In this email I thought a little refresher on the anatomy of the mandibular first molar would be exciting. It becomes even more exciting when some anatomy is missed.

A classic study on root canal morphology (Skidmore and Bjorndal, 1971 Oral Surg) found that the canal configuration of the teeth studied resulted in these following percentages: 7% (6.7%) had two canal systems, 65% (64.5%) had three canal systems, and 29% (28.9%) had four canal systems. In 60% (59.5%) the mesial canals remained divided throughout the length of the root. In the other 40.5% the mesial canals joined in the apical third of the root and had a common foramen. In 38.5% of the distal roots with two canals, the canals remained separate, each having an apical foramen. In the other 61.5% the two distal canals unite and terminate in a common apical foramen.

The number in ( ) is the actual percentages. I like to round the numbers as a memory aid. Different studies result in variance of the above percentages.

As a rule of thumb, 30-40% of mandibular first molars have at least four canal systems.

The take home message is always plan on at least four canal systems in a mandibular first molar, and be aware that they might all have separate POE’s (Portals of Exit).The importance of planning for the occurrence of two distal canal systems is the influence it has on access preparation, in allowing adequate space for visualization and exploration. Often the DB canal can be like a “flying buttress” and sweeping the DB aspect of your access toward the buccal is a good way to check the anatomy. A high percentage of endodontic failure is incomplete removal of the canal contents and untreated anatomy.