s, in the present study, the DR1 file was used in three samples and DR2 was used in one sample and then discarded, in accordance with the manufacturer's recommendations ([@ref29]).

According to the results of the present study, the amount of gutta-percha and sealer residue was lowest in the XP group and highest in the CI group (P\<0.001). There was no significant difference in the amount of gutta-percha and sealer residue in the EA and IS groups (P\>0.05). Thus, the null hypothesis of our study was rejected. We attribute the findings to the different efficiencies of the activation processes.

In the current study, none of the methods applied in the groups were capable of removing the entire gutta-percha and sealer from the root canals. This finding is similar to the results of previous studies, which used different systems, instruments and/or methodology ([@ref30], [@ref31]). As noted above, in the present study, the amount of gutta-percha and sealer residue in the root canal walls was significantly lowest in the XP group. A detailed literature review showed that there were no retreatment studies conducted with the XP-endo Finisher NiTi file in oval root canals. Thus, it is not possible to compare the findings obtained in the XP group with those reported in earlier studies. In a recent study, Alves et al. ([@ref32]) reported that the usage of XP-endo Finisher significantly improved filling material removal in retreatment process in mandibular molar's mesial roots. Also, a recent study that evaluated the efficacy of four final irrigation protocols on the reduction of hard-tissue debris accumulated within the mesial root canal system of mandibular first molars ([@ref33]). The authors reported that XP-endo Finisher and passive ultrasonic irrigation was statistically more effective than CI and Self-adjusting File (ReDent-Nova, Ra'anana, Israel). The lower amount of gutta-percha and sealer residue in the XP group compared to that of the other groups might stem from the fact that after the file passes from the martens