INTRODUCTION
An understanding of inner dental anatomy and its variations is essential to ensure successful outcomes in endodontic treatment. This is especially significant for the treatment of C-shaped canal configurations, which generally require modifications of the usual instrumentation, irrigation and obturation techniques 1 . When variations of the tooth morphology remain undetected by the clinician, a therapeutic failure can be expected.
The most commonly reported anatomic configuration in maxillary first molars is the presence of 3 roots (2 buccal and 1 palatal) and 4 canals: 2 in the mesiobuccal root, 1 in the distobuccal root and 1 in the palatal root 2 . A maxillary first molar should be considered a four-canal tooth until proved otherwise; however, a clinician should be aware of the possibility of variations in the number of roots, from 1 to 5 3, 4 ; the number of root canals, from 1 to 8 4-8 , or C-shaped root canal configuration which may or may not be split into two or more canals 9 . The first documented case of a maxillary first molar with a C-shaped root canal was published by Newton & McDonald in 1984 10 . According to De Moor 11 , the probability of observing a C-shaped canal in the first maxillary molar is 0.091%, although it would be questionable to use the term prevalence in his work, since only the fusion of the distobuccal and palatal roots were considered. In 2006, Cleghorn et al. observed C-shaped canals in 0.12% of maxillary first molars 2 .
Maxillary second molars usually have 3 roots: 1 mesiobuccal, 1 distobuccal and 1 palatal, with one canal in each root 12, 13 , and these roots may be fused 14 . The presence of an extra canal in the mesiobuccal root (MB2) is not as frequent in maxillary second molars as it is in first molars 15 . Different authors have reported variations in the number of roots and canals, and in presence of C-shaped root canals 6, 16-18 . Because maxillary and mandibular molars have different anatomical features and numbers of roots, C-shaped root canals in them are classified differently. Martins et al. developed a classification for maxillary molars in 2016 1 , since until that date, the definition of a C-shaped maxillary molar configuration was not clear, and no standardization was available.
Cone-beam computed tomography (CBCT) is sufficiently precise for morphological study of the number of roots and canals for diagnostic purposes.
It is a useful diagnostic tool in endodontic practice because of its non-destructive in vivo application. Additionally, the radiation dose is lower and the resolution is higher than in conventional computed tomography scans, making it more advantageous for clinical application 19 .
The aim ofthis study was to determine the prevalence and the characteristics of C-shaped root canals in maxillary first and second molars, assessed by the CBCT scans taken at the Department of Diagnostic Imaging, School of Dentistry of the University of Buenos Aires (FOUBA), in the Autonomous City of Buenos Aires, Argentina.
MATERIALS AND METHOD
Sample selection
This was a retrospective, observational, cross-sectional, descriptive study which assessed a total 332 CBCT images obtained between August and September 2020. Informed consent was provided by all patients included in this study. The protocol was approved by the Ethics Committee of the School of Dentistry of the University of Buenos Aires (CETICA/FOUBA 006/2020).
Digitalized CBCT images of maxillary first and second molars were collected from a data bank in Diagnostic Imaging Department. During 2020, a total 80,000 patients received care at the FOUBA during the COVID-19 pandemic, and 4,000 CBCT scans were taken, adding up a total of 220,000 services provided.
Inclusion criteria
Maxillary CBCT scans showing at least one molar other than the third molar and developed apices.
Exclusion criteria
Teeth with crown-root decay involving pulp chamber floor, previous endodontic treatment, root resorption, artifacts preventing appropriate visualization in images, including crown post/core and/or crown, or faulty radiographic technique.
Radiographic technique and image assessment
The assessed images were acquired with a Planmeca ProMax® 3D Max CBCT system (Planmeca OY, Helsinki, Finland), with 88 kV and 9.0 mA, exposure time 12.07 seconds and voxel size 150-200 pm. The images were assessed with the software corresponding to the tomograph in shifts of only two hours a day, to prevent visual strain and any misinterpretation of images. The collected data were entered in ad-hoc data recording sheets.
C-shaped root canal classification
This study was based on the classification proposed by Martins et al. 1 , which says that a canal is considered as C-shaped in a maxillary molar when it exhibits root fusion and 3 consecutive axial cross sections with an upper-C (UC) 1 or 2 configuration in the fused root. The UC configuration system for maxillary molars is a modification of the Fan et al. 20 classification for C-shaped mandibular molars, and 5 configurations were observed in axial slices: UC1, continuous large C-shaped canal system; UC2, continuous C-shaped canal with 2 main canal lumens in the extremities connected by a large isthmus; UC3, 2 separate root canals; UC4, a single round or oval root canal; UC5, no canal lumen 1 .
In turn, depending on which roots were fused, they were classified into 5 types: Type A: Fusion between the mesiobuccal and palatal roots, forming a semilunar mesiopalatal root canal. Type B: Fusion between the mesiobuccal and distobuccal root canals, forming a semilunar buccal root canal system; the concavity of the semilunar shape may be turned to the palatal (subtype B1) or buccal (subtype B2) root. Type C: Fusion between the distobuccal and palatal roots, forming a large semilunar distopalatal root canal. Type D: Presence of a large palatal root canal, forming a semilunar shape; this type has been previously described as a fusion between 2 palatal roots. Type E: Fusion among the 3 roots; this configuration resembles the mandibular C-shaped anatomy, with a large semilunar mesiopalatal canal merging together with an independent distobuccal canal at a single apical foramen (subtype E1) or with a large semilunar distopalatal canal connecting with a mesiobuccal canal at a single apical foramen (subtype E2) 1 .
The classification was developed by observing axial slices at five levels: a) coronal-2 mm apical to the canal orifice openings in the chamber floor; b) apical-2 mm above the anatomic apex; c) middle-middle distance between “coronal” and “apical”; d) one third-middle distance between “coronal” and “middle”; e) two thirds-middle distance between “middle” and “apical”.
Statistical analysis
CBCT images were examined by two FOUBA endodontists trained in the observation of tomography slices and updated by means of the continuous critical reading of scientific reports related to the subject matter of this work. Cohen’s kappa unweighted coefficient was used to measure interobserver agreement. The kappa coefficient (k) with a 95% confidence interval (CI95) was obtained. A Z-test was applied to analyze the difference between the coefficient obtained and the zero value, with a significance level of 5%: p-value < 0.05 indicates that the Cohen’s kappa coefficient differs significantly from zero. The Cohen’s kappa coefficient value was computed according to the criteria proposed by Altman 21 . The assessment was done in software R version 4.0.3 22 using the packages irr 23 and psych 24 .
The age of the patients was described by the following measurements: mean, standard deviation (SD), median, first quartile (Q1), third quartile (Q3), minimum and maximum. The remaining data were described by absolute frequencies (AF) and percentages. The score method was used to estimate 95% confidence intervals (CI95) for percentages 25 . The Chi-square test or Fisher’s exact test were used to evaluate the association between categorical variables, as required. When all the expected frequencies were higher than or equal to 5, the Chi-square test was utilized. If this condition was not fulfilled, the Fisher’s exact test was used. A 5% significance level was set up. The following software was used: Calc, de Apache OpenOfficeTM v. 4.1.6 26 and R v. 4.0.3 22 .
RESULTS
The results of Cohen’s kappa test for reliability revealed significant agreement for the assessment of C-shaped canal configuration (k = 0.97; CI95: 0.91 to 1.00; Z = 13.7; p < 0.05; N = 200).
Out of the 332 CBCTs assessed, 120 (36%; CI95: 31% to 41%) were from patients who met the selection criteria. Out of the 120 patients included in the study, 69 were women (58%; CI95: 49% to 66%) and 51 were men (43%; CI95: 34% to 51%). Age ranged from 19 to 79 years, with mean (SD) 40 years (13) and median (Q1-Q3) 38 years (32-48).
A total 272 maxillary molars were assessed; 120 were maxillary first molars (44%; CI95: 38% to 50%) and 152 were maxillary second molars (56%; CI95: 50% to 62%). A significant association was found between tooth group and presence of C-shaped canal configuration (Chi-square=4.44; df=1; p<0.05; Fig. 1). Out of 120 maxillary first molars, 5 had C-shaped canal configurations (4%; CI95: 2% to 9%), whereas 115 did not (96%; CI95: 91% to 98%). Out of 152 maxillary second molars, 17 had C-shaped canal configurations (11%; CI95: 7% to 17%), whereas 135 did not (89%; CI95: 83% to 93%).
The age of patients with C-shaped maxillary first molar canals ranged from 32 to 39 years. The age of patients with C-shaped maxillary second molar canals ranged from 20 to 55 years, with median (Q1-Q3) 37 (24-38) and mean (SD) 34 (11) ( Fig. 2 ).
Out of the 5 maxillary first molars with C-shaped canal configuration, 4 belonged to men (80%; CI95: 38% to 96%) and 1 to a woman (20%; CI95: 4% to 62%). Out of the 17 maxillary second molars with C-shaped canal configuration, 7 belonged to men (41%; CI95: 22% to 64%) and 10 to women (59%; CI95: 36% to 78%).
Out of the 22 teeth with C-shaped canals, 11 corresponded to the right side (50%, CI95: 31% to 69%) and 11 to the left side (50%, CI95: 31% to 69%).
In only one of the 2 patients who had both maxillary first molars, a bilateral C-shaped configuration canal was observed, while in the other patient, the C-shaped configuration was seen in tooth 14 (left). In the remaining patients (N=2), the C-shaped configuration was seen unilaterally, 1 in tooth 3 and the other in tooth 14. Regarding the 8 patients who presented both maxillary second molars, 4 had bilateral C-shaped configuration canals (N=8 teeth) (50%; CI95: 22% to 78%) ( Fig. 3 ), while the rest did not (50%; CI95: 22% to 78%). In 4 patients out of 9 with unilateral second molars, C-shaped canals were seen in tooth 2 (44%; CI95: 19% to 73%), and in 5 patients, these canals were seen in tooth 15 (56%; CI95: 27% to 81%).
A significant association was found between the C-shaped type according to Martins’ classification and the tooth group (Fisher’s exact test: p<0.05; Fig. 4). Out of 5 C-shaped maxillary first molars, 2 were type E2 (40%; CI95: 12% to 77%), 2 were type C (40%; CI95: 12% to 77%) and 1 was type B1 (20%; CI95: 4% to 62%). In the single case of bilateral C, both teeth (3 and 14) had the same configuraron. Out of 17 C-shaped maxillary second molars, 4 were type A (24%; CI95: 10% to 47%), 7 were type B1 (41%; CI95: 22% to 64%), 5 were type B2 (29%; CI95: 13% to 53%) and 1 was type C (6%; CI95: 1% to 27%). Out of 4 patients with bilateral C-shaped canals, 2 had the same configuration on both sides, while the other 2 did not.
The UC1 and UC2 categories were the most frequent, except in the apical third, where UC4 was the most frequent. Out of the 5 C-shaped maxillary first molars, 1 was UC1 type (20%; CI95: 4% to 62%) and 4 were UC2 type (80%; CI95: 38% to 96%). Out of 17 C-shaped maxillary second molars, 12 were UC1 type (71%; CI95: 47% to 87%) and 5 were UC2 type (29%; CI95: 13% to 53%). Despite these differences, no statistically significant association was found between the UC configuration and the tooth group (Fisher’s exact test: p=0.12). Root fusion was observed in both groups of teeth, but not with the C-shaped configuration.
DISCUSSION
The present study, based on a retrospective assessment of CBCT images, provides a description of the C-shaped anatomy of the first and second maxillary molars, estimated for the first time in an Argentine subpopulation.
The term “C-shaped” in maxillary molars is used to describe root canals with large semilunar canal shape that can represent a whole root canal or a partial fusion of two or more canals 7,10,11,27, 28 .
In different studies, several methods were used to study root canal morphology, including radiographic technics, clearing technique, spiral computed tomography, sectioning technique and microcomputed tomography, all of which have some limitations. The CBCT scans reveal anatomic details of external and internal anatomy, being an important tool for diagnosis and treatment in
The prevalence of C-shaped canals in a Portuguese population was 1.1% for first molars and 3.8% for second molars. Martins et al. observed higher prevalence in women 1 . In a Korean population, prevalence was 0.8% in first maxillary molars and 2.7% in second maxillary molars 29 . Mashyakhy et al. reported that the prevalence of C-shaped canals in a Saudi Arabian population was 0.6% in first maxillary molars (only 2 out of 354) and 1.1% in second maxillary molars (4 out of 372), and the only types found were C and B 18 . A prevalence of 4.9% in second maxillary molars was reported by Yang et al. in a Chinese population after the assessment of 309 extracted teeth by the clearing technique 30 . Ordinola-Zapata et al. evaluated 100 second maxillary molars with fused roots by micro-computed tomography (micro-CT), and found C-shaped configuraron in 22 specimens (22%) in a Brazilian population 31 . These differences are likely due to participant ethnicity and age, sample size, study methods and the criteria applied to classify the C-shaped canals.
C-shaped maxillary molars may have low prevalence, but they are of high anatomical complexity because of a large isthmus connecting the root canals that are expected to be separate 1 . Concerning clinical implications, the percentage found in the present study provides heretofore unknown information about the prevalence and characterization of C-shaped canals. This is important since ignoring this morphology might lead to endodontic treatment failure. CBCT scans can enhance the understanding