Patients who have undergone root canal treatment often hear their doctors recommend a crown after root canal treatment, but they do not understand why a crown is necessary. Root canal treatment is a treatment method for endodontic disease and periapical disease, while crown is a dental restoration method. Whether crown restoration is needed after root canal treatment depends on the degree of tooth destruction, if the tooth is so damaged that it cannot be filled with filling material or cannot get enough strength to restore chewing function after filling, crown restoration is needed, otherwise filling with permanent filling material is enough. 1.Because the teeth that need root canal treatment are generally carious (worm teeth) and defective, at this time, although the root canal treatment is completed, the remaining tooth tissue is very fragile, very easy to fracture, crack, and lose the normal function, and it is impossible to restore the function with materials alone. 2. Root canal treatment is a process of removing and disinfecting the debris and bacteria inside the root canal as much as possible by using drugs and instruments. However, if this disinfection process is done carefully, a lot of bacteria-infected teeth must be removed, as well as tooth tissue that has to be trimmed. Therefore, once root canal treatment is performed, most of the teeth may not be strong enough to support chewing and may fracture, and it is impossible to restore the function with materials alone, and even if it is possible to restore the long-term effect, it is not good. 3. After root canal treatment, the tooth loses the nutrient supply from the pulp, just like a tree that loses its roots, slowly withers and becomes fragile, causing the tooth to become more brittle and easy to split. Due to these three reasons, the supporting force of the whole tooth is obviously weakened and may be insufficient to support the chewing pressure under certain specific conditions and prone to splitting of the crown, so a crown should be made in time to protect it.