Root canal treatment is required when a tooth has become non vital (pulp death) or the pulp painful to the point where it must be removed. The pulp is the living part of the tooth containing nerves and blood vessels. The pulp can become irritated (pulpitis) due to decay, trauma, a deep filling or a cracked tooth. If this irritation becomes severe enough (especially if not treated early enough), it can lead to pulp death and this will eventually result in the formation of a tooth abscess. In either case the pulp tissue must be removed form the tooth and the remaining space (root canals) cleaned, disinfected and filled with a root canal filling material (gutta percha). The tooth then may be restored with a filling or sometimes a crown. Some teeth (near the front) have only one root canal, whereas other teeth can have 2-4 root canals. Teeth with multiple root canals will require longer visits. Usually root canal treatment will be done over 2-3 visits.
Non-vital teeth or teeth that have an abscess do not require local anesthetic as the pulp is already dead. In other cases, local anaesthesia is provided to ensure a pain free experience. Occasionally if a pulp is very inflamed, it may not respond fully to local anesthetic. In case like this, where the patient is experiencing discomfort , then a minimal amount of work is done and the tooth dressed with a sedative dressing which should settle the pulpal pain down and allow the tooth to be treated painlessly at the following visit.
In summary, a root canal treatment is a painless procedure that can take several visits and is quite time consuming. It allows the patient to save a tooth that otherwise would require an extraction.