In the past, a common misconception about bad teeth (residual roots and crowns) is to pull out the teeth that cannot be filled and take the approach of “extraction followed by veneering”, especially for the middle-aged and elderly people who have fewer teeth left in their mouths, most of them think that if all the teeth are pulled out and a full denture is made, there will be no more teeth to be repaired once and for all. Moreover, the medical cost of making a full denture is low, which is economical and affordable. In fact, permanent teeth are the second and last set of teeth of human beings, and they will not regenerate after extraction. Teeth play a very important role in the oral cavity and are an indispensable part of the body, so we should preserve them effectively and cherish them to improve the quality of life. I. What is crown and root remnant? Dental defect refers to different degrees of damage or abnormalities in the texture and physiological anatomical appearance of the hard tissues of the teeth. When the hard tissue of the tooth is partially missing and large, it is called a crown; when the hard tissue of the tooth is completely missing or nearly completely missing, it is called a root. Second, the evaluation of the residual root and crown 1, according to the mobility of the residual root and crown for classification: Ⅰ, loose only to the labial lingual side loose: loose amplitude <1mm Ⅱ, loose to the labial lingual side and the near and distal middle loose: loose amplitude 1mm-2mm Ⅲ, loose to the labial lingual side and the near and distal middle loose, and accompanied by vertical direction loose: loose amplitude >2mm 2, according to the residual root and crown The first category of the broken end of the root is located on the gingiva The second category of the broken end of the root is flush with the gingival margin The third category of the broken end of the root is located under the gingival margin What are the benefits of preserving the root and crown of the tooth? After the tooth is missing, the tooth root and crown are left behind, the doctor will consider the problem of retaining the root and crown when the tooth is inserted, the doctor will try to retain the root and crown, there are many advantages of retaining the tooth root and crown, so what are the benefits of retaining the tooth root and crown? 1, the retention of residual roots and crowns can slow down the resorption of alveolar bone and maintain the height of the alveolar bone: although the mechanical stimulation received by the residual roots and crowns in the chewing movement may be weaker, it can still delay the alveolar bone resorption of the affected teeth. If the crown or root is removed, the physiological stimulation such as mastication disappears and the alveolar bone in the area will then undergo disuse atrophy. If the alveolar bone is less resorbed, it is beneficial for future denture restoration; if the alveolar bone is too much resorbed and even becomes low, it will cause great difficulties for future denture restoration. 2. The retention of residual roots and crowns preserves the periodontal membrane, maintains the physiological function of teeth, and makes eating more fragrant: the residual roots and crowns are extensive defects in the hard tissues of the teeth, but the periodontal membrane tissue still exists and can play a normal physiological role and can withstand a certain amount of chewing pressure. There are many receptors around the roots of the teeth, and their function is to feel the pressure from chewing food, and this pressure information makes us feel the “pleasure” of chewing. Once the roots of the teeth are removed, the receptors disappear, meaning that the pleasure of chewing no longer exists. This innate receptor is very valuable and is a non-renewable resource as far as the current technology is concerned, so it is best to preserve it until the root is broken. 3, the retention of residual roots and crowns helps patients’ psychological health: some patients’ residual roots and crowns are not effectively preserved, forming a state of missing teeth, which not only affects the main physiological functions of the oral cavity, but also the alveolar bone is easily resorbed, the face looks old, inaccurate pronunciation and air leakage, easily causing psychological disorders, reluctance to participate in social activities, forming psychological autism, directly or indirectly affecting their life This will directly or indirectly affect their quality of life and work efficiency. 4. For some elderly people who cannot tolerate tooth extraction (such as those suffering from hypertension, diabetes, heart disease and other systemic diseases), coupled with the fear in their hearts, most of them are unwilling to remove the residual roots and crowns in their mouths, and can save the residual roots by turning the harmful roots into harmless roots through root canal treatment. 5. The preservation and restoration of residual roots and crowns is important to maintain the stability of the dental row. The more the residual roots and crowns are preserved, the better the integrity of the dentition can be maintained. 6, retaining some important position of the residual root, residual crown, in the anterior teeth can play a role in restoring the aesthetic, to avoid the activity of the restoration affect pronunciation; posterior teeth of the residual root, residual crown, such as the first and second molar residual, root residual crown has important restoration significance, can avoid the free end of the missing active denture repair. 7.When most of the teeth are missing in the whole mouth, retaining the residual roots and crowns is beneficial to the retention of the denture and increase the stability of the denture to improve the restoration effect. 8.Compared with fixed prosthesis or implant prosthesis after root extraction, it has the advantages of short treatment time, no trauma and low cost. 9, located in the back of the mouth root, can provide the necessary support for the movable denture, when the movable denture chew food, chewing force can be transferred to the hard root, rather than directly loaded to the soft oral mucosa, so as to avoid the pain and instability problems after setting movable teeth. Fourth, the retained root and crown should have the following conditions: ① the root is not loose; ② there is no lesion in the periapical tissue or the lesion can be cured; ③ the root surface of the root should be 1-2mm above the gum; or the root surface of the root is flush with the gingival margin; or the broken end of the root is located within 3.5mm below the gum. ④ The angle of the tooth root is normal (relative to the site). Those who do not meet the above conditions should generally be extracted. V. For the retention of the residual root and crown, there are currently four main methods: I. Gingival resection; The indications for gingival resection are: 1. Gingival hypertrophy, hyperplasia, crown appears short, pseudo-periodontal pockets exist, or gingival margin is hypertrophic and uneven, and after basic treatment, fails to restore the normal form. 2, shallow and moderate periodontal pockets on the palatal side (suprabony pockets). 3, periodontal pockets involving the root bifurcation, with sufficient attached gingiva. 4, chronic periodontal abscesses located near the coronal side. 5, the third molar coincidental gingival flap cover, can erupt and have a synergistic relationship. Gingival tissue covering too much during cavity preparation or crown and bridge restoration, which affects filling or restoration. Second, crown lengthening, surgical removal of gingiva and part of the alveolar bone, so that the clinical crown becomes longer; the indications for crown lengthening are mainly 4: 1, restoration needs. A clinical crown that is too short cannot provide sufficient retention and therefore requires surgical lengthening of the crown. On the one hand, the dentin shoulder collar is formed to improve the retention and fracture resistance of the restoration [1]; on the other hand, the crown edge is not extended too far below the gingiva and violates the biological width. 2. Aesthetic needs. All or individual anterior crowns are too short and the aspect ratio is not coordinated; too much gingiva is exposed when smiling, Cunliffe believes [1] that gingival exposure of not more than 1 mm is appropriate; the gingival margin of the central incisors, lateral incisors and cuspids are not in a coordinated relationship [2]. 3, Suitable for coronal analysis, residual roots, and root surface gingival up to within 3 .5-4 mm below the gingiva. 4. Suitable for lateral root canal penetration or root resorption at the cervical 1/3, and the tooth still has retention value. The indications for the concomitant traction of the root: orthodontic traction of the root of the anterior tooth damaged to 4 mm below the gingival margin before restoration. Dental hemisection is one of the treatment measures for the preservation of molar teeth. It is suitable for the preservation treatment of molar teeth with root fracture, root longitudinal fracture, proximal (distal) deep caries and periodontal disease, etc. After the conventional endodontic and periodontal treatment, dental hemisection is used to remove the severely diseased crown and root and retain the less damaged root and part of the crown to maximize the preservation of tooth tissue and create conditions for restoration. The selection criteria for the indication of hemisection: 1) caries-derived or medically-derived pulp chamber floor perforation with perforation diameter ≥3 mm; 2) root bifurcation lesion of grade II or above, root bifurcation exposure or estimation that crown lengthening will lead to root bifurcation exposure; 3) dental caries to subgingival on the crown side of the proximal or distal mesial side of the tooth, estimation that it is not suitable to be preserved even by crown lengthening; 4) serious destruction of periapical tissue on the proximal or distal mesial side of the tooth is not suitable to be preserved. The above 4 articles can be met as long as one of them is met. The following 3 items must be met: 1) pre-preserved roots with supporting alveolar bone ≥ 1/2 of the root length and loosening