A root canal is needed due to an infection in the tooth. This is usually due to an extremely deep cavity or, less often, due to a blow to the tooth. A tooth could have a deep cavity and the doctor might not know that the tooth needs a root canal because nothing shows in an x-ray and the patient has no pain. Then, a week or a year or many years later, something may show up in an x-ray or the patient could report pain. If that happens, root canal treatment would be done at that time.