A crown that has been improperly fit will not sit well below the gum line of the tooth it covers. Food particles become trapped in this area, allowing the tooth to become the target of bacteria and decay, since it is difficult to brush or keep this area clean. This will result in a cavity forming on the tooth, having a root canal' to remove the nerve endings of the tooth and adding a new crown. Also as crowns age the crown margins will become open as a patient's gums tend to recede, becoming a food trap and place for patients to keep clean.
A cavity starts as a pin hole in a tooth and is caused by bacteria developing and resulting in decay. When the cavity is ignored, it gets larger and causes what was once a small spot of decay to grow and get larger or undermine underneath a crown. When the cavity is not cleaned out by the dentist, the decay eventually reaches the nerve and cause a toothache. At this point, the nerve will need to be removed during a root canal and the tooth replaced with a crown or replace the old crown with a new crown.
Removing the roots of a badly decayed tooth eliminates the pain and swelling within the tooth. This is is known as having a root canal procedure. The roots of the tooth are completely removed, and the top of the tooth is prepared for further care by the dentist.