The success of root canal treatment is dependent on cleaning and shaping the root canal system (RCS) as an important step. It also affects the efficacy of root canal irrigation and the quality of obturation. An instrumented canal should have a uniform conical shape with decreasing diameter from the coronal towards the apical region, providing adequate space for the activity of irrigating solutions [1]. Preserving the original canals and root structure is crucial for successful root canal preparation [2]. Instrumentation is more challenging in severely and abruptly curved root canals [3].

This is done by enlarging and shaping the canal to allow for adequate chemical debridement while preserving the radicular anatomy.

It is particularly difficult to instrument the apical third due to its complexity both in canal shape and in ramification [1, 4]. Regardless of the instrumentation technique, cleaning and shaping procedures inevitably lead to dentin removal from the canal walls. However, excessive dentin removal in a single direction within the canal rather than in all directions equidistant from the main tooth axis causes what is known as canal transportation [5, 6]. Apical transportation can cause the apical foramen to move from its anatomical position to an iatrogenic position in the root canal wall, causing further aggravation and causing treatment failure [7]. Apical transportation is responsible for impairing the root structure’s integrity, decreasing its fracture resistance, and leading to perforation of root canal walls when using larger files [5].

Innovative nickel-titanium (NiTi) instruments have been developed to enhance the efficacy of root canal treatment. The use of more flexible NiTi rotary files instead of stainless-steel hand files can reduce canal transportation and errors [8]. Improvements in mechanical properties of endodontic files focused on properties like cutting efficiency, flexibility, and fracture resistance.

Due to their higher flexibility, NiTi files can reduce the risk of procedural errors in narrow curved canals [9]. Thus, they can preserve the canal shape and decrease canal transportation. In NiTi files, deformation can be reversed up to 10%, while conventional alloys only have a maximum rate of 1%. Moreover, NiTi files have lower wear and deformation than stainless steel (SS) files and have excellent anti-corrosive properties as well [10].

The Neoniti (Neolix, Châtres-la-Forêt, France), which is introduced as a single file rotary system, is manufactured from the heat-treated wire alloy which provides these files with cyclic fatigue resistance and controlled memory, improving their canal preservation properties [11]. In addition, The electrical discharge machining (EDM) technology produces a rough surface, which improves their abrasiveness and cutting efficiency [6, 12, 13].

Neoniti contains a heterogeneous rectangular cross-section and multiform tapers in a single file that contains a C1 file as an orifice shaper (with size 25#, taper at the tip of 0.12 mm and length of 15 mm) and three A1 (with #20, #25 and #40 tip sizes) instruments [14].

Various techniques are employed to evaluate the effectiveness of NiTi files in preserving the original path of the central canal. These methods include radiography, sectioning as per Bramante’s technique, longitudinal tooth clearing, high-resolution computed tomography (CT), micro-CT, and cone-beam CT (CBCT) [15–17]. CBCT is an effective method for measuring dentine thickness, canal transportation, and centering ability used in many recent studies [18, 19]. CBCT is known for its high-resolution imaging capabilities, making it ideal for assessing root canal morphology, detecting fractures, and observing alterations in the RCS after preparation [20].

In this study, CBCT was chosen for its non-invasive nature, allowing for precise and repeatable three-dimensional evaluations at a smaller field of view of the RCS without causing damage to the specimens [7]. On the other hand, information regarding apical transportation and centering ability following root canal instrumentation with Neoniti A1 with/without Neoniti C1 is limited; therefore, this study’s objective was to evaluate these two features in curved root canals using CBCT.

This study compared the apical transportation and centering ability of curved canals instrumented with Neoniti A1 single-file system with/without pre-flaring with Neoniti C1 using CBCT. The mesial and mesiobuccal roots of molar teeth with an apical size corresponding to a #10 K-file were used in this study to better simulate the use of rotary files in the clinical setting; because in oval canals, the file is engaged with the canal wall in narrower diameter of the canal, and does not engage with all root canal walls equally [23]. The present results indicated that both groups showed some degree of apical transportation. Canal transportation was lower in the C1 + A1 group compared with the A1 group at all levels, although the difference was not statistically significant. In other words, coronal flaring of the canals before instrumentation had no significant effect on the occurrence or amount of apical transportation. Although the C1 + A1 group showed superior centering ability at 3 mm from the apex.

Glossen et al. [24] reported similar results. They manually prepared the root canals by the balanced force technique and reported that although primary flaring enhanced root canal instrumentation, it had no significant effect on canal transportation [24]. Uzunoglu and Turker [25], Fallatah and El Sherief [26], and Türker and Uzunoğlu [27] found no significant difference in canal transportation between different groups at different levels from the apex, which was in line with the present results. Zanette et al. [18] used the ProTaper Next file with and without creating a glide path and found no significant difference in the amount of canal transportation between the two groups.

Due to the advantages of the Neolix files, such as high flexibility, having a non-cutting tip, positive rake angle, and lower screw-in forces (which is an unfavorable property of rotary files especially in high tapers), canal transportation in both groups in the present study was < 0.03 mm. The greater the amount of canal transportation from 0.03 mm, the poorer the prognosis of treatment would be. Uzunoglu and Turker [25] used One Single Shape and ProTaper Next rotary systems and showed that canal transportation was < 0.03 mm in both groups, which supported the present findings.

In the present study, the minimum amount of canal transportation was recorded at 5 mm (0.0032), 7 mm (0.0074), and 3 mm (0.0110) from the apex in the C1 + A1 group. In the present study, the amount of canal transportation at 5 mm from the apex in the A1 group (0.0445) was higher than that in the C1 + A1 group (0.0032), which indicates greater transportation at the danger zone in the A1 group. It should be noted that most NiTi instruments are used with the crown-down technique such that the thicker part of the file is engaged in the coronal part of the canal before reaching the apical region, and interferences are eliminated as such. Accordingly, the risk of canal transportation decreases. However, the file fatigue increases as such, and the clinical service of the file is compromised. To overcome this problem, it may be useful to use lubricants for real-time torque generated during intracanal instrumentation [28].

The glide path and coronal flaring were advised to use especially in curved root canals to reduce procedural errors such as ledging and file breakage, in contrast, excessive removal of dentin during coronal flaring may result in weakening the root structure, strip perforation, predisposing teeth to fracture, and treatment failure [21].

AbuMostafa et al. [29] concluded that glide path and coronal flaring had an insignificant effect on the dentin volume removal and percentage of untouched walls in curved canals. Hawi et al. [30] reported that coronal flaring could be clinically useful in increasing the centering ability, especially of the austenitic files in the apical third.

According to the present results, the centering ratio was higher in the C1 + A1 group than in the A1 group at all levels from the apex but this difference was only significant at the 3 mm level (P = 0.013). Also, the maximum centering ratio was noted in the C1 + A1 group at the apex (0.37).

Selvakumar et al. [19] compared the centering ratio in three groups K3 file with 0.02% taper, K3 file with 0.04% taper, and SS K file. They reported a higher centering ratio in group 1 and at the apex (1 ± 0.22 mm). Despite the use of different files and methodologies, their results were under the present findings. Schäfer and Zapke [31] reported that the centering ratio of K3 files was much higher than that of K FlexoFiles. Canoglu et al. [23] found no significant difference in the centering ratio of rotary and hand files in any part of the root, which was different from the present results.

A study by Yamamura et al. [32] does not support the use of one rotary file system over the other when comparing transportation and centering ability.

The current study found no significant difference in the mean dentin thickness before instrumentation, the amount of removed dentin, and the final dentin thickness of the mesial and distal canal walls between the two groups at any level from the apex. This finding was in line with that of Mohammadzade Akhlaghi et al. [33], and Garala et al. [34]. In the present study, the maximum amount of residual dentin in both groups was noted at the distal wall at 7 mm from the apex. So, it could be suggested to use this single-file system to shape the curved canals, decrease the incidence of instrument separation, and straighten canal curvature, changing in wall and time-saving [14].

Use of CBCT, evaluation of natural teeth (which increases the generalizability of the results to the clinical setting), and use of white dental stone and sawdust for mounting of the teeth (for repeatability and simulation of the jawbone on CBCT scans) were among the strengths of this study. CBCT allows multiple imaging before and after root canal instrumentation although the resolution of this tool is lower than the one of micro-CT [35]. In addition, the micro-CT imaging technique, despite its higher resolution, is well known to be time-consuming and expensive, especially for a high number of samples [36]. Two approaches are utilized to measure canal transportation using CBCT scans. Some research has involved overlaying pre- and postoperative images to identify alterations in the canal path resulting from the preparation process [37]. Other studies have calculated the distance between the external surface of the root and the internal canal wall at both the mesial and distal sides using pre- and postoperative cross-sectional images at three specific levels. Relevant formulas were then applied to quantify the changes in the canal path [38]. Our study adopted this latter method, measuring the distance from the external root surface to the internal root canal wall at three points (1 mm, 3 mm, and 7 mm from the apex) to assess changes in the apical and middle thirds of the root canal, as these areas are more prone to procedural errors.

Despite the attempts to standardize the teeth, extracted teeth cannot be perfectly standardized in terms of canal size and root curvature, which was a limitation of the present study. Further studies are required to compare different rotary files and also with hand files regarding apical transportation with/without pre-flaring.

Many factors can affect shaping ability including instrument size and taper, as well as the design and type of alloy [30]. Also, anatomical complexities need to be evaluated in clinical conditions [39]. As well as evaluation of cyclic fatigue with and without coronal flaring that impacts on life span of the files [40]. Further studies (in vitro and in vivo) are warranted to confirm and generalize these results. In a systematic review aimed at answering the question of whether the use of martensitic root canal instruments could result in less apical transport compared to austenitic instruments during root canal therapy, they concluded that martensitic root canal instruments had less apical transport than austenitic instruments during the shaping phase of root canal therapy, and this behavior could be attributed to the increased flexibility of martensitic instruments. Although, they considered that further research is necessary to provide additional evidence to support the use of martensitic instruments in clinical practice [41].

Another limitation of this study is the limited sample size. Conducting the study with a large sample size is needed to confirm the study results.