A high-resolution computed tomographic study of changes in root canal isthmus area by instrumentation and root filling.

INTRODUCTION The aim of this study was to obtain a three-dimensional analysis of the isthmus area of the mesiobuccal root canal system in mandibular molars using high-resolution micro-computed tomography (μ-CT) scanning and to measure the amount of debris and root filling material in the isthmus after instrumentation/irrigation and root filling. METHODS Mandibular molars with two separated mesial root canals (10 teeth) were scanned by using the Skyscan 1172 μ-CT system (Skyscan, Aartselaar, Belgium) before and after instrumentation and after filling using the Thermafil root filling technique. An isthmus was defined as the ribbon-shaped or thin connecting structure between two root canals after instrumentation. The characteristics of the isthmuses were quantitatively monitored during the whole treatment. The images were segmented and quantified. The surface area of the isthmus, volume of debris after rotary instrumentation, and volume of the filled space in the isthmus after obturation were evaluated. RESULTS Of the seven mesial roots, two had isthmus/anastomoses somewhere along its length in the apical 5 mm, and five had an isthmus that was continuous all the way from the coronal part to the apical part. The average percentage of isthmus surface area and isthmus volume after instrumentation was 21.4% and 9.4% of the whole root canal system, respectively. About 35.2% of the isthmus volume was filled with apparent hard tissue debris after instrumentation/irrigation. The average percentage of volume of filling material in the isthmus areas was significantly lower (57.5%) than in the main root canals (98.5%, p < 0.001). CONCLUSIONS A considerable amount of dentin debris is produced and packed into the isthmus area during rotary instrumentation of mesial canals of lower molars despite continuous irrigation during and after instrumentation. The debris may partly prevent penetration of the filling material and sealer into the isthmus area.