Root canal therapy is the most effective method to treat pulpitis and periapical periodontitis. Usually, after root canal treatment, there is no toothache in theory. However, some patients still feel dull pain after root canal treatment. Then, the following experts from Jingzhou Chuxing Stomatological Hospital will introduce it in detail. 1. Root canal omission: The root canal system of teeth is very complicated, and the root canals of some teeth often vary. If a root canal is missed in the process of root canal treatment and the perfect root canal treatment is not completed, it may become a focus, leading to residual pulpitis or periapical periodontitis. 2. The pulp is not cleaned: the root canal is not cleaned completely during root canal treatment, and there is still vibrant pulp in the root canal, which causes pain under the joint action of mechanical injury, drug stimulation and inflammation. 3. Insufficient root canal filling: When root canal filling is insufficient, the apical seal is not good, and tissue fluid can penetrate into the root canal, leading to bacterial reproduction in the root canal and causing acute and chronic periapical periodontitis. 4. Periapical tissue damage: The pain caused is also a common cause. When the root canal is cleaned and enlarged, chronic apical inflammation is stimulated and acute periapical periodontitis occurs in the affected teeth.