The isthmuses of the mesial root of mandibular molars: a micro-computed tomographic study

Aim To investigate the prevalence of root canal isthmuses in the apical 5 mm of the mesial root of mandibular molars by means of micro‐computed tomography (MCT) and to describe the morphology of the isthmuses. Methodology Twenty extracted mandibular first molars that had fully formed roots were selected. The mesial roots were sectioned from the distal roots and subjected to MCT. The number of sections showing isthmuses in each of the apical 5 mm of the root canals was recorded. In each one of the apical 5 mm of the 20 roots examined, 40 sections were observed, giving a total of 800 sections observed in each group. Data were analysed as a contingency table using the chi‐square statistic to test the null hypothesis that location of the sections in each of the apical 5 mm and presence of the isthmus were independent. Results Isthmuses were found to be present at all levels with prevalence figures between 17.25 and 50.25%. The chi‐square test indicated a significant difference in the distribution of isthmuses with section (P = 0.001). It was found that sections in the first millimetre from the apex had fewer isthmuses than expected and that sections in the third millimetre from the apex had more isthmuses than expected under the null hypothesis. Calcifications were found to be present in most isthmuses, occasionally lateral canals originated from the central part of the isthmuses. Conclusions Isthmuses were present in the vast majority of roots observed. The third millimetre from the apex showed more isthmuses than expected. The results of clinical and surgical endodontic procedures performed in the mesial root of mandibular molars may be affected by this aspect of the root canal anatomy.