Appropriate cleaning and shaping and three-dimensional (3D) obturation of the root canal system lead to a successful endodontic treatment. To achieve this, complete knowledge of the internal anatomy of root canals is necessary. ¹ , ² , ³ Anatomical variations cause difficulties in the process of proper debridement of root canals. Mandibular incisors, premolars, and second molar are reported among teeth with the highest incidence of root canal variation. For instance, the incidence of the second canal in mandibular incisors was reported 27.5% by Vertucci, ⁴ 15% by Miyashita, ⁵ and 26.2% by Al-Qudah and Awawdeh. ⁶ The incidence of canal variation in mandibular premolars was reported 16.5% by Liao et al., ⁷ 12.9% by Yu et al., ⁸ and <8% by Ok et al. ⁹ Mandibular first molar is typically reported as a two-rooted tooth in ≥85% of cases, ¹⁰ whereas mandibular second molar shows a wider range of anatomic variation. The incidence of C-type mandibular second molar was reported 43.3% by Chen et al., ¹¹ and it is reported as the most common teeth with C-type canal configuration. ¹²

Ethnic background can affect root canal anatomy; therefore, documenting the incidence of root canal variation seems necessary. There are plenty of studies conducted on the Iranian population to report anatomical classification of root canals, ² , ³ , ¹³ , ¹⁴ but their results are somehow controversial and their report is not comprehensive. Many of these studies used conventional radiograph that is less accurate than 3D imaging ¹⁵ or clearing and staining technique that requires tooth extraction and is not applicable in the clinic.

Therefore, this study evaluated the number and shape of mandibular premolar roots and canals and classified their Vertucci classification using cone-beam computed tomography (CBCT) images of an Iranian sample population.

This in vivo study was performed on CBCT images of patients aged 20–70 years who had attended a dental radiology center in Isfahan. All CBCTs had been retrospectively taken solely for clinical purposes. No X-ray was emitted to patients for this study. All CBCTs had been taken with the same unit (Soredex, Tuusula, Finland), with a similar field of view (8 cm × 5 cm), focal size (0.3 mm), current (10 mA), peak kilovoltage (90 kVp), and time (0.4 s). Inclusion criteria were availability of at least three premolar teeth for each patient, and full patient information was obtained. Exclusion criteria were teeth with previous root canal treatment, resorption, open apex, agenesis, and dental fractures. A total of 213 patients with full arch mandibular CBCT scans were included.

All measurements were done by two observers (an oral radiologist and an endodontic resident), using an OnDemand3D software program (Cybermed Inc., Korea Rep.). CBCT images were examined in coronal, sagittal, and especially axial dimensions Figure 1. The radiographic measurements were repeated 1 week later for the evaluation of inter- and intraobserver reliability. Figure 1

CBCT images in axial view (a) mandibular first premolar with one canal, (b) mandibular premolar with one large canal, (c) mandibular premolar with two root canals and cortical bone destruction due to periapical lesion, (d) mandibular premolars with three root canals, (e) coronal view of a mandibular second premolar with periapical lesion, (f) sagittal view of a mandibular second premolar with periapical lesion.

The statistical analysis showed high inter- and intraobserver reliability (κ = 0.987) (P ≤ 0.05) Table 1.{Table 1}

There were two cases of disagreement between observers; in these two particular cases, readings from the maxillofacial radiologist were selected as the correct result.

Of 213 enrolled patients, 106 were male and 107 were female. Patients' average age was 32.6 ± 13.23 years. Of these patients, 153 (71.84%) were aged between 20 and 49 years, whereas 60 (28.16%) were aged between 50 and 70 years.

Among 773 assessed teeth, 389 (50.32%) were first premolar and 384 (49.68%) were second premolar.

Number of roots and canals

Table 2summarizes the number of roots and canals in each tooth. 88.69%, 10.54%, and 0.77% of first premolars had one, two, and three roots, respectively. 88.8%, 9.11%, and 2.09% of second premolars had one, two, and three roots, respectively Table 2. There were no significant differences between males and females in terms of the number of roots in first (P = 0.175) and second premolars (P = 0.195).{Table 2}

Vertucci classification

Table 3presents Vertucci classes and Vertucci modifications. In both first and second premolars, the most common classes were Type I followed by Type V. The Chi-square did not show significant differences between males and females in terms of Vertucci classes in the first (P = 0.305) or second premolar (P = 0.315).{Table 3}

The findings of the current study showed that 88.69% of mandibular first premolars had one root and 81.49% of them had one root canal. Our findings were almost similar to other studies conducted in Iran. Khedmat et al. ¹⁸ reported that 88.5% of mandibular first premolars had a single root canal. Sobhani et al. ¹⁹ reported that 87.3% of these teeth had one root canal, whereas Rahimi et al. ²⁰ stated that 70.6% of mandibular first premolars had one canal. In studies conducted in India and India ²¹ , ²² and Spain ^,[23] the prevalence of single canal mandibular first premolar was reported 88.4%, 80.8%, and 83.3%, respectively. Table 4summarizes some studies analyzing mandibular first premolar roots, and Table 5summarizes studies analyzing mandibular first premolar root canals. The most common canal configuration in the first premolars in our study was Vertucci Type I which was similar to other studies conducted in Iran and the rest of the world. ¹⁸ , ¹⁹ , ²⁰ , ²¹ , ²² , ²³ , ²⁶ , ²⁵ , ²⁹ , ³⁰ , ³² , ³³ , ³⁴ {Table 4}{Table 5}

Data on mandibular second premolar are less than the first premolar. Our study showed that 88.80% of mandibular second premolars had one root canal and 83.6% of them had a single root canal.

Our findings were not consistent with Rahimi et al. findings ²⁰ since they reported all of their second premolar samples had a single root and 80.5% of them had a single canal. Iyer et al. ²⁸ in a study conducted in Syria reported that 97% of second premolars had one root and the rest of them had two roots. Singh and Pawar ²³ reported that 92% of second premolars had one root and 8% showed two roots. Table 4summarizes some studies analyzing mandibular second premolar roots, and Table 5summarizes studies analyzing mandibular second premolar root canals. The most common canal configuration in the second premolars in our study was Vertucci Type I, which was similar to other studies conducted in Iran and the rest of the world. ²⁰ , ²⁴ , ²⁵ , ²⁶

The findings of the current study on the number of two- and three-rooted second premolars were inconsistent with other studies, which may be a result of different sample sizes and racial differences.

Since a thorough knowledge of root canal anatomy is necessary for successful root canal treatment and almost one out of ten mandibular premolars has additional canal in their root canal system, accurate evaluating of preoperative radiographs, taking CBCT images if periapical radiograph demonstrates atypical anatomy, and probing the root canal system with fine and precurved files should be done to negotiate the entire canal system. 35

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Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.