Initial
root canal therapy does not always have success and periradicular
inflammatory lesions might remain or develop after the treatment. The causes
of 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 leakage
5. In the study by
Friedman and Mor et al., 9 was reported that when there was no prior apical periodontitis, the
commonness of healed cases after both initial root canal therapy and orthograde retreatment ranges
from 92% to 98% up to 10 years after therapy .
When prior apical periodontitis exists, the rate of healing falls to
between 74% and 86% regardless of whether primary treatment or nonsurgical retreatment was carried
out 13. The
retreatment files have been proposed
for removing gutta-percha and sealer from root canals and many studies
reported their efficacy and safety. The  retreatment NiTi files were
faster than the manual technique 18, 21. The aim of
each study is to obtain clinically relevant and meaningful results. To achieve
this, standardization of the investigation method is essential 13, 24. The present
study focused on evaluating and comparing the efficacy of 4 NiTi files in the
removal of root canal filling material. For this Study, 60 single distal root canal
of extracted mandibular molar teeth were used and treated with a step-back. The
advantage of the study by Ersev et al., 8 is the
application of many samples (120 teeth). Lateral compaction is the most popular
obturation technique, both in practice and as taught in most institutions 15. In our study lateral compaction was used
for 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 filling
materials, different techniques were used and the remaining filling materials were
evaluated after retreatment. The benefit
of these studies is in that they exhibit the use of various filling
materials and techniques. The
disadvantage 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-down
instrumentation 24. The results of
many studies have shown that retreatment systems required less time to remove
gutta-percha and sealer from root canals 2, 14, 16. In the study
by Alves et al., 3 the efficacy of ProTaper
universal, protaper retreatment, Mtwo universal and Mtwo retreatment were
evaluated. The results of this study have shown that remaining material was
found in all the systems and there was no statistically significant difference
between the groups (p=0.9). As a
result, all these systems are suitable for retreatment of root canal
therapy. In the present study, all gutta-percha and sealer were removed with four
different methods. The samples
were split with a diamond disc longitudinally. In others studies, where micro-CT was used for evaluation, the remaining filling
material, 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 software
was used for evaluation the preparation of dentin walls in root canals in dentistry
7. In the present study, like many others studies,
such as those by
Ersev et al., 8, Fruchi et al., 11, Rios et al., 20, Roeding et al., 21, Zuolo et al., 26 verified that removal of filling materials
in all root canals was not complete. In our study, no significant difference
was found between groups with the statistical analyze of one-way ANOVA. All of
the retreatment systems can remove filling material from the root canals, meaning
all files can be recommended for retreatment of root canal therapy.