In cases where decay is left untreated for a period of time, bacteria can get into the nerve of the tooth and cause the nerve to be infected. This will result in a tooth ache. At this point a root canal treatment will be required to relieve the pain. Doing a root canal treatment means that you will be able to keep the tooth. Depending on the complexity of your case, your root canal treatment may need to be performed over several visits.
It is strongly advised that a crown is placed on the tooth after the root canal treatment has been completed to preserve the tooth structure and ensure its ability to function.