On occasion, a previously root canal treated tooth may flare up. If the tooth is deemed restorable and worth keeping, one may pursue root canal retreatment. Here, the endodontist would remove the old root filling and redo the cleansing of the root canal system. The tooth is then refilled when there are signs of recovery from the re-infection. Sometimes, it may be deemed necessary for the endodontist to do a minor surgical procedure to retreat the tooth via its apex.