MB2: THE MISSED CANAL
A source of great frustration associated with the endodontic therapy of maxillary molars is whether or not a second mesiobuccal canal (MB2) exists, and to what extent practitioners should go in pursuit of locating it. Failure to find and to fill a canal influences the prognosis of endodontic treatment. The frequency of second canal in mesiobuccal root of maxillary molars is quite high, so time should be devoted in its location and treatment. The present article describes the possible location of these canals and various methods proposed to help in locating the fourth canal. It is almost axiomatic to accept the fact that the root system of the mesiobuccal root of maxillary molars frequently has a root canal system containing more than one canal. This fact should lead to an awareness that has to be reflected in our routine practice of clinical endodontics.
Clinicians should consider treating the mesiobuccal (MB) root of maxillary molars as if they always have two canals until proven otherwise. Untreated mesiobuccal 2 (MB2) canals have been implicated in therapy-resistant endodontic infections. Magnification has been found to increase the detection rate of MB2 canals from 17.2% with the naked eye, to 62.5% with loupes and 71.1% using the surgical operating microscope. But what leads to missing out these MB2 canals while the other 3 are mostly easily located? Well this is usually due to diffuse calcifications, anatomic variations, pulp stones associated with this canal, while in retreatment cases the access to the MB2 may be totally blocked out due to obturation materials and or sealers and mostly by dentinal debris in addition to the previously mentioned reasons.
Weine’s classification has been used to describe four common configurations of the maxillary MB root. Type I is a single canal from orifice to apex, Type II has two orifices that converge to one, Type III has separate and distinct canals from orifice to apex, and Type IV begins as one canal and diverges into two separate canals (Figures 1 and 2). Type II and III canals comprised almost 95% of all teeth in Kulild’s study. Gilles et al noted two orifices in the MB root in 81% of maxillary first molars and 59% of second molars. This data reminds clinicians to search for two canal openings the majority of the time.
There are several reasons other than mentioned before as to why the MB2 canal is challenging to locate and negotiate. Developmentally, there is often a dentinal cornice, or rounded growth of dentin, found in the middle of the mesial surface of the pulp chamber which conceals entry into the MB2 canal. Likewise, during development due to dentin deposition, the ML area of the MB root first moves slightly mesially and lingually, exiting at a distance of about 1.8mm from the MB canal. The MB1 canal normally departs the pulpal floor with only a slight mesial inclination. However, the MB2 canal usually has a marked mesial incline immediately apical to its orifice in the coronal 1 to 3mm. When an attempt is made to instrument the MB2, the tip of the file tends to catch against the mesial wall of the canal, preventing apical progress. Finding and instrumenting the MB2 canal can be made more difficult due to the fact that the canal is usually smaller and can become calcified over time when exposed to irritants such as mesial proximal caries and deep restorations.
PROTOCOL FOR LOCATING THE MB2 CANAL
Road Map. Using a rhomboid access (Figure 3), follow the road map that the developmental grooves form on the pulpal floor. The MB2 canal is palatal and often mesial (under the marginal ridge) to a line drawn between the MB1 and palatal canal (Figure 4).
Remove Mesial Shelf. After locating the MB1 canal, remove the mesial dentin shelf which represents the roof of the pulp chamber overlying the MB2 orifice. Enter the MB2 canal from a flat pulpal floor angling a precurved file from the distal toward the mesial.
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