When it is needed
Severe pain that wakes you at night, sensitivity that lingers after cold or heat, swelling of the gum near the tooth — all signs that the nerve is inflamed or infected. At that point a filling is no longer enough; either the canal is cleaned or the tooth comes out.
How it is done
Under local anaesthetic the tooth is opened, the infected nerve removed, and the canals cleaned and filled. Usually one to two visits. Afterwards the tooth is protected with a filling or crown, because without a nerve it becomes more brittle.