Dentin thickness of C-shaped root canal walls in mandibular premolars based on cone-beam computed tomography: a retrospective cross-sectional study

Article information

Restor Dent Endod. 2025;50.e18

Publication date (electronic) : 2025 May 15

doi : https://doi.org/10.5395/rde.2025.50.e18

Elif Aslan^1,, Ali Canberk Ulusoy¹, Bilge Hakan Sen², B. Guniz Baksi¹, Erinc Onem¹, Ali Mert³

¹Department of Oral and Maxillofacial Radiology, School of Dentistry, Ege University, Izmir, Türkiye

²Department of Endodontics, School of Dentistry, Ege University, Izmir, Türkiye

³Department of Statistics, School of Science, Ege University, Izmir, Türkiye

Citation: Aslan E, Ulusoy AC, Sen BH, Baksi BG, Onem E, Mert A. Dentin thickness of C-shaped root canal walls in mandibular premolars based on cone-beam computed tomography: a retrospective cross-sectional study. Restor Dent Endod 2025;50(2):e18.

^*Correspondence to Elif Aslan, DDS, MS Department of Oral and Maxillofacial Radiology, School of Dentistry, Ege University, 35100 Bornova, Izmir, Türkiye Email: aslanelif090@gmail.com

Ali Mert's current affiliation: Department of Engineering Sciences, School of Engineering and Architecture, Izmir Katip Celebi University, Izmir, Türkiye

Received 2025 February 20; Revised 2025 March 31; Accepted 2025 March 31.

Abstract

INTRODUCTION

The C-shaped canal morphology is an anatomical variation in which independent root canals are merged or connected through isthmuses or fins, forming a C-shaped appearance at cross-sectional views. As reported previously, it is a well-defined anatomical variation frequently related to mandibular second molars, and it has been thoroughly studied, including its anatomic, histologic, and radiologic features [1–3].

C-shaped canal morphology in mandibular molars is often associated with the fusion of the roots and a longitudinal radicular groove on the root surface [2,4]. The radicular groove is described as a developmental depression area that occurs because of root fusion and creates a danger zone, increasing the risk of stripping perforation during endodontic preparation [5]. It has been reported that the root canal wall facing the radicular groove was thinner than the other walls in C-shaped mandibular molars [3]. In accordance with this finding, histological evaluation of C-shaped mandibular second molars demonstrated that the dentin thickness between the root canal and the external surface of the root was less than 1 mm in some sections [6].

Although limited in number, recent case reports and scientific studies have also described the presence of C-shaped canal morphology in mandibular premolars [7–11]. The prevalence of C-shaped canal configuration in mandibular premolars varies depending on ethnicity, and its prevalence was reported to be between 1.1% to 10.9% for mandibular first premolars and 0.6% to 2% for second premolars [9–11]. While the Chinese population demonstrated a lower prevalence of C-shaped canal morphology [9], the prevalence rates were relatively higher in the Argentine population [11]. Morphologic features of C-shaped canals in mandibular premolars have been found similar to C-shaped mandibular molars, including longitudinal radicular groove [5,7]. Due to the smaller size of mandibular premolar teeth, the longitudinal radicular groove is particularly important and requires considerable attention with regard to endodontic mishaps [5]. Considering anatomical complexities including isthmuses, transverse anastomoses, irregularities, concavities, and thin dentin thickness, endodontic treatment of C-shaped root canals presents a clinical challenge. Removal of pulp and necrotic tissue, working length determination, and proper irrigation and disinfection will be key problems during root canal instrumentation and shaping. Consequently, the awareness of dentin thickness in premolars with c-shaped canals is particularly important to maintain the integrity of the canal walls.

To our knowledge, there are few studies evaluating the root dentin thickness of C-shaped mandibular first premolars [12–15]. Given the low prevalence of C-shaped morphology in mandibular second premolars, the number of scientific reports evaluating the prevalence and morphology of C-shaped canals in such teeth is even scarce [11,16]. In fact, no study can be found investigating the root dentin thickness of C-shaped mandibular second premolar teeth. Therefore, the aim of this study was to measure the dentin thickness of C-shaped canals in both mandibular first and second premolars at coronal, middle, and apical root levels using cone-beam computed tomography (CBCT) images.

METHODS

This study was approved by the Medical Research Ethics Committee of Ege University (No. 22-2.1T/33) and followed the principles of the Declaration of Helsinki. A general formal consent is obtained from every patient who applies to our clinic, stating that the data can be used in an anonymized fashion for scientific purposes.

A total of 2,024 CBCT volumes from 1,061 patients of the Turkish population (654 females, 407 males; mean age, 31.57 ± 15.35 years) taken for various reasons between the years of October 2018 and March 2020 at the Department of Oral and Maxillofacial Radiology were examined retrospectively. Images of patients showing at least one C-shaped mandibular premolar tooth were included in the study. Teeth with previous endodontic treatment, internal and/or external root resorptions, immature apices, periapical lesions, full-crown restorations, and CBCT images with severe artifacts were excluded. CBCT images had been previously obtained using a Kodak 9000 3D device (Kodak Carestream Health, Trophy, France) at 70 kV, 10 mA, and 10.8 seconds exposure time, using a 50 × 37-mm small field of view (FOV), and a 76-μm isotropic voxel resolution.

The diagnosis of C-shaped configuration was made according to the description of Fan et al. [7] and teeth with a partial or complete external radicular groove on the mesiolingual or distolingual root surface and C1 or C2 canal configuration at any axial root level (coronal, middle, and apical) were considered as having C-shaped canal morphology. According to Fan et al. [7], C1 canal configuration was identified as a continuous “C” with no separation or division, whereas C2 was discontinuous in the “C” outline resembling a semicolon (Table 1). When a C1 or C2 configuration was detected at any axial root level, coronal and sagittal CBCT slices were also examined to confirm the root level of the C shape (Figure 1). Forty-nine premolar teeth of 37 patients were determined to have a C-shaped canal configuration. Images of five first premolars and three second premolars were excluded due to the presence of severe artifacts and low image resolution interfering with the correct measurement of dentin thickness.

Table 1.

Classification of C-shaped canal configuration in mandibular premolar teeth according to Fan et al. [7]

Figure 1.

Cone-beam computed tomography (CBCT) image illustrating the diagnosis of C-shaped configuration. (A) Coronal and (B) sagittal CBCT images showing a C-shaped mandibular first premolar. Axial CBCT images showing (C) a C4-type configuration at the coronal level, (D) a C2-type configuration at the middle level, and (E) a C3-type configuration at the apical level. CEJ, cementoenamel junction.

Concisely, 39 small FOV CBCT volumes of 41 C-shaped mandibular premolars (32 first and nine second premolars) of 33 patients with a mean age of 31.8 ± 14.6 years were used to measure dentin thicknesses at three different root levels (coronal, middle, and apical) on axial CBCT slices.

CS 3D Imaging Software (Kodak Carestream Health) was used for the measurements of dentin thicknesses. The root lengths of premolars were measured from the cementoenamel junction (CEJ) to the root apex and divided into three equal parts. The first level beginning from the CEJ to the middle third of the root was named as the coronal level, the third level beginning from the apex to the point where the middle third ends was named as the apical level and the level between the coronal and apical levels was named as the middle level (Figure 2A). At each root level where C-shaped canal morphology was present, the dentin thicknesses were measured on three consecutive axial CBCT slices: midpoint of each root level, 1 mm below the midpoint, and 1 mm above the midpoint (Figure 2B).

Figure 2.

Coronal cone-beam computed tomography images showing (A) the coronal, middle, and apical root levels beginning from the cementoenamel junction (CEJ) to the root apex and (B) the dentin thickness measurement slices and midpoints of coronal (MP-c), middle (MP-m), and apical (MP-a) root thirds.

The dentin thicknesses measured on three consecutive CBCT slices were calculated as the mean wall thickness and recorded along with the C-shaped canal type (C1 or C2).

Dentin thickness measurements were made on eight separate dentin walls at all root levels, demonstrating a C-shaped canal configuration. For each root level presenting with C1 or C2 configuration, primarily, thicknesses of buccal (Bu), lingual (Li), distal (D), and mesial (M) walls were measured. Then additional thickness measurements were done at both buccal and lingual canal walls as the distances between: (1) mesial end of the buccal canal and the external root surface facing the radicular groove (M1), (2) distal end of the buccal canal and the external root surface facing the radicular groove (M2), (3) lingual canal and the external mesial root surface facing the radicular groove (M3), (4) mesial end of the buccal canal and external mesial root surface (M4), (5) lingual canal and external distal root surface (D1) and (6) distal end of the buccal canal and the external distal root surface (D2) (Figure 3) [12].

Figure 3.

Axial cone-beam computed tomography images showing (A) the root dentin thickness measurements of buccal (Bu) and lingual (Li) walls, (B) mesial and distal ends of the buccal canal facing the radicular groove (M1, M2), (C) between mesial ends of lingual and buccal canals and the external mesial root surface (M3, M4) and (D) between distal ends of lingual and buccal canals and external distal root surface (D1, D2).

Two investigators performed the measurements separately, and the mean of their measurements was used for statistical analysis.

Analysis of variance and Kruskal-Wallis tests were used to compare the dentin thickness measurements of different dentin walls. The independent samples t-test was used to compare the dentin thickness measurements of the first and second premolars, and C-shape canal types (C1 and C2). For all groups, a p-value of <0.05 was considered significant.

RESULTS

Among 32 first premolars, C-shaped morphology was present at the coronal level of three teeth, at the middle level of 29 teeth, and at the apical level of 16 teeth. All nine second premolar teeth showed a C-shaped configuration at the middle level, and four of them also continued at the apical level. No C-shaped morphology was found at the coronal level of the second premolars. Both first and second premolars exhibited an external radicular groove on the mesiolingual surface of the root.

In general, mean dentin thicknesses for first premolars were listed as Bu, Li, D2, M4, M2, M1, D1, and M3 beginning from the thickest to thinnest at all three root levels. For the second premolars, the list was similar to the first premolars; however, M1 was the thinnest wall, followed by M3 and D1 at all root levels.

Root dentin thickness at coronal level

Table 2 shows the mean dentin thicknesses of C-shaped first and second premolars at the coronal, middle, and apical levels of the root. No difference was found between the mean dentin thicknesses of any walls at the coronal level of the first premolars (p > 0.05). Since no C-shaped morphology was found at the coronal level of the second premolars, no comparison could be made.

Table 2.

The root dentin thickness measurements of C-shaped first and second premolars at coronal, middle, and apical levels of root dentin

Comparison of the first and second premolars for root dentin thickness of C1- and C2-type canals

The mean dentin thicknesses of the C1- and C2-shaped first premolars are presented in Table 3. Among 32 C-shaped first premolars, six canals exhibited C1-type and 26 canals showed C2-type canal configuration (Figure 4A and D, 4B and E). The buccal wall was the thickest, followed by L and D2 walls at all levels of C1- and C2-type premolar roots; however, the difference was significant only at middle and apical levels of the C2-type first premolars (p < 0.05). At the apical level, the M1 and M3 walls of C1-type first premolars were thinner than those of C2-type first premolars (p = 0.036 and p = 0.02, respectively). Since all of the nine second premolars showed only C2 canal configuration (Figure 4C and F), no further comparison could be done.

Table 3.

The root dentin thickness measurements of C1 and C2-shaped first premolars

Figure 4.

Cone-beam computed tomography (CBCT) images depicting the most prevalent C-shaped canal configurations in mandibular first and second premolars. Coronal and axial CBCT images showing (A, D) mandibular first premolar with a C2-type canal configuration, (B, E) mandibular first premolar with a C1-type canal configuration, and (C, F) mandibular second premolar with a C2-type canal configuration.

DISCUSSION

The C-shaped canal morphology is a challenging anatomical variation associated dominantly with mandibular second molars. However, it can be seen in mandibular premolar and maxillary molar teeth as well [8,17]. Although few studies have investigated the C-shaped canal morphology in premolar teeth, it has been generally accepted that the C-shaped configuration has a lower prevalence in mandibular premolars as compared to mandibular molar teeth. Yet, similar anatomical characteristics were reported [5,7]. A longitudinal radicular groove on the external root surface, which is associated with the presence of C-shaped canal anatomy, is a common feature in both C-shaped mandibular molar and premolar teeth [4,7]. It is acknowledged that the proximity of C-shaped canals to radicular grooves, as well as the increase in the depth of the groove, creates danger zones resulting in decreased thickness in dentin walls of the root canal, creating a higher risk for stripping perforation [18,19]. Therefore, knowledge of the minimum thickness of dentin walls close to the radicular groove is critical to avoid complications and failures during endodontic instrumentation.

The C-shaped canal morphology, including the structural features of danger zones, has been extensively evaluated in maxillary and mandibular molars [20,21] and maxillary premolars [22,23]. On the other hand, there are only a couple of studies that have evaluated the prevalence and morphology of C-shaped canals in mandibular first and second premolar teeth [8–10]. However, to our knowledge, the presence of danger zones, in other words, the assessment of root dentin thickness in C-shaped mandibular first premolars, has been rarely studied [12–14], and no study has investigated the root dentin thickness in C-shaped mandibular second premolars. Furthermore, all of the above-mentioned studies have used micro-computed tomography (micro-CT) and ex vivo methodology for the evaluation of C-shaped canals. Therefore, the present study is the first to evaluate the root dentin thicknesses of C-shaped second premolar teeth along with the first premolars using clinical CBCT images.

According to the results obtained, the cross-sectional morphology of the root canals of mandibular premolar teeth showed a wide range of anatomical variations from the coronal to the apical root level. According to the results, C-shaped morphology was rarely observed in the coronal level of first premolars and not seen at the coronal level of second premolars. C2 was predominant in the middle and apical root levels of both first and second premolars. These findings suggest complex canal morphology in C-shaped premolars is compelling for clinicians both from a diagnostic and practical perspective. Anatomically complex canals or additional root canals in mandibular premolar teeth may be easily overlooked in periapical radiographs. Moreover, the C-shaped canals cannot be easily detected with two-dimensional images [24]. Even though the sudden narrowing of the root canal and a change in density as well as taurodont appearance [25] resulting from the deeply placed pulp chamber [26] in periapical images give a clue about the canal bifurcation (groove) signaling a thin root dentin, definitive diagnosis of C-shaped canal and details regarding morphology requires three-dimensional imaging [27]. In this context, CBCT is the primary three-dimensional imaging method widely recommended for the diagnosis of anatomically complex root canal systems since the clinical use of micro-CT is not suitable [28]. However, it should be noted that, as used in the present study, only high-resolution images with sub-millimeter spatial resolution obtained with a limited FOV are applicable for the detection of C-shaped canals and for the accuracy of measurement of dentin wall thicknesses.

Our results revealed that the thinnest dentin wall in the first premolars was M3, while it was M1 wall for the second premolar teeth. This finding supports the previous results that all C-shaped premolars had an external radicular groove in the mesiolingual position, and the minimum dentin thickness was dominantly found at the mesial walls in the middle and apical root thirds [12,14]. Similar to the presented findings, previous studies have demonstrated that D1 and M3 walls were significantly thinner than the other dentin walls for the C-shaped mandibular first premolars and that the mesial wall was significantly thinner than the distal wall at the middle and apical thirds of C-shaped first premolars [12,14]. This finding is also valid for second premolar teeth however, no significant difference was found between dentin thicknesses of M3 and D1 walls at the apical third of C-shaped second premolars. The discrepancy in the results may be due to the use of different classification methods for C-shaped canals, as well as due to the quality of the images used to evaluate C-shaped morphology and measure dentin thicknesses. Many previous studies have demonstrated that the spatial resolution of the images has a significant impact on measurement accuracy [29–31].

In accordance with previous findings, the presented results revealed that danger zones for mandibular first premolars are D1 and M3 walls [13]. Given the low prevalence of C-shaped morphology in premolar teeth [10,16], no studies can be found in the literature that have investigated the root dentin thickness of mandibular second premolars. Our findings revealed that in addition to the D1 and M3 walls, dentin thicknesses of M1 and M2 walls were thinner than 0.5 mm and therefore present high potential danger zones for mandibular second premolars at the apical root third. Even though the sample size of C-shaped second premolars was low in this study, our findings provide important information regarding the complex morphology of C-shaped canals and the thinness of M1, M2, M3, and D1 walls of mandibular second premolars. Although more studies with higher sample sizes are required to support these findings, it is still possible to recommend meticulous choice of endodontic instrument type and use of advanced equipment such as operating microscopes and dental loupes to sufficiently visualize and locate root canals with complex anatomy and avoid perforation in these thin dentin zones of second premolars. It may be advisable to further recommend the use of conservative shaping methods, meticulous irrigation, and warm gutta-percha obturation techniques during endodontic treatment of C-shaped premolar teeth [32–34].

According to the classification of Fan et al. [7], C1 and C2 were the most common C-shaped canal configurations often observed at the middle and apical levels. Using Fan’s classification, Gu et al. [12] did not find any difference in root dentin thicknesses of C1 and C2 canal configurations. Contrary to this finding, we demonstrated a significant difference in dentin thicknesses of M1 and M3 walls at the apical level of C2-shaped first premolars. This finding is important considering that the C-shaped canal in mandibular premolar teeth may not continue as a single C-shaped form from the CEJ up to the apex [7,25]. In this case, C-shaped morphology may go easily undetected and may create an increased risk of stripping perforation, inadequate irrigation and disinfection, and accordingly, incomplete obturation, which may all lead to endodontic treatment failure.

The mean dentin thickness of C-shaped canals was reported to range from 0.25 to 0.66 mm in the apical and middle portions of the root [35]. In our study, these values varied from 0.36 mm to 1.44 mm for the same regions. As these measurements have been done before root canal instrumentation, one can expect that the root canals would be thinner, particularly in apical regions, after instrumentation. If conservative techniques such as anti-curvature filing and rotary instrumentation with low tapers are not employed, excessive dentin may be removed, causing complications from ledge formation to perforation [36–38]. In addition, root canal filling of C-shaped teeth also poses a problem. It has been speculated that compaction forces during obturation can exceed the resistance of the root canal wall when the remaining thickness of dentin is 0.2 to 0.3 mm after instrumentation [39]. This may inevitably cause a root fracture. Therefore, warm gutta-percha techniques instead of cold lateral compaction will be safer and more suitable for the three-dimensional filling of C-shaped root canals [38,40].

CONCLUSIONS

The findings suggest that C-shaped canals in mandibular first and second premolars were predominantly found in the middle third of the root. In general, while the buccal and lingual dentin walls were the thickest, the mesial dentin walls facing the external radicular groove were the thinnest. The results revealed that potential danger zones for C-shaped mandibular first and second premolars are predominantly the mesial and distal walls of the lingual canal. However, the mesial walls of buccal canal facing the external radicular groove were extremely thin at the apical root third of second premolar teeth, demonstrating additional danger zones.

Clinicians should be aware of this variation in dentin thicknesses of C-shaped root canals in mandibular premolars. CBCT imaging and further precautions are recommended to avoid complications during endodontic treatment of C-shaped root canals of mandibular premolar teeth.

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