D16

#16. Maxillary molars have from one to three roots and from two to four root canals. From an occlusal view the pulp chamber is situated rather mesially, which has to be taken into account when cutting the access cavity.
The upper first molar is perhaps the most variable tooth when it comes to root canal morphology, and provides quite a challenge in endodontics. There are usually three roots with three or four root canals. Dentists are quite familiar with the mesiobuccal, distobuccal and palatal canals, but not with the fourth canal, which is known as the mesiocentric or mesiopalatal, mb2 or accessory mesiobuccal canal. This fourth canal is usually difficult to find just by clinical inspection and is not apparent in the radiograph. However, finding all canals is necessary for successful therapy.
The distobuccal canal is often easy to locate and instrument. It is typically rather straight or curves only slightly mesially, or sometimes distally.
The palatal canal always looks straight radiographically but often has a buccal curvature. If this curvature is not identified by careful exploration with files it can lead to perforation 2 - 4 mm before the apex. Moreover, in radiographs a file will still appear to be in the canal but in reality it is only superimposed onto the canal. The palatal canal is often 1 - 2 mm longer than the buccal canals. Two palatal roots in the upper first molar have been reported in the literature.
The mesiobuccal root is the most challenging to treat. The root is usually curved all the way to the apex, which increases the risk of tip perforation and strip perforation. The distal surface of the root is concave which increases the risk of strip perforation.
The mesiopalatal canal is present in well over half of cases, with some authors reporting over 90% incidence. The canal orifice is difficult to find because it is typically situated near the mesial wall of the pulp chamber. While the other three canals can readily be found, the fourth canal must always be actively looked for with suitable instruments. The orifice is usually located 1 - 3 mm palatally from the mesiobuccal canal. In most cases the mesiopalatal canal joins the mesiobuccal canal before the apex.

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