Like any other medical procedure, endodontic treatment cannot be guaranteed. Endodontically treated teeth can fail to heal for a number of reasons:
It was not possible to treat narrow or curved canals well enough or the canals were not fully cleaned during the initial procedure.
The tooth may have additional complicated anatomy that was not found on the initial treatment.
The final restoration was not placed quickly enough
The final restoration leaked due to a poor fit, fracture or recurrent decay around it.
In these cases it may be possible to carry out the treatment again, a procedure called endodontic retreatment.
Retreatment is usually more complicated than initial root canal treatment as the tooth is normally fully restored with a permanent restoration. This can range from a simple restoration to a full coverage restoration such as a crown or as part of a bridge. In addition to this a post may have been placed inside the root prior to a final restoration being placed. This creates difficulty as access to the root canals is more difficult.
Additionally the canals will have been filled with root filling material and hence this has to be removed before they can be instrumented and cleaned again. All of these obstructions make the process more complicated.
Illustrations reproduced with permission from the American Association of Endodontists