Forty-eight simulated L-shaped canals were used in this study (Endo Training Block-S; Dentsply Maillefer, Ballaigues, Switzerland, with 0.02 taper, 0.15 mm apical diameter, 17 mm length, and 40 curvature). The patency of the canals was confirmed by passing a size 10 K-file just beyond the apex, and the unity in the angles and length of the curvatures were confirmed before distribution of the blocks by taking pictures of the samples on a photography stand. After assuring that the samples are standard, they were randomly divided into three groups (n =16 canals/group) and were numbered.
Instrumentation of L-shaped canals
A new instrument was used for each canal in all groups. Glyde-Prep (Dentsply Maillefer, Ballaigues, Switzerland) was used as a lubricant before using each instrument, and saline was used for irrigation during preparation. The canals were instrumented using the protocols suggested by the manufacturer described in the following sections without glide path preparation or additional use of hand files except for recapitulation with a size 10 K-file.
The OneShape file (tip size, 25; apical taper, 0.06) was used in a full clockwise rotation generated by an X-Smart motor (Dentsply Maillefer, Ballaigues, Switzerland), and the speed and torque were adjusted to 400 rpm and 4 Ncm. The files were used in a slight pecking motion according to the manufacturer’s instructions. The flutes of the instrument were cleaned after each retrieval of the file from the simulated canal.
The Hero Shaper files were used following the yellow sequence with file size 25 taper 0.6 used as a modification to the protocol to achieve the same apical size and taper of prepared canals as that of other groups. According to the manufacturer's torque guide, the motor was set at the speed of 400rmp with the torque of 1 to 2 set for each instrument. The instrument sequence was as follows:
- A 0.06 taper size 20 instrument for 2/3 of the WL.
- A 0.04 taper size 20 instrument for the full WL.
- A 0.04 taper size 25 instrument for the full WL.
- A 0.04 taper size 30 instrument for the full WL.
The Revo-S files were used up to the size 25 and taper of 0.06 in a full clockwise rotation with a rotational speed of 400 rpm generated by the X-Smart motor (Dentsply Maillefer, Ballaigues, Switzerland), and the torque was adjusted to 2 Ncm. The files were used in a slight pecking motion according to the manufacturer’s instructions. The instrument sequence was as follows:
- A 0.06 taper size 25 instrument (SC1) 2/3 of the WL.
- A 0.04 taper size 25 instrument (SC2) for the full WL.
- A 0.06 taper size 25 instrument (SU) for the full WL.
An experienced operator prepared all canals, and a total of 48 L-shaped canals were prepared. Canals were irrigated during preparation by using saline. A new instrument was used to prepare four canals only, and the flutes of all instruments were cleaned after retrieval of the instruments from the canals during instrumentation or after three pecks.
Image analysis and assessment of canal preparation
All canals were injected with black ink (Parker Quink, Parker, France) to obtain a clear pre-operative image. The canals were photographed using a digital camera (Sony Alpha DSLR-A100 camera with DSLR-A100 macro lens, Sony, Japan) on a fixed stand with constant settings. The canals were rinsed with saline before and after instrumentation. The canals were subsequently filled with red ink (Parker Quink, Parker, France) and were photographed again under identical conditions.
The pre-and post-instrumentation images were superimposed into a composite image using a computer software program (Adobe Photoshop Elements 7.0, Adobe Systems Incorporated, San Jose, CA, USA). A measuring template was superimposed on the composite images. The amount of resin removal due to instrumentation was measured using ImageJ 1.46r software (Wayne Rasband, National Institutes of Health, USA) perpendicularly to the surface of the canal at 22 measuring points (11 on each side of the canal). The measurement points (MP) were arranged in 1-mm steps: points 0 corresponded to the canal's endpoint, 7 to the beginning of the curve, and points 7 to 10 belonged to the straight portion of the canal. A second examiner who was blinded to all experimental groups carried out the canal shapes' assessments before and after instrumentation.
The canal preparation time, which included total active instrumentation, cleaning of the instruments' flutes, and irrigation, was recorded. The amount of time spent to change the files or adjust stoppers was excluded to facilitate the files' efficacy. Canal aberrations were determined by two clinicians blinded to the canal preparation instruments by using composite images. Assessments were performed based on an apical zip, narrowing, ledge, and the danger zone. The canal aberrations were defined according to Ersev et al.
Statistical evaluations were performed with SPSS software (IBM SPSS Statistics 21, SPSS Inc., Chicago, USA). The normality of the data was verified for each set of measurements by using the Kolmogorov-Smirnov test. The results were statistically analyzed using one-way analysis of variance (ANOVA) and the posthoc Tukey test. ANOVA and the posthoc Tukey were also used to analyze the preparation times, and the Chi-square test was used to analyze the incidence of canal aberrations. The significance level was set at P<0.05.