When a root canal treatment fails, the first course of action is to attempt to redo the root canal treatment. A tooth may not heal as expected after the initial root canal treatment because;
The procedure is very similar to the original root canal treatment. The root canal filling is removed and the tooth is cleaned and shaped very well till the very tip. The procedure may need more than one visit for the infection to resolve. When this happens, the root canal is filled permanently once again and the tooth is reviewed.
Since the nerve had been previously removed, no anaesthetic will be required during the procedure.
If nonsurgical retreatment is not an option or not all the infection has resolved, then endodontic surgery should be considered were an incision in the gum is made to allow access to the tip of the root. The infected part is removed and a filling is placed to close off the cavity. If these are not an option then the tooth will need to be removed.