Micro-CT Analysis of Solvent Use and Apical Enlargement in the Retreatment of S-Shaped Root Canals of Mandibular Molars

Aim: To evaluate the endodontic retreatment of S-shaped mesial root canals of mandibular molars using the Reciproc Blue R25 and R40 instruments, with or without the use of an orange oil solvent, and to assess the effect of additional apical enlargement with the HyFlex EDM 50/0.03 instrument on root canal cleaning after retreatment. Methodology: Eighteen mesial root canals with double curvature from mandibular molars were prepared up to size 40/0.04 using HyFlex EDM instruments and obturated. Specimens were anatomically matched and allocated into two groups (n = 9) according to the retreatment protocol: Reciproc Blue R25 and R40 without solvent and Reciproc Blue R25 and R40 with orange oil solvent. Following filling removal, all canals underwent additional apical enlargement using the HyFlex EDM 50/0.03 instrument. Micro-computed tomographic scans were performed before and after obturation, after filling removal, and after apical enlargement. The percentage of remaining filling material, canal transportation, and centering ability were quantified by root canal thirds and in all root canal. Data were analysed using Student's t-test or Mann-Whitney U test for intergroup comparisons and paired Student's t-test or Wilcoxon signed-rank test for intragroup analyses (α = 0.05). Results: No significant differences were observed between retreatment protocols, regardless of the use of orange oil solvent, for any evaluated parameter (p > 0.05). Additional apical enlargement with the HyFlex EDM 50/0.03 instrument significantly reduced the overall volume of remaining filling material in both groups (p < 0.05). However, complete removal of the filling material was not achieved in any specimen. Apical enlargement did not result in significant changes in canal transportation or centering ability (p < 0.05). Conclusion: The use of an orange oil solvent did not improve filling material removal or influence canal shaping in S-shaped mesial root canals of mandibular molars. Additional apical enlargement with the HyFlex EDM 50/0.03 instrument enhanced root canal cleanliness without compromising canal anatomy, although residual filling material persisted.