We found the prevalence of isthmuses to be roughly consistent with the findings of previous studies, although we detected a lower than expected incidence of MM canals. There may be a few different potential explanations for this finding. For one, MM canals are defined differently in various studies. For example, Mehrnaz Tahmasbi  defined MM canals as we did in the present study, identifying them based on the presence of a distinct round cross-sectional area in radiographic image located in-between the MB and ML canals without regard for whether or not an isthmus was present. In other studies, however, MM canals were considered to be present if 3 canals were evident in the mesial roots. While most studies define isthmuses based on the presence of thin ribbon-shaped communications visible in between the MB and ML canals, others such as Hsu and Kim  define types of isthmuses, with a type I isthmus being defined as having either 2 or 3 canals with no noticeable communication, types II and III isthmuses containing 2 and 3 canals, respectively, each with clear communication between the main canals, type IV isthmuses having canals extending into the isthmus area, and type V isthmuses having extensive connections throughout the region. By this definition, MM canals would thus be classified as a form of isthmus, thereby potentially throwing off prevalence rates for these two types of features in certain studies.
We chose to define isthmuses and MM canals based on the criteria outlined in the study of Mehrnaz Tahmasbi, making effort to distinguish between isthmuses and true MM canals. We classified isthmuses and true MM canals as specific kinds of communication between the MB and ML canals, and once we established prevalence rates of these features, we analyzed them separately.
An additional factor potentially complicating comparisons between studies is that a range of different techniques have been employed to explore the morphology of mandibular molar root canals, and different approaches have their unique advantages and disadvantages. Previously used methods include plastic casts , staining and clearing , an operating microscope , and micro–CT imaging [3, 4, 15]. These different methods have all yielded different prevalence rates with respect to isthmuses and MM canals. Past findings and relative advantages of some of these approaches are detailed in the following paragraphs.
Microscope-based examinations have determined MM canal frequencies to be from 1 – 37.5% [7, 8, 10, 16, 17]. In a study by Sunil Kim , a total of 72/106 teeth exhibited isthmuses, with no evidence of such communications in the remaining 34 teeth. In that same study, cumulative 4-year survival rates were 61.5% and 87.4% when isthmuses were present and absent, respectively. In certain clinical studies, including the above research, MM canals were determined to be present based on the detection of 3 canals upon treatment. However, as certain canal conformations such as a Type IV canal can be present without any true MM canal in evidence, this approach is insufficient as a means of accurately determining the rates of MM canal prevalence.
With respect to micro-CT, this approach has more success than microscopic and clearing-based techniques for accurately observing fine anatomic structures such as MM canals in the resultant 2D and 3D images. Indeed, in some studies highly complex configurations of mandibular first molar root canals were detected, defying standardized classification efforts . When patients were separated by age and assessed for isthmus presence, frequencies for individuals aged 20 - 39 years, 40 - 59 years, and ≥60 years were 50%, 41%, and 24%, respectively .
Mehrnaz Tahmasbi used CBCT-based imaging and reported that an MM canal prevalence of 26% in the mesial roots of mandibular first molars, while the frequency of isthmuses was 64.7%. Another study employing CBCT imaging detected an MM canal prevalence rate of just 0.35% (5/1435 teeth), while 26/1435 teeth showed evidence of only a single mesial canal (1.81%) . These and other results suggest that CBCT imaging results may be less accurate than micro-CT based imaging strategies, however as micro-CT images are often only available from older patients or those with serious periodontal diseases, the results from such studies cannot be construed as being representative of the general population. CBCT is primarily utilized in pre-treatment clinical contexts, making it an extremely valuable technique for establishing the presence of isthmuses and MM canals in areas of operative interest. Indeed, given its superiority over traditional periapical radiography, it has been proposed as an ideal means of identifying root canals . For these reasons, we selected CBCT imaging for use in the present retrospective analysis.
We found frequencies of communications containing isthmuses and MM canals to be 59.54% in males and 57.61% in females. Incidence rates of these anatomical features on the left and right sides were 61.21% and 56.25%, respectively. Neither of these differences was significant, and these prevalence rates were consistent with previous studies . We did, however, identify a significant relationship between the presence of additional mesial-lingual roots and the incidence of MB-ML communication. Mandibular first molars most typically have a total of 2 roots and 3 canals in the mesial and distal roots . Different populations seem to exhibit different rates of additional mandibular molar roots, with rates of 22.7 % in Japanese patients , 15 % in patients from Hong Kong , and 29 % in those from China . A lower rate of just 3.64% was detected in a Turkish Cypriot population. We found in our assessment of a Chinese population that the incidence of additional distal roots was 20.4%. Of the 73 individual analyzed cases in which a separated distal- lingual root was evident, 41.1% of these showed evidence of either an isthmus or MM canal. A significantly higher rate was detected in those cases without this separated distal-lingual root. The reason for this significant variation between these two groups remains uncertain and warrants further investigation.
Our findings indicate that younger individuals are significantly more likely to have MM canals and isthmuses than their older counterparts, with an incidence of MB and ML communication in mandibular first molars of 71.4% in those age 20 or less. In their micro-CT reconstruction-based study of isthmus anatomy, Gu et al.  found that isthmuses were present significantly more often in younger patients (20-39 years old; 50%) relative to older patients (60+ years old; 24%), with an increasing ratio of partial to complete isthmuses increasing with age  . This was also true in a study conducted by Ali Nosrat , and are consistent with our results in the present study. Together these results indicate that clinicians should take additional time to explore and assess the pulp chamber floor area between MB and ML canals to identify any isthmuses which may be present, especially in younger patients.
When assessing types and lengths of isthmuses, we diverged from the Vertucci standard and instead divided roots into three portions: the cervical, middle, and apical sections. As different sections of the isthmus can have different effects and clinical significance, and this stratification can have important clinical implications. In clinical settings, particularly using microscopic enhancement, cervical sections can be readily dealt with using ultrasonic tips. Such isthmuses were evident in 47.89% of cases, suggesting that isthmuses in the cervical third should be routinely treated under the microscope via standard RCT approaches. The apical portion of the RCS has been considered critical for therapeutic and pathogenic reasons , with cleaning isthmuses in this region being essential to effective long-term treatment outcomes. Even when appropriate irrigation and preparation techniques are used, preventing and controlling infections in this site can be challenging. We determined that 17.36% of cases in our study exhibited isthmuses in the apical third region, although limitations in the CBCT technology may mean that the true prevalence rate is even higher as the apical structure is relatively subtle. Fortunately, treatment of this region via apical surgery typically yields favorable clinical outcomes. The treatment of the apical area by apical surgery often achieves ideal clinical results, although surgical success rates are highest in teeth that do not have an isthmus relative to those with a prepared isthmus. Given these success rates and the fact that there is a risk of weakened roots following preparation, efforts must be made to improve isthmus preparation techniques for isthmuses located in the apical region. Enhanced efforts to understand how such isthmuses affect surgical outcomes will make it possible to ensure better patient results . Isthmuses located in the middle third region are relatively more difficult to effectively clean in the process of apical surgery or RCT. Substantial amounts of tissue may be destroyed to access isthmuses in this location, potentially significantly increasing fracture rates. We found that the prevalence rate of such isthmuses was 18.48%, which is fairly low. While it is important to clean isthmuses located in the middle third section, they are not as prevalent as those in cervical or apical regions. Given the risks of operating in this site, conservative measures should be taken during treatment.
Given the relatively strict definition which we used, we detected a very low prevalence of true MM canals by CBCT. Our and others’ results suggest that true MM canals should only be identified when three distinct independent canals are present. Detecting and managing isthmuses presents as an important but complex challenge. Navigating both isthmuses and MM canals can be accomplished using a combination of imaging approaches, tactile search techniques, and an operative microscope to carefully and accurately navigate these delicate and complex structures. Future studies assessing long-term patient outcomes are needed to better explore the effects of MM canal preparation on non-surgical endodontic treatments in mandibular first and second molars.