Simulated curved canals made of clear polyester resin (Endo Training Block 02 taper, REFA 0177; Dentsply Maillefer, CH-1338 Ballaigues, Switzerland) with 35°. The diameter and the taper of all simulated canals were equivalent to an ISO standard size 15 root canal instrument. Canals were 17 mm long, the straight part being 12 mm and the curved part 5 mm. Prior to instrumentation, the specimens were divided into three experimental groups (n = 10) and were drilled on one side with a diamond bur to ensure repositioning accuracy in subsequent superimposition of the pictures and a coloring solution (Caries Marker, coloured caries indicator, VOCO, Cuxhaven, Germany) was injected into the canals.
The Blocks were placed with a black background in a reproducible position and the simulated canals were prepared with any of the three systems: AK, GTX and TF.
Pre- and post-instrumentation canal pictures were taken in a standardized manner using a digital camera EOS 400 Digital (Canon Inc., Tokyo, Japan) with a macro-objective “Tamron SP AF 60 mm F/2 Dill Macro 1:1” (Tamron Co., Ltd., Saitama, Japan) and stored directly in a computer.
The instruments were set into permanent rotation with a 6:1 reduction hand-piece (Sirona, Germany) powered by a torque-limited electric motor VDW Silver (VDW, Germany). The individual torque limit and rotational speed of each file which recommended by the manufacturers were entered and stored manually by the operator in the Dr’s Choice program.
FileCare (EDTA, VDW, München, Germany) was used as lubricant, and a total of 5 ml water was used repeatedly after the use of each instrument. Each instrument was used to enlarge one canal only. All of the canals were enlarged by the same operator who was experienced with all three systems. Once the instrument had achieved to the end of the canal and had rotated freely, it was removed.
The following instrumentation sequences were used with the different systems:
TF instruments were used in a crown-down manner at a speed of 500 rpm as recommended by the manufacturer. A small assorted pack (25/.08, 25/.06, and 25/.04) was used. The preparation sequence was as follows: a 15, K-File was used to create a guide path; an 8% taper, size-25 instrument was used at (11 mm); a 6% taper, size-25 instrument was used at 14 mm; and a 4% taper, size-25 instrument was used at the full WL (17 mm).
GTX instruments were used in a crown-down manner at a speed of 300 rpm as recommended by the manufacturer. The preparation sequence was as follows: a 15, K-File was used to create a guide path; a 6% taper, size-20 instrument was used at (11 mm); a 4% taper, size-20 instrument was used to the full WL (17 mm).
AK instruments were used in a crown-down manner at a speed of 250 rpm as recommended by the manufacturer. The red assorted pack (25/.06, 25/.04, and 25/.02) was used. The preparation sequence was as follows: a 15, K-File was used to create a guide path; a 6% taper, size-25 instrument was used at (11 mm); a 4% taper, size-25 instrument was used at 14 mm; and a 2% taper, size-25 instrument was used at the full WL (17 mm).
Assessment of canal preparation and analysis of data
Assessment of canal curvature modifications was carried out with the image analysis software (GSA Image Analyser Software development and Analytics Bansemer and Scheel GbR, Germany). A composite image of each canal was produced using the software from the pre- and final post-instrumented images. The area between canal configuration before and after instrumentation (material removed by instrumentation) was determined both for the inner and outer curvature using the Image Analyser program. Ten concentric circles spaced 1 mm apart were sectioned the composite image with their centers targeted over the apical end of the pre- instrumented canal, i.e. a radius of the first circle was 1 mm from the apical point of the canal and a radius of the last circle was 10 mm from the apical point. This resulted in a total of 20 segments (10 segments of the outer curvature and 10 segments of the inner curvature). The segments of all canals (material removed) were measured automatically with the GSA Image Analyser program in two dimensions as a surface area (mm2).
The cutting efficiency of instruments (the total amount of material removed at both the inner and outer canal walls) was evaluated in three parts of the root canal starting from apex: apical part which is the most curved part of the canal (segments 1–4), middle part (segments 5–7) and coronal part (segments 8–10).
Furthermore, based on the composite images, assessments were made according to the presence of different types of canal aberrations, such as apical zip, elbow, ledge and perforation. The canal aberrations were defined according to Thompson & Dummer .
After preparation of the blocks, all instruments were examined under a microscope with 15 × magnifications (Carl Zeiss OPMI Pro Ergo, Germany) for signs of deformation.
After preparation, canal length was measured using an ISO size-15 stainless steel hand K-file and Endo gauge. The K-file was placed in the canal and the length that it reached was marked by adjusting the rubber stop of the file to the upper surface of the resin block which served as reference surface. The change of working length was determined by subtracting the canal length after preparation from the original canal length (17 mm). The time for canal preparation including the total active instrumentation, instrument changes within the sequence, photography and irrigation was recorded.
Wilcoxon test was used to compare the material removed from the inner and outer canal walls of one group. To compare canal transportation among the groups, cutting efficiency and working time, Kruskal- Wallis and Mann–Whitney U-tests were used at a confidence interval of 95%) [SPSS, version 19.0 (IBM Corporation, USA)].