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Objectives C-shaped root canal configuration is an unusual root canal morphology and one of the most often anatomical variations in root canal systems of second mandibular molars. The purpose of this study was to assess the prevalence, sym-metry and configuration of C-shaped canals in first and second mandibular molars in an Israeli population using cone beam computed tomography (CBCT).
Materials and methods One thousand twenty Israel patients’ CBCT scans were screened and evaluated. First and second mandibular molars with C-shaped canals were identified. Prevalence configuration along the root and symmetry were analyzed, and new radicular groove classification was proposed.
Results Total of 1465 mandibular second molars and 1229 mandibular first molars were evaluated. The overall preva-lence of C-shaped canals in first and second mandibular mo-lars was 0.16 and 4.6%, respectively. No significant difference was found by gender or side of occurrence. The unilateral prevalence of C-shaped canals in mandibular second molars was 55%. C-shaped canal configuration changed along the root in 63% of the cases.
Conclusions The prevalence of C-shaped canals in first and second mandibular molars in the Israeli population is low in comparison to other subpopulations. Clinicians should be aware of the possible asymmetry, different configuration along the root, and the location of danger zone of this anatom-ic variation during clinical work.
Clinical relevance CBCT is an efficient three dimensional radiographic examination for evaluation of C-shape root canal configuration. CBCT may help the clinicians during clinical work for considering appropriate cleaning, shaping, and obtu-ration technique according to the characteristic of C-shape root canal configuration.
Keywords Cone beam computed tomography . C-shaped canal . Mandibular molar . Prevalence
Anatomical variation of canals is well documented in the lit-erature . C-shaped canal is an anatomical variation mostly found in mandibular second molars with prevalence between 2.7–52% in different populations [2–9]. C-shaped canals were rarely found in other teeth—maxillary first molars [10, 11], mandibular first molars [12, 13], third molars,  and man-dibular premolars . Asymmetry of C-shaped canal in man-dibular second molars ranged between 26.1 to 68.3% in dif-ferent subpopulations [16–18]. The main anatomical feature of C-shaped canal is the presence of a fin or web connecting the individual canals [19, 20]. Moreover, configuration can vary along the root length [19, 21], and C-shaped canals had thin lingual canal wall . Therefore, this ribbon-shaped canal with danger zone may presents unique challenges to clinician during cleaning, shaping, and obturation [23, 24].
C-shaped canals are not diagnosed easily on 2-dimentional X-ray [20, 25, 26]. Cone beam computed tomography (CBCT) is a noninvasive method that permits cross-sectional observation of teeth, enables easy diagnosis of C-shaped canals and investigation of different anatomic features like grooves and notches [5, 27].
Fig. 1 CBCT axial images of mandibular second molars with C-shaped canal. C-shape canal classifications. a Type 1. b Type 2. c Type 3. d Type 4
Data about the prevalence, symmetry, and configuration along the C-shaped root in first and second mandibular molars in an Israeli population was never investigated. This data may be useful for clinical management.
The purpose of this study was to investigate the prevalence, configuration along the root and symmetry of C-shaped canals in the Israeli population, and to create a classification for the radicular groove in first and second mandibular molars.
The study was approved by the Ethics Committee of Medical Corps, IDF (IDF-1258). One thousand twenty patients’ retro-spective full-mouth middle field of view dental CBCT (Alioth, Asahi Roentgen IND, Japan) records, which had al-ready been recorded at an X-ray institute in Israel from 2009 to 2012, were examined.
The patients (447 male and 573 female) were referred to this institute and required a tomographic examination by CBCT as part of their dental examination, diagnosis, and treat-ment planning.
The CBCT images were taken using an ASAHI Alioth CBCT device with 360° X-ray tube head rota-tion. All CBCT scans were reformatted using the stan-dard manufacturer’s settings so that the exposure param-eters of each scan were constant: tube voltage 85 kV, tube current 6 mA, and field of view 80 mm×80 mm, with an isotropic resolution of 0.155 mm. All CBCT exposures were performed with the minimum exposure necessary to obtain adequate image quality.
Personal details including the age and sex of all the patients were recorded. The CBCT images were displayed using OnDemand3D software (CyberMed, Irvine, USA) in a darkroom. The contrast and brightness of the images were adjusted using the image processing tool in the software to ensure optimal visualization. All CBCT anonymized images were analyzed simultaneous-ly by two graduate endodontic residents to reach a con-sensus for the interpretation of the radiographic find-ings. In cases of disagreement, a third, definitive eval-uation was conducted by an endodontist with 10 years of experience. All the evaluators were calibrated by analyzing together random 20 cases of mandibular molars with C-shaped canals using three planes (sagittal, axial, and coronal) slices based on the same criteria and variants.
All dental CBCT which had been recorded from 2009 to 2012 were evaluated. Inclusion criteria for CBCT images were the presence of mandibular molars with fully formed apex. Exclusion criteria for CBCT images were the presence of root canal treatments, posts, or crowns.
First and second mandibular molars were analyzed using cross-sectional slices in the coronal, middle, and apical thirds of the root and were classified in each third according to mod-ified Fan et al. classification of C-shaped canals in mandibular second molars (2004) as follows:
Fig. 2 CBCT axial images of mandibular second molars with C-shaped canals. Radicular groove classifications. a Type 1. b Type 2. c Type 3. d Type 4. e Type 5. Arrows indicate buccal side
Table 1 Analysis of the gender and side distribution of C-shaped ca-nals in mandibular molars in the Israeli population
|No. of teeth||C-shaped canal||P valuea|
|2nd molar Gender||Female||818||42 (5.1%)||0.366|
aNon-parametric Pearson’s chi-squared test
Type 5 of Fan’s classification was not included in our re-search because CBCT does not enable clear visualization of the apical 2 mm of the root canal. A radicular groove classifi-cation of each C-shaped root was created and teeth were clas-sified as follows (Fig. 2):
The Pearson’s chi-squared test was applied to determine statistically significant differences by sex and side of occur-rence. A P value of less than 0.05 was considered significant.
In all 1020 patients, 573 females and 447 males aged 13 to 89 years (average 43.05 years) were included in this study.
A total of 1465 mandibular second molars and 1229 man-dibular first molars were evaluated. C-shaped canals were found in only two mandibular first molars (0.16%), in the same patient. The prevalence of C-shaped canals in mandib-ular second molars was 4.6%. C-shaped canals (Fig. 1a–c) were found in 68 mandibular second molars in 49 patients (Table 1). Seven teeth had a pyramidal canal along the entire root (Fig. 1d) and were excluded from the C-shaped statistics. No statistically significant differences were detected for sex (P > 0.05) or for side of occurrence (left vs. right side, P > 0.05) of C-shaped canals for mandibular second molars (Table 1).
Symmetry was defined as a bilateral occurrence of C-shaped canal and asymmetry was defined as a unilateral C-shaped canal. Therefore, symmetry/asymmetry was evaluated in patients that had both right and left mandibular second molars. Forty-two of the patients with C-shaped canals had both right and left mandibular second molars. In those 42 patients, 45% (19/42 patients) of the C-shaped mandibular second molars occurred bilaterally, while 55% (23/42) had a unilateral C-shaped canal (Fig. 3). Seven patients had a uni-lateral C-shaped canal in a mandibular second molar with a missing homologous tooth; these patients were not included in the statistics of symmetry/asymmetry.
Table 2 indicates that the C-shaped configuration can vary at different levels of the root. Seven teeth had a pyramidal canal along the entire root and were excluded from the C-shaped statistics. In 25 (37%) C-shaped canals (type 1–3), the configuration did not change along the entire root, while in 43 (63%) C-shaped canals, the configuration changed along the root. In coronal third, type 1 was found in 22 (51%) C-shaped canals, while type 2 and type 3 were found in 10 (23.2%) each. In middle third, type 1 was found in only 4 (9.3%) C-shaped canals, while type 2 and type 3 were found in 18 (42%) and 19 (44%) retrospectively. In apical third, type 1 was found in 23 (53.5%) C-shaped canals, while type 2 and type 3 were found in 5 (11.6%) and 8 (18.6%) retrospectively. Out of 68 mandibular second molars with C-shaped canals, 54 (80%) had a longitudinal groove on the lingual root surface (groove type 1 + 2), whereas the other 13 (19%) had a longitu-dinal groove on the buccal root surface (groove type 3 + 4). Thirty-three of the teeth with a lingual longitudinal groove had a buccal notch (groove type 2), whereas 7 of the teeth with a buccal longitudinal groove had a lingual notch (groove type 4) (Table 3). Pyramidal teeth showed no grooves or notches (groove type 5) (Fig. 2e).
Fig. 3 C-shaped mandibular second molars. a Symmetric. b Asymmetric
Table 2 Classification of C-shaped mandibular second molars by modified Fan’s classification
|Type 1||Type 2||Type 3||Type 4||Total|
|Same configuration along entire root||20 (62.5%)||2 (6.3%)||3 (9.4%)||7 (21.8%)||32|
|Coronal third||22 (51.1%)||10 (23.2%)||10 (23.2%)||1 (2.3%)||43|
|Middle third||4 (9.3%)||18 (41.9%)||19 (44.2%)||2 (4.6%)||43|
|Apical third||23 (53.5%)||5 (11.6%)||8 (18.6%)||7 (16.3%)||43|
C-shaped canal anatomy was first reported in the endodontic literature by Cooke and Cox in 1979 . The most widely accepted theory for the formation of this unusual ribbon-shaped canal is the failure of Hertwig’s epithelial root sheath to fuse either at the buccal or at the lingual root surface .
Frequencies of this variation ranged among 39 to 52% in China [2–4], and maximum frequencies of 2.7–10.6% have been found in other populations [5–9]. The sample size in these researches ranged between 75 to 1146 mandibular sec-ond molars. In our research, 1465 mandibular second molars and 1229 mandibular first molars were examined and evaluated.
C-shaped canals are rarely found in teeth other than the mandibular second molars. Some case reports and studies have reported C-shaped canals in maxillary first molars [10, 11], mandibular first molars [12, 13], third molars , and mandibular premolars . Silva et al. investigated 234 first molars and 226 second molars and found that the incidence of C-shaped canals in first molars was 1.7% . In the present research, 1229 mandibular first molars were evaluated, and the preva-lence of C-shaped canal was 0.16%.
Two-dimensional radiograph does not provide data about the cross-sectional morphology of the root canal system. Therefore, diagnosis of C-shaped canals on a two-dimensional radiograph is challenging [20, 25, 26, 30]. Radiographic hints are present in some cases and include fusion of the roots, the convergence of instruments at the apex, and the appearance of furcation perforation by instruments on a working-length radiograph [26, 31]. Because C-shaped canals can have a different configura-tion along the root (Table 2), it is not always possible to identify them after opening the pulp chamber [9, 20]. CBCT is a three-dimensional imaging technique that facil-itates and improves understanding the complex morphologically.
A few classifications of C-shaped canals have been published based on anatomic and radiographic appear-ance. Based on clinical observations, Melton et al. clas-sified C-shaped canals into continuous C-shaped canals, semicolons, and two or more discrete and separate canals . Fan et al. scanned each tooth from the anatomical apex to the crown at 0.5-mm intervals using micro-computed tomography (μCT) and classified C-shaped ca-nals as follows: an uninterrupted BC^ with no separation or division, a semicolon resulting from a discontinuation of the BC^ outline, two or three separate canals, and one round or oval canal in which no canal lumen could be observed . Our study was based on CBCT examina-tion, which does not allow high-quality assessment of the apical 2 mm of the root. Therefore, Fan’s classification was modified, and type 5 was excluded from this study. C-shaped canal morphology was noted in just 4.6% of mandibular second molars, which is in the range reported in Caucasians populations [4, 8, 9].
C-shaped canals are known to present a complex canal anatomy . This anatomy has many variations that may change along the length of the root [19, 32]. Our research supports these data and showed that C-shaped canal configu-ration varied in 63% of cases. In our study, we found that type 1 was most frequent in coronal and apical thirds of C-shaped canals while type 2 and 3 were found more in the middle third of C-shaped canals. This finding is in agreement with previous studies [19, 32].
Root canal morphology and anatomical variations tend to have an asymmetrical appearance [33, 34]. Sabala et al. found that 26.1% of fused-root mandibular second molars are unilateral . A recent study has found that 29.4 and 19% of first and second mandibular molars, respectively, showed asymmetry . In a Brazilian population, 68.3% of C-shaped canals were asymmetrical . Our research revealed that 55% of C-shaped canals were asymmetrical. Therefore, clinicians must be aware of possible asymmetry root canal configuration when treating two homologous molars in the same patient and choose an appropriate cleaning, shaping, and obturation technique [23, 24, 35].
Table 3 Analysis of the radicular groove location
|Taype 1||Taype 2||Taype 3||Taype 4||Taype 5||Total|
|2nd molar C-shaped canal||21 (32%)||32 (48%)||6 (9%)||7 (10%)||1 (1%)||68|
According to the literature, almost all cases of C-shaped molars have a lingual longitudinal radicular groove. Moreover, in most cases, the dentin surrounding the canal is uneven, and the thin part is known as a Bdanger zone^ is located in the mesiolingual direction [20, 22, 36]. Clinically, the thin dentin in the danger zone is prone to perforation and requires appropriate instrumentation technique . In our study, relatively high incidence of buccal groove was found (19% of cases) and in these cases, the danger zone is located in buccal direction. Because of the important clinical influence of location of the longitudinal radicular groove and the danger zone, in the present study, we propose a new classification for grooves and notches in C-shaped canals.
The existence of danger zones in teeth with different loca-tions of grooves and notches and the influence of different cleaning, shaping, and obturation techniques may be issues of interest for further studies.
The prevalence of C-shaped canals in first and second man-dibular molars in the Israeli population in this study was 0.16 and 4.6%, respectively. C-shaped canal configuration varied along the root in 63% of cases and was asymmetrical in 55% of cases. CBCT is a three-dimensional imaging technique that facilitates and improves understanding of the C-shape canal characteristic that may influence cleaning, shaping, and obtu-ration techniques. The location of grooves and notches was different, and a new classification system has been proposed.
Conflict of interest The authors declare that they have no conflict of interest.
Funding This study was not supported.
Ethical approval This article does not contain any studies with human participants or animals performed by any of the authors. The study was approved by the Ethics Committee of Medical Corps, IDF (IDF-1258). The study was performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments.
Informed consent The Ethics Committee approved informed consent dismissal.