The present study found that the percentage of root canal filling removal, time consumed, and complications were significantly different between the file groups. Based on these results, the null hypotheses were rejected.
In the present study, the angle of curvature in the original root canals ranged from 21°–29° with a mean of 23.5°. This corresponds to the average angle of curvature in mesial root canals of mandibular molars (23°–27°) [14, 15]. Thus, our specimens can be considered as the typical mesial root canals in mandibular molars that would be encountered in the clinic.
Retreatment in curved root canals studies usually reported the original canal curvature of their specimens. However, a previous study found that the angle of curvature in the root canal was significantly reduced up to 3% after instrumentation [16]. The angle of curvature in the root canals in the present study was also reduced 5.9% after instrumentation. Therefore, the present study reported the angle of curvature in the root canals after instrumentation and obturation that were between 21.95°–22.25° with a mean of 22.1°. The percentages of root canal filling removal in the coronal- and middle-thirds in this study were high at 97–99%. The ratio of the coronal- and middle-third volumes to the whole canal volume was 89%, therefore, the percentage of root canal filling removal in the whole canal ranged from 93–98%. The coronal- and middle-thirds of the curved root canals are usually straight and large, thus the rotary file can operate easily in these thirds. Our findings were higher than previous studies that found that the percentage of root canal filling removal in the coronal- and middle-thirds ranged from 78–96% [4, 17]. The differences might be caused by two factors. First, the previous studies’ specimens had a greater mean angle of curvature (35.5°–42.5°). Second, our study inspected the cleanliness of the root canal using a DOM during retreatment. Thus, we encourage the use of a DOM for inspecting the remaining root canal filling after retreatment.
The percentage of root canal filling removal in the apical-third ranged from 56–93% that was 6–42% significantly less than the coronal- and middle-thirds. These findings corresponded to those of previous studies in which the percentage of root canal filling removal in the apical-third was 68–84% [4, 17]. The significant difference in the percentage of root canal filling removal in the apical-third resulted from a 37% difference between the X2c and VR groups. Because the apical part of the curved root canal is usually curved and small. Therefore, the percentage of root canal filling removal in the apical part of the curved root canal can be an indicator for the efficacy of NiTi rotary files and motions.
Many previous studies used motor running time to compare the rotary systems [5, 18, 19]. This is because the motor running time reflects the time that the instrument is active inside the canal. However, the motor running time and total time in the experimental groups ranged from 30–97 sec and 89–297 sec respectively. The specimens required less than 5 min to perform the retreatment procedures, and the differences between the groups might not be clinically meaningful.
The present study found instrument complications in the VR and X2a groups. There were file deformations and separations in VR and X2a groups. The complications in the X2a group were most likely due to the adaptive motion, because there were no file complications in the X2c group.
In mechanical instrumentation, clockwise rotation generates stress in the file as it penetrates the root canal dentin. In contrast, counterclockwise rotation helps release the stress generated in the rotary file. However, during retreatment, clockwise rotation generates stress in the file while it penetrates the root canal filling material and counterclockwise rotation generates additional stress on the other side of the rotary file as it penetrates the root canal filling on the other side. Therefore, reciprocating and adaptive motions have a greater likelihood of instrument fracture due to torsional failure based on their respective motion parameters than continuous rotation [20–22].
This study found no deformation or separation of the X2c and RB files. These results correspond to a previous study that reported no instrument complications in the ProTaper NEXT group [4]. In contrast, a previous study found two ProTaper NEXT X2 fractures during curved canal retreatment. These fractures might have occurred because the rotary file was used in three root canals before being replaced [1].
Our pilot study evaluated a retreatment protocol using TF adaptive files (ML1; tip size 25; 8% taper) in adaptive motion. In that study, the ML1 files were deformed and could not completely remove the root canal filling. Moreover, a previous curved root canal retreatment study reported that 100% of TF adaptive files using adaptive motion had defects after retreatment [11]. Therefore, the ML1 file was excluded from the present study and was replaced with X2 files in adaptive motion.
The strength of the study is that using one file per root canal can indicate the true efficacy and efficiency of the NiTi rotary file. The results from the ProTaper NEXT group using continuous rotation and adaptive motion suggest that using continuous rotation is more efficacious and efficient than adaptive motion in curved root canal retreatment when using the same rotary file system. Single file retreatment using X2c or RB in a small curved root canal can be done with high efficacy, efficiency, and cost-effectiveness.
The limitation of the current study was that the cleanliness of the retreatment procedure in the present study may be due to the approximate sizes of the NiTi rotary files and root canals. Therefore, the results of this study can only be generalized to retreating teeth with small curved root canals, such as the mesial root of the mandibular molars and the buccal roots of the maxillary molars [14, 15, 23]. However, X2c and RB can be used for penetrating to the working length of a curved canal in the original canal path without complications. Therefore, X2c and RB can be used for creating the path in large or oval root canals up to the working length, and then a bigger rotary file or sequent rotary files should be used for complete root canal filling removal.