Posterior tooth root pain is supposed to be a case of periapical tooth inflammation. In the early stages, the affected tooth has mild pain, which can be temporarily relieved if the affected tooth is clenched. In addition to the increase in inflammation, the patient can feel the pain by tapping on the affected tooth. When biting, the affected tooth contacts earlier than the other teeth. The pain of periapical periodontitis is felt as spontaneous, persistent pain, limited in scope, and the patient can clearly indicate which tooth is in pain, which is essentially different from pulpitis. If this stage is not treated effectively, an acute periapical abscess tends to form, with significant percussion pain and persistent throbbing pain. It is possible for the pus to break through the periosteum and reach the submucosa. Once the tympanic membrane is broken through, the pain is not as pronounced, but there is submucosal swelling.