Matching the Dimensions of Currently Available Instruments with the Apical Diameters of Mandibular Molar Mesial Root Canals Obtained by Micro-computed Tomography

Introduction: This micro-computed tomographic study investigated the original anatomic diameters of the apical portion of mandibular molar mesial canals and matched them to the dimensions of instruments that are currently available and commonly used for apical preparation. Methods: One hundred eight mandibular molar mesial roots with Vertucci type IV configuration were scanned by micro-computed tomographic imaging, and the largest anatomic diameter of both mesial canals at 1, 2, 3, and 4 mm short of the apical foramen was measured. Canal diameters were compared with the dimensions of 10 endodontic instruments for simulation of preparation 1 mm short of the apical foramen. Accordingly, the instruments with a diameter larger than the largest canal diameter were regarded as having the potential to prepare 100% of the canal walls at each specific point or over the 4-mm apical segment. Results: At 1 and 2 mm short of the apical foramen, a 45/.02 instrument had a larger diameter than the mesiobuccal canal diameter in 73% and 55% of the teeth, respectively. Corresponding figures for the mesiolingual canal were 65% and 55%. When the entire 4-mm apical segment was considered, a 40/.06 instrument had better results, being larger than the apical canal in about 20% of the specimens. Overall, 78% of the apical canals would not be completely prepared by any instrument. Based on the mean (median) anatomic diameters, the adequate instrument dimensions would be 40/.10 (40/.08) for mesiobuccal canals and 45/.08 (40/.09) for mesiolingual canals. Conclusions: The dimensions of the available instruments are not compatible with complete apical preparation in the majority of cases. The ideal instrument size/taper to include all the canal walls in apical preparation is too large and may not be safe for clinical use.