Why Would an Old Root Canal Start Causing Pain?

Hi:
There might be infection that is not showing up on the X-ray yet.  Sometimes infection takes up to 3-6 months to show up on an X-ray.  Another reason could be cracked tooth.
Best,
Dr. Maddahi

Teeth that have already been treated with root canal therapy can at times become reinfected due to a variety of reasons. One of the more common causes is simply time, with time the materials used to seal the canal system can break down and fail, creating a potential space in which bacteria may possibly enter again and re-colonize leading to a re-occuring infection of the same tooth. Regardless of the reason of re-occuring infection or pain, if the tooth is mostly intact and has a good prognosis of further survival, re-treatment of the tooth with another root canal therapy is indicated. 
Keep in mind whenever re-treatment is performed, the prognosis is poorer  for the tooth the second time around. As such, careful consideration should be made on whether to re-treat an already root canal treated tooth or perhaps consider completely replacing the tooth if the prognostic factors and chance of success of a re-treatment seem to be low. Only your doctor can make this assessment and you should speak about all your options with your treating doctor. Good luck!

Thanks for the great question. There are several possible reasons for an old root canaled tooth to start hurting. Most likely reason a leakage between outside restoration covering access to the canal. Nothing lasts forever. That's the reason why 2/year dental check ups are so important. Prevention is a key to success and longevity. Another reason could be a fracture of the root, which actually comes down to the first reason-leakage. It also could be adjacent teeth that hurt but feels like it is one with a root canal. Clinical examination with an experienced doctor will help to diagnose the issue properly.
Best of luck to you.

There are several reasons.  The first of which is that the root canal  treatment may be failing and need retreatment or possibly a minor surgery to save the tooth.  Root canals are very successful, exceeding 90% today.

Another reason is that the tooth may have cracked under the crown...you did have a crown done over the root canal tooth, didn't you?  Most root canal teeth do need restorations that are stronger than just fillings to strengthen the weaker root canal tooth.  Root canaled teeth fracture easier and more frequently non-root canal teeth.

A third reason is that you may have a gum problem or periodontal abscess around that tooth, that has no relationship to the root canal treatment.

A fourth reason is that you may have had some movement of your teeth (most likely due to periodontal disease) and you are hitting harder on the root canal tooth than the others.

OK, you got the run down of possibilities.  What did your dentist say after the x-ray and he examined your tooth?  If he can't give you an answer, then suggest you have your tooth evaluated by an endodontist (root canal specialist).

There are several possibilities for the circumstance you describe.  Root canals can fail over time and an abscess can develop or get larger.  There could be decay that has developed under/around the restoration causing pain.   A root canal tooth is also vulnerable to fracture over time.  It could also be referred pain from another structure including adjacent teeth or even your jaw joint.  You will need an - x-ray, thorough exam and history taking to determine the source of pain and possible infection.  If indeed it is a root canal that is bad, it would be best to remove the tooth and replace it with an implant