Micro-computed tomographic assessment of the variability and morphological features of root canal system and their ramifications

Abstract Objectives This study assessed the incidence and variability features of root canals system (RCS) and their ramifications according to Pucci & Reig (PR) (1944) and the American Association of Endodontists (AAE) (2017) by micro-computed tomography (μCT). Methodology 500 representative extracted human teeth of each tooth group (n=50) (maxillary/mandibular central and lateral incisors, canines, first and second premolars and molars) were scanned by μCT with a resolution of 26.70 μm. The reconstructed cross-sections images and the visualization of the continuous slices in the transversal axis were performed using DataViewer software. RCS were classified according to Pucci & Reig (main canal, collateral canal, lateral canal, secondary canal, accessory canal, intercanal, recurrent canal) and AAE (main canal, accessory canal, lateral canal). The apical deltas were assessed for both classifications. The prevalence of apical deltas was evaluated using the Chi-squared test (p<0.05). Results According to PR, a higher incidence of lateral canals was observed in maxillary canines (10%), central incisors (8%) and first premolars (6%). Using AAE, the highest incidence of lateral canals was observed in the mandibular first premolars (85%), first and second molars (84%), lateral incisors (67%), canines (59%), and in maxillary first premolars (52%). Regarding accessory canals, the PR showed a frequency in 2% of the maxillary lateral incisors and maxillary and mandibular first premolars and 3% of mandibular first and second molars. On the other hand, the AAE showed the highest incidence of accessory canals in 86% of the maxillary first premolars, 71% in mandibular lateral incisors, 69% in mandibular first premolars, 65% in mandibular canines, and 56% in maxillary canines. The PR showed the lowest incidence of apical deltas for all dental groups when compared with AAE (p=0.004). Interestingly, distal canals in maxillary molars showed a significant discrepancy between classifications (p=0.027). Conclusions μCT enabled accurately describing the RC system and related ramifications, adding to the PR and AAE classifications, with some discrepancies reported for maxillary molars. Clinical Relevance This μCT study enabled a thorough description of the variability among root canals and their ramifications, including clinically relevant details on the presence and location of lateral canals and accessories in all human tooth groups, beyond the currently existing classification systems.

include in a clear, objective and didactic way the description of the types of root canals in all human tooth groups. 12,25,28 The definition proposed by PR is more complete and classifies the root canal system in main canal, collateral canal, lateral canal, secondary canal, accessory canal, intercanal, recurrent canal and apical deltas. 25 On the other hand, the AAE classification is simpler and more objective, classifying the anatomical variations of the root canals in main canal, accessory canal, lateral canal and apical deltas. 28 In order to enable visualization and characterization of RCS, micro-computed tomography (μCT) has become a reference standard enabling a fast and detailed, noninvasive, nondestructive, and high precision three-dimensional analysis of the RCS, ensuring the consistency of the results obtained. 4,5,8,11,14,17,23,[29][30][31] Many studies have used μCT to describe the RCS

Micro-CT Analysis
Thus, to acquire μCT images, teeth were placed in an individually customized apparatus and the scanning was made separately in a SkyScan 1174 μCT device

Statistical analysis
The data were examined for normal distribution

Pucci & Reig classification of root canals
The percentage of root canals types found according to the PR classification are shown in Table   1. The assessment evidenced the presence of a main canal in 100% of the assessed dental groups, except for the second mesiobuccal canal in the maxillary first and second molars, which showed an incidence of 87% and 75%, respectively. There was a higher incidence of lateral canals in the maxillary canines (10%), followed of the maxillary lateral incisors and maxillary and mandibular first and second premolars and between 3% and 4% of mandibular first and second molars.
The highest incidence of apical deltas was found in 6% of the maxillary canines, in 4% of maxillary second premolars, in 6% of the mesiobuccal root of maxillary first molars and in 7% of the mandibular first premolars. In addition, recurrent canals were found in 2% of the maxillary lateral incisors and intercanals in 2% of the maxillary first molars and mandibular canines ( Table 1, Figures 1-3). mesiolingual roots of first and second molars (84% and 76%, respectively), lateral incisors (67%), canines (59%), and palatal roots of maxillary first premolars (52%). Apical deltas were found more frequently in 12%, 9%, 7% and 6% of the maxillary and mandibular first molars, mandibular central incisors, mandibular first premolars and maxillary and mandibular canines, respectively (  Figures 1-3).

American Association of Endodontists classification of root canals
The PR showed the lowest incidence of apical deltas for all dental groups when compared with AAE (p=0.004). When teeth group was assessed, only distal canals in maxillary first and second molars showed statistical difference between the two classification systems (p=0.027).   On the other hand, the AAE 27 classification evidenced an incidence of accessory canals between 10% and 86% and lateral canals in 4-85% of the assessed teeth, which can be found similarly in other studies. 9,11,32,34,35,38 The presence of apical deltas ranged between 2% and 7% for the Pucci & Reig 25 classification and between 2% and 12% for AAE 28 , with a higher prevalence in the distobuccal root canals of maxillary molars. Although most studies do not mention the type of classification used, the data obtained in this study for apical deltas agree with what has been previously established in the literature. 1, 6,8,12,14,18,31,34,[38][39][40] This study showed a high incidence of apical deltas in all dental groups assessed in the literature. 6

Conclusions
This study concluded that μCT enabled an accurate description of the variability of the RC system and

Availability of data and material
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Funding
This study was not financed by any funding agency.