Cone-beam computed tomographic analysis of middle mesial canals and isthmus in mesial roots of mandibular first molars-prevalence and related factors.

CONTEXT: The internal anatomy of mandibular first molars has been studied in depth in different populations. However, limited information is present in differentiating a true middle mesial (MM) canal from an isthmus. AIMS: The primary aim of this study was to identify the prevalence of a true MM canal and isthmus by retrospectively analyzing cone-beam computed tomography (CBCT) images in vivo. The secondary aim was to determine any correlation between related factors such as sex and age.
MATERIALS AND METHODS: CBCT images of 130 patients with the age group of 13-70 years were selected. Findings of MM canals and isthmus were recorded along with variables such as age and sex. Prevalence was compared using the Chi-square test (P < 0.05).
RESULTS: Out of 143 mandibular first molars, the prevalence of the MM canal was 18.2%. There was no statistically significant difference between sex and prevalence of the MM canal and isthmus. The prevalence of isthmi in the mesial roots was 78.4%. Their presence was significantly higher in the apical third area (37.1%) (P < 0.05). Both MM canal and isthmus were seen significantly higher in the age group of 31-50 years (P < 0.05).
CONCLUSIONS: This study showed a high prevalence of MM canals (18.2%) and isthmus (78.4%). The prevalence of the MM canal and isthmi was significantly higher in the middle age groups. This knowledge may direct the clinicians in locating them leading to good endodontic prognosis.

INTRODUCTION

The primary aim of endodontic treatment is to eradicate irritants which include necrotic pulp tissue, microorganisms and their byproducts. Missed root canals configurations are one of the factors that have been related to a significantly higher prevalence of endodontic treatment failures.[1] Mandibular first permanent molars are teeth that most often require root canal treatment because it is the first permanent tooth to erupt in the oral cavity, more prone to caries and showing a high degree of anatomic variability.

Cone-beam computerized tomography (CBCT) was introduced in the field of endodontics in 1990 by Tachibana and Matsumoto.[2] CBCT provides fine details about the root canal morphology and is more sensitive to detection of supplemental canals than are radiographic images.[3]

Both genes and race are decisive factors which affect the morphology of the mandibular first molar.[4] Numerous studies have explored the morphology of the mandibular molar, but the prevalence of a true middle mesial (MM) canal in the mandibular molar fluctuates in literature. Furthermore, a true MM canal is not clearly distinguished from an isthmus between the mesiobuccal (MB) and mesiolingual (ML) canals.

Limited data are available in this context in the Saudi subpopulation. Awareness of these anatomical variations in mandibular first molar is a key to successful root canal treatment. Hence, the primary aim of this study was to identify the prevalence of MM canal and isthmus in mandibular first molars. The secondary aim was to determine any correlation with variables including sex and age.

MATERIALS AND METHODS

Image selection

CBCT images of 130 patients within the age group of 13–70 years were selected. Sample calculation was performed using 95% confidence interval to have a precision of 5%. All the images were taken as part of the dental examination for diagnosis and treatment planning purpose. Inclusion criteria for CBCT images were fully erupted first mandibular permanent molars with mature apex and high-resolution images. Exclusion criteria were open apices, root resorption, calcifications, root canal treatments, posts, crowns, developmental disorders, pathologies, and history of orthodontic treatment.

Imaging method

The CBCT unit used in this study was GALILEOS Comfort (Dentsply-Sirona Dental Systems, Galileos, Bensheim, Germany). It had a tube voltage of 85 kVp; tube current of 5–7 mA; field of view 15×15×15 cm3; isotropic voxel size 0.3–0.15 mm and exposure time of 14 s to 2–6 s. Images were examined using the scanner's proprietary software (Sidexis XG 3D Viewer; Germany) in an Intel Core i5–4460 at 3.20GHz (Intel Corp, Santa Clara, CA, USA) PC workstation running Windows XP professional SP-2 (Microsoft Corp, Redmond, WA, USA). The level of the images was adjusted using the image processing tool in the software to ensure optimal visualization.

Imaging analysis

All mandibular first molars were thoroughly examined in the three planes (axial, sagittal, and cross-sectional) at 1.0 mm intervals by continuously moving the toolbar from the floor of the pulp chamber to the apex. Patients were divided based on sex and into three age groups as follows: 13–30, 31–50, and 51–70 years. In the axial view, an isthmus was recorded when a narrow ribbon-shaped communication was visualized between the MB and ML canals. The MM canal was recorded when a radiolucency with a distinct round cross section was visualized between the MB and ML canals regardless of the presence or absence of an isthmus. The findings were divided into six categories as follows: Cervical third (C3); cervical third to middle third (C3-M3); cervical third to apical third (C3-A3); middle third (M3); middle third to apical third (M3-A3) and apical third (A3).

Data analysis was performed with the help of the Statistical Package for Social Sciences Version 22 (SPSS Inc., Chicago, IL, USA). Differences in the prevalence of the MM canal and isthmus based on sex and age were compared using the Chi-square test with a level of significance as P < 0.05.

RESULTS

Out of 130 cases, 82 cases (39 females and 43 males, with a mean age of 33.9 years) fulfilled the inclusion criteria. Of these 82 cases, 143 mandibular first molar images were analyzed.

Middle mesial canal

Out of the 143 mandibular first molars, 26 had MM canals. The prevalence of the MM canal was 18.2%. In 12.4% of mandibular first molars, MM canals were located in the C3, 11.2% in M3, and significantly lower in A3 area (4.2%) (P < 0.05) [Tables 1 and 2]. Among the 26 MM canals identified, 14 had a separate orifice from the MB and ML canals and 12 had merged with either MB or ML canals. The 14 MM canals with separate orifice were found to be located closer to the ML canal than as compared to MB canal. They were seen running from C3 to M3 area and finally merging with either MB or ML canal in A3 area [Figure 1a]. Only 2 MM canals had an orifice located in between MB and ML canal orifice and ended in a separate apical foramen [Figure 1b and c]. The remaining 12 MM canals were seen dividing from MB or ML canals in either the M3 or A3 area sharing a common orifice with them in C3 area.

Table 1

Number and percentage of middle mesial canal and isthmus in different locations

Table 2

Number and percentage of middle mesial canal and isthmus in different axial slices

Figure 1

(a) Cone-beam computerized tomography image showing middle mesial canals merging with mesiobuccal canal in apical third area in cross-sectional view. (b) Cone-beam computerized tomography image showing independent middle mesial canals with separate apical foramen in cross-sectional view. (c) Cone-beam computerized tomography image showing middle mesial canals with separate canal orifice in cervical third area in axial view

Isthmus

Out of the 143 mandibular first molars, 112 had isthmi. The prevalence of isthmi in the mesial roots was 78.4%. In 23.8% of mandibular first molars, isthmi were present in the C3, 15.4% in M3, and significantly higher in A3 area (37.1%) (P < 0.05) [Tables 1 and 2]. 19.6% of mandibular first molars showed a continuation of isthmus from C3 to A3 area [Figure 2].

Figure 2

Cone-beam computerized tomography images showing isthmus in cervical third, middle third, and apical third areas in axial view

There was no statistically significant difference between sex and prevalence of MM canal and isthmi, but it was significantly higher in the age group of 31–50 years (P < 0.05).

DISCUSSION

The mesial roots of mandibular first molars display a great variation in canal configuration.[5] In 1974, Vertucci and Williams and Barker et al. described the presence of an independent MM canal.[67] Pomeranz et al. classified MM canals into three morphologic categories as a fin, confluent, and independent.[8] Mortman stated that the MM canal is not an extra canal but rather the sequelae of instrumenting the isthmus between the MB and MLcanals.[9] However, a true MM canal was not differentiated from an isthmus in these classifications. Hence in this study, we tried to identify true MM canals and true isthmi as described by Tahmasbi et al.[10]

Recently, CBCT imaging has gained considerable importance in endodontics for various purposes, including the detection of additional canals in teeth.[11] Matherne et al.[12] suggested the use of CBCT imaging in identifying the root canal system. Hence, in this study, we used the advantage of this technology in locating MM canals and isthmus.

Root canals are formed as secondary dentin apposition occurs during tooth development resulting in vertical partitions inside the root.[13] A third root canal may be formed in mandibular molars by this process.[14] The literature states that the presence of MM canals in the mesial root of the mandibular molars is 0.95%–15%.[1516] The results of our study showed that the prevalence of MM canals was 18.2% which is higher as compared to earlier findings. This might be attributed to the use of CBCT as a diagnostic tool used in searching MM canals and variation in the ethnicity of the population studied.

We found a higher prevalence of MM canals in C3 (12.4%) and M3 (11.2%) area as compared to the A3 area. In general, the MM canal joins the MB or the ML canal in the A3 area. Very few cases have reported the presence of three independent canals in their mesial root.[151617] Our findings are consistent with these studies as we found the low prevalence of MM canals in the A3 area (4.2%). A separate apical foramen for a MM canal is a rare finding.[18]

In this study, the prevalence of MM canals (10.5%) was seen significantly higher in the age group 31-50 years. Most of the cases have reported MM canals in patients who are in the middle age group of 30–60 years.[19] This period seems to be a transition period for canal differentiation. During this period, secondary dentin deposition in the mesiodistal direction within the canal at the cervical, middle and apical thirds causes canal separation. It has been suggested that the form and number of root canals are principally determined by the deposition of secondary dentin, and these partitions cause extensive differentiation in root canals, which were originally in a simple form.[20] Thus, a possibility of additional canals should be expected in patients above the age of 30 years.

The results of this study showed that the overall prevalence of isthmus was 78.4% which is considerably higher than some of the previous studies.[2122] These differences may be partially attributed to the sample size used. Earlier studies[4] have conducted in vitro clearing technique, whereas we evaluated in vivo CBCT images which truly shows intercanal communications. Our findings are consistent with those of Peiris et al. and Villegas et al., who found a higher prevalence of isthmus.[1923] The presence of a higher percentage of isthmi (37.1%) in A3 area as seen in this study may be of clinical significance, because it may be difficult to debride and fill these ramifications adequately which can lead to endodontic failure.

Gu et al.[24] studied the isthmus anatomy of 36 mandibular first molars in vitro using microCT reconstructions. They showed a significantly higher prevalence of 50% isthmi in patients aged 20–39 years compared with 24% isthmi in patients older than 60. These findings are consistent with the results of our study. We found the prevalence of isthmi (39.8%) significantly higher in the age group 31–50 years. Our findings are further supported by Peiris et al. who stated that the prevalence of intercanal communications was low at the young and old ages, but high at the intermediate ages. He found that young individuals had single large root canals and with age, the deposition of secondary dentin caused numerous intervening stages, which resulted in a network of transverse anastomosis and communications between the separate canals.[19]

As the age advances, further deposition of secondary dentin takes place which results in the narrowing of canals, the disappearance of transverse anastomosis and lateral canals.[20] These findings are further supported by our study where the prevalence of isthmus was seen only 16.7% after 50 years of age.

The total prevalence of MM canals and isthmi in C3 area was found to be 33.6%. These spaces were present in between MB and ML canals. Clinically, this raises the need for adequate disinfection of these areas which are easily accessible due to their presence in the C3 area. The total prevalence of MM canals and isthmi in A3 area was found to be 41.3% which was significantly higher than other groups. Microorganisms present in these untouched areas may remain unaffected by endodontic disinfection procedures or can be clogged with bacteria in treated cases.[25] Hence, proficient delivery and activation of irrigants to reach these inaccessible areas are mandatory to achieve proper disinfection. The use of activated 5.25% sodium hypochlorite for cleaning these ramifications and use of thermoplasticized gutta-percha for three-dimensional obturation can aid in the good endodontic outcome.

CONCLUSIONS

The root canal morphology of mesial roots of 143 mandibular first molars was analyzed using CBCT images which showed a high prevalence of MM canals (18.2%) and isthmus (78.4%). When present, MM canals were seen running from C3 to M3 area and finally merging with MB or ML canal in A3 area, or they were seen dividing from either the M3 or A3 of the MB or ML canals sharing a common orifice with them in C3 area. The prevalence of isthmi was significantly higher in the A3 area (37.1%). The prevalence of the MM canal and isthmi was significantly higher in the middle age groups (31–50 years).

This data can assist the clinician in the detection of MM canals and isthmus particularly in C3 area which is easily negotiable. The isthmus in the A3 area acts as a portal of exit and can cause failure of endodontic treatment both surgically and non-surgically if not cleaned adequately. The clinician should be ready to explore these spaces in the middle-aged patients.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.