Evaluation of dentine thickness of middle mesial canals of mandibular molars prepared with rotary instruments: a micro‐CT study

Aim To evaluate, using micro-CT, the remaining dentine thickness after preparation of the mesiobuccal (MB), mesiolingual (ML) and middle mesial (MM) canals of mandibular first molars with the ProTaper Next rotary system. Methodology Eleven mesial roots of mandibular first molars having three independent canals from the furcation level up to at least 5 mm towards the apical direction were selected. Preparation of MM canals was performed in two steps using ProTaper Next X2 (step 1) and X3 (step 2) instruments, whilst MB and ML canals were prepared in a single step up to X3 instruments. The roots were scanned (pixel size of 10 µm) before and after each step, and the dentine volume was calculated. Postoperative models of the roots were coregistered with their preoperative dataset and colour-coded cross-sections of the roots were used to measure the smallest dentine thickness surrounding each canal at 1.0-mm intervals from the furcation level up to 5 mm in the apical direction, in both mesial and distal aspects of the roots. Changes in the remaining wall thickness between mesial canals were analysed with repeated-measures anova and post hoc Tukey test. Significance level was set at 5%. Results Mean percentage reduction of dentine volume after steps 1 and 2 was 4.66% and 5.16%, respectively. Overall, pre- and postoperative dentine thickness of the MM canal walls, in both mesial and distal aspects of the root, were significantly thinner than those of MB and ML canals (P < 0.05). No significant difference was observed when comparing dentine thickness in the mesial and distal directions of MM canal after preparation step 1 (0.88 ± 0.18 mm and 0.73 ± 0.26 mm, respectively) or 2 (0.83 ± 0.17 mm and 0.67 ± 0.26 mm, respectively) (P > 0.05). Dentine thickness values less than 0.5 mm were observed mostly towards the distal aspect of the MM canal. Mesial roots were not associated with strip perforations after canal preparation procedures. Conclusions A significant decrease in the thickness of canal walls at all levels evaluated in the mesial roots of mandibular molars suggests that files with small tapers should be used in preference to instruments with large tapers to prepare mesial canals in mandibular molars.