments and instrumentation techniques. SEM images have been used to evaluate the effects of preparation methods on root canal surface, cleaning efficacy of various root canal instruments, and formation of dentinal defects in endodontic practice. ⁶ Khoshbin et al. found that Neolix caused significantly least number of cracks when compared with Mtwo and ProTaper file system. ⁴ According to Das et al., the incidence of crack observed in root dentin was greater after instrumentation with OneShape as compared to HEDM and ProTaper Next.

To the best of the author's knowledge, the literature lacks data on the effect of above mentioned three SFSs on dentin. Hence this study was conceptualized to compare dentinal crack formation in root canal walls following instrumentation with Neolix NiTi, One Shape, and OneCurve SFSswith continuous rotation, to the conventional Hand File system under SEM.

This SEM study was performed after Institutional Ethical Committee (IEC) clearance (CDCRI/DEAN/ETHICS COMMITTEE/ENDO/PG-01/19). Freshly extracted, single-rooted, and single canalled 80 mandibular first premolars were collected. Teeth with apical foramen no larger than size #15 K-file with a maximum root curvature of 25°C were included in the study. Teeth with internal or external root resorption, external surface cracks (observed under ×25), root caries, open apex, or canal calcification were excluded (n = 10). The teeth were cleaned with an ultrasonic scaler and then disinfected using 2.5% sodium hypochlorite. They were stored in distilled water to prevent dehydration throughout the study. The crowns of all teeth were decoronated such that the remaining standardized root length is 17 mm that enabled straight-line access to the canal Figure 1. A silicon impression material was used for coating the external root surface so as to simulate periodontal ligament space. Care was taken to instrument the root canals immediately after setting of polyvinyl siloxane so as to avoid loss of properties of the impression material which is simulating the periodontal ligament Figure 2a. All roots were subsequently embedded in acrylic blocks Figure 2b. All samples were randomly divided into 3 experimental groups (n = 20 for each group) and 2 control groups (n = 5 for each group) as follows: Figure 1

Crowns of all teeth were de-coronated so as to maintain the standardized root length as 17 mm.

Evaluation of crack formation at various levels revealed that apical third (P = 0.012) and coronal third (P = 0.002) showed significant incidence of crack formation compared to middle third (P = 0.46) in all 5 groups Table 1. In Group I, 14 (70%) samples had a crack in the coronal third; however, there was no statistically significant difference in the incidence of crack at all 3 levels (P = 0.12). In One Shape file group, maximum cracks were seen in apical third (11 [55%]) but it was nonsignificantly different from other levels [Graph 1]. Moreover, in Group III, there was a statistically significant difference in crack formation with the highest incidence at the middle third (P = 0.042).{Table 1}[INLINE:1]

Root canal preparation using endodontic files is frequently associated with the formation of some dentinal defects. ⁷ Most of the NiTi instruments with different designs result in incomplete cracks, craze lines, or even VRF, and such defects should be prevented as it necessitates tooth extraction. ⁵ , ⁸ A significant association exists between the amount of dentin removed and crack formation, and excessive widening of the canal increases the risk of VRF. ⁷ Even in the absence of VRF, the presence of cracks can compromise the outcome of endodontic treatment. ¹ The current study assesses the frequency of microcrack formation using 3 different SFS with continuous rotation and conventional Hand File system with the help of SEM. SFS with continuous rotation was used because of their economic viability, unique design, and presterilized usage with zero cross-contamination. ⁹

Mandibular premolars were selected for the study because of the high prevalence of VRF as reported by Tamse et al. ¹⁰ Acrylic blocks and a silicone impression material were used to simulate bone and periodontal ligament respectively. Periodontal ligament simulation acts as a major stress absorber and sways the outcome of such studies. ¹¹ In this study, the apical master file was standardized using a file with a tip diameter equivalent to size 25. After canal preparation, all file systems showed dentin crack formation which was in accordance with previous studies. ¹¹ , ¹² In this study, OneShape file showed maximum number of dentinal cracks in the apical third (11 [55%]) and minimum in the middle third (4 [20%]). Das et al. reported similar results in their study. ¹³ It has a triangle cutting edge in the apical part, 2 cutting edges in the coronal part, and a cross-section that progressively changes from 3 to 2 cutting edges between the apical and coronal parts. ¹³ This design may affect shaping forces on root dentin these forces may cause root fracture. ¹⁴ This could be the probable reason for more cracks in the apical region. No statistically significant difference (P = 0.070) was seen between crack formation at apical, middle, and coronal third for OneShape. Neolix NiTi showed maximum cracks in the coronal third when compared to other file systems which was in accordance with Harandi et al. ⁵ The file is generated using a wire-cut electrical discharge machining process and has rectangular non-similar cross-sections all along its length this file has larger taper (0.08) and increased rotational speed (500 rpm) when compared to the other two file system which generate more stress thus causing dentin damage as suggested by Bier et al. ⁸ This result was contradictory to the results of Harandi et al. and Elham et al. ⁴ , ⁵ No statistically significant difference (P = 0.12) was seen between crack formation at apical, middle, and coronal third for Neolix NiTi.

OneCurve showed maximum cracks in the middle third and minimum in the apical third. OneCurve files are composed of a NiTi alloy that undergoes a patent protected heat treatment (C. Wire), which provided a shape-memory effect. It has the same tip size (size 25) and the constant taper (0.06) of their predecessors but has a different shape design. 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. ¹⁵ The fie design could be the reason for more cracks in the middle third. It was seen that the experimental groups showed more crack formation than positive control which was in accordance with previous studies that stated rotary instruments cause more dentin damage. ¹⁶ , ¹⁷ Significant difference was seen (P = 0.042) between crack formation at apical, middle, and coronal third for OneCurve. A significant difference was seen in the apical third (P = 0.012) and coronal third (P = 0.002) when comparing all file systems.

It should be noted that this study had an in vitro design, though periodontal ligament was simulated using polyvinyl siloxane impression material, it doesn't provide the exact intraoral environment. As the extracted tooth becomes brittle, the incidence of a crack in such teeth increases. Thus, the generalization of results to the clinical setting must be done with caution. Future clinical studies are required to obtain more reliable results.

Within the limitation of this study, it can be concluded that all file systems produce some amount of crack. There was a significant difference in crack formation in apical and coronal third. OneCurve showed a minimal incidence of crack when compared to Neolix Niti and OneShape. Neolix NiTi showed the maximum number of cracks. OneCurve file system can be a choice for canal preparation over Neolix Niti and OneShape in such oval canals.

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Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.