Introduction

The scientific and technological advances in the last decades have been responsible for developing new materials, devices, instruments, kinematics, and techniques, which have substantially modified the modus operandi of Endodontics. However, the inability or impossibility of identifying root canals prevents effective processes for cleaning, disinfection, and filling the root canal system (RCS), compromising the main objectives of the endodontic therapy, i.e., to keep or restore the health of periapical tissues1,2.

Previous studies reported difficulties managing different anatomical complexities due to the limited information provided by periapical radiographs3,4. Cone-beam computed tomography (CBCT) allows multiplanar imaging analyses, favoring more predictable treatment plans and favorable prognoses3,4. CBCT has also been important in providing a more comprehensive understanding of the RCS complexity5,6,7,8.

Permanent mandibular molars are often affected by anatomical variations9. An extra root in these teeth refers to an anatomical variation named radix molaris. When the extra root is located on the lingual surface, it is named radix entomolaris; when the extra root is present on the buccal side, it is referred to as radix paramolaris10. In the studies by Song et al.,11 and Zhang et al.,12 the incidence of radix molaris was higher in Asian and lower in Caucasian populations, respectively. These and other previous research pointed out that race and ethnicity play a fundamental role in the incidence of radix molaris13,14. Hence, more studies are needed to investigate the correlation among these factors in different countries and regions15,16.

Highly populated countries and/or those with large territorial extensions usually present great ethnic and racial heterogeneity17. The greater the population density, territorial extension, and ethnic heterogeneity of a country or geographic region, the greater the difficulties in obtaining reliable samples to accurately determine some population characteristics, such as the anatomy of the RCS. For this reason, different research has been performed to study this variable in subpopulations from the same country, such as Saudi Arabia18,19,20,21, India22,23, and Brazil24,25,26,27. One of the main criticisms regarding studies conducted to investigate the incidence of anatomical variations in subpopulations is that their findings represent only the specific population analyzed. Although this limitation is methodologically valid from a scientific perspective, the relevance of such studies should not be overlooked. Their results can serve as foundational data for future investigations employing more sophisticated methodological designs and rigorous, statistically robust analytical approaches, such as those used in systematic reviews and meta-analyses13,28,29,30,31.

This cross-sectional study investigated the prevalence of radix molaris in mandibular molars from a subpopulation of Brazil’s Northeast region using CBCT exams.

Methods

Ethical approval and informed consent

This study was authorized and approved by the Research Ethics Committee of the Centro Universitário Christus – UNICHRISTUS, Fortaleza, Ceará, Brazil (5.442.892), and it was performed considering the principles of the World Medical Association Declaration of Helsinki “Ethical Principles for Medical Research Involving Human Subjects,” (amended in October 2013)32. Informed consent was obtained from each patient or guardian of patients under 18 who participated in the study.

Sample size calculation

The sample size for conducting the current research was calculated based on the number of the subpopulation studied – Brazil’s Northeast region (54.644.582 million people)33. To obtain a 95% confidence interval, the following formula was applied – ([Np(1 − p)]/[(d2 /Z2 1 − α/2*(N − 1) + p*(1 − p)])34,35,36,37, resulting in a minimum sample size of 1025 exams.

Sample selection and image acquisition

Initially, 1420 CBCT exams (909 from women and 511 from men) were randomly screened. These scans were originally performed in the Radiology Department of the Centro Universitário Christus (UNICHRISTUS), Fortaleza, Ceará, Brazil, for purposes unrelated to this study, including treatment planning for implant surgery, assessment of tooth impaction, orthodontic therapy, and trauma evaluation.

The images were acquired using a Prexion 3D imaging device (Prexion, Inc., San Mateo, USA) operating at 90 kVp and 4 mA, with a voxel size of 0.125 mm and a slice thickness of 1 mm. According to the evaluators ' necessity, two fields of view (FOV) were used, ranging from 5 × 5 cm to 10 × 10 cm. All scans were conducted following the manufacturer’s guidelines to ensure minimal radiation exposure while maintaining image quality17.

CBCT images were included when they exhibited adequate contrast and density to insure proper visualization of root canal morphologies, and when they featured permanent mandibular first or second molars.

Exclusion criteria comprised the absence of bilateral first and second mandibular molars, incomplete root formation, root resorption, and image artifacts that could hinder the proposed analysis, such as endodontic treatment, crown-bridge prostheses, or intraradicular posts29 So, if a tooth presented a condition that made the analysis difficult, it was excluded. Of the 1420 CBCT examinations initially screened, 328 were excluded. Consequently, the final sample consisted of 1092 exams (690 from women and 402 from men), comprising 3315 teeth (1541 first and 1774 s mandibular molars).

Images analysis

Three endodontists with experience in CBCT analysis underwent a calibration process using 40 standardized CBCT scans, totaling 100 teeth. Each evaluator analyzed 10 scans per session. To minimize fatigue, the calibration was divided into four sessions, with 10 scans assessed per day. The entire process was completed over a 10-day period. Tomography sections were visualized in axial, coronal, and sagittal planes using the PreXion 3D Viewer software (Prexion, Inc., San Mateo, USA) on a Dell Precision T5400 workstation (Dell, Round Rock, TX, USA), with a 17-inch Dell LCD screen with a resolution of 1280 × 1024 pixels, 85 Hz, and 0.255 mm dot pitch operated at 24 bits, in a dark room. Each evaluator analyzed these images twice within a 30-day interval. They were free to adjust the contrast and brightness of the images using the software’s image processing tool to ensure optimum visualization. The long axis of teeth was determined, and images were examined in cross-sections up to the apical third of the root by moving the toolbar from the pulp chamber to the apex. The evaluators were also allowed to manipulate the visualization settings and tools (such as noise reduction or specific filters) to improve the quality of the output image. So, the teeth morphologies were analyzed according to the standard classification of radix molaris.

To ensure methodological consistency and minimize variability associated with evaluator fatigue, the entire sample of 1092 CBCT exams was analyzed using previously established standardized procedures. The dataset was stratified into three subsets of 364 scans each, and three previously calibrated evaluators independently assessed all exams. Each evaluator analyzed 14 scans per day, with mandatory rest intervals every three days to maintain diagnostic accuracy and reduce visual strain. As the evaluation was performed in duplicate, the complete assessment process spanned a period of seven months. All findings, including the presence, classification, and anatomical distribution of radix molaris, were systematically recorded alongside demographic data in a structured Excel database and subsequently subjected to appropriate statistical analyses. Additionally, a comparative table summarizing similar studies in other populations was included to contextualize the present results.

Statistical analysis

Data were expressed as absolute and percentage frequencies and associated with the classification previously quoted using Pearson’s chi-square and Fisher’s exact tests. Statistical comparisons were performed according to the side of radix molaris location, tooth, type (entomolaris and paramolaris) and patients gender. SPSS software version 26.0 for Windows (IBM Corp., Armonk, United States of America) was used to perform the statistical analysis (P < 0.05)34,35,36,37.

Results

Inter- and intra-observer agreement demonstrated Fleiss’ Kappa coefficients of 0.8938 and 0.9039, respectively. The overall inter-observer reliability observed after the definitive image evaluations was similar to that obtained during the calibration process (Fleiss’ Kappa coefficient: 0.87)38. These data support the effectiveness of the calibration process and, consequently, the reliability of the findings obtained in the present study.

The present study analyzed 1092 CBCT exams (690 from women and 402 from men) and 3315 teeth (1541 and 1774 first and second mandibular molars, respectively).

Of the 1092 CBCT exams studied, 59 (5.40%) presented radix molaris (23 men [5.72%] and 36 women [5.22%]) (P > 0.05) and in 1033 exams (94.60%) were no observed radix molaris morphology (Table 1).

Table 1 General occurrence of radix molaris, location, side, tooth, and type (entomolaris or paramolaris) according to the patient’s gender.
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Among the 3315 teeth investigated, 71 (2.14%) presented radix molaris (45 first molars [63.38%] and 26 s molars [36.61%]) (Fig. 1) (P < 0.05) (Table 2). 3244 teeth (97.86%) presented no radix molaris morphology. The prevalence of radix molaris was significantly higher in the first molars (2.92%) compared with the second molars 1.47% (P < 0.05) (Table 2).

Fig. 1
figure 1

CBCT axial slices showing a radix paramolaris (RP) in a second mandibular molar (white arrows).

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Table 2 Influence of the tooth (first or second molar) on the prevalence, type, and side of radix molaris.
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No statistically significant differences were identified (P > 0.05) after comparing the prevalence of radix molaris in the two hemiarches (Table 1).

Bilateral radix molaris were only observed in the first molars (P < 0.05) of 12 CBCT exams who presented this characteristic (20.34%) (Fig. 2) (Table 2).

Fig. 2
figure 2

CBCT axial slice showing the bilateral occurrence of radix entomolaris in mandibular first molars (white arrows).

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Of the 71 radix molaris, 66 (92.96%) were classified as radix entomolaris and 5 (7.04%) as radix paramolaris (Table 2).

Table 3 shows absolute frequencies and percentages (%) found in studies that evaluated the prevalence of radix molaris in different populations worldwide.

Table 3 Absolute frequency and percentages of the prevalence of radix molaris in first and second permanent mandibular molars in different studies.
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Discussion

Over the years, different methods have been used to study human root canal anatomy, such as radiographs53, longitudinal and cross-sections54,55,56, staining and tooth-clearing techniques57, and the injection of different materials into the root canals, such as silicon, inlay casting wax, and acrylic resin, trying to replicate its internal anatomy58. Although these approaches were instrumental in the past, more modern and accurate techniques have been employed nowadays59,60.

Currently, CBCT is an imaging modality that enables the detailed analysis of internal tooth morphology and represents a significant advancement in Endodontics. Several studies have associated the use of CBCT with improvements in the diagnosis, planning, and execution of endodontic therapies. CBCT imaging is indicated, among other applications, for the investigation and identification of anatomical complexities. However, not all CBCT scans are adequate for detecting subtle anatomical variations, such as radix molaris. Scans obtained with a large field of view (FOV) and larger voxel sizes tend to have reduced sensitivity, making the detection of discrete endodontic anatomical variations less likely. Therefore, to ensure precise diagnosis in Endodontics, the use of CBCT with a small FOV is recommended, as was employed in the present study.

Micro-computed tomography (micro-CT) is another method that has been used to study numerous matters in Dentistry. Specifically, in Endodontics, it has been employed to investigate root canal anatomy59 and distinct techniques used for removing gutta-percha61, cleaning, shaping62, and filling the root canals63. However, micro-CT cannot be used clinically. Hence, the current study was performed to determine the prevalence of radix molaris in mandibular molars from the Northeast region of Brazil using CBCT exams, considering its potential to be used in vivo, allowing thus the analysis of potential correlations among the anatomical complexity investigated herein and demographic data, such as age and gender60.

The prevalence of radix molaris in the Middle East and European countries ranged from 1.20 to 6.20%43,45 and from 1.67 to 8.60%15,46, respectively. Since in Asiatic countries of Mongoloid origins, such as China, Japan, and Korea, the incidence of radix molaris is greater than 25%, this anatomical feature is no longer considered an anomaly but rather a populational genetic peculiarity12,52. In the studies by Rodrigues et al.,26 and Caputo et al.,24 the prevalence of radix molaris in Brazilian subpopulations (Southeast and Central-West regions) was 2.58% and 2.87% in this order. The higher index found in the current research (5.40%) reiterates the country’s high population heterogeneity.

In the present scientific investigation, the prevalence of radix molaris in the first and second molars was 2.92% and 1.47%, respectively. This finding is in line with previous research conducted in Brazil and other countries12,14,25,30,40,42,49. Our results also rectify a higher prevalence of radix molaris in first mandibular molars12,14,30,40,42. Few studies showed a higher prevalence of radix molaris in second mandibular molars46,51.

A higher prevalence of radix entomolaris (92.96%) than radix paramolaris (7.04%) was also observed herein, which agrees with most previous research, rectifying the supernumerary root on the buccal surface is rare12,30,40,45,46,51. However, Martins et al.49 observed a significantly higher prevalence of radix paramolaris (35.30%) in a Portuguese population. Previous research in Brazilian subpopulations (Southeast and Central-West regions of the country) did not identify radix paramolaris9,26,27. This result may be associated with the Portuguese colonization process that occurred most effectively in the Brazilian Northeast region.

The current findings also showed that although the prevalence of radix molaris has been higher in male patients, no statistically significant differences were identified between genders. Previous studies found similar results15,30,46. However, in the studies by Duman et al.14 and Talabani et al.41, statistical differences were relevant for patients from distinct genders in Turkey and Iraq, respectively.

The major limitation of the present study is that it was performed on a specific region of Brazil using a small sample, considering the country’s population contingent. Nonetheless, developing scientific evidence in Dentistry is a complex and gradual process. The inherent complexity of dental procedures and the multitude of factors and variables capable of influencing their outcomes render dental research particularly challenging64,65. Correlation analyses among the root canal anatomy and demographic data seem even more complex once the heterogeneity of results occurs as a consequence of diverse methodological components, such as sample size, analytical processes, subjective factors potentially capable of influencing the results, and population diversity25. The latter is strongly influenced by the colonization and immigration processes fostered by economic dynamism.

Hence, the findings presented herein, and the results of similar studies should not be dismissed as limited or lacking scientific value. Instead, they contribute to developing more comprehensive and reliable information that extends beyond the investigated populations. This understanding is essential for future research efforts, including studies conducted in randomly chosen countries13,30, across one or more continents29, or globally28. Such research can facilitate simple or complex comparisons, considering the potential influences of various factors, such as demographic and economic conditions, on the anatomy of the RCS. These comparisons can be achieved through well-designed methodologies and advanced statistical approaches, including systematic reviews and meta-analysis31.

Conclusions

The prevalence of radix molaris in the Brazilian subpopulation studied was 5.40% regardless of the gender of the patients. In 20% of cases, radix molaris was present bilaterally only in the first molars, which also had a higher prevalence of the anatomical variation studied than the second molars. Most teeth associated with supernumerary roots (93%) were classified as radix entomolaris.