by Richard Mounce, DDS
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The most challenging root for endodontic treatment at the level of location, shaping, avoidance of iatrogenic events, and obturation is the fourth (MB2) canal of the upper first molars. Not surprisingly, the upper first molar has the highest endodontic failure rate of any due to its complexity.
MB2 management is complicated by several factors. One of these is the degree of fluting on the furcal side of the root that predisposes the tooth to perforation if the final taper of the preparation is excessive. In addition, as mentioned above, MB2 canals, in which patency cannot be obtained easily, represent a significant challenge even for the most experienced clinician.
The complexity of MB2 is proven in the endodontic literature1, which shows that two canals are present in the chamber of 95.2% of the MB roots. Also, 71.1% of these had two apically patent canals. The disparity between located canals at the orifice level and the apically patent canals illustrates the challenge provided by some roots which cannot be negotiated despite the best techniques and equipment, such as the surgical operating microscope (SOM) (Global Surgical, St. Louis, Mo.). The SOM significantly improves a clinician's ability to locate and enlarge MB2 canals.
Location of the MB2 is only one component of its management. Prior to placing any instruments into the MB2, access must be straight line. This often requires significant deviation from the stylized pictures of “ideal” access in endodontic textbooks.
The clinician should push the access walls back so that each of the canals can be seen in one mirror view, and the MB2 allows a hand K file (HKF) or rotary nickel titanium (RNT) file to enter the root without canal wall deflection. Attempting to conserve tooth structure by compromising the access size will diminish MB2 tactile control and increase chances of iatrogenic events, especially blockage and ledging.
The MB2 canal of an upper first molar is usually found just to the mesial off a straight line between the MB1 and the palatal canal. While two independent canals are possible without an isthmus, an isthmus is the most common anatomy encountered.
After access, the exact location of the MB2 may not be clear in some cases. Placing a drop of sodium hypochlorite onto the chamber floor can alert the clinician to the location of the canal from the bubbles that arise from the orifice. Unroofing the MB2 orifice may be necessary, and if it is it will require dentin removal along the isthmus until the main MB2 canal is clearly visualized.
A note of precaution. If an upper first molar is accessed and the MB2 canal is not found, the treatment of the other canals should stop until it is located or conclusively determined that the canal is not present. If there is any compromise with regard to available time, clinician experience, instrumentation, lighting and magnification, etc., in the management of the MB2 canal, the case should be referred.
Once determined, the orifice of the MB2 should be entered with a precurved No. 6-8 HKF. Precurving is simple and predictable with the EndoBender pliers.* The file is gently inserted as far as it will go, which is ideally to the estimated working length. As soon as the file resists advancement with gentle pressure, it is withdrawn, a new file is selected and precurved, and the canal is irrigated and reentered.
If the clinician can reach the estimated working length, the working length is taken electronically and the canal is shaped at this depth from a No. 6-15 HKF with emphasis placed on maintaining apical patency. The clinician can generally feel a pop at the minor constriction (MC) of the apical foramen as the file passes through the apex. Using a reciprocating handpiece attachment such as the M4* can save time and effort in the early enlargement of the MB2.
In any event, if the HKF does not want to move passively to the apex or the canal cannot be accessed, the clinical case is far more complex and will require additional techniques. For example, if the initial precurved No. 6 HKF does not move passively to the apex, it can be reinserted in different orientations in an attempt to locate the correct path that will allow negotiation.
The clinician will usually feel a “give” with one orientation of the HKF. As a result, if progress is made, the file is taken further apically until patency is established. Once the canal is open, patent, and negotiable to the MC to the size of a No. 15 HKF, the canal can be shaped with RNT instruments, which in my hands are the Twisted File*.
1. Kulild JC, et al. J Endod 16:311, 1990k.
*SybronEndo, Orange, Calif.
Dr. Richard Mounce lectures globally and is widely published. He is in private practice in endodontics in Vancouver, Wash. Contact him at RichardMounce@MounceEndo.com.