This is the procedure to remove the nerve from the canal in the tooth and to seal it afterwards. There are different reasons for root canal treatment including the following:

1. Decay in the tooth involving the pulp, nerve and blood vessel.
2. Irreversible pulpitis due to trauma or an old leaking restoration.
3. Acute pulpitis
4. Fracture into the pulp.
5. Re-preparation of an old root canal treatment.

The aim is to remove all the putrifaction from the canal so that it is prepared for a root canal filling.  Due to the nature of tooth decay, bacteria are involved  and the bacterial infection in the canal must be treated to prevent this from spreading into the apex of the tooth involving bone. The dentist files the canals with profile rotating instruments and  constantly washes the canals with sodium hypochloride or ozone water. After a session of filing and cleaning the canal, and the patient is symptom free, the canal is then filled with a rubber material called Thermafill.

This procedure is done with proper local anaesthetic and can be completely pain free.  In some severe septic cases the anaesthetic is not very effective and the patient may have discomfort or some pain initially, this is rare though.
The root canal in more than 90% cases is usually successful, but if there is a complication of an abscess it can be treated surgically.