Initialroot canal therapy does not always have success and periradicularinflammatory lesions might remain or develop after the treatment. The causesof such failures are most often microorganisms that have either withstood the conventional treatment procedures or reinfected the root canal system at later phases through coronal leakage5. In the study byFriedman and Mor et al., 9 was reported that when there was no prior apical periodontitis, thecommonness of healed cases after both initial root canal therapy and orthograde retreatment rangesfrom 92% to 98% up to 10 years after therapy .

When prior apical periodontitis exists, the rate of healing falls tobetween 74% and 86% regardless of whether primary treatment or nonsurgical retreatment was carriedout 13. Theretreatment files have been proposedfor removing gutta-percha and sealer from root canals and many studiesreported their efficacy and safety. The  retreatment NiTi files werefaster than the manual technique 18, 21. The aim ofeach study is to obtain clinically relevant and meaningful results. To achievethis, standardization of the investigation method is essential 13, 24. The presentstudy focused on evaluating and comparing the efficacy of 4 NiTi files in theremoval of root canal filling material. For this Study, 60 single distal root canalof extracted mandibular molar teeth were used and treated with a step-back. Theadvantage of the study by Ersev et al.

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, 8 is theapplication of many samples (120 teeth). Lateral compaction is the most popularobturation technique, both in practice and as taught in most institutions 15. In our study lateral compaction was usedfor obturation of root canals, as in the studies by Alves et al., 3 and Zuolo et al., 26. In the studies by Ersev et al., 8 and Khedmat et al.

, 14 the samples were filled with various fillingmaterials, different techniques were used and the remaining filling materials wereevaluated after retreatment. The benefitof these studies is in that they exhibit the use of various fillingmaterials and techniques. Thedisadvantage of our study is that only one technique (lateral compaction)was used for root canal filling. The remaining gutta-percha and sealer are removed during a crown-downinstrumentation 24. The results ofmany studies have shown that retreatment systems required less time to removegutta-percha and sealer from root canals 2, 14, 16. In the studyby Alves et al.

, 3 the efficacy of ProTaperuniversal, protaper retreatment, Mtwo universal and Mtwo retreatment wereevaluated. The results of this study have shown that remaining material wasfound in all the systems and there was no statistically significant differencebetween the groups (p=0.9). As aresult, all these systems are suitable for retreatment of root canaltherapy. In the present study, all gutta-percha and sealer were removed with fourdifferent methods. The sampleswere split with a diamond disc longitudinally. In others studies, where micro-CT was used for evaluation, the remaining fillingmaterial, meaning the samples did not have to be split 21, 22. In our study, like others studies,such as those by Akhavan et al.

, 4 and Bhagavaldas et al., 6 the samples were photographed under a stereomicroscope.The remaining filling materials were measured with AutoCAD software.

AutoCAD softwarewas used for evaluation the preparation of dentin walls in root canals in dentistry7. In the present study, like many others studies,such as those byErsev et al., 8, Fruchi et al., 11, Rios et al.

, 20, Roeding et al., 21, Zuolo et al., 26 verified that removal of filling materialsin all root canals was not complete. In our study, no significant differencewas found between groups with the statistical analyze of one-way ANOVA.

All ofthe retreatment systems can remove filling material from the root canals, meaningall files can be recommended for retreatment of root canal therapy.