Introduction

For successful endodontic treatment, the root canal system must be completely chemo-mechanically cleaned, prepared, and obturated (1). As is well known, these processes, which are essential for endodontic treatment, are affected by root canal anatomy (2). Therefore, knowledge of the main canals, additional canals or other anatomical variations for the elimination of incomplete instrumentation causing apical periodontitis may increase the success of endodontic treatment (1-3).

Mandibular permanent first molars (ManFMs) are frequently treated endodontically and usually have two roots, (distal and mesial) and three canals (two mesial and one distal). A common variation in ManFMs is the presence of a third root, which may be either buccal (radix paramolaris) or lingual [radix entomolaris (RE)] (1,4). This additional root may frequently cause procedural errors during endodontic treatment because it is smaller and separate from the other roots or fused with these roots. Radix paramolaris and RE are also curved. Previous studies showed that this anatomical variation needed to be considered in various dental procedures, such as surgical extractions, periodontal treatment, orthodontic treatment, and root canal treatment (1,4).

Mandibular central incisors (ManCIs) usually have a single root and a single canal. However, an additional canal or root is a common anatomical variation among ManCIs (2). To ensure the success of endodontic treatment and other dental procedures, extensive evaluations of this anatomical variation should be conducted prior to treatment (2,3).

Many studies have investigated the root canal anatomy in different populations using extracted teeth and periapical and panoramic radiographs (5,6). Although periapical and panoramic radiographs are important diagnostic tools for evaluating root canal configurations, they are not completely reliable due to limitations, such as distortion and superposition in the images obtained. Furthermore, studies of root canal configurations of extracted teeth are not always possible for legal and ethical reasons (6). Recently, cone-beam computed tomography (CBCT) has been widely used in dentistry because it allows clinicians to display an area in three different planes. The combination of CBCT images in the sagittal, coronal, and axial section eliminates the superposition of anatomical structures, giving the clinician a more complete view of the true three-dimensional morphology of root canal systems (7,8).

In the literature it was reported that the existence of RE in ManFMs may be associated with the presence of additional canals in ManCIs (9). Thus far, no study has investigated this association in a Turkish population. The aim of this study was to investigate the association between the existence of RE in ManFMs and the presence of complicated root canal configurations of ManCIs in a Turkish population.

Materials and Methods

Study Group

Due to the retrospective design of the study, the consent was not required from the patients. The study was approved by the Local Ethics Committee of Bolu Abant İzzet Baysal University (decision no: 2018/132 date: 06.09.2018). The study was performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments. The CBCT images included in this study were obtained from the Faculty of Dentistry of Bolu Abant İzzet Baysal University. In total, 1,250 CBCT images taken between 2014 and 2018 were obtained. Only CBCT images of good image quality in which the root canal configurations and canal orifices could be easily determined were included in the study. Only teeth without dental anomalies, periapical pathologies, root resorption, previous hemisection or root amputation treatment, large metallic restorations, coronal or post-core restorations, and root canal treatment were included. In all the included ManFMs and ManCIs, complete root formations were present bilaterally.

The final group included images obtained from 534 patients [319 females (59.7%) and 215 males (40.3%)] and 1,068 ManFMs and 1,068 ManCIs.

Imaging Procedures

The CBCT scans were obtained using an I-Cat imaging system (Imaging Sciences International, Hatfield, PA, USA) at 120 kVp and 15 mA, with a voxel size of 0.2 mm and an exposure time of 8.9 sec.

The presence or absence of RE and the existence of complicated ManCI canal configurations, including the type of configuration, were investigated using i-CAT vision Q 1.9 imaging software (Imaging Sciences International LLC. Hatfield, PA, USA). Tomography slices of 0.2 mm in the coronal, axial, and sagittal views were produced. To determine the number of canals and roots and the canal configurations in axial tomography slices, we investigated all sections throughout from the bottom of the pulp chamber to the root apex.

Morphological Analysis and Classification

Two observers evaluated all the CBCT images: an oral and maxillofacial radiologist with 6 years of experience and an endodontist with 6 years of experience. Observer calibration was achieved by having the two observers evaluate 20% of the CBCT scans at the beginning of the study. Intra-observer reliability was assessed by calculating the correlation coefficients (for observer one and observer two; 0.857 and 0.861, respectively). The observers were permitted to use image enhancement filters to change the intensity, contrast, or brightness of the images. A consensus was established in cases where there was inconsistency between the observers in the determination of morphological differences. Interobserver reliability was assessed when all the measurements were performed. The correlation coefficient was 0.908.

The presence of RE in the ManFMs was evaluated according to previous studies (9-11). The RE status of the patients was classified as follows (Figure 1 A-D):

No RE: RE was not found in either the left or right side;

Unilateral RE: RE was found in just one ManFM on either the left or right side, and no RE was detected in the other ManFM on the other side;

Bilateral RE: RE was found in both the right and left sides of ManFMs.

To determine the root canal configurations of ManCIs, a series of axial images and cross-sectioned images were examined from the cemento-enamel junction to the root apex. The ManCI canal configurations were then classified in accordance with a system used in previous studies (9,12), as follows:

A single canal: the presence of one root and one canal.

A complicated canal: the occurrence of more than one root and canal.

The symmetric distribution arrangement of complicated ManCI canals was categorized as follows: bilateral single, unilateral complicated or bilateral complicated (Figure 2 A-C).

The root canal configuration of the teeth was classified according to Vertucci’s classification (13).

Statistical Analysis

In each analysis, frequencies and percentages were calculated at tooth and patient levels. The chi-square test was used for examining differences in categorical variables, such as side (left/right) and sex (male/female), root canal configuration (single/complicated), and RE group (no RE, unilateral RE, or bilateral RE) (9). To assess the effect on the canal configurations of ManCIs, a multivariate logistic regression analysis was used and was simultaneously adjusted for other variables (side, age, and sex). The statistical analysis was performed using SPSS for Windows (Version 22.0; SPSS, Inc., Chicago, IL, USA). The statistically significant level was set at p<0.05.

Results

As shown by the results, 18.6% (199/1,068) of ManFMs had RE, and 22.8% (244/1,068) of ManCIs had a complicated root canal configuration. Only Vertucci’s type I and type III canal configurations were seen in ManCIs (Figure 3). In 74.2% of ManCIs, the canal configuration was type I, and it was type III in 25.8% of ManCIs.

Based on the CBCT images, 11.7% (63/534) of the patients had bilateral RE, and 12.1% (65/534) had unilateral RE (Figure 4A). Among the patients, 15.9% (86/534) had ManCIs with a bilateral complicated canal configuration, 73% (394/534) had ManCIs with a bilateral single canal, and 10% (54/534) had ManCIs with a unilateral complex canal configuration (Figure 4B).

Table 1 shows the prevalence of RE in ManFMs and complicated ManCI canal configurations according to side and sex. The prevalence of RE was higher in males (24.7%) than in females (13.8%) (p=0.002). The ratios of complicated ManCI canal configurations and RE in ManFMs were similar on the right and left sides (p=0.215 and p=0.753, respectively).

There was a significant sex-related difference in the prevalence of RE in ManFMs (p=0.003) (Figure 5A), whereas there was no significant sex- or side-related related difference in the prevalence of complicated ManCI root canal configurations (p=0.443) (Figure 5B). There was a positive correlation between complicated ManCI root canal configurations and ManFMs with RE (p<0.001, r=0.248) (Figure 5C).

The multivariate analysis was adjusted for categorical variables, such as sex and sides. The adjusted odds ratios for the frequency of complicated ManCI root canal configurations in the bilateral RE group and unilateral RE group were 0.18 (95% confidence interval, 0.103-0.322; p<0.001) and 1.54 (95% confidence interval, 0.764-3.129; p=0.226), respectively (Table 2).

Discussion

Anatomical variations in root canal anatomy can cause difficulties in diagnosis and treatment. To ensure the success of dental treatments, the practitioner must have knowledge of the root canal anatomy. Although many studies have investigated the root canal anatomy of different tooth groups in different populations, only a few studies have investigated the relationship between variations in root canal anatomy (9,14).

In previous studies, the prevalence of RE in ManFMs differed, depending on the population (15,16). The prevalence of RE in ManFMs was reported to be higher in populations of Mongoloid origin (Chinese, Japanese, Korean, and Taiwanese) (5.8-43.6%) as compared with that in white populations (0.68-4.2%) (4). In this study, the prevalence of RE in ManFMs in a Turkish population was 18.6%. Although the etiology of RE is not known, previous research reported that external factors in the odontogenesis phase, penetrance of an atavistic gene, or a polygenetic system may influence the etiology (17). Studies also suggested that the presence of RE depended to a high degree on racial genetic factors (17,18). Therefore, we think that genetic differences between populations most likely explain the discord in the findings of the present study as compared with those in the literature.

A previous research reported root canal anatomy variations of ManCIs (19), with a number of studies reporting that ManCIs most commonly have type I or type III Vertucci root canal configurations (12,20). In line with these results, in this study, ManCIs with type I (74.2%) and type III (25.8%) root canal configurations were the most common.

Wu et al. (9) reported that there was a correlation between the presence of RE in ManFMs and complicated ManCI root canal configurations in a Taiwanese population. Similarly, we found a correlation between the presence of RE in ManFMs and complicated ManCI root canal configurations in this Turkish population. No previous studies have investigated this relationship in a Turkish population. Based on the findings of the present study, if RE should be present, and there should be a suspicion that complicated canal configuration in ManCIs. Therefore, the results of this study can benefit clinical management.

A number of studies on symmetry in dental structures found that many anatomical variations were symmetrical and irrespective of the region (17,19,21). In patients with RE, Wu et al. (9) reported a 54.6% prevalence of the bilateral type in ManFMs. In this study, among those with RE, the prevalence of the bilateral type was 49.2%, and the prevalence of RE did not show a significant difference between the sides. In contrast, Schäfer et al. (15) detected no cases of bilateral RE in ManFMs in a German population. Similarly, other studies reported that anatomical variations differed significantly, depending on the side (16,18). According to some studies, the prevalence of RE was greater on the right side (16,22) whereas others found that it was greater on the left side (19). As stated previously in the literature (17), genetic variations between different populations most likely explain this difference.

Roots or canals in the same buccolingual plane in both ManFMs and ManCIs may cause superposition on two-dimensional radiographs. As a result, additional roots or canals may not be detected. To overcome this limitation and detect additional roots and canals, radiographs may be taken at different angles (23). However, the latter increases radiation exposure and is time consuming. In the identification of additional canals and roots, CBCT has been shown to provide more accurate results, without causing superposition and distortion as compared with two-dimensional radiographs (10). The voxel size is one of the most important factors that affects the image resolution in CBCT, with the smaller the voxel size, the better the quality and detail of the resulting image. However, a reduction in the voxel size in CBCT devices increases the exposure to radiation. In this study, CBCT images with a voxel size of 0.2 mm were obtained. This size is considered the maximum ideal size for evaluating root canal anatomy (23,24).

In a previous study, the presence of RE in ManFMs was reported to be related to a C-shaped canal configuration and radicular grooves in mandibular first premolar teeth (14). Zhang et al. (25) evaluated the morphological characteristics of RE in a Chinese population, and De Moor (26) classified RE according to the RE type. The relationship between the presence of RE and root canal configurations of teeth other than ManFMs, in addition to the morphological characteristics of RE, should be determined in a Turkish population in future studies.

Conclusion

The results revealed a correlation between the presence of RE in ManFMs and the existence of complicated ManCI root canal configurations in this Turkish population. RE should be considered to be present in the presence of a complicated root canal configuration.

Ethics

Ethics Committee Approval: The study was approved by the Local Ethics Committee of Bolu Abant İzzet Baysal University (decision no: 2018/132 date: 06.09.2018).

Informed Consent: Retrospective study.

Peer-review: Externally and internally peer-reviewed.

Authorship Contributions:

Surgical and Medical Practicies: Z.U.A., Concept: D.G.B., Z.U.A., Design: D.G.B., Z.U.A., Data Collection or Processing: D.G.B., Analysis or Interpretation: Z.U.A., Literature Search: D.G.B., Z.U.A., Writing: D.G.B., Z.U.A.

Conflict of Interest: No conflict of interest was declared by the authors.

Financial Disclosure: The authors declared that this study received no financial support.