Coronal root canal morphology of permanent two-rooted mandibular first molars with novel 3D measurements

Aim To measure the coronal root canal morphology of permanent mandibular first molars using 3D coordinates for more precise conservative endodontic cavity preparation. Methodology In total, 57 cone beam computed tomography (CBCT) images of sound mandibular molars with fully formed apices without previous endodontic treatment were taken from 33 patients aged 16 to 75 years. The CBCT machine (MCT-1[EX-2F], J. Morita Manufacturing Corp, Kyoto, Japan) provided 14-bit greyscale images with the voxel size of 0.125 mm. All images were taken at 80 kV and 5.0 mA, with a 17-s exposure time, which were then reconstructed in 3D models and viewed by an endodontist. The gender of the patients and the tooth position, and number of root canals, were recorded. The landmarks of coronal root canals were determined, then the distribution of landmarks, maximum curvature in the axial direction and curvature directions in the horizontal direction of coronal root canals were measured. Distributions of landmarks were analysed using a spatial statistics method. Data about curvature were compared using a t test. Results Overall, the distribution of root canal orifices and the centre of the canal primary curve were more centralized than other landmarks. The landmarks were located more mesiobuccally to the centre of the occlusal plane of mandibular first molars. Specifically, the measurements of the maximum curvature of coronal root canals in the axial direction were as follows: in 3-canal teeth, the average angles of curvatures were 22 degrees,23 degrees,15 degrees for mesiobuccal (MB), mesiolingual (ML) and distobuccal (DB) canals, respectively; in 4-canal teeth, the average angles of curvatures were 24 degrees,26 degrees,15 degrees,22 degrees for MB, ML, DB and distolingual (DL) canals, respectively. The degrees of coronal root canal curvatures in the horizontal direction were as follows: in 3-canal teeth, the average angles of curvatures were -8 degrees,47 degrees,-2 degrees for MB, ML and DB canals, respectively; in 4-canals teeth, the average angles were -11 degrees,50 degrees,-28 degrees,45 degrees for MB, ML, DB and DL canals, respectively. Statistically, there was no reliable side or sex difference in any of the measurements (P > 0.05). Conclusions In permanent two-rooted mandibular first molars, the coronal canals clustered in the direction of the mesiobuccal side of the occlusal surface of the teeth.