Caries is the most common problem in the oral cavity. When the caries involves the pulp, root canal treatment is usually needed, which is commonly called “killing the nerve”. After the root canal treatment, the doctor will usually recommend the patient to have a full crown restoration. Many patients do not understand this, thinking that this practice is “superfluous”, but in fact, this recommendation is entirely from the perspective of the benefit of the patient. The specific reasons for this are as follows: 1. The hardness of the tooth decreases after pulp necrosis: After pulp necrosis, the tooth cannot obtain a continuous supply of calcium from the pulp, so the hardness of such teeth will continue to decrease slowly. 2.The degree of force of tooth with tooth defect is changed: After the tooth is treated with root canal due to decay or other reasons, the tooth will be empty all the way to the pulp cavity, even the filling cannot replace the tooth for force. If a complete tooth is a fist, which can be subjected to great force, then for a hollow tooth, the surrounding tooth body becomes a scattered finger, and if it is subjected to the same great force as before, it is likely to be unable to bear it, and then cause the tooth body to split. 3, in addition, some patients with serious dental defects need to restore the bite and aesthetic function through full crown restoration: the risk of tooth splitting is increased by the decrease in hardness and uneven force, and such cases are often seen in the clinic, and the teeth that could have been retained have to be extracted. In order to prevent these situations, we recommend that patients undergo restorations as soon as possible after root canal treatment and, if necessary, staking may be required to strengthen the tooth. Of course, full crown restorations are not the only option, there are also inlays, high inlays and other types of restorations that need to be designed by the doctor according to the patient’s reality.