EIGHTEEN PERCENT, 36 percent, 54 percent, 71 precent, 93 percent, . . . these are various percentages given for the presence of MB2s (also known as mesiopalatal canals, MPs) in maxillary molars in studies published in various dental journals in the past fifteen years. The bottom line is that this fourth canal is out there, so here are some helpful tips for finding it.
The first thing to do is to modify the traditional triangular access to a rhomboidal shape. From the mesiobuccal orifice, extend the wall about 1.8 mm (average distance between MB1 and MB2) mesiopalatally with a long-shanked small round. However, finding the MB2 can be frustrating, as you may know, because the orifice is often very small and calcified due to growth of dentin in the pulp chamber or exposure to irritants such as mesial proximal caries and deep restorations.
One thing that you can do to help yourself find the MB2 is to place few drops of NaOCl or EDTA in the pulp chamber and look for bubbles coming out of the MB2. However, the opening that is the source of the bubbles can often be difficult to see with the naked eye, so you should use a loupe or a microscope. Another thing that you can do is to look for developmental grooves on the pulpal floor. You can also stain the chamber with methylene blue dye, which may reveal a discolored dot at the opening of the MB2. A final tip is to use the light from the high-speed handpiece (with the water off). Turn your head light off or to the side and place the handpiece on the outer buccal or lingual wall. Press on the rheostat so that the high-speed lights up and look for a discolored dot in the mesial area.
Once you find the MB2, open the orifice with ultrasonic tips or a long-shanked small round bur to get straight-line access for instrumentation. Then use a size 6 or 8 file to negotiate the canal.
I think the most important thing is to enhance magnification, so loupes and a microscope are important tools that will really help you. Also allow sufficient time, because now that you know that the fourth canal can be present, you should at least be looking for it
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