However, about 40 percent of the time, the lower incisors can have two canals. Upper first premolars have two roots and two root canal systems in about 98 percent of all cases, but occasionally they have only one system. Rarely, they look like miniature molars and have three roots. The second premolars have one root with one canal approximately 50 percent of the time with the other 50 percent comprising two roots with two canals. The majority of lower premolars have one root with one canal, but the lower first premolars can have two systems 25 percent of the time.
Molars are generally more complicated.
Upper first molars typically have three distinct roots, each containing a root canal. In about 67 percent of all cases, one of the roots of the upper first molar can house two canals. Occasionally, the second canal is very difficult to detect and can be missed during root canal treatments. This may lead to continued pain and discomfort in the tooth until the fourth canal is found and properly treated. The second molar can have a fourth canal about 25 percent of the time, but may also exhibit fewer canals than the first molar.