Oral Health Group

The Notorious MB2 Canal: How To Find And Treat It

May 7, 2019
by Manor Haas, DDS, Cer. Endo, MSc(D), FRCD(C)

One of the most infamous canals we chase in endodontics is the second mesio-buccal (MB2) canal of maxillary molars. It’s often referred to as the “fourth canal” and is one of the most frustrating aspects of maxillary molar root canals.

We have known of the presence of the MB2 canal for over a century. In 1917, Professor Walter Hess injected root canals of human teeth with vulcanized rubber and then removed the hard tooth tissues by decalcification. By doing so, he clearly showed the presence of the MB2 canals in maxillary molars (Fig. 1).1,2 Perhaps due to the difficulty in finding these canals clinically, or the lack of proper tools needed to look for them, many dentists have performed root canals without treating the MB2.


Fig. 1

Dr. Hess showed the presence of MB1 and MB2 canals and the communications between them (blue arrows).

In this article, I will explain why the MB2 canal is important, where it is located, and how to treat it.

Why the Mb2 Canal is Important to Treat
One often hears colleagues state that treating the MB2 canal is not very important. They may argue that their root canal treatments have been successful, even without treating the MB2 canal. That may be the case empirically, but we certainly know better. Leaving a canal untreated has its consequences. And, let’s not forget the obvious. Root canal treatment is considered incomplete until all, not some, of the tooth’s canals are treated.3 If I may put it differently, please ask yourself the following. If it was you or your family getting the root canal, would you be okay with a partially completed root canal?

In vital cases (i.e. irreversible pulpitis), incomplete root canal treatment may translate into ongoing pain such as to cold/hot, following root canal treatment. In necrotic teeth (with infected canals), leaving an MB2 untreated may be even more significant. The reason is because leaving bacteria in the MB2 canal simply increases the chance of endodontic failure and apical pathology.4 A case in point is seen in Figures 2 and 3.

Figs. 2 & 3

Note the apical lesion (red arrows) over and around an untreated MB2 canal in tooth #26.

Furthermore, some may argue that MB2 orifice is sealed from any bacteria in the pulp chamber because it is often covered by a solid layer of dentin (the dentin shelf). This may be one of the reasons why many may not worry about an untreated MB2. But, as we’ve known from Dr. Hess’s work a century ago, there are communications between the MB1 and MB2 canals in the form of isthmuses or anastomoses.1,5 Figure 1 highlights the communications between the MB1 and MB2. So, if the MB1 canal has bacteria in it, then you can expect the MB2 to also have bacteria in it.

Nowadays, with CBCT imaging, it is easier to identify endodontic failures due to untreated MB2 canals. In Figures 2 and 3, a MB lesion is seen centered around the apical exit point of an untreated MB2 canal. Such cases could and should be treated by means of non-surgical root canal retreatment, or apicoectomy, and with use of a dental microscope.

Hence, an untreated MB2 canal is a main reason for endodontic failures in maxillary molars.3,4 So, hopefully everyone will take note of how important treating the MB2 canal is. And, one should consider referring to an endodontist if the MB2 canal can’t be located or treated. There’s nothing wrong with performing a pulpectomy to manage the patient’s symptoms and referring to a specialist if/when needed.

Frequency of Mb2 Canals
How often is the MB2 present? Well, the following is the best answer. It’s always there. Unless, you looked for it at the right location, with the right tools and technique and couldn’t find it. In other words, almost always. More specifically, studies put the numbers at around 90 to 95 per cent for maxillary first molars and about 90 per cent for maxillary second molars.2,6 What seems to matter clinically is how one goes about looking for the MB2. A case in point is the use of magnification. It’s been shown that with dental loupes and a dental microscope, the MB2 canal was located in 41 per cent and 94 per cent, respectively.6 With no magnification, the frequency of finding the MB2 canal may drop to as low as 17 per cent.7

It should also be mentioned that the MB1 and MB2 canals may run separate from each other or converge prior to reaching the apex (Figs. 4 & 5).2

Figs. 4 & 5

Separate MB1 and MB2 canals (left) and converging MB1 and MB2 canals (right).

Location of Mb2
The MB2 canal is located palatal to the MB1 canal. One needs to be well aware that is not in the direction of the palatal canal, but towards the palatal aspect of the tooth. This is key. There may be exceptions, but for the most part, the MB2 orifice is about 1 to 3 mm from the MB1 orifice. In maxillary second molars, the MB2 tends to be closer to the MB1 than in first molars. In fact, by the time the two MB canals are fully instrumented, they may form a figure-8 or ovoid orifice

In middle aged or older patients, the MB2 is typically located under a layer of dentin that sits on the pulp floor. This layer is called the “dentin shelf”.2 As such, if a dentin shelf is present, it needs to be removed in order to uncover the MB2 orifice. Otherwise, one will not be able to access the MB2 canal, no matter how diligently they look inside the pulp chamber (Cases 1 & 2).

Case 1: Step by step of locating and uncovering the MB2 canal in a maxillary first molar ( #26). Images are viewed under high magnification with a dental microscope. 

Fig. 1A

Fig. 1B

Fig. 1C

Fig. 1D

Fig. 1E

Fig. 1F

Case 2: Step by step of locating and uncovering the MB2 canal in a typical maxillary second molar (#27). Upon initial access, it appears the canals are in line with each other.

Fig. 2A

Fig. 2B

Fig. 2C

Fig. 2D

Fig. 2E

Fig. 2F

Treating the MB2
First, the obvious. As per what has already been mentioned, know where to look for the MB2 canal. Second, if the orifice is covered by the dentin shelf, remove it by grooving the part of the floor/shelf in a direction away from the MB1 orifice. This is where two tools come in handy. One is magnification with illumination such as loupes with a good LED light or a dental microscope (Figs. 6-8).8 Two is a slow-speed handpiece with either a surgical length (long) round bur or a small bur the length of a gates glidden bur, called a Mueller bur (Fig. 11). Both serve the same purpose. They aide in the removal of the dentin shelf in order to unroof the MB2 orifice. The direction of the grooving should be away from the MB1 canal and in a palatal direction. I’ve mentioned this already but it needs to be stressed. If the tooth is very calcified, the MB2 orifice tends to be deeper and as such you may need to groove, about 1 to 2 mm (or more) apically.3

Fig. 6

Dental loupes with light (Designs for Vision, left) and a dental operating microscope (Carl Zeiss, middle and Global Surgical, right) are used to locate MB2 canals.

Fig. 7

Fig. 8

I must add that performing the above is not simple. If you groove the floor in the wrong direction, too mesial or too distal, you may perforate the tooth. A common error is grooving the floor towards the palatal canal/orifice. You must never groove in that direction as doing so would mean grooving the floor over the furcation and may result in a perforation. And, of course, it will not help in locating the MB2 canal since it’s not located over the furcation. Instead, you must groove from the MB1 canal towards the palatal aspect of the tooth, not the palatal canal (Figs. 9 & 10). If you groove too mesially, then you risk perforating the mesial wall of the endo access.

Fig. 9

A red “X” marks site of the tooth furcation. Grooving the floor over the furcation risks perforating the tooth and is also not the site of the MB2 canal. The blue arrow in figure 6 shows how the MB2 is palatal to the MB1. The red arrows in Figures 9 and 10 show the incorrect direction to groove the floor.

Fig. 10

Good magnification and illumination is so important as it helps visualize your landmarks and also in seeing if the floor is “blushing” from grooving too deep.

When using a slow speed bur, the head of the handpiece will tend to obstruct the view of the bur tip. This may increase the risk of perforation as you may be drilling and grooving blindly. On the other hand, a great benefit of an endodontic ultrasonic tip is that it makes it possible to visualize the ultrasonic tip when it’s grooving the dentin shelf. This in turn makes it possible to accurately groove the floor and reduces the risk of perforating the floor (Figs. 11-13).

Fig. 11

Instruments for grooving/ unroofing the MB2 canal. Slow speed handpiece with a Mueller bur (left), a surgical length (long) round bur (top-left). Endodontic ultrasonic with blunt tip (right).

Fig. 12

Note how the handpiece head limits visibility into the pulp chamber and of the bur tip.

Fig. 13

Unobstructed view of the pulp floor with an ultrasonic tip.

The MB2 orifice is often very small and may be seen as a little black dot on the pulp floor. Since the MB2 tends to be calcified in heavily restored teeth, or middle aged and older patients, it can be difficult to negotiate. As such, it is recommended to start with small stiff stainless-steel hand files (i.e. #6,8 and/or 10). Following the use of small hand files, nowadays it is recommended to use glide path NiTi mechanized files to further open the canals (Figs. 14 & 15). In a February 2018 Dentistry Today article about managing calcified canals, I elaborated on instrumenting small canals.9

Fig. 14

Examples of glidepath NiTi files: WaveOne Gold Glider (Dentsply Sirona Endodontics, left) and ESX Scout (Brasseler USA).

Fig. 15

Although the MB2 canal may be small and often much smaller than the MB1 canal, it should still be fully instruments. This often means instrumenting it to the same master size as the MB2.

In summary, it goes without saying that a fundamental aspect of root canal treatment is that of treating all, not just some, of the canals in the involved tooth. As such, it is imperative to treat the MB2 canal in maxillary molars. One should expect to find the MB2 canal in nearly all maxillary molars. However, the canal is often difficult to locate and negotiate since it is often hidden under a dentin shelf and tends to be calcified. Having the right tools and knowing where to look for this infamous canal is paramount.

Oral Health welcomes this original article.

Disclosure: Dr. Haas reports no disclosures.


  1. Hess W. The anatomy of the roo-canals of the teeth of the permanent dentition. Dental College of the University of Zurich, 1917.
  2. Vertucci FJ. Root canal anatomy of the human permanent teeth. Oral Surg Oral Med Oral Pathol. 1984;58:589-99.
  3. Hargreaves, K. Pathways of the Pulp, 11th edition, Elsevier Inc, 130-208, 2016.
  4. Akbar I. Radiographic study of the problems and failures of endodontic treatment. Int J Health Sci. 2015 Apr;9(2):111-118.
  5. Weller RN, Niemczyk SP. Incidence and position of the canal isthmus. Part 1. Mesiobuccal root of the maxillary first molar. J Endod 1995 Jul;21(7):380-3.
  6. Schwarze T, et al, Identification of second canals in the mesiobuccal root of maxillary first and second molars using magnifying loupes or an operating microsocope. Aust Endod J, 2002 Aug;28(2):57-60.
  7. Buhrley LJ et al, Effect of magnification on locating the MB2 canal in maxillary molars. J Endod 2002 Apr;28(4): 324-7.
  8. Gorduysus MO, et al. Operating microscope improves negotiation of second mesiobuccal canals in maxillary molars. 2001 J Endod 27, 683-686.
  9. Haas M, Clinical tips for instrumenting calcified canals. Dent Today, 2018 Feb;37(5).

About The Author
Dr. Haas is a certified specialist in endodontics and lectures internationally. He is a Fellow of the Royal College of Dentists of Canada and is on staff at the University of Toronto Faculty of Dentistry and the Hospital for Sick Children. He maintains a full-time private practice limited to endodontics and microsurgery in Toronto. He can be reached via the website: www.HaasEndoEducation.com.

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