Mandibular first molars with mature apices extracted for purposes other than this study e.g. periodontal disease and extensive caries were collected. All patients consented to the use of their extracted teeth for research purposes prior to extraction at the department of oral and maxillofacial surgery, and signed an informed consent form for this purpose.The study protocol was approved by the ethics committee of Hamadan University of Medical Sciences (IR.UMSHA.REC.1397.464).
Periapical radiographs were obtained from the teeth using MinRay intraoral digital radiography system (Soredex, Tuusula, Finland) and photostimulable phosphor plate sensor (Optime, Soredex, Tuusula, Finland). Teeth with calcification, internal or external root resorption, root fracture, severe curvature, or curves in two different directions were excluded. Eventually, 44 extracted human teeth that met the eligibility criteria were selected.
The angle of curvature was measured using the Schneider’s method [15]. The Scanora software (Soredex, Tuusula, Finland) was used in order to measure the root curvature. For this purpose, a line was drawn along the longitudinal axis of the tooth. A second line was drawn from the apical foramen to the first point of curvature. The angle formed between the two lines was measured by a caliper, and the curvature angle was determined as such. Teeth with mesial root curvature between 10° to 20° in the mesiodistal plane were enrolled and divided into two groups with similar degree of curvature.
The teeth were disinfected with 5.25% sodium hypochlorite. Access cavity was prepared using a #4 high-speed round carbide bur (Dentsply Maillefer, Ballaigues, Switzerland). A #10 K-file (Dentsply Maillefer, Ballaigues, Switzerland) was introduced into the mesiobuccal canal until the file tip was visible at the apex. Next, the working length was determined as the distance between the occlusal reference point and 1 mm shorter of the length of the #10 K-file when its tip was visible at the apex. Next, the teeth were decoronated such that the working length was standardized at 19 mm. The teeth were then mounted in acrylic resin blocks in order to enhance radiography and ensure the reproducibility of CBCT images.
All teeth were scanned prior to canal preparation using Cranex 3D CBCT system (Soredex, Tusuula, Finland) with the exposure settings of 90 kVp, 10 mA and 12 s time. Next, a 15 K-file was used for glide path preparation. All canals were prepared randomly by the same operator using an endodontic electric motor (X-Smart Plus motor; Dentsply Maillefer, Ballaigues, Switzerland) according to the manufacturer’s instructions regarding the speed and torque of the files.
In group A (n = 22), the root canals were prepared using ProTaper Universal rotary system (Dentsply Maillefer, Ballaigues, Switzerland). First, SX and then S1 and S2 files were used to flare the orifice and coronal and middle thirds of the mesiobuccal canal in order to create a straight-line access. Next, the canals were prepared by F1 and F2 files to the working length.
In group B (n = 22), XP-endo Shaper (Dentaire, La Chaux-de-Fonds, Switzerland) was used for root canal preparation. In order to simulate the body temperature, the teeth were kept in water at 37 ± 1 °C during root canal preparation [7]. The file tip was introduced into the canal and then the instrument was activated during rotation, and long and slow up-and-down movements were performed. When the file reached to the working length, up-and-down motions were repeated five times to the working length and then the file was removed from the canal while rotating.
After using each file in both systems, recapitulation was performed using a #15 K-file, and the canals were rinsed with saline. Each file was used for preparation of four canals. Next, the samples underwent CBCT with the same exposure settings as those applied prior to instrumentation.
The OnDemand 3D Dental software (Cybermed, Seoul, South Korea) was used to measure the degree of root canal transportation and centering ratio at 3, 4 and 5 mm distance from the apex of the mesiobuccal canal before and after root canal preparation. The shortest distance between the mesiobuccal canal wall and the external root surface in the mesial, distal, buccal and lingual was measured and recorded. The measurements were made on CBCT scans taken before and after instrumentation as follows [16]:
The degree of canal transportation at each level was calculated using the following formula:
$$\left( {{\text{m}}1 - {\text{m}}2} \right) - \left( {{\text{d}}1 - {\text{d}}2} \right)\,{\text{in}}\,{\text{the}}\,{\text{mesiodistal}}\,{\text{plane}}\,{\text{and}}\,\left( {{\text{b}}1 - {\text{b}}2} \right) - \left( {{\text{l}}1 - {\text{l}}2} \right)\,{\text{in}}\,{\text{the}}\,{\text{buccolingual}}\,{\text{plane}},$$
where d1 is the shortest distance between the distal margin of the root and the distal margin of the un-instrumented canal, d2 is the shortest distance between the distal margin of the root and the distal margin of the instrumented canal, m1 is the shortest distance between the mesial margin of the root and the mesial margin of the un-instrumented canal, m2 is the shortest distance between the mesial margin of the root and the mesial margin of the instrumented canal, I1 is the shortest distance between the lingual margin of the root and the lingual margin of the un-instrumented canal, I2 is the shortest distance between the lingual margin of the root and the lingual margin of the instrumented canal, b1 is the shortest distance between the buccal margin of the root and the buccal margin of the un-instrumented canal, and b2 is the shortest distance between the buccal margin of the root and the buccal margin of the instrumented canal.
The answer of 0 in the aforementioned formula indicated absence of apical transportation.
The canal centering ratio was calculated at each level using the following formula:
$$\begin{aligned} & \left( {{\text{m}}1 - {\text{m}}2} \right)/\left( {{\text{d}}1 - {\text{d}}2} \right){\mkern 1mu} {\text{or}}{\mkern 1mu} \left( {{\text{d}}1 - {\text{d}}2} \right)/\left( {{\text{m}}1 - {\text{m}}2} \right){\mkern 1mu} {\text{in}}{\mkern 1mu} {\text{the}}{\mkern 1mu} {\text{mesiodistal}}{\mkern 1mu} {\text{plane}}\,{\text{and}} \\ & \quad \left( {{\text{b}}1 - {\text{b}}2} \right)/\left( {{\text{l}}1 - {\text{l}}2} \right){\mkern 1mu} {\text{or}}{\mkern 1mu} \left( {{\text{l}}1 - {\text{l}}2} \right)/\left( {{\text{b}}1 - {\text{b}}2} \right){\mkern 1mu} {\text{in}}{\mkern 1mu} {\text{the}}{\mkern 1mu} {\text{buccolingual}}{\mkern 1mu} {\text{plane}}. \\ \end{aligned}$$
In this formula, smaller values were placed in the numerator and the answer of 1 indicated excellent centering ability (Fig. 1a, b).
The images were assessed twice by an expert oral and maxillofacial radiologist and a dentist separately with a 2-week interval. Both observers were blinded to the group allocation of samples. They were both allowed to adjust the brightness, contrast, and magnification of images to obtain the best visual results. The mean and standard deviation values were calculated for both groups. The Kolmogorov–Smirnov two-sample test was applied to analyze the normal distribution of data. Since the data were normally distributed, independent t-test was used to compare the two groups. P < 0.05 was considered statistically significant.