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The Case of the Elusive MB2 Canal

As live demonstrations go, this case was unforgettable; as a teaching case about managing mesiobuccal (MB) root anatomy, it was classic. It was an upper first molar (Figures 1 and 2) with 4 canals and 4 apical portals of exit, as seen in the distally-angled periapical (PA) radiograph and confirmed with my cone beam computed tomography (CBCT) (J. Morita) machine (Figure 3). Oh, did I mention that the demo patient was a personal friend? No pressure.
After access was cut with a pilot-tip LAX diamond bur (SybronEndo), I ran a 15-.06 Vortex File (DENTSPLY Tulsa Dental Specialties), spinning at 300 rpm, halfway into each of the MB1, distobuccal (DB), and palatal canals. The LAX diamond bur was lightly used again to touch up the line angle drops into each of the orifices. Now the MB2 hunt began!
Figure 1. Maxillary first molar with severely calcified pulp chamber and abrupt apical curvature of the
mesiobuccal (MB) root.
Figure 2. Conventional X-ray image shot low and from distal to mesial. Note the second periodontal ligament line behind the MB root and the hint of a second root apex.
Figure 3. Cone beam computed tomography axial cross-section slice showing the MB root containing 2 canals. Figure 4. Photograph of mesial aspect of access preparation. Note the adjacent light and dark-colored dentin indicating where MB2 might enter the pulp chamber.
Figure 5. BUC-1 ultrasonic tip (Obtura Spartan) cutting in a mesioapical direction, beginning at the MB1 canal orifice and arcing toward the palatal orifice. Note the lighter dentin being removed as the most probable site for the MB2 orifice is approached. Figure 6. LAX guided-access bur (SybronEndo) removing the overhang created by the mesioangular cut with the BUC-1 ultrasonic tip.

I washed the access with my air/water syringe and saw the typical grayish coloration of the dentin on the pulp chamber floor and the lighter-colored dentin above (Figure 4). Then I used an ultrasonic handpiece (Obtura Spartan) to cut a curvilinear trough straight into the intersection of light and dark dentin, arcing from the MB1 orifice around the mesial toward the palatal orifice (Figure 5).
MB2 canals enter most upper molar pulp chambers at a distal angle between 30° and 50° off the mesial access wall, so we must hunt for them in a mesioapical direction. I use ultrasonic tips for this purpose because of the exceptional view I have of the cutting site (there is no handpiece head in the way). I specifically use a BUC-1 (Obtura Spartan) for cutting the first 3 to 4 mm of the MB trough because of its round-ended tip. Sharp-ended tips cut a groove that mimics the MB isthmus, possibly leading the clinician astray and increasing the chances of perforation.
Typically, after removal of this lighter-colored dentin, we see the color change at the edge of the MB2 groove and an isthmus line, and hopefully the MB2 orifice. MB isthmus lines are the holy grail of MB2 hunts, as that line is a signpost to the near-center of the MB root, and also because its palatal extent usually points directly to the hidden MB2 orifice.
I then used the LAX high-speed diamond bur to cut off the dentin overhang that was limiting my visual access to the cutting site, so that further progress could then be made with the BUC-1 (Figure 6). So, it is basically a 2-step MB2 hunting trick: (1) Use the well-controlled ultrasonic tip to gain new ground, and then, (2) Use the LAX bur to quickly and safely unroof it to optimize the visual and procedural access for further exploration. Rinse and repeat until you find the MB2 canal, or until you choose to end the hunt.

1. You have decided that there is no MB2 canal. Be careful here, as it is so easy to decide that there is no MB2 canal when the case has already taken more clinical time than you expected and you have nothing to show for the effort. If you think this could be the voice in the back of your head, see reason No. 3 below. If you haven't taken a distally-angled radiograph showing a file in the known canal to be centered in the root structure, you are not allowed to use this excuse.
If you have a CBCT machine in your office, you will simply and surely know how many canals are in each root before the pulp chamber is invaded. When the computed tomography (CT) imaging shows that for sure there is no MB2, there is often a small thrill of euphoria about the case ahead. Conversely, when the CT imaging tells you that for sure your patient's tooth has an MB2 (or maybe an MB3 as well) and you cannot for the life of you find it/them, having CT-based anatomic knowledge is not so good.
2. A decision that there is an MB2 canal, but despite your best efforts, you cannot find it and you are calling off the hunt. This may be the wisest treatment plan if you have tried and failed to find the elusive canal on 2 separate appointments. This is way better than perforating the MB root. However, check out No. 3.
3. You run out of patience and new ideas about how to find this canal that you know has its own apical portal of exit. The recommended finish to this first treatment appointment is to fill the DB and palatal canals with sealer and gutta-percha, fill the MB1 canal with CaOH, and close the tooth with sponge and cavit. Often, with a fresh viewpoint, the MB2 canal is found immediately into the next appointment.

After the dentin shelf was cut back to create a better sight-line to the groove, I was able to see an MB isthmus line projecting in a palatal direction from the MB1 orifice but was unable to achieve a "catch" in the dentin with my DG16 Endodontic Explorer (Figures 7 to 9). In my experience, it is a nonstarter to attempt entering a calcified orifice with any file before feeling tug-back from a DG16 Endodontic Explorer placed there. In this case, I even placed the tip of a 15-.06 Vortex rotary file into each of the explorer penetrations (Figure 10), unfortunately without advancement.

Figure 7. DG16 Endontic Explorer (Hu-Friedy) pressed firmly into the intersection of light and dark dentin, specifically into the debris-filled isthmus groove coursing from the MB1 orifice toward the palatal. No "catch" or
tugback was felt at this location.
Figure 8. DG16 Endodontic Explorer probing closer to the MB1 orifice as MB2 canals can enter the pulp chamber quite close to the MB1 orifice. No catch.
Figure 9. DG16 Endodontic Explorer probing into the palatal extent of the MB groove; still no catch. Figure 10. A 21-mm 15-.06 Vortex rotary file (DENTSPLY Tulsa Dental Specialties) being run in the palatally-located explorer divots. Note the curved flexure. This is the angle that MB2 canals will enter the pulp chamber so pushing on and flexing the rotary file in this manner will increase the chances of
penetrating into an occluded MB2. No dice.
Figure 11. Computed tomography (CT) imaging showing the MB root resliced saggitally through the MB1 and MB2 canals. Note the faint MB2 canal branching off the MB1 canal about 4 mm apical to that canal's orifice. Figure 12. Postoperative x-ray of upper molar treated as a live demonstration at the Chicago Midwinter Meeting. Note the MB2 bifurcating off the MB1 canal halfway up the root and then bifurcating yet again before its terminus.

Rather than continue in this fruitless vein, I changed paths—negotiating the other 3 canals to length with 15-.06 and 15-.04 Vortex files, and shaping them afterward with GTX Files (DENTSPLY Tulsa Dental Specialties). Leaving the MB2 hunt while working in the other canals can result in MB2 orifices opening up due to the NaOCl and ethylenediaminetetraacetic acid soaking in the pulp chamber. Unfortunately, in this case no further evidence of the entry point to the fourth canal was apparent.
Feeling somewhat desperate, I did what I should have done before cutting access in this tooth: I resliced the CT volume exactly across the MB1 and MB2 canals as seen in the axial CT view and then took a careful look at the MB root from a mesial direction (Figure 11). What I saw was a faint canal form branching from the previously shaped MB1 canal, about a third of the way up from the orifice. Ahhhh!
As an aside, this reminded me of another live demo case I did at the Chicago Midwinter Meeting many years before, where the same exact MB anatomy was present (Figure 12); the difference this time being that I was able to visualize it in 3-dimensional (3-D) CT space and treat it with intention rather than hope. This was way more fun!

With that said, I must add that CBCT is not needed to successfully treat complex anatomy. When using conventional PA radiographs exclusively, it is critical to capture ideal x-ray images of the tooth/teeth in question from every possible angle, the most important views of upper molars being a very straight-on image and a shallow, distally-angled image.
Clinicians can often, by chance or skill, negotiate branching canals; however, in other teeth, even with CT imaging it is at times impossible to find and enter them. This is why effective irrigation and 3-D obturation of root canal systems are such critical precursors to consistent endodontic success—especially in molars. As such, irrigants remove debris, kill microbes, and they set the stage for the fill, because 3-D obturation can easily fill any lateral anatomic aberration that has been cleaned out by the chemicals.
When second canals are suspected but not yet found in a given root, clinicians can confirm or deny their presence by placing a file in the first canal, and for molars, then take another distally-angled image. If the file appears to be off-center between the root edges, there is usually another canal to be found. If the file is centered apically and off-center more coronally, there are most likely canals that merge apically and exit the root structure as one canal. When you see the file off-center apically, you know this second canal is a must treat situation, because leaving a diseased root canal untreated dramatically lowers the prognosis for healing.

My next move was to place a moderate bend (~35o) in the tip-half of an No. 08 C-file (DENTSPLY Maillefer) (Figure 13), rotate the mark on the side of the stop toward the bent file tip and then run this bent file tip up and down the palatal side of the shaped MB1 canal, hunting for a lateral orifice leading to the MB2 canal. Suddenly, I felt the file "catch" in the canal about 4 to 5 mm beyond the MB1 orifice and the stop mark was pointing to the distolingual. Gotcha! I knew from experience to be very careful not to pull the file tip back short of the branching point before carefully working it for some time in a watch-wind, push-pull, push-pull motion. This is one of the few situations in which doing 30 or 40 push-pull strokes at length and beyond will actually improve the outcome of treatment.
In this fashion, I was eventually able to work the .08 C-File (with Prolube [DENTSPLY Tulsa Dental Specialties] filling the access cavity) through the MB2 canal, all the way to its terminal point (Figures 14 and 15) as indicated by my Root ZX Apex Locator (J. Morita)—the cool new mini-Root ZX with a detachable cord caddy that has just been introduced in the US and Canadian markets.

Figure 13. A 25-mm No. 08 C-file (DENTSPLY Maillefer) with a moderate curve in its tip. In this case a more acute bend—appropriate for negotiating around an apical impediment—would cause the file to buckle after engaging the MB2 canal orifice on the palatal surface of the shaped MB1 canal. Figure 14. No. 08 C-file at length in the MB2 canal as indicated by apex locator reading. Note the straight apex locator probe adjusting the stop to the MB cusp as reference point. This apex locator probe is much more convenient to use than the spring-loaded electric test probe most dentists use.
Figure 15. Conventional x-ray showing file in the MB2 canal as well as a better-defined MB2 root apex. Figure 16. Illustration of a nickel titanium rotary file dramatically over-bent in EndoBender Pliers (SybronEndo) so that a residual bend of 30° to 40° remains after the file springs back toward straight.
Figure 17. X-ray taken after the MB canal system was downpacked with an Elements/ System-B Heat Source (SybronEndo), showing the MB2 canal filled off the MB1 by the hydraulic movement exerted on the sealer and gutta-percha by the continuous wave of condensation technique. Figure 18. Postoperative x-ray showing the MB and palatal canal systems filled with multiple lateral canals seen.
Figure 19. Straight on x-ray view showing all canals filled. Note the similar apical accuracy shown in the GTX Obturator fill of the DB canal. Note the conservative mesially angulated access cavity through which all rotary files entered canals with no interference.

All that was left at that point was to enlarge the length of this file path to at least a No. 15 k-file size. The strategy I used in this case was and is my favorite in tight, highly curved canals: I cut initial shape with small Vortex files (15-.06 and 15-.04), then work No. 08 and No. 10 k-files to length, then I use the new Rotary PathFiles (DENTSPLY Tulsa Dental Specialties) to finish negotiation procedures in this canal.
While my preferred rotary negotiating files are still the small-tipped Vortex instruments (usually no hand file negotiation needed), these PathFiles are especially helpful in the apical half of highly curved canals as they increase in tip size by small increments—.13, .16, and .19—and they have only a .02 taper, so cyclic fatigue is not an issue in tortuous canals. To get the PathFiles started into the branching MB2 canal, the tips of each PathFile were bent with an EndoBender plier (SybronEndo) (Figure 16), and after insertion into the branching orifice, each of them quickly spun to length, reminding me that I did not need to by a new endo motor to use proprietary reciprocating files when this was so simple. Then I prebent the tip of a 20-.04 GTX File (DENTSPLY Tulsa Dental Specialties) with the EndoBender plier to get it started into the branching MB2 canal, and it actually cut to length. Getting a 20-.06 GTX rotary file to length proved to be a challenge though, so I used a 20-.06 GT hand file to complete the shape. NaOCl was freshened in the canals every 5 minutes for 20 more minutes and then it was time for the fill.
Despite achieving my shaping objective for both of these MB canals (a 20-.06 in both canals), I was only able to fit a gutta-percha cone in one of the canals at a time due to the limited coronal enlargement that could be safely cut in such a narrow, curving root. I chose to conefit the straighter, longer MB1, dried the canals and coated the walls of the MB2 with sealer on a bent No. 15 file. This was to increase the lubricity of the heat-softened gutta-percha as it was condensed into the MB2 canal branch; I then cemented the MB1 cone to length.
Using a prefit, .08 taper continuous wave (CW) electric heat plugger, I was able to fill both canals by down-packing with the CW of centered condensation technique using the EOU System-B Heat Source (SybronEndo) (Figure 17). I then back filled these canals with the EOU extruder and a No. 1 CW hand plugger.
In each of these live-patient demonstrations, I show all of the filling techniques I routinely employ each week in my clinical practice, so I used the single cone backfill method. I down-packed the palatal canal the usual way, but rather than firing the CW plugger for a separation burst of heat so as to remove the gutta-percha alongside the plugger, I let the condensed gutta-percha cool for 10 seconds and after the double-click sound from the EOU unit was heard, I applied apical force on the plugger (no more heat!) as I rotated it back and forth to break it loose from the previously condensed gutta-percha.
After working the plugger back and forth to tease it out of the canal, a tapering hole, the size of the plugger, was left in the gutta-percha. Using Autofit Backfill Cones (SybronEndo) is easy; I chose a backfill cone having the same taper as the plugger. Then I set the .08 taper Backfill Cone into the No. 50 die hole in a gutta-gauge, clipped the tip excess with a blade, coated it with sealer, and then placed it into the single-cone backfill space.
The DB canal was filled with a GTX Obturator (DENTSPLY Tulsa Dental Specialties), thereby finishing the case barring placement of the sponge and cavit (Figures 18 and 19).

These challenging cases always beg the question, "Does it really matter if we find and treat MB2 canals?" Many dentists doubt it, as they have seen few failures despite their rare discovery and treatment of them. However, anecdotal evidence is always suspect, and rightly so, as we tend not to see our own failures.
Early in my career, I asked an endodontist from Los Angeles (LA) what his best tricks were for finding MB2 canals (this was before microscopes and ultrasonic handpieces). His response floored me. He said, "You know, I don't really worry about fourth canals in upper molars. I seldom see them, I seldom treat them, and I seldom have any problems related to them."

Figure 20. CT images sliced through both MB canals of both MB roots on contralateral upper first molars. Note the untreated MB2 canals in both teeth as well as the diseased soft tissue in the maxillary sinuses above those roots.

The very next week I saw a patient (in my Santa Barbara practice, 90 miles north of LA) who had had endodontic therapy the month before and was miserable because her tooth still gave her fits when she drank hot coffee. Conventional radiographic imaging revealed a very nice treatment result in the MB1, the DB, and the palatal canals, but the shallow, distal x-ray angle showed the MB fill to be off-center—confirming a second canal in the MB root. When I asked her who did the root canal therapy, she named the endodontist from LA who had claimed that he never had any repercussions from ignoring MB2s—not one week before.
I couldn't make this stuff up if I tried. The take-away here is that when your cases fail, you may not hear about it, especially if your patient thinks you are a good person and doesn't want to hurt your feelings.
Several years ago, as I was preparing for a debate on managing the apical third of root canals, I called my friend and widely-respected academic colleague, Dr. Phil Lumley (director and head of the Dental School at the University of Birmingham, England), and asked him what he thought were the most important issues to consider when treating these delicate anatomic spaces we work in.
He laughed and replied (I am paraphrasing), "All the time endodontists spend arguing about what we should do at the end of root canals is somewhat ironic, considering how many MB2 canals are left untreated. Rather than arguing about very specific apical extents of treatment, we should be paying more attention to teaching dentists to find all of the canals in a given tooth, as leaving diseased canals untreated will result in a far greater percentage of failure." Phillip was right (Figure 20).
Those of us who treat root canal systems, rather than just primary canals, know that the number of lateral canals seen filled at the end of a case is a matter of anatomic chance; assuming effective irrigation and 3-D filling methods are used. However, the more carefully you work, the luckier you get, and the final radiographs of this case were proof of saying that when preparation (effective irrigation and 3-D filling) meets opportunity (wild anatomy).
So it was a case to remember.
If you haven't trained up (a lot) and/or haven't bought all of the equipment needed to treat these complex cases and please send them to your local endodontist.

Dr. Buchanan is a Diplomate of the American Board of Endodontics, a Fellow of the American and International Colleges of Dentistry, and is on the part-time faculties of the graduate Departments of Endodontics at the Uniersity of Southern California and University of California, Los Angeles. He is the founder of Dental Education Laboratories, a hands-on teaching lab in Santa Barbara, Calif, and 2010 marked his 30th year of a career dedicated to endodontic practice, teaching, and development of new endodontic tools and techniques. He has also won international awards for his video and DVD content. He can be reached at (800) 528-1590 or via the e-mail address at This email address is being protected from spambots. You need JavaScript enabled to view it..


Disclosure: Dr. Buchanan has designed and holds numerous patents on dental products, including the GTX System, BUC tips, LAX burs, EOU/System B, and the new J. Morita Apex Locator with cord caddy, all mentioned in this article and used for clinical practice.

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