The primary purpose of a root canal is to treat the dead or dying nerve of a tooth. The nerve could be dying from some kind of trauma. Of which, decay is only one source of that trauma. The trauma can include the act of removing the decay, blunt force trauma that could take years to manifest, fractured teeth into the pulp, or cracked teeth. The bottom line is any trauma to the tooth, including the smallest, simplest filling can cause a nerve to go bad.