Middle mesial root canals in mandibular molars: prevalence and correlation to anatomical aspects based on CBCT imaging

Abstract

Objectives:

To determine the prevalence of middle mesial root canal (MMC) in a Brazilian subpopulation, verify whether its presence is related to anatomical characteristics of the tooth, and propose a classification using cone-beam computed tomography (CBCT).

Methods:

CBCT scans of 284 patients were evaluated by 2 radiologists to determine the presence of the MMC in mandibular first and second molars. Subsequently, the mesiodistal and buccolingual measurements of the mesial roots were obtained; the measurements between the root canals, and from MMC to the mesiobuccal canal and to the mesiolingual canal were also obtained. The MMC was classified according to its relationship with mesiobuccal and mesiolingual canals. The data were analyzed using χ² and Fisher’s exact test, multiple logistic regression analysis, Student’s t-test, κ and intraclass correlation coefficient (p < 0.05).

Results:

The intraexaminer agreements for the presence of the MMC and the measurements were considered almost perfect (0.953 and 0.999, respectively). Of 216 mandibular first molars, 11.1% had the MMC, and of 228 mandibular second molars, only 1.75% had the MMC. The presence was significant higher in the mandibular first molar (p < 0.0001). The buccolingual measurement and the measurement between mesiobuccal and mesiolingual canals were higher in teeth with MMC (p = 0.024 and p = 0.005, respectively). It was possible to classify the configuration of MCC into four types: independent (60.7%), confluent (14.3%), mesiolingual confluent (14.3%), and mesiobuccal confluent (10.7%).

Conclusions:

The prevalence of MMC is more pronounced in mandibular first molars and anatomical measurements, such as greater measurement between mesiobuccal and mesiolingual canals, may alert clinicians to its presence. It was found four types of configurations of MMC.

Introduction

The mandibular first molars are often affected by pulp infections that require endodontic procedure. ^1,2 The knowledge of the root system is a basic requirement for endodontic treatment success. ¹ Tooth submitted to an endodontic treatment needs a complete cleansing and obturation of all canals that compose its root system, since there is a significant correlation between missed canals and the presence of apical periodontitis. ^3–5 The most frequent root canal systems morphology is well-documented in the literature, but some anatomical variations can be found, and the dentist must know the possible variations of this complexity. ¹

Normally, mandibular molars have three root canals, being two in the mesial root and a single in the distal root. ^6,7 The middle mesial root canal (MMC) is a variation that can be found in mandibular molars. This canal is located between the mesiobuccal canal (MBC) and the mesiolingual canal (MLC). The incidence of this variation is variable in the literature. Some studies reported incidence ranging from 8 to 46%. ^8–14 The varied prevalence can be explained by the different methods of assessment and population studied.

The MMC can be classified into three types: fin, confluent and independent. ¹⁵ This classification was proposed based on periapical (PA) radiography. However, in some cases, the cone-beam computed tomography (CBCT) may be indicated for endodontic assessment ^16–18 when PA radiography is considered limited. ^19–21 When the justification and optimization principles are respected, the CBCT can be an important exam that can help the endodontic treatment planning. ^16–18 Considering the importance of knowing the prevalence of anatomical variations of the canal systems to help professionals be aware of it, the aims of this study were to evaluate the prevalence of the MMC in a Brazilian subpopulation, and verify whether its presence is related to sex and anatomical measurements of the tooth. Moreover, this study aimed to propose a classification of the MMC using the three-dimensional evaluation of the CBCT scans.

Methods and materials

This cross-sectional study was approved by the local Institutional Research Review Board (protocol number #44182621.5.0000.5418).

Sample selection

All CBCT scans performed for endodontic reasons between September 2019 and December 2020 at a private radiology center were examined. The requests for CBCT scans were independent of the present study and not related to the evaluation of the presence of MMC. One of the inclusion criteria was a request for endodontic treatment due to the CBCT protocol; all scans were acquired using the Veraview X800 unit (Morita, Kyoto, Japan) operating at 90 kV, 10 mA, 4 × 4 cm of field of view (FOV), and 0.08 mm voxel size.

All CBCT scans containing at least one mandibular molar (being first or second) of patients between 18 and 70 years old were included in this study. Teeth with non-completed root formation, signs of pulp calcification, dental anomalies, root fractures and scans severely affected by artifacts were excluded from this study.

Prevalence of middle mesial root canals

The evaluation of the presence of MMC was performed by two oral and maxillofacial radiologists with at least 3 years of experience in CBCT evaluation in consensus using OnDemand 3D software (Cybermed Inc., California) in a dimmed-light and quiet room. This step was done by two examiners due to the complexity of the root systems in molars and to decrease suspicion about the presence or absence of MMC. In case of disagreement, a third oral and maxillofacial radiologist with more than 10 years of experience in CBCT was requested to reach a consensus. The examiners used all the multiplanar reconstructions and enhancement tools to review the mandibular molars root anatomy. Information about the sex and age of the patients was recorded.

Anatomical characteristics

After assessing the prevalence, the teeth with MMC were evaluated for their anatomical characteristics. This evaluation was carried out by one oral and maxillofacial radiologist in the same way, conditions, and software used in the first step.

Mesiodistal and buccolingual measurement of the mesial root

The mesiodistal and buccolingual measurements of the mesial root were obtained in the axial plane. The position on this plane was standardized at the level of tooth furcation visualized on the sagittal plane. To confirm the level of the axial plane, the mesial root should be visualized individually. (Figure 1). The buccolingual measurement was made between the most external lingual and buccal surface of the root (Figure 2a). The mesiodistal measurement was made between the mesial and distal surface of the root aligned with the MMC (Figure 2b).

Figure 1.

(a) Sagittal CBCT plane showing the set position for the measurements; (c) Axial CBCT plane showing the mesial root individually visualized where the measurements were performed. CBCT, cone beam CT.

Figure 2.

(a) Buccolingual measurement. (b) Mesiodistal measurement.

Measurement between root canals

At the same plane and position, the measurement from the MMC to the mesiobuccal canal (MBC) and to the mesiolingual canal (MLC) was performed separately. The measurement was made between the center of the orifice of the canals (Figure 3a and b). Subsequently, the measurement between the MBC and MLC was also obtained (Figure 3c).

Figure 3.

Axial CBCT showing the measures between the canals of the tooth. (a) Measurement between MMC and MBC. (b) Measurement between MMC and MLC. (c) Measurement between MBC and MLC. CBCT, cone beam CT; MBC, mesiobuccal canal; MLC, mesiolingual canal; MMC, middle mesial root canal.

Configuration of the MMC

The MMC was classified according to its relationship with MBC and MLC. This step was carried out with a dynamic evaluation with all multiplanar reconstruction. It was found four types: (i) independent: separate orifice and independent apical foramen; (ii) confluent: separate orifice but joins with mesiobuccal and mesiolingual with a common apical foramen; (iii) mesiolingual confluent: separate orifice, but joins only with mesiolingual; (iv) mesiobuccal confluent: separate orifice, but joins only with mesiobuccal (Figures 4 and 5).

Figure 4.

Schematic illustration of the MMC according to the three-dimensional classification. MMC, middle mesial root canal.

Figure 5.

Coronal CBCT plane showing the four types of MMC found. (a) independent; (b) confluent; (c) mesiolingual confluent; (d) mesiobuccal confluent. CBCT, cone beam CT; MMC, middle mesial root canal.

Control cases

Teeth without MMC, paired for sex and molar type against teeth with MMC, were selected to perform the anatomical analysis to compare both teeth with and without MMC. In the teeth without MMC, the measurements obtained were the mesiodistal and buccolingual length of the mesial root, and between MBC and MLC.

Intraexaminer agreement

After 30 days of the completion of the evaluation, 30% of the sample was reassessed under the same conditions to verify the intraexaminer agreements.

Statistical analysis

All the statistical analyses were performed using the SPSS software v. 24.0 (IBM, Armonk, NY) with a significance level set at 5%. The prevalence of MMC was represented by frequency and percentage. The associations between the presence of MMC and sex and age were analyzed using χ² and Fisher’s exact test. χ² test was also used to compare the prevalence of MMC in first and second molars. Multiple logistic regression analysis assessed whether one or more measurements could be a predictor of the presence of MMC. The measurements of the teeth with and without MMC were compared by Student’s t-test. The intraexaminer agreement for the prevalence was analyzed using the κ test and interpreted according to the Landis & Koch. ²² The intraexaminer agreement for the measurements was evaluated using the intraclass correlation coefficient (ICC). The power analysis was calculated considering the sample size and the prevalence of MMC for analysis of the prevalence; the χ² or Fisher value and the degree of freedom for χ² and Fisher’s exact test; the regression coefficient and standard error for multiple logistic regression analysis; and the means and standard deviations for Student’s t-test. All analyses achieved a statistical power of 90%.

Results

After applying the inclusion criteria, CBCT scans of 284 patients were included in this study, being 103 men and 181 women (Figure 6). The mean age of patients was 49.27 ± 13.41 (min. 18 and max. 70). The number of mandibular first molars was 216 and the MMC was found in 24 teeth (mean age 44.37 ± 11.62; min. 27 and max. 68), corresponding to a prevalence of 11.1%. For mandibular second molars, 228 teeth were evaluated, and only 4 presented the MMC (mean age 34.75 ± 8.30; min. 28 and max. 45), corresponding to 1.75% of prevalence. The presence of MMC was significantly higher in mandibular first molars (p < 0.0001) (Table 1). Table 2 shows the distribution of the presence and absence of MMC according to patients' sex. The presence of MMC was not associated with the patients’ sex in both mandibular first and second molars (p = 0.229 and p = 0.569, respectively) nor with age (p = 0.303 and p = 0.143, respectively) (Table 3).

Figure 6.

Flowchart describing the sample selection process. CBCT, cone beam CT.

Table 1.

Distribution of the presence and absence of MMC according to the teeth

Tooth	Presence	Absence	Total
Mandibular first molar	24 (11.11%)	192 (88.89%)	216 (100%)
Mandibular second molar	4 (1.75%)	224 (98.25%)	228 (100%)
Total	28 (6.30%)	416 (93.70%)	444 (100%)

MMC, middle mesial root canal.

the distribution is significant different according to the tooth (p < 0.0001, Fisher’s exact test).

Table 2.

Distribution of the presence and absence of MMC according to sex

Sex	Mandibular first molar		Mandibular second molar
	Presence	Absence	Presence	Absence
Male	6 (7.7%)	72 (92.3%)	1 (1.3%)	78 (98.7%)
Female	18 (13.0%)	120 (87.0%)	3 (2.0%)	146 (98.0%)
Total	24 (11.1%)	192 (89.9%)	4 (1.57%)	224 (98.43%)

MMC, middle mesial root canal.

Sex did not influence the distribution of presence and absence of MMC in mandibular first and second molars (p = 0.229 and p = 0.569, χ² and Fisher’s exact test, respectively).

Table 3.

Distribution of the presence and absence of MMC according to age

Age	Mandibular first molar		Mandibular second molar
	Presence	Absence	Presence	Absence
18–30	4 (19.1%)	17 (80.9%)	2 (0.7%)	28 (93.3%)
31–40	7 (17.1%)	34 (82.9%)	1 (2.9%)	33 (97.1%)
41–50	5 (10.2%)	44 (89.8%)	1 (1.9%)	53 (98.1%)
51–60	5 (10.4%)	43 (89.6%)	0 (0.0%)	47 (100.0%)
61–70	3 (3.3%)	54 (94.7%)	0 (0.0%)	63 (100.0%)

MMC, middle mesial root canal.

Age did not influence the distribution of presence and absence of MMC in mandibular first and second molars (p = 0.303 and p = 0.143, respectively, χ² test).

Table 4 shows the mean of the measurements performed in teeth with and without MMC. The buccolingual measurement was significantly higher in teeth with MMC (8.79 mm) than in those without MMC (8.44 mm) (p = 0.024). Still, the measurement between the MBC and MLC in teeth with MMC was significantly higher (3.47 mm) than in teeth without MMC (2.94 mm) (p = 0.005). The mesiodistal measurement was similar for teeth with and without MMC (3.16 mm for both) (p > 0.05) (Table 4). The measurements from MMC to MBC and MLC showed that MMC is closer to MLC.

Table 4.

Mean and SD of the measurements in teeth with the MMC and without the MMC in mm

Tooth	Measurements
	Buccolingual	Mesiodistal	Between MBC and MLC	Between MMC and MBC	Between MMC and MLC
With the MMC (n = 28)	8.79 (0.32)	3.16 (0.11)	3.47 (0.34)	1.91 (0.37)	1.74 (0.42)
Without the MMC (n = 38)	8.44 (0.38)	3.16 (0.10)	2.94 (0.32)	-	-
p-valuea	0.024	0.9962	0.0005	-	-

MBC, mesiobuccal canal; MLC, mesiolingual canal; MMC, middle mesial root canal; SD, standard deviation.

^aaccording to Student’s t-test.

The multiple regression analysis showed that measurement between MBC and MLC as a predictor of the presence of MMC, with a four times greater chance of a tooth having MMC with greater measurements between MBC and MLC (p = 0.014) (Table 5).

Table 5.

Multiple logistic regression analysis according to the measurements

Measurements	Regression coefficient	SE	Z	p-value	OR	IC 95%
Mesiodistal	−0.389	0.949	−0.409	0.681	0.677	0.11–4.36
Buccolingual	0.350	0.593	0.589	0.555	1.419	0.44–4.54
Between MBC and MLC	1.433	0.587	2.442	0.014	4.194	1.33–13.25

MBC, mesiobuccal canal; MLC, mesiolingual canal; OR, odds ratio; SE, standard error.

According to the classification, independent was found in 60.7% of the teeth with MMC, followed by confluent and mesiolingual confluent (14.3% for both) and mesiobuccal confluent with 10.7% (Table 6).

Table 6.

Frequency and percentage of the classification according to the teeth

Classification	First molar	Second molar	Total
Independent	14 (58.3%)	3 (75%)	17 (60.7%)
Confluent	3 (12.5%)	1 (25%)	4 (14.3%)
Mesiolingual confluent	4 (16.7%)	0 (0%)	4 (14.3%)
Mesiobuccal confluent	3 (12.5%)	0 (0%)	3 (10.7%)
Total (n)	24 (100%)	4 (100%)	28 (100%)

The intraexaminer agreements for the presence of MMC and for the measurements were almost perfect (0.953 and 0.999, respectively).

Discussion

The MMC is an anatomical variation relevant to mandibular molars, and its identification is important to successful endodontic treatment. The non-obturation of all root canals is a factor that could persist the symptoms of apical periodontitis in patients submitted to endodontic treatment. ^3,4 The known anatomical of the root system is important to plan the endodontic treatment and some variations of the common anatomy can be found in teeth and make their treatment difficult. So, it is important to know the prevalence of these variations to understand the probability of the clinicians finds challenging endodontic treatment. This study assessed the prevalence of MMC in a Brazilian subpopulation in mandibular first and second molars and the anatomical aspects of the teeth and found 11.1 and 1.75%, respectively.

The presence of the MMC was first introduced by Pomeranz in 1981; the prevalence of this third canal in mesial root of mandibular first molars is being referred in the literature differently, which makes its study important. Qiao et al ¹⁴ reported the lowest prevalence of MMC in mandibular first molars, being 3.41% in a Chinese subpopulation,¹⁴ followed by Kuzekanani et al ¹³ , who found 8.1% in the Kerman subpopulation. ¹³ The closest prevalence to that of the present study was found by Weinberg et al ¹² and Akbarzadeh et al ¹¹ , who found 13.73 and 14.7% in American subpopulations, respectively. ^11,12 Higher prevalence values were also reported in the literature, ranging from 22.0 to 37.5%. ^8–10 Tahmasbi et al ¹⁰ reported a 26% prevalence of MMC in an American subpopulation as well. ¹⁰ As the present study, these five previous studies have also used CBCT scans for analysis; however, the spatial resolution varied among the studies.

In contrast, other studies have assessed the prevalence of MMC based on their findings during root canal treatment. Nosrat et al ⁸ found a 22% prevalence of MMC using a dental operating microscope, while Azim et al ⁹ found MMC in 37.5% of their sample with a similar method, both in American subpopulations. ^8,9 The main source of discrepancies among the prevalence of different studies may be the studied population. Other important hypotheses are the method used to evaluate the presence of MMC and the sample size, which varied among the studies. Although we have raised the method as a possible cause for variation because it appears that the prevalence with dental operating microscope tends to be higher, a study showed that dental operating microscope and CBCT imaging are equally effective in detecting the presence of MMC. ⁹ Considering the studies that used CBCT scans, the voxel size could also influence the results, since a recent study found that smaller voxel size increased the detection of second MBCs in maxillary molars. ²³ It is important to note that the present study used high spatial resolution CBCT scans, which may favor the visualization of the root system. In our study, we provided the prevalence of the MMC in a Brazilian subpopulation. This prevalence can be applied to similar populations. However, since comparation of present prevalence with those of previous studies showed that it may vary among different populations, further studies should be carried out to detect the prevalence rates of MMC in different countries.

We found that the prevalence of MMC in second molars was significantly lower, at only 1.75%. This significance may alert the clinicians to the possibility of an anatomical variation to mandibular first molars, which are the teeth most subjected to endodontic procedures. ^1,2 Only a few studies have reported the prevalence of MMC separately for mandibular second molars, and the values also varied considerably, being 8%, 16% and 60%, ^11,14,23 probably due to the factors raised for the first molars. Of those, only one tested whether the distribution of MMC cases was different between molar types; although the raw data pointed out that the prevalence was higher in first molars than in second molars (22% vs  16%), the difference was not statistically significant, which is opposite to our findings.

It is important to differ the MMC from the presence of the isthmus. During the root formation process, mandibular molars present a single canal, with the MLC and MBC joined by the isthmus. ^8,10,11 When root formation is completed, mandibular molars usually present separate MLC and MBC, with or without the isthmus. However, if the isthmus calcifies, a true third canal, the MMC, is formed. In our study, we differentiated the MMC from the isthmus by evaluating the entire trajectory of the MMC. The fact that we included patients older than 18 also aimed to avoid this misinterpretation. This differentiation is important because the treatment is different for these two types of variation. In addition, the isthmus presents a higher frequency when compared to the MMC. ^8,10,11

The CBCT is a suitable imaging method to evaluate teeth with complex anatomy. In endodontics, the use of CBCT is commonly requested when patients had persisted symptoms and the intraoral radiography is limited in diagnosing. The justification principle needs to be clear for the clinicians before this request, i.e. the benefits succeed the potential risks. ^16–18 In cases of mandibular first molars with persisted symptoms after endodontic treatment, the dentist can suspect of the presence of a non-obturated canal. The intraoral radiography can be the first imaging method to assess the origin of the persisted apical periodontitis, but in some cases, this method could be limited and the CBCT should be considered. Regardless, other technical errors could also be the origin of persisted apical periodontitis, as underfilling canal and even vertical root fractures. ^3,4 Therefore, the clinicians need to know the indication of the exams, clinical signs, previous medical history to be more accurate about the diagnosis.

The MMC classification by Pomeranz et al ¹⁵ was the first and is currently used. ¹⁵ The authors classified this canal using intraoral radiographs, which could be limited to an appropriate visualization of the root canal system. The CBCT allows a multisectional and dynamic evaluation of the root canal system and, when compared to bidimensional radiography, CBCT has a more accurate visualization for endodontic examination. ^20,21 Still, CBCT allows enhancement of brightness, contrast, and application of filters during the evaluation. This dynamic evaluation allows clinicians to explore more accurately the morphology of the root system. ^19–21 In this study, the evaluation was performed in all multiplanar reconstructions and the application of filters, enhancement of brightness and contrast was used for the examiner to achieve an appropriated visualization. Thus, this evaluation was carefully performed to visualize the root canal path, the pulp chamber orifice and apical orifice to define the proposed classification. The use of three-dimensional examinations in the present study allowed the Pomeranz’s classification to be further developed since it was possible to evaluate the type of confluence that the MMC showed.

In the present study, the MMC presented four different types with a variable frequency. The most frequent was Independent, with 60.7%, and the less frequent was mesiobuccal confluent. The independent MMC represents a canal with a separate orifice and independent apical foramen. This configuration represents less complexity when compared to canals with ramifications and confluences. ¹ Few studies have evaluated the MMC configuration and found that the confluent type was the most prevalent. ^13,14 However, it appears that the configuration was evaluated using periapical radiographs rather than using 3D imaging as the present study. Assessment of the canal system using 2D imaging may mask independent canals by the overlapping nature of 2D and may lead to misinterpretations. Some studies, based on CBCT imaging, reported that the more complex the root system configuration, the greater the occurrence of endodontic technical errors. ^4,24,25 Further research is encouraged to verify if the complexity of MMC is related to the occurrence of endodontic technical errors. Furthermore, applying this classification in different imaging methods, such as micro-CT and different populations could be considered.

This study evaluated the anatomical aspects aiming to correlate the presence of MMC with some of them, because this topic is still controversy in the literature. While one study concluded that there does not seem to be a correlation between the presence of MMC and mesial intracanal distance, ⁹ other investigation found that distance is shorter in teeth with MMC. ¹⁰ The latter finding does not seem reasonable, because the distance between MBC and MLC should be larger to fit the third canal, or at least it should be similar to that of teeth without MMC. Our data reinforced our hypothesis about the need for more space in the tooth to fit the third canal, since both mesial root width and the distance between MBC and MLC were statistically significantly greater in teeth with the MMC, and the intracanal distance was also indicated as a predictor of the presence of MMC in the regression analysis. It is not possible to establish a measurement at which MMC will be present, but these findings may alert the clinicians when the presence of MMC should be suspected during the three-dimensional evaluation of the CBCT scan. In the clinical routine, the use of the operating microscope can also evaluate the distance between the regular canals and alert the clinicians.

Although the present study used high-resolution scans, CBCT examination is not the gold-standard for the presence of MMC; other imaging methods, such as micro-CT and nano-CT could be more specific in defining the anatomy. Nonetheless, these methods are exclusive to laboratory analysis and could not be used in patients such as those in the present study. However, studies with extracted mandibular molars could be performed to analyze MMCs and describe their anatomy in these imaging modalities. Despite the limitations, the prevalence of MMC is relevant for clinicians’ knowledge because it could predict the possibility of encountering this variation during the endodontic procedure.

Conclusions

The presence of MMC is a significant variation to be found in mandibular first molars, with a prevalence of 11.1%; however, it is an uncommon variation to be found in mandibular second molars, with a low prevalence of 1.75%. No association was found between its presence and patients’ sex. Anatomical characteristics, such as the measurement between MBC and MLC and the buccolingual measurement of the mesial root, are significantly higher in teeth with MMC, which may alert clinicians to its presence. Finally, the proposed classification showed four types of MMC with different levels of complexity, being the independent, the most prevalent.