Symmetry of root and root canal morphology of mandibular incisors: A cone-beam computed tomography study in vivo

Background Aim of this study was to analyze the root canal configuration in mandibular central and lateral incisors in vivo using cone-beam computed tomography (CBCT) imaging. Material and Methods A total of 487 mandibular central incisors and 491 mandibular lateral incisors from 250 patients were examined using CBCT imaging, previously taken for diagnosis and treatment. The number of roots, root canal system configuration, presence of apical confluences, distance between confluences and radiographic root end, symmetry between left and right elements were recorded and statistically analyzed. Results All the examined teeth presented only one root. No significant differences were found between the prevalence of two root canals in mandibular central incisors (219 teeth, 45%) compared to lateral incisors (211 teeth, 43%). Conclusions The percentage of Vertucci type II configuration was higher than expected, being more frequent than type III. Type I was the prevalent, while other configurations were present but rare. Key words:Cone-beam computed tomography, mandibular incisors, root canal anatomy, confluences.


Introduction
The study of the anatomy is very important in endodontics, since most of the errors that occur during an endodontic treatment are related to failure in respecting the canal anatomy (1); the existence of an untreated canal may be a reason of failure (2). To achieve these goals, clinical examination and diagnostic imaging are both essential elements of preoperative diagnosis and treatment planning (3). Ideally, internal anatomical comple-xities, such as the number of canals, their shapes and trajectories, including the presence of confluences and bifurcations should be assessed before undertaking instrumentation. The study of endodontic anatomy can be divided into ex vivo methods, performed on extracted teeth, and in vivo methods, performed on patients (4). Amongst these various methods, CBCT recently become the most valuable tool for researching endodontic anatomy in vivo, e528 because it is a non-invasive tool that can provide images displayed in coronal, sagittal and axial planes. Moreover, it defines precisely the position of the tooth and allow studies about symmetry (5). Tooth configurations are usually classified according to Vertucci and the great majority of studies used this classification; more recently Ahmed and Dummer (6) proposed a new classification. It is a more accurate method, based on in vitro microCT evaluations of extracted teeth. Therefore it requires higher resolution of images which is not always achievable by in vivo CBCT to avoid unnecessary high radiation doses to the patient. The aim of the present study was to analyze in vivo the root canal configuration in mandibular central and lateral incisors using CBCT and Vertucci classification, to assess clinically relevant anatomical features.

Material and Methods
Sample selection: A total of 487 mandibular central incisors and 491 mandibular lateral incisors were examined. Teeth were selected from the CBCT examinations of 250 patients (130 males and 120 females) with an age ranging between 18 and 79 years. Images were obtained from CBCT examinations as part of diagnosis and treatment planning of patients who required large field of view for other reasons. The research was approved by the Ethical Committee of Policlinico Umberto I, Rome, Italy (ref. 582/17). The samples were selected according to the following criteria: • available CBCT images of lower incisors with complete root formation; • absence of root canal treatment, crowns and posts; • CBCT images without scattering. Teeth with immature apexes and root resorption were excluded. Image acquisition: CBCT images had been taken using the GXDP-500 system (Gendex Dental, Biberach, Germany), operating at 90 kVp and 7 mA, with an exposure time of 23 s and a voxel size of 0.2 mm3, with a field of view of 13x9x13 cm, with an estimated dose of about 5 mSv, allowing measurements to an accuracy of 0.2 mm. Image evaluation: Through the use of Horos™ software (The Horos Project, 64-bit medical image viewer, GNU Lesser General Public Licence, version 3.0) three-dimensional reconstructions were analyzed to evaluate the parameters of interest. Images were reworked according to the axial, sagittal and coronal planes. CBCT images were viewed on reconstructions according to the axial plane, scrolling the cursor in the coronal-apical direction before, and then in the apical-coronal direction, to get a detailed view of the root canal system of examined teeth. This action was repeated 3 times, and when the images in the axial plane were not clear, the tooth was also inspected in the three-dimensional reconstruction.
The software had a specific tool which allowed precise measurements (~0,01 mm). The following parameters were evaluated: number of roots, root canal system configuration, presence of apical confluences, distance between confluences and radiographic root end, and symmetry between left and right elements in the same individual. The classification of the canal morphology was done according to the Vertucci's criteria ( Fig. 1): • Type I: single canal from the pulp chamber to the apex; • Type II: two different canals emerge from the pulp chamber but converge to the apex; • Type III: a canal emerges from the pulp chamber, divides into two within the root and emerges into one at the apex; • Type IV: two different canals from the pulp chamber to the apex; • Type V: a single canal emerge from the pulp chamber and divides into two at the apex; • Type VI: 2 different canals emerge from the pulp chamber, join at the middle of the root and then divide again into two with two different apical foramina; • Type VII: one canal in the pulp chamber that divides into two and rejoins within the root, and redivides into two canals at the apex; • Type VIII: 3 separate canals from the pulp chamber to the apex. Statistical Analysis: The results were analysed statistically using SPSS 20.0 (SPSS, Inc., Chicago, IL, USA) with the significance set at p<0.05. One-way ANOVA was used for the association between the variables along with the post hoc tests, Tukey's HSD and Games-Howell. The t-test was used to compare the mean distances from confluence to radiographic root end.
• No statistical differences were found between central and lateral incisors, except for type VII (P<0,05). Symmetry of root canal morphology between left and right elements of the same individual: • Symmetry was found in 44,6% of cases in mandibular central incisors and in 44,8% of cases in mandibular lateral incisors.
• No statistical differences were found between central and lateral incisors (p<0,05).
Our results were similar with the research by Kayaoglu et al. (45) and lower than other studies performed on molars (5).

Conclusions
The percentage of mandibular incisors presenting a complex anatomy (two canals with different configurations) is higher than previously reported. These results highlighted that only an accurate preoperative radiographic exam could reveal and identify complex canal configuration in mandibular incisors.