Sometimes a tooth that has undergone endodontic treatment fails to heal or continues to cause discomfort. On rare occasion, a tooth that had initially responded well to root canal therapy can become painful or diseased months or years later, usually due to bacterial contamination inside, causing root-end inflammation.  In such situations, the tooth often can be maintained with a second endodontic treatment, or retreatment.  This requires "undoing" the initial procedure, which may include removal of a crown, buildup, post, root filling material; and recleansing and enlarging of the entire root canal system.  Then the canal spaces are repacked, and a new buildup and crown may be necessary. Occasionally a root canal can be retreated through an existing crown, where that restoration can be repaired and preserved. Retreatment cases are frequently more technically demanding than first-time-in root canal therapy, and often carry a less favorable prognosis.  There are instances in which extraction, rather than retreatment, may be the treatment of choice.