In order for teeth to remain alive and healthy, each tooth is supplied by nerves and nutrients through a chamber called the root canal. As decay spreads downward and into a decayed tooth, the decay encroaches on the pulp chamber of that tooth, which contains the nerve. This is often what leads to a toothache. In order remove all of the decay from the tooth, sometimes it is necessary to remove the nerve of the tooth entirely, and replace the empty canal with a root canal filling.
Unfortunately root canals are not always entirely effective in removing all bacterial contents from the chamber of the tooth. Even if only a little bit of bacteria persist in the root of the tooth, it can sometimes lead to recurrent infection and pain. A root canal specialist (endodontist) is skilled in re-treating the canal chamber by removing the previous root canal filling, and intensely cleaning the chamber before filling it again and hoping for improved results.