How can we tell whether we have a separate MB2 orifice? It's simple, but you literally have to "Get in the Groove"!
Recognize that MB2 is usually found palatal and frequently mesial to an imaginary line drawn between MB1 and the Palatal orifice. It actually is not that difficult to find IF YOU HAVE MAGINFICATION! Start by slowly running a small slow speed round bur (#2, #1 or LN bur) or an ultrasonic troughing instrument along the pulpal floor groove that runs from MB1 toward the palatal canal. Don't work too aggressively. These grooves are our roadmap and we need to preserve them until we locate MB2. If the groove is deep enough you will notice that as you remove it with a bucco-palatal motion of the bur, the center portion of the groove will often begin to disappear and the groove will split into buccal and palatal portions. (See Diagram A)
Continue to remove the groove toward the palatal. As you do this, the orifice may appear as a small dot at the most palatal extent of the groove, frequently filled with white dentin mud (if you work dry). Continue to remove both buccal and palatal halves of the groove until you are left with only two distinct orifices. (See Fig. B) (In rare occasions there may even be THREE orifices in this root!)
NOTE: When only a single MB canal is present, proceeding as above will result in you noticing that there is no "palatal" extension of the groove and no second orifice. As you remove this groove it will ONLY appear adjacent to the MB orifice and the whole groove can frequently be completely eliminated as we move the bur buccally…leaving a single MB orifice. That is known as a "fin" and can sometimes be mistaken for an MB2. A fin can also be a variable depth .(Diagram C and D below) In a final Endo film, this "fin" often shows up as a small white line in the cervical area of the MB1 fill. It is NOT a true separate canal. They key is recognizing whether or not you have a second canal is that fins disappear completely as we work them and do NOT leave a separate orifice. If the fin does not disappear as you work it palatally, you probably will find the orifice of MB2.
Another way of recognizing a fin is that placement of an .06 or .08 file into one often results in an abrupt 90 degree file bend pointing toward the MB. The file cannot be advanced. In a true MB2 canal you should be able to advance a file at least part way into the canal (and all the way to the working length if it was bleeding originally!)
Use NaOCl! If you still aren't sure whether you have a separate orifice, lightly irrigate the orifice and just barely cover the pulpal floor dentin with NaOCL (full strength 5.25%). Watch closely to see if there are any bubbles forming at a location of where MB2 is supposed to be. The bubbling reaction of the tissue in the canal with the NaOCl can frequently assist you in finding the MB2 orifice. (Here is where having a microscope is a big advantage.)
How can I tell if MB1 and MB2 join together at some point?
I am frequently asked how I can tell if canals join. This is actually two separate questions:
1. Do they join? And 2. Where?
MB1 and MB2 frequently have a common foramen. But as we have seen in last month's literature review, many have separate foramina and this can occur with almost equal frequency to those that join together at some point. We need to know whether these canals join BEFORE we shape both canals (NOT AFTER!), ESPECIALLY if you use rotary instrumentation.
Separate or Joined Foramina? - Before/During Instrumentation
The most obvious method of deciding whether canals join at some point is to place two files in the canals and see if one contacts the other. (We can use an apex locator to determine this or a film) Another way is to place a gutta percha cone in MB1 (once it has been shaped) and a file in MB2. Remove both and examine the Gutta Percha cone to see where the file has pierced it.
Separate or Joined Foramina? - After Instrumentation BUT both of the methods above do NOT account for anastomoses BETWEEN the canals. This is best accomplished with the simple irrigation test after final cleaning and shaping. Fill one canal (MB1 for example) with NaOCl. Be careful not to place any in MB2. Now use a paper point to length in MB2. If there is a communication anywhere along the canal (ie/ not JUST at the apex), the paper point will act as a wick and the fluid level in MB1 will drop immediately. If MB1 has no communication with MB2 no fluid level change will occur.
The importance of locating the junction MB2 frequently joins MB1 short of the foramen. This is an important concept to understand. One of the reasons we clean and shape MB1 first (beside the fact that it is most often easier!) is that it can result in physically "shortening" MB2. Shorter working lengths are easier to clean and shape. In other words, if MB1 joins MB2 you only have to clean and shape MB2 to its junction with MB1. In fact, if you establish the working lengths of MB1 and MB2 at separate times, it is possible for you to "overwork" the junction, ledge, perforate, tear or strip the common portion of the canal.
Another reason to be precise about the junction of MB1-MB2 is that Ni-Ti rotary instruments do NOT respond well to abrupt changes in direction. If you clean MB1 first and attempt to place a Ni-Ti rotary file past the junction in MB2, file breakage can occur with alarming ease. Not only have you broken an instrument in MB2, but you may have made it impossible to adequately renegotiate and fill the apical portion of MB1 - the canal that actually reaches the foramen. (Fig. A)
The keys to successful treatment of maxillary molars lie in understanding the overall anatomy of the complex MB root canal system before, during and after cleaning and shaping procedures have been accomplished.