Mandibular first molars with immature apices extracted for purposes other than this study e.g. periodontal disease and extensive caries were collected. In the surgical ward, before extracting a tooth, the patient completes and signs an informed consent form stating that the extracted tooth will be used in research.The study was approved in the ethics committee of Hamadan University of Medical Sciences (IR.UMSHA.REC.1397.464). Periapical radiographs with photostimulable phosphor plate sensor (Optime, Soredex, Tuusula, Finland) were obtained of the teeth using MinRay intraoral digital radiography system (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 teeth that met the eligibility criteria were selected.
The curvature angle was measured using the Schneider’s method . In order to measure the root curvature, Scanora software (Soredex, Tuusula, Finland) was used. 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. Teeth with mesial root curvature between 10° to 30° in the mesiodistal plane were enrolled and were divided into two groups with similar degree of curvature (Fig. 1).
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 measured from the occlusal reference point to 1 mm shorter of the length of #10 K-file when its tip was visible at the apex. Next, the crowns were shortened such that the working length was standardized at 19 mm. In order to enhance the radiography and for CBCT images reproducibility, the teeth were mounted in acrylic resin blocks.
All teeth were scanned prior to canal preparation using Cranex 3D CBCT system (Soredex, Tuusula, Finland) with the exposure settings of 90 kVp, 10 mA and 12 s time. Next, a 15 K-file was used for preparation of a glide path. All canals were prepared by the same operator using an endodontic electric motor (X-Smart Plus motor; Dentsply Maillefer, Ballaigues, Switzerland) according to the manufacturer’s instructions for the speed and torque of 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 . 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 working length, up-and-down motions were repeated five times to 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 were subjected to CBCT with the same exposure settings as the CBCT prior to instrumentation.
OnDemand 3D Dental software (Cybermed, Seoul, South Korea) was used to measure the mesiobuccal canal before and after root canal preparation and the degree of root canal transportation and centering ratio were measured at 3, 4 and 5 mm distance from the apex. The shortest distance between canal wall and external root surface in the mesial, distal, buccal and lingual was measured for the mesiobuccal canal. Measurements were made on CBCT scans taken before and after instrumentation as follows :
The degree of canal transportation at each level was determined using the following formula:
(m1-m2)-(d1-d2) in the mesiodistal plane and (b1-b2)-(l1-l2) in the buccolingual plane,
Where d1 is the shortest distance between the distal margin of the rootand the distal margin of un-instrumented canal, d2 is the shortest distance between the distal margin of the root and the distal margin of instrumented canal, m1 is the shortest distance between the mesial margin of root and the mesial margin of un-instrumented canal, m2 is the shortest distance between the mesial margin of root and the mesial margin of instrumented canal, I1 is the shortest distance between the lingual margin of root and the lingual margin of un-instrumented canal, I2 is the shortest distance between the lingual margin of the root to the lingual margin of instrumented canal, b1 is the shortest distance between the buccal margin of the root and the buccal margin of un-instrumented canal and b2 is the shortest distance between the buccal margin of the root and the buccal margin of instrumented canal.
The answer of 0 in the aforementioned formula indicates absence of apical transportation.
The canal centering ratio was determined at each level using the following formula:
(m1 − m2)/(d1 − d2) or (d1 − d2)/(m1 − m2)in the mesiodistal plane and (b1 - b2) / (l1-l2) or (l1-l2) / (b1-b2) in the buccolingual plane.
In this formula, smaller values are placed in the numerator and the answer of 1 indicates excellent centering ability (Figs. 2A,B).
The mean and standard deviation values were calculated for both groups. Independent t-test was used to find significant differences between the two groups. P < 0.05 was considered as significant.