Clinical Research

Prevalence of Middle Mesial Canals in Mandibular Molarsafter Guided Troughing under High Magnification:An In Vivo InvestigationAdham A. Azim, BDS,* Allan S. Deutsch, DMD,† and Charles S. Solomon, DDS†

Abstract

Introduction: A limited number of in vivo studieshave discussed the prevalence of middle mesial canalsin root canal systems of mandibular molars. The re-ported results have varied between 1% and 25%, withno detailed description of the depth and direction oftroughing needed to identify such small canal orifices.The objective of the present study was to determine(1) the prevalence of a middle mesial canal before andafter troughing by using a standardized troughing tech-nique, (2) the pathway of the middle mesial canal inrelation to the mesiobuccal (MB) and mesiolingual(ML) canals, and (3) its correlation with the patient’sage. Methods: Ninety-one mandibular molars from87 patients were included in this study. The patient’sage and tooth number were recorded. After accesscavity preparation, a standardized troughing techniquewas performed between MB and ML canals to searchfor a middle mesial canal by using a dental operatingmicroscope. If a middle mesial canal was located, itwas recorded as separate or as joining the MB orthe ML canals. Results were statistically analyzed byusing Z test and logistic regression. Results: A middlemesial canal was found in 42 of 91 mandibular molars(46.2%). Six middle mesial canals were located afterconventional access preparation (6.6%). The other 36were located after standardized troughing (39.6%).The results were statistically significant (P < .001).There was a higher tendency to locate the middle mesialcanal in second molars (60%) versus first molars(37.5%). Younger patients had a significantly higherincidence of a middle mesial canal (P = .004). Conclu-sions: The middle mesial canal was present in 46.2%of mandibular molars. High magnification, troughing,and patient’s age appeared to be determining factorsin accessing the middle mesial canal. (J Endod2015;41:164–168)

From the *Department of Endodontics, College of Dentistry, UnCollege of Dentistry, Columbia University, New York, New York.

Address requests for reprints to Dr Adham A. Azim, DepartmenAvenue, Memphis, TN 38163. E-mail address: aazim@uthsc.edu0099-2399/$ - see front matter

Copyright ª 2015 American Association of Endodontists.http://dx.doi.org/10.1016/j.joen.2014.09.013

164 Azim et al.

Key WordsAccessory canals, anatomy, dental operating microscope, mandibular molars, mesialroot, middle mesial canal, troughing

Root canal system (RCS) disinfection is of paramount importance in endodontictreatment (1). Efforts are made during instrumentation to remove infected

pulp tissues and debris from the RCS spaces. However, achieving a sterile environ-ment is impossible because of presence of lateral canals, ramifications, and interca-nal communications (2). Complex anatomy is often seen in the mesial root ofmandibular molars (1, 3, 4). Few clinical studies have investigated the incidenceof middle mesial canals in mandibular molars (5–8). Pomeranz et al (7) classifiedthe middle mesial canal into 3 possible canal configurations: fin, confluent, or in-dependent. Despite reports of a high prevalence of intercanal communications inmandibular molars, reaching 83% (9, 10), success in locating and accessing amiddle mesial canal has been very low, ranging between 1% and 25% (5, 6, 11).Failure to locate, cleanse, and shape these RCSs may result in persistent apicalperiodontitis (12).

Troughing in the floor of the pulp chamber, with visualization via dentaloperating microscope, has been proposed by several authors to locate accessoryRCSs (5, 6, 8, 13–16). However, the technique describing the depth and directionof troughing, particularly to identify middle mesial canals, has not been thoroughlyexplained. Lack of such information may result in iatrogenic mishaps whileattempting to locate accessory RCS orifices (13). Therefore, the aim of the presentstudy was to do the following:

1. Record the incidence of middle mesial canals in mandibular molars before and aftertroughing by using a standardized troughing approach

2. Characterize the pathway of the middle mesial canal3. Determine whether a correlation exists between the prevalence of a middle mesial

canal and the patient’s age

Materials and MethodsPatient Selection

The study was approved by the institutional review board of Columbia University,New York, NY. Eighty-seven consecutive patients from a mixed ethnic population, with91 teeth referred for root canal therapy of their mandibular first (56 teeth) and/or sec-ond (35 teeth) molars, were included in this study. The age of patients ranged between

iversity of Tennessee Health Science Center, Memphis, Tennessee; and †Division of Endodontics,

t of Endodontics, University of Tennessee Health Science Center, College of Dentistry, 875 Union

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Figure 1. Illustration of troughing technique after access opening in the mesial root of extracted mandibular molar. (A) MB and ML canals after access prep-aration. (B) Orifice opening of MB and ML canals. (C) Troughing within 2 mm between the MB and ML canals. (D) Locating the middle mesial canal by using a handfile. (E) Middle mesial canal after complete instrumentation.

TABLE 1. Prevalence of Middle Mesial Canal before and after Troughing inMandibular First and Second Molars

ProcedureCases

with MMTotal no.of cases MM % P value

Troughing <.001Before 6 91 6.6After 36 91 39.6

Mandibular molar .06First molar 21 56 37.5Second molar 21 35 60

Total 42 91 46.2

MM, middle mesial.

Bold text indicates significance.

Clinical Research

9 and 71 years, with a mean age of 34 years. All teeth had complete rootformation and closed apices.

Clinical ProcedureAll treatments were performed by the first author by using the

dental operating microscope (OPMI pico; Carl Zeiss Meditec, Inc, Dub-lin, CA) at magnifications ranging between �6 and �11. Anesthesiaand rubber dam isolation were achieved before initiating treatment.Conventional access cavity was prepared to remove the pulp chamberceiling (Fig. 1A). Canals were identified, and the number of RCS orificeswas recorded at the time. If a middle mesial canal was not located afteraccess cavity preparation, the orifices of the mesiobuccal (MB) andme-siolingual (ML) canals were enlarged up to size 25/04 by using rotaryfiles (Endo Sequence; Brasseler USA, Savannah, GA) (Fig. 1B). Trough-ing was then initiated in the isthmus area between the MB andML canalsby using size #2Munce Discovery Bur (CJM Engineering, Santa Barbara,CA). Troughing was performed at the expense of the mesio-axial wall,away from the furcation area, moving the entire isthmus mesially andapically (Fig. 1C). Dentin between the MB and ML canals was removed

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no more than 2 mm apical to the pulpal chamber floor. The depth oftroughing was controlled by using the diameter of the bur head (1mm in diameter) as a guide. After troughing, a fine explorer wasused to check for a negotiable middle mesial canal in the isthmusline betweenMB andML canals. If it was found, the orifice was coronally

Incidence of Middle Mesial Canals in Mandibular Molars 165

Figure 2. Preoperative and postoperative radiographs of 4 possible scenarios of middle mesial canal in mandibular molars. (A and B) Independent middle mesialcanal with separate orifice and apical foramen. (C and D) Middle mesial canal with apical fin connecting MB, middle mesial, and ML canals. (E and F) Confluentmiddle mesial canal joining with ML canal. (G and H) Confluent middle mesial canal joining with both MB and ML canals. (B) was performed by the same clinicianin the same manner described but was not part of the study. The image is placed for demonstration purposes.

Clinical Research

flared with rotary files, Shaper X (SX) or Shaper 1 (S1) (Dentsply TulsaDental Specialties, Tulsa, OK). Size #15 files (Dentsply Maillefer, Bal-laigues, Switzerland) were placed in the 3 mesial canals, and periapicalradiographs were taken to determine whether the middle mesial canal

166 Azim et al.

was separate or joining with any of the other RCSs (Fig. 1D). To be re-corded as amiddle mesial canal, the canal had to be negotiable to within5 mm of the radiographic apex. All mesial canals were enlarged to aminimum size of 35/04 (Endo Sequence) by using standard needle

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Clinical Research

irrigation of sodium hypochlorite throughout instrumentation. AfterRCS obturation, a cotton pellet was placed in the access cavity, followedby Cavit (3M ESPE, St Paul, MN), or the tooth was restored with a defin-itive restoration. Multiple postoperative angulated radiographs weretaken to determine the configuration of the middlemesial canal by usingthe classification by Pomeranz et al (7).

Statistical AnalysisA Z test was used to determine the impact of troughing on locating

the middle mesial canal and the prevalence of middle mesial canal inmandibular first and second molars. Logistic regression analysis wasused to correlate between the patient’s age and presence of a middlemesial canal. This analysis was performed for the following:

1. First molars only2. Second molars only3. First and second molars combined

ResultsOf the 91 mandibular molars treated, 42 (46.2%) had negotiable

middle mesial canals. Six of those (6.6%) were located after conven-tional access cavity preparation. The other 36 (39.6%) were locatedafter troughing betweenMB andML canals. The Z test showed troughingto be a significant factor in locating a middle mesial canal (P < .001).There was a trend in locating middle mesial canals more often inmandibular second molars. However, results were not statistically sig-nificant (P= .06). There were 21 middle mesial canals (60%) detectedin mandibular second molars (n = 35) and 21 (37.5%) in mandibularfirst molars (n = 56). Results are shown in Table 1.

Of the 42 located middle mesial canals, 4 were independent withseparate orifices and apical foramina (9.5%) (Fig. 2A and B). In 5 teeth,the middle mesial canal was a fin that apically communicated with bothMB and ML canals (12%) (Fig. 2C and D). In the other 33 teeth, themiddle mesial canal was confluent, joining with either MB or ML(Fig. 2E and F), or all 3 canals were joining into 1 RCS (78.5%)(Fig. 2G and H).

Regression analysis showed a significant effect of age on theincidence of middle mesial canal, with decreased prevalence as ageincreased. That applied in all 3 analyses: mandibular first molar(P = .01), mandibular second molar (P = .02), and all molars com-bined (P = .004).

DiscussionThis study highlights the importance of access modification in ad-

dressing potential communications present in the mesial root ofmandibular molars. Establishing a troughing technique to access themiddle mesial canal will assist clinicians in achieving better disinfectionof the RCS and possibly better outcome, without excessive damage tothe remaining tooth structure. In this study, each tooth served as itsown control. This allowed validation of the troughing approach andits impact in locating negotiable middle mesial canal orifices. Thetroughing procedure (13, 16) requires minimal dentin removalbetween the MB and ML canals in a mesio-apical direction awayfrom the furcal danger zone (13, 16, 17). Troughing at that levelrequires clear visibility, specialized instruments, and caution toavoid perforation and its subsequent complications (18). In this study,no perforations occurred in any of the treated teeth. Troughing wascontrolled to within 2-mm depth by using a 1-mm-diameter roundbur head as a depth guide. Further apical extension may jeopardizethe visibility and access to the site and possibly lead to root perforation(13). The rationale for using long shank round burs for access modi-

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fication instead of the commonly used ultrasonic tips (13, 16, 19) wasbased on the first author’s personal preference. Using such burs fortroughing allows controlled cutting, with larger debris particlesformation that can be easily removed by using standard needleirrigation.

In this study, troughing was a significant factor in accessing a mid-dle mesial canal (P < .001). Before troughing, the incidence was 6.6%,which coincides with the results presented by Baugh and Wallace (11).After troughing, the prevalence increased by almost 40%. The patient’sage also appears to significantly affect the incidence of locating a middlemesial canal (P = .004). The results are in agreement with previous re-ports (7, 20, 21). This may be due to the ongoing calcification processthat occurs in the pulp chamber and RCS with age (22–24). Such highincidence of middle mesial canal reported in this study cannot be readilyexplained but may stem from using high magnification, the troughingtechnique used, and the relatively young patient population. Previousstudies were mostly performed on extracted teeth (5, 6, 8), with noreference to the age of the teeth at the time of extraction. It isconceivable that the lower incidence in previous reports (5, 6, 8) isrelated to using teeth from an older patient population.

It has been proposed by Hess (25) that themesial roots of mandib-ular molars develop as 1 RCS, which tend to compress at the middleportion, forming 2 RCSs that are often communicating. In this study,1 of 8 patients younger than the age of 15 had a single large mesialRCS. This finding disagrees with the results of Peiris et al (21), who re-ported that in their patient population between the age of 11 and 15, onelarge mesial canal was common. The incidence of a middle mesial canalin mandibular second molars was higher than in first molars. However,the results were not statistically significant (P = .06). These results aresimilar to the findings of Karapinar-Kazandag et al (6). This may resultfrom the later eruption of mandibular second molars, allowing betteraccess to intercanal communications before compression and calcifica-tion (22–24). This finding is not in agreement with the findings of deCarvalho and Zuolo (8) that first molars had a higher incidence of mid-dle mesial canal compared with second molars.

When communication between the MB andML RCSs is present, themiddle mesial canal may not always be a true separate RCS but ratherspaces that still require debridement and disinfection. Regardless ofwhat form these spaces may take, accessing and debriding accessorycanals will improve the mechanical efficiency of irrigation and willattenuate the microbial population within the RCS (25). Leaving thesespaces untreated may lead to infection and treatment failure (12, 13,26). It can be argued that mechanical instrumentation of such spacesmay weaken the remaining tooth structure. However, previousstudies have shown that standard mechanical instrumentation has nosignificant effect on reducing tooth stiffness (27). Further studies arerequired to determine the effect of instrumenting the middle mesialcanal on the strength of the remaining tooth structure. This study hasdemonstrated that troughing in the mesial root within a 2-mm depth ap-pears to be an effective and safe approach to examine the mesial rootsfor potential middle mesial canals. This procedure should be consid-ered as an accessory step when accessing mandibular molars.

AcknowledgmentsThe authors thank Dr James Kulild for his help with the manu-

script.The authors deny any conflicts of interest related to this study.

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