Prevalence of radix entomolaris and distolingual canals and their association with the incidence of middle mesial canals in mandibular first molars of a Saudi subpopulation

Abstract

This study aimed to determine the prevalence of radix entomolaris (RE), distolingual canals (DLCs), and middle mesial canals (MMCs) in mandibular first molars within a Saudi subpopulation and to explore potential associations among these variations using cone-beam computed tomography (CBCT). A retrospective cross-sectional analysis was performed on 642 CBCT scans (1,284 mandibular first molars) from patients aged 18–65 years. Each molar was examined in axial, coronal, and sagittal views to identify distolingual canals (DLCs), radix entomolaris (RE), and Middle mesial canals (MMCs) bilaterally. Prevalence was analysed according to gender and age groups. Binary logistic regression assessed associations among variations and demographic predictors. DLCs were the most common variation, detected in 31.2% of left and 28.7% of right mandibular first molars. REs were found in 8.3% (left) and 9.7% (right), while MMCs were less frequent (2.6% left; 1.6% right). Age was significantly associated with the prevalence of DLCs, with higher rates observed in the 18–25, 31–40, and 41–50 year age groups (p < 0.05). Similarly, RE prevalence showed a significant age-related pattern, being more frequent in the 18–25 and 41–50 year groups (p < 0.05). MMCs were very rare on both sides, peaking at 1.1% on the left (31–40 years) and 0.6% on the right (26–30 and 31–40 years), with no statistically significant differences across age groups. A near-significant association was observed between left-sided DLCs and the presence of MMCs (OR = 3.33; p = 0.065). Gender was not significantly associated with any variation.DLCs and REs are relatively common in mandibular first molars in this Saudi subpopulation, while MMCs are rare but clinically relevant. Patients with 41–50 year age group are more likely to exhibit DLCs and REs, and the presence of a DLC may indicate a higher likelihood of an MMC. Incorporating CBCT evaluation in multiple planes is essential for accurate detection and successful endodontic management.

Introduction

A comprehensive understanding of the normal root canal configuration and its variations is essential for successful root canal treatment¹. Failure to identify all root canals during endodontic procedures can lead to treatment failure². Mandibular first molars typically have two roots and three root canals; however, variations involving four or five canals and additional roots have also been reported^3,4. The complexity and diversity of mandibular first molar anatomy can be influenced by factors such as ethnicity and genetic background³.

Carabelli⁵ first identified the accessory distolingual root in 1844, later termed the radix entomolaris (RE). This root is typically characterised by a pronounced curvature and a shorter length compared with other roots. It may be fused with or entirely separate from the main distal root⁶. Although the RE is generally shorter than the distobuccal (DB) root, it can sometimes present as completely distinct from the DB root or fused with it⁷. When an additional root is located mesiobuccally, this rare anatomical variation is known as the radix paramolaris^8,9.

The most common additional canals in mandibular first molars are the middle mesial canal (MMC) and the distolingual canal (DLC)¹⁰. The MMC in mandibular molars was first reported in 1974^11,12. It is located between the mesiobuccal and mesiolingual canals and can be classified as independent, confluent, or fin type¹³. The DLC is positioned more lingually than the main distal canal, and its identification often requires modification of the traditional access cavity¹⁴. Cone-beam computed tomography (CBCT) with three-dimensional (3D) reconstruction is a sensitive and reliable diagnostic tool for detecting anatomical variations in mandibular first molars¹⁵.

The prevalence of REs varies according to tooth type, population, and geographic region. They are more commonly found in the first molar than in the second molar¹⁶. East Asian populations demonstrate a higher prevalence. Globally, the prevalence of REs in mandibular first molars has been reported as 3%¹⁷ and 5.6%¹⁸.

In the Saudi Arabian population, several studies have investigated the prevalence of REs or extra roots in mandibular molars; however, their findings have been inconsistent. The overall prevalence of extra roots in first molars has been reported as 3.05%, without specifying the location or type of root¹⁹. Reported prevalence rates for REs in mandibular first molars include 4.3%²⁰, 4.7%²¹, and 6.6%⁵. The prevalence of MMCs has been estimated at 4.2% based on a systematic review of studies involving Saudi populations²², whereas another study reported a prevalence of 2.6% in a Saudi population²³.

Understanding the complex root canal anatomy of mandibular first molars is essential for achieving endodontic treatment success. Among the various anatomical variations, the presence of an RE, DLC, or MMC poses significant clinical challenges because they are often overlooked during treatment. Although the prevalence of each variation has been studied individually, their potential anatomical or developmental interrelationships remain unclear. Previous research has typically examined these variations in isolation across different populations. Furthermore, most studies have lacked bilateral analysis and predictive correlation assessments. Given that root canal morphology can vary significantly across ethnic groups, investigating these variations within specific demographics is important. This gap limits clinical insight into potential relationships that could enhance diagnostic accuracy. Therefore, this study aimed to provide a comprehensive evaluation of the prevalence of RE, DLC, and MMC in mandibular first molars within a large sample of the Saudi subpopulation and to explore their interrelationships to support more effective endodontic treatment planning.

Materials and methods

Study design

This cross-sectional observational study used CBCT scans to assess the anatomical features of the mandibular first molars in a Saudi subpopulation. It was conducted at a leading dental centre in the Ha’il region of Saudi Arabia. The study protocol was approved by the Medical Ethics Committee of the University of Ha’il, Ha’il, Saudi Arabia (No. H-2025-614). Because the study used previously acquired CBCT images with no direct patient involvement, informed consent was not required. All patient data were anonymised to ensure confidentiality and privacy.

Sample size calculation and collection

Raosoft^® Sample Size Calculator (Raosoft Inc.) was used to calculate the sample size, considering 95% as the confidence level, 5% as the margin of error, and a population of 431,000 (the 2025 Ha’il population). The sample size was 384. A total of 642 scans were used in this study to ensure the accuracy, reliability and generalisability of the results.

High-resolution CBCT images were collected between January and May 2025. A nonprobability purposive sampling technique was employed to ensure that only scans meeting specific diagnostic and anatomical requirements were included.

The inclusion criteria required that CBCT images clearly display mandibular first molars with completely developed roots in patients aged between 18 and 65 years. To be included, scans had to:

• Provide clear visualisation of the entire crown and root structure, including apical areas, without distortion.

• Have no radiographic artefacts or obstructions that could hinder assessment (e.g., metallic restorations, crowns, posts).

• Show teeth free from previous endodontic treatment, extensive restorations, periapical pathology, root resorption, or canal calcification.

• Meet a minimum image resolution and field of view sufficient for multiplanar assessment (axial, coronal, sagittal) of the root canal system.

CBCT scans were excluded if they failed to meet any of these criteria, including those with poor image quality. A total of 4,250 CBCT scans were initially retrieved and reviewed from the radiographic database as part of the screening process. After applying the inclusion and exclusion criteria, 642 high-quality CBCT scans were included in the final analysis.

CBCT procedures and techniques

CBCT scans were acquired using the Carestream CS 8100 3D system (Carestream Dental LLC, Atlanta, USA), a high-resolution imaging unit specifically designed for dental and maxillofacial applications. The device used a complementary metal–oxide–semiconductor (CMOS) sensor and incorporated dental volumetric reconstruction (DVR) technology to generate 3D reconstructions with high diagnostic precision. The X-ray generator operated within a range of 60 to 90 kilovolts (kV) and 2 to 15 milliamperes (mA), with a frequency of 140 kHz, enabling flexible exposure settings tailored to patient size and diagnostic need. Scan durations varied between 3 and 15 s, depending on the selected field of view (FOV) and resolution settings. The machine offered multiple FOV options (4 × 4 cm, 5 × 5 cm, 8 × 5 cm and 8 × 8 cm), allowing for targeted imaging of specific anatomical regions while minimising radiation exposure. The smallest achievable voxel size was 75 micrometres (µm), providing high spatial resolution essential for detailed evaluation of root canal morphology. All CBCT images were reviewed and analysed using CS 3D Imaging Software (Carestream Dental LLC, Atlanta, USA), which enabled multiplanar reconstruction and precise assessment of internal root canal structures. Each mandibular first molar was evaluated in axial, coronal, and sagittal views, with multiplanar reconstructions used to visualise complex morphology from different orientations. This approach ensured accurate detection of DLCs, REs, and MMCs and allows for reproducibility and comparability with other CBCT-based anatomical studies.

Recorded anatomical features

Before the evaluation, the examiners (EA, EAA) underwent a structured calibration training programme designed to ensure accuracy, consistency and adherence to standardised assessment criteria. This training was carried out under the close supervision of three senior observers (AAM, AFA and AA), each possessing more than 10 years of experience in clinical and radiological interpretation. These senior observers provided ongoing mentorship and were actively involved in the assessment of mandibular first molars. Where discrepancies arose, all observers participated in collaborative case discussions to achieve a consensus. Each differing case was re-examined jointly in all three CBCT planes (axial, coronal, sagittal) using the same imaging software, and relevant diagnostic criteria were reviewed. The final classification was based on unanimous agreement among the examiners. If consensus could not be reached initially, the case was re-evaluated until agreement was obtained. Inter-examiner reliability was measured using Cohen’s Kappa statistic, demonstrating a near-perfect agreement across all evaluation parameters (Kappa = 0.93). The presence of REs, DLCs and MMCs in both the right and left sides, as well as in relation to different age groups and gender, was recorded.

For classification purposes, an RE was defined as an additional distolingual root separate from the main distal root, regardless of the number of canals within it. A DLC was defined as an additional canal located within the main distal root, positioned lingually to the distobuccal canal. In cases where a tooth presented with an RE, the canal within the RE was not counted as a DLC to prevent overlapping classification. This ensured that each variation was recorded exclusively according to its anatomical category.

Statistical analysis

Data analysis was conducted using SPSS version 26 (IBM Corp., Armonk, NY, USA). Descriptive statistics, including frequency distributions and cross-tabulations, were used to summarise demographic characteristics and the prevalence of anatomical variations. The associations between anatomical features (DLCs, REs and MMCs) and demographic variables (age and gender) were assessed using the Chi-square test and Fisher’s exact test, as appropriate. To further explore these relationships, binary logistic regression analysis was performed to evaluate whether age and gender significantly predicted the presence of each anatomical variation. Additionally, separate binary logistic regression models were constructed to determine whether the presence of a DLC or RE could serve as predictors for the occurrence of an MMC in mandibular first molars. A p-value of less than 0.05 was considered statistically significant.

Results

A total of 642 CBCT scans were analysed, comprising 278 men (43.3%) and 364 women (56.7%). The participants were categorised into six age groups, with most aged 18–25 years (27.1%) and 31–40 years (26.8%; Table 1).

Table 1.

Demographic distribution and prevalence of anatomical variations in mandibular first molars among the study population.

Variables	N (%)
Gender
Male	278 (43.3%)
Female	364 (56.7%)
Age
18–25 years old	174 (27.1%)
26–30 years old	105 (16.4%)
31–40 years old	172 (26.8%)
41–50 years old	122 (19.0%)
51–60 years old	56 (8.7%)
> 60 years old	13 (2.0%)
Distolingual Canal 36
Present	200 (31.2%)
Absent	442 (68.8%)
Radix Entomolaris 36
Present	53 (8.3%)
Absent	589 (91.7%)
Middle Mesial Canal 36
Present	17 (2.6%)
Absent	625 (97.4%)
Distolingual Canal 46
Present	184 (28.7%)
Absent	458 (71.3%)
Radix Entomolaris 46
Present	62 (9.7%)
Absent	580 (90.3%)
Middle Mesial Canal 46
Present	10 (1.6%)
Absent	632 (98.4%)

Figures 1, 2 and 3 illustrate the anatomical variations observed in this study. Figure 1 shows a DLCs, Fig. 2 displays a RE, and Fig. 3 highlights a MMCs. DLCs were the most commonly observed variation. In the left mandibular first molar (#36, FDI notation), DLCs were present in 31.2% of cases, followed by REs in 8.3% and MMCs in 2.6%. In the right mandibular first molar (#46, FDI notation), DLCs were observed in 28.7% of cases, REs in 9.7% and MMCs in 1.6%.

Fig. 1.

Axial CBCT images (a-h) demonstrating the presence of a distolingual canal (arrows) in mandibular first molars.

Fig. 2.

Axial CBCT (a-h) images display radix entomolaris (arrows) in mandibular first molars.

Fig. 3.

Axial CBCT images (a-f) demonstrate the middle mesial canal (arrows) in mandibular first molars.

No statistically significant gender differences were found in the occurrence of DLCs, REs or MMCs in either molar (Table 2). For instance, a DLC in #36 was present in 14.5% of men and 16.7% of women (p = 0.271), and an RE was found in 3.7% of men and 4.5% of women (p = 0.761). However, age showed a significant association with the presence of DLCs, which were more common in younger and middle-aged patients, particularly those aged those aged 18–25, 31–40, and 41–50 years, before declining in older groups (p < 0.05). RE also exhibited a significant age-related distribution, with higher prevalence in the 18–25 year and 41–50 year groups, while being rarely detected in individuals over 60 years of age (p < 0.05). The prevalence of MMCs was very low on both sides, with the highest prevalence on the left at 1.1% in the 31–40-year age group and on the right at 0.6% in the 26–30 and 31–40-year groups with no statistically significant differences across age groups.

Table 2.

Distribution of distolingual Canal, radix entomolaris, and middle mesial Canal in mandibular first molars by gender and age group.

Mandibular First Molar Findings		Gender			Age
		Male	Female	P value	18–25 years old	26–30 years old	31–40 years old	41–50 years old	51–60 years old	> 60 years old	P value
Left	Distolingual Canal	93 (14.5%)	107 (16.7%)	0.271	42 (6.5%)	30 (4.7%)	58 (9.0%)	53 (8.3%)	13 (2.0%)	4 (0.6%)	0.010
	Radix Entomolaris	24 (3.7%)	29 (4.5%)	0. 761	13 (2.0%)	6 (0.9%)	8 (1.2%)	15 (2.3%)	8 (1.2%)	3 (0.5%)	0.023
	Middle Mesial Canal	9 (1.4%)	8 (1.2%)	0.416	4 (0.6%)	3 (0.5%)	7 (1.1%)	2 (0.3%)	1 (0.2%)	0 (0.0%)	0.785
Right	Distolingual Canal	87 (13.6%)	97 (15.1%)	0.197	40 (6.2%)	30 (4.7%)	52 (8.1%)	49 (7.6%)	10 (1.6%)	3 (0.5%)	0.014
	Radix Entomolaris	27 (4.2%)	35 (5.5%)	0.967	13 (2.0%)	6 (0.9%)	11 (1.7%)	19 (3.0%)	10 (1.6%)	3 (4.8%)	0.005
	Middle Mesial Canal	5 (0.8%)	5 (0.8%)	0.667	1 (0.2%)	4 (0.6%)	4 (0.6%)	1 (0.2%)	0 (0.0%)	0d (0.0%)	0.239

Binary logistic regression was performed to assess the influence of gender and age on anatomical variations (Tables 3 and 4). For the left mandibular first molar (#36), neither gender nor age significantly predicted the presence of DLCs or RE, although age showed a non-significant trend toward lower RE prevalence in older individuals (p = 0.067). MMCs were very rare, and while age appeared as a significant predictor with an extremely large odds ratio, this reflects model instability due to their low prevalence. Similarly, for the right mandibular first molar (#46), gender did not significantly affect the occurrence of DLCs, RE, or MMCs. Age was not a significant predictor for DLCs and showed only a non-significant trend for RE (p = 0.058). MMCs were again rare, and the extremely large odds ratio for age indicates model instability. Overall, both age and gender had limited predictive value for these anatomical variations.

Table 3.

Binary logistic regression analysis for anatomical variations in the left mandibular first molar (#36).

Effect	Estimate	SE	95% CL		P-value
			Lower	Upper
#36-distolingual canal
Gender	0.164	176	0.835	1.662	0.351
Age	− 0.257	0.632	0.224	2.670	0.685
#36-Radix Entomolaris
Gender	− 0.063	0.298	0.523	1.686	0.834
Age	-1.343	0.733	0.062	1.099	0.067
#36-Middle Mesial canal
Gender	0.480	0.496	0.611	4.271	0.333
Age	18.101	1.132	7891483.951	668275676.029	0.000

Table 4.

Binary logistic regression analysis for anatomical variations in the right mandibular first molar (#46).

Effect	Estimate	SE	95% CL		P-value
			Lower	Upper
#46 distolingual canal
Gender	0.222	0.179	0.878	1.775	0.216
Age	0.100	0.688	0.287	4.255	0.885
#46-Radix Entomolaris
Gender	− 0.151	0.280	0.497	1.487	0.589
Age	-1.386	0.732	0.060	1.050	0.058
#46-Middle Mesial canal
Gender	0.358	0.642	0.406	5.038	0.577
Age	18.174	1.422	4816561.425	1267442282.507	0.000

An additional binary logistic regression analysis was conducted to evaluate whether the presence of DLCs or REs predicted the occurrence of MMCs in the mandibular first molars (Table 5). The analysis summarises the regression models performed separately for the right and left sides. On the right side, neither DLCs (Exp(B) = 1.385, p = 0.515) nor REs (Exp(B) ≈ 0, p = 0.997) demonstrated a statistically significant association with MMC presence. However, on the left side, the presence of a DLC was associated with more than a threefold increase in the odds of detecting an MMC (Exp(B) = 3.331), with a p-value approaching statistical significance (p = 0.065). In contrast, REs again showed no meaningful predictive value on the left side (Exp(B) ≈ 0, p = 0.997), likely due to their low occurrence. These findings suggest a potential, though not statistically confirmed, association between the presence of a DLC on the left side and the presence of an MMC. REs did not appear to be a relevant predictor in either model.

Table 5.

Binary logistic regression analysis predicting middle mesial Canal presence using distolingual Canal and radix entomolaris.

Side	Predictor	B	Exp (B)	Wald	SE	df	p-value	95% CI for Exp(B)
Right side	DLC	0.326	1.385	0.424	0.501	1	0.515	3.696
	RE	-17.572	0.000	0.000	5520.148	1	0.997	0.000
Left side	DLC	1.203	3.331	3.407	0.652	1	0.065	3.331
	RE	-16.648	0.000	0.000	5092.522	1	0.997	0.000

Discussion

This study evaluated the prevalence of three key anatomical variations in mandibular first molars (DLCs, REs and MMCs) in relation to age and gender distribution, using a large CBCT dataset. Overall, the findings underscore the clinical relevance of root canal anatomical variability, which is essential for successful endodontic treatment and planning^24,25.

DLCs were observed in 31.2% of mandibular left first molars (#36) and 28.7% of right first molars (#46), highlighting a moderately high prevalence. These results are consistent with previous studies, particularly in Asian and Middle Eastern populations, where the incidence of additional canals in mandibular molars is reportedly higher due to ethnic variations in root canal morphology^26–28. Clinicians should be aware of the high likelihood of DLCs, especially since missed canals are a leading cause of endodontic failure.

REs, supernumerary roots typically found distolingually, were found in 8.3% of teeth #36 and 9.7% of teeth #46 in this study. This prevalence is slightly lower than the ranges reported in some East Asian populations (ranging between 10% and 30%) but aligns with findings from Middle Eastern cohorts^29–31. The clinical importance of the RE lies in its tendency to be overlooked on standard radiographs, emphasising the diagnostic value of CBCT in detecting such anatomical anomalies.

MMCs, although the least common variation, were identified in 2.6% of teeth #36 and only 1.6% of teeth #46. These findings reaffirm that MMCs, although uncommon, are not negligible and must be actively sought, particularly in patients presenting with persistent symptoms after initial treatment. Previous studies using micro-CT and CBCT have reported similar MMC prevalence rates ranging from 1% to 15%, depending on methodology and population^32,33. During access cavity preparation, dental operating microscopy (DOM) improves illumination and depth perception, enhancing detection of accessory canals such as MMCs³⁴. Ultrasonic troughing along the mesial isthmus, combined with tactile exploration and dye staining, further aids canal identification^32,35. Pre-operative CBCT, interpreted in axial, coronal, and sagittal planes, assists in detecting RE and DLC variations, guiding access modification³⁶. Selecting a limited field of view and optimal voxel size follows radiation dose-optimization principles^36,37. Integrating DOM, ultrasonic troughing, and multiplanar CBCT improves detection of complex anatomy and reduces the risk of missed canals^32,34–36.

This study confirms the notable prevalence of anatomical variations such as DLCs and REs in mandibular first molars and emphasises the rare but important occurrence of MMCs. Awareness of these variations, combined with the use of advanced imaging such as CBCT, is crucial for improving treatment outcomes.

The findings of this study offer valuable insight into the demographic factors that may influence the occurrence of these variations, reinforcing the necessity for individualised anatomical assessment in endodontic practice. No associations were observed between gender and the presence of DLCs, REs or MMCs on either the left or right mandibular first molars. For instance, the presence of DLCs on the left side was slightly more frequent in women (16.7%) than in men (14.5%). Similar non-significant trends were observed for REs and MMCs. These findings align with previous studies that have found no strong correlation between gender and root canal anatomy, including the presence of additional canals or roots^38,39. The lack of gender-based variation suggests that clinicians should not rely on patient gender as a predictor of root morphology complexity. In contrast, age showed relationships with DLCs and REs in both left and right mandibular first molars. The prevalence of DLCs was highest among individuals aged 31–40 years, followed by the 41–50-years and 18–25–years age group. Similarly, REs were significantly associated with age, with the 41–50-years age group showing the highest frequency. These findings may reflect anatomical detectability differences across stages of tooth development and progressive calcification. These might be that root canals are wider, pulp chambers larger, and dentinal walls thinner, making accessory canals such as MMCs and additional roots easier to visualise both clinically and radiographically^32,40. With increasing age, deposition of secondary dentin and calcification of the pulp space can partially or completely obscure these anatomical features, reducing their visibility on CBCT or under magnification^34,41. Such age-related morphological changes have been documented as contributing to a lower detection rate of accessory canals in older populations. Interestingly, MMCs showed a low prevalence across all age groups and no statistically significant age-related differences. This is consistent with previous literature indicating that MMCs are uncommon and often challenging to detect without high-resolution imaging or magnification tools^32,34. Despite the low detection rates (≤ 1.4%), their clinical relevance remains high due to the potential for persistent infection if overlooked during treatment. The relationship between age and canal or root variants also aligns with the hypothesis that canal morphology is more complex in in younger and middle-aged patients, particularly in the 18–25, 31–40, and 41–50 year groups but becomes less detectable with age due to secondary dentin deposition and canal calcification⁴⁰. The gradual decline in prevalence after the 40-year mark supports this age-related anatomical remodelling.

This study examined whether demographic factors, specifically gender and age, served as significant predictors of three key anatomical variations in mandibular first molars: DLCs, REs and MMCs. The statistical analysis provided insight into potential associations, revealing clinically meaningful trends that can enhance diagnostic accuracy and inform endodontic treatment planning, while also contributing valuable data to anatomical and epidemiological research. For both teeth #36 and #46, neither gender nor age showed a statistically significant association with the presence of a DLC. For #36, gender had an odds ratio (OR) of 1.178 (p = 0.351), and age was not significant (p = 0.685). Similarly, for #46, gender (p = 0.216) and age (p = 0.885) remained non-significant predictors. These findings are consistent with previous studies indicating that the presence of a DLC is not reliably predicted by demographic factors, but rather by ethnic and population-related anatomical trends. In particular, Asian and Middle Eastern populations tend to exhibit a higher prevalence of additional canals, including DLCs, independent of gender or age^26,28. This reinforces the notion that individualised radiographic assessment, rather than demographic assumptions, is essential for detecting canal variations. For REs, gender was not a significant predictor in either tooth (#36: p = 0.834; #46: p = 0.589). For the left molar (#36), age had an estimate of -1.343 (p = 0.067), and for the right molar (#46), the estimate was − 1.386 (p = 0.058). Although these trends suggest that RE prevalence may decrease with increasing age, the associations did not reach statistical significance. This indicates that while age may have a potential influence on the occurrence of RE, the low frequency of this anatomical variation in the sample likely limits the statistical power to detect a definitive effect. Overall, age shows a possible but non-significant trend toward lower RE prevalence in older adults, and further studies with larger samples may be needed to confirm this pattern. This pattern aligns with the concept that ageing is associated with secondary dentin deposition, canal obliteration or morphological changes that may obscure accessory roots such as RE in conventional radiographs or even CBCT imaging^30,31,41. Clinically, this highlights the importance of early detection and detailed imaging in younger patients, especially in regions or populations where REs are more prevalent^30,31,41. The findings also show that the prevalence of MMCs in mandibular first molars was very low across all age groups. For left side, frequencies ranged from 0.0% in individuals over 60 years to 1.1% in the 31–40-year age group. In the second set, frequencies were similarly low, ranging from 0.0% to 0.6%. The overall low prevalence, particularly in older age groups, likely contributes to the instability of regression models when age is included as a predictor, as reflected by large effect estimates with wide confidence intervals in other analyses. These findings align with previous studies reporting that MMCs are uncommon anatomical variations, and their occurrence does not show a clear trend with age. However, the frequencies itself reinforces a known clinical reality: MMCs are more frequently found in younger patients and may become undetectable with age due to canal calcification or fusion^32,34. On the other hand, gender was not a significant predictor for MMC in either tooth (#36: p = 0.333; #46: p = 0.577), aligning with other reports that have found no consistent gender-based predisposition³⁸.

This study further investigated the potential association between two anatomical variations (DLCs and REs) and the presence of MMCs in mandibular first molars. Binary logistic regression analysis was employed to determine whether the presence of DLCs or REs could significantly predict the occurrence of MMCs. Identifying such relationships is clinically relevant because MMCs are often difficult to detect during routine endodontic procedures. A better understanding of these anatomical correlations may enable clinicians to more accurately anticipate the presence of MMCs, thereby improving root canal exploration, cleaning and overall treatment outcomes. In the left mandibular first molar, the presence of a DLC was found to be a potentially meaningful predictor of MMC presence (B = 1.203, Exp(B) = 3.331, p = 0.065). Although this association narrowly missed the conventional level of statistical significance (p < 0.05), the OR suggests that teeth with a DLC were over three times more likely to have an MMC. This may reflect a general trend towards increased anatomical complexity in certain molars. Previous studies have shown that molars with accessory canals or roots are more likely to exhibit additional configurations in other parts of the root system^32,35. In contrast, REs were not significantly associated with MMC presence (p = 0.997), and the model yielded a coefficient suggesting almost no predictive value. This is likely due to both the rarity of REs in most populations and the fact that they occur in a different root (distal) from where MMCs typically arise (mesial). As such, their anatomical independence from the mesial system likely explains their lack of influence. For the right side, neither DLCs (p = 0.515) nor REs (p = 0.997) were significantly associated with the presence of MMCs. The OR for DLCs (Exp(B) = 1.385) indicates a slight increase in likelihood, but the wide confidence interval and non-significant p-value suggest a weak and unreliable effect. Similarly, REs showed no predictive value for MMC presence on the right side. This asymmetry between the left and right molars may reflect natural anatomical variation and sampling limitations. Previous studies have reported slight side differences in root canal morphology and canal configuration prevalence, though the evidence remains mixed^38,40.

This study has several limitations. The study’s sample size, although adequate for logistic regression, may limit the generalisability of the results. Additionally, the population studied may have unique ethnic or demographic characteristics that affect anatomical variation, restricting extrapolation to other groups. The observational, cross-sectional nature of the study prevents assessment of longitudinal changes, such as age-related canal calcification or morphological changes over time. Finally, even with calibration and experience, the interpretation of CBCT images may be subject to observer bias or error, which can affect the detection consistency of subtle anatomical features.

Clinical implications

The prevalence of anatomical variations such as DLCs, REs, and MMCs in mandibular first molars highlights the need for heightened clinical vigilance during endodontic diagnosis and treatment. Missed canals or roots remain a common cause of treatment failure, making early and accurate identification essential. CBCT offers a valuable adjunct to conventional radiography by enabling three-dimensional visualisation of root canal systems in multiple planes (axial, coronal, sagittal), which improves detection of accessory roots and canals before treatment begins. For complex cases where variations are more prevalent—CBCT imaging can guide modifications in access cavity design, the use of dental operating microscopy, and ultrasonic troughing to locate hidden canals. Incorporating CBCT findings into treatment planning can help clinicians anticipate anatomic complexity, reduce the risk of missed anatomy, and improve overall treatment outcomes.

Conclusions

This study underscores the intricate anatomical variations found in mandibular first molars, emphasising DLCs, REs and MMCs. DLCs and REs are relatively common in mandibular first molars in this Saudi subpopulation, whereas MMCs are rare but clinically significant. Individuals aged 41–50 years show a higher likelihood of exhibiting DLCs, REs, and MMCs. Additionally, the presence of a DLC appeared to increase the likelihood of finding an MMC, especially in the left mandibular first molar, though further studies are needed to confirm this association. Conversely, REs did not serve as a predictor for MMCs, suggesting separate developmental pathways for distal and mesial root structures. These results highlight the critical need for comprehensive anatomical evaluation using advanced imaging modalities such as CBCT and enhanced clinical visualisation techniques to better identify accessory canals and improve the success of endodontic treatments.

Author contributions

AAM and AFA contributed to the concept of the research, study design, data collection, supervision, statistical analysis, writing the original draft, and reading and editing the final paper. EAA, AA, and EAA contributed to data collection and writing the original draft. Every author evaluated and approved the final manuscript.

Funding

This work was supported by "This research has been funded by Scientific Research Deanship at University of Ha’il - Saudi Arabia through project number RG-25 049".

Data availability

Data are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Ethics approval and consent participate

The study protocol was approved by the Medical Ethics Committee of the University of Ha’il, Ha’il, Saudi Arabia (No. H-2025-614). Informed consent was waived for this retrospective study by the Medical Ethics Committee of the University of Ha’il, Ha’il, Saudi Arabia. All methods were performed in accordance with the declaration of Helsinki.