The performance of PTR, WOG, and RB systems in removing iRoot SP and GP from curved root canals was compared for the first time, and the portions of the curved part of root canal were defined, thereby analyzing the ability of three approaches to negotiate the curved part and the reasons for blockage. Meanwhile, a resolution higher than in previous studies, that is 12.8–19.9µm, was applied to observe the occurrence of complications like the formation of ledge and microcrack [13, 14, 25]. Three types of instruments, belonging to rotary system and reciprocating system, were applied in this study. The result suggested that RB had superior ability to negotiate the upper portion smoothly and obtained more reduction at apical third than other groups, and there formed more dentinal microcracks in PTR group in this study. However, no significant differences were observed among groups in filling material reduction, ledge formation, and iRoot SP resistance.
Most studies presented that the straight and curved root canals obturated with AH plus regained satisfied patency post retreatment by using mechanical Ni-Ti instruments [11, 12, 14, 17]. The WL of most straight root canals obturated with bioceramic sealer EndoSequence BC could be regained, and the apical patency reached 80%-100% during retreatment [10, 25]. Our results revealed that the straight part of canals could be negotiated smoothly by all three approaches, but the curved part existed blockage and made it difficult to reestablish the patency. Meanwhile, 58.82% (PTR), 64.71% (WOG), and 82.35% (RB) of the samples in each group reestablish canal patency successfully. The discrepancy may be attributed to different design concept of these instruments and high strength of iRoot SP which resulted in blockage of the curved part and loss of patency in some samples .
The etiology of canal blockage includes calcification, instruments separation, or ledge formation. This study indicated that the portion which cannot be negotiated was located in the curved part through micro-CT scan images, and the probable reasons included ledge formation and iRoot SP resistance encountered by instruments. However, no statistical differences were found among groups. The iRoot SP resistance could be related to the direct contact between iRoot SP and the file tips, which have no cutting ability in all three approached. The higher-hardness iRoot SP obturated in the irregularly shaped area of the curved root canal increased the difficulty to negotiate . Blockage caused by ledges impeded the instrument to travel along the direction of original canals to reach ideal WL . The extreme flexibility and rebound tendency of Ni-Ti instruments caused a bypass to become difficult. Our result suggested that canals in RB group were negotiated more compared with other groups in the upper component. This finding was consistent with that of Romeiro1 et al . It proved that RB possesses more excellent ability of exploration, and effectively follows the root canal path to reach apex.
Almost all studies presented that the reduction rate in the apical third of each group is significantly lower than that in the middle and cervical thirds [14, 25, 27]. This result is consistent with our conclusion. Material reduction rate, as an indirect indicator of patency regaining, is related to the cross-sectional design, taper, tip diameter, and movement mode of the instrument, which reflects retreatment efficiency [16, 27]. Our study presented that no statistical difference was found in the total reduction among three groups, and the apical third in the RB group was higher than that of others probably due to its S-shaped cross-sectional design that results in sharp cutting edges and larger chip space . In addition, compared to two Ni-Ti systems (0.07 in WOG25 and 0.06 in PTN X2), the greater taper size at the tip of the RB system (0.08 in RB25) could contribute to this reduction of filling materials  .
The formation of dentinal microcracks occurred typically during the process of root canal shaping, cleaning, filling and retreatment [15, 29, 30]. Studies revealed that dentinal microcracks formed when using PTR and RB during root canal retreatment [15, 31], but limited evidence presented this phenomenon in WOG. At present, the formation and increased rate of new dentinal microcrack during retreatment procedure are still controversial [15, 32, 33]. Our results displayed that all three approaches produced microcracks in the root wall, which could be relevant to the increased heat production during retreatment due to the high-strength of iRoot SP and strong bond between iRoot SP and dentine. At the same time, the higher resolution of micro-CT scanning brought more detective numbers than that of previous studies. The increasing dentinal microcracks in PTR was more than that in other groups, this may be related to the continuous rotational force and constant torque applied by the PTR rotary system on the root canal walls, and the lower flexibility of PTR than other two systems [34, 35]. However, additional studies are necessary to evaluate the changes in original microcracks morphology from cross-sectional images after retreatment.