Embedded Files
MB2 is often found close to MB1. If a line is drawn from the MB1 to the Palatal canal orifice, MB2 can often be found along this line, around 1/3 of the way along. There is often an overlying shelf of dentine which has to be removed by a goose neck bur and/or ultrasonics, which then allows scouting with a DG16 probe and small (no. 6/8/10) file to locate MB2. Once located, MB2 is refined and shaped like all other canals.
The literature regarding MB2 incidence is varied. Much depends on whether the studies were in-vitro or in-vivo and if magnification was used. Most papers suggest a figure of between 60-80% of molars have 2 or more mesial canals. The path of extra MB canals can vary, sometimes MB2 will coalesce with MB1, and sometimes it terminates with its own apex, with many variations in between.
MB2 is very curved at the coronal limit of the canal. The shelf of dentin that grows over and obscures the minute orifice has the effect of moving the canal entrance distal to the true path (yellow arrow). When the exploratory file (.06/.08) is inserted, it immediately runs into a wall of dentin on the mesial aspect of the canal. The extremely narrow confines of these typically small canals will not allow the file to negotiate the typically abrupt curvature. Excessive vertical work merely crumples the file tip and risks ledging the orifice.
In order to create access, the file tip is worked to the point where resistance is met, pulling the file to the mesial to remove the overhanging dentin. When the file first engages, only 2 or 3 flutes of the file may engage. As these few flutes are worked, movement of the MB2 orifice to the mesial becomes evident. Once the operator gains experience, this mesial shift can done with ultrasonic troughing tips to provide a more developed mesial incline into the canal. Ultrasonics can be alternated with a small round burs; this makes the orifice more visible with magnification and illumination. It should be noted that a thin film of water or alcohol enhances visibility at this point.
When the initial access glide path is sufficient, the file begins to increase its penetration of the canal space. This will coincide with an increasingly vertical file orientation. A significant number of small files will be utilized for this procedure as they will tend to buckle. You want to avoid a lot of work with the file tip early on, following the safer watch winding approach to prevent ledging within the canal. Slow and incremental removal of the cervical ledge (blue arrow) obscuring the true path of the MB2 canal will enable accurate negotiation of the full canal length with time and patience.
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