Loose teeth are currently clinically indicated for extraction and are by far the most common cause of tooth extraction. Generally, it is natural for a dental clinician to make a decision to extract a tooth as soon as he or she sees a loose tooth. Macroscopically speaking, the most common cause of loose teeth is long-term chronic periodontal disease that is not well treated, which eventually leads to periodontal membrane damage and extensive alveolar bone resorption. However, there are many details that need to be more clearly understood, which is helpful for the design of clinical loose teeth fixed. 1, the causes of loose teeth 1.1 inflammation Any inflammation that injures the periodontal membrane may cause loose teeth. Acute attacks of chronic periapical periodontitis and acute attacks of long-term chronic periodontitis, which are common in clinical practice, may be accompanied by symptoms of tooth loosening. Specifically, when inflammation attacks the periodontium, the periodontium, a connecting fiber that connects the dental bone to the alveolar bone, is in a state of extreme congestion and swelling. At this point, the fibers lose their elasticity, as if they were a piece of twine immersed in water, and are much longer than normal, and have lost their function of pulling and fixing the connection. Once this state of congestion and swelling spreads to most or the entire periodontal membrane, the tooth begins to loosen. If the inflammation is widespread, it can lead to loosening of the tooth from degree 0 to degree III. With the improvement of clinical diagnosis and treatment, both periapical inflammation and periodontal inflammation can be accurately diagnosed and treated in a timely manner. If the affected tooth is only loose due to a single periodontal inflammation, once the inflammation is under control or healed, the tooth looseness will then return to normal. This is because the periodontal fibers have regained their ability to connect and hold. For this type of tooth loosening, the prognosis is good. However, if tooth loosening is combined with other related factors in addition to inflammatory factors, the problem becomes complicated. 1.2 Dental trauma The three-dimensionality of dental forces: Dental trauma as a concept of oral disease is familiar to most dental professionals. However, most dentists have in their mind the concept of “coaptation trauma” only from the masticatory trauma to the coaptation teeth. In fact, this type of coaptation trauma is only a part of all coaptation trauma. Many doctors have the experience of treating periodontal disease and at the same time adjusting the dentition, that is, resharpening the cusps of the affected teeth or the opposing teeth, or even disengaging the dentition contact, which improves the looseness of some teeth, but most of the teeth do not have significant improvement in looseness. Obviously, at this point, the coaptation trauma did not occur on these cusps. So what are the other causes and ways of occurrence of coaptation trauma? The so-called coaptation trauma is understood here as a non-equilibrium force on the affected tooth, which causes a long-term, continuous abnormal pull on the periodontium (ligament). Together with the periodontal inflammatory effect, this causes the periodontal membrane to be unable to resist these forces, and the fibers are damaged or even pulled off. When a tooth is stressed, it is stressed in three dimensions (Figure 2). This is the reason why some molars have evolved to differentiate into three roots. These three directional forces are: Vertical root-oriented force: This refers to the force in the same direction as the long axis of the tooth. It is currently considered to be the main chewing direction force. Buccolingual horizontal force: refers to the buccolingual force perpendicular to the long axis of the tooth. Proximal-distal-medial horizontal force: refers to the proximal-distal-medial force perpendicular to the long axis of the tooth. This force is more influential. It is a force that is transmitted from tooth to tooth. Therefore, once the teeth are not aligned, this force is transmitted to the reversed teeth, which is very damaging to the periodontium because the teeth are not in the normal position to receive and transmit this force. And this direction of force is the most neglected one. Any force applied to a tooth is a combination of these forces. Therefore, trying to remove the dentinal trauma by grinding away the cusps or disengaging the dentinal contacts is only removing the vertical root-oriented forces, and that has only a 30% chance of being clinically effective. Therefore, when considering the adjustment to remove these coaptation forces, all three directions of coaptation forces should be taken into account. In this way, the dental trauma can be truly removed and clinically satisfactory results can be achieved. Significance of the dental arch: In the course of human evolution, the anterior dental arch has an arch-shaped morphology, which has positive implications in terms of counteracting the coaptation forces. In general, the long axis of the human teeth is not perpendicular to the horizontal plane of the jaws, but tilted forward (Figure 3). Therefore, when occlusion occurs, the vertical root-oriented force is also not perpendicular to the horizontal plane of the jaw, but creates a forward dentition force. This means that the vertical root force is partially converted into a proximal-distal-medial horizontal force. The proximal-distal-medial horizontal force of the posterior teeth is transmitted along the central sulcus, through the adjacent surfaces between the teeth to the anterior teeth, and finally to the anterior arch, where it is counteracted by the same force transmitted to the anterior teeth at the same time on the opposite side, thus allowing the teeth to remain unharmed even when subjected to a force of 20-40 kg or more. Some trained people who can drag cars or heavier objects with their teeth are using the arch form to counteract the huge external forces and achieve a performance effect. This is practically the same as holding a raw egg in your hand with force, but the egg will not be crushed. These situations occur on the premise that the central sulcus or incisal ridge of each tooth within the arch is evenly situated on the arch line. If the central sulcus or incisal crest of a tooth does not sit on the arch line due to torsion, displacement, etc., this means that the tooth cannot transmit the proximal and distal mesial forces in a balanced manner, which means that most of these forces are absorbed by the tooth itself and cannot be transmitted to the next tooth well. In the long run, the periodontal membrane is damaged and with the onset of periodontal disease, the tooth becomes loose. This is reflected in the X-rays. In addition, as we age, tooth wear increases. This abrasion does not only occur on the cemental surfaces but also on the adjoining surfaces. In most cases, the wear on the adjoining surfaces varies proportionally to the proximal and distal mesial horizontal forces and remains in equilibrium on the dental arch. However, some teeth lose this equilibrium after the occurrence of abrasion on the adjoining surfaces, making the force transmission gradually deviate from the arch line and cannot be well counteracted, and eventually, one day, loosening of the tooth occurs under the induction of periodontal disease. Damage to the periodontal membrane: The concept of periodontal membrane is a sensory impression obtained by viewing it with the naked eye, especially in x-rays where the image seen resembles a membrane structure, which is actually a fibrous ligament structure. Therefore, sometimes it is more accurate to call it periodontal ligament. Due to its connecting function, the periodontal ligament is buried in the alveolar bone and dental bone as calcified fibers, and then gradually calcifies and weakens to become a true fibrous connective tissue. However, when subjected to abnormal torsion, it is not the non-calcified fibrous tissue itself that is damaged, but the calcified fibrous portion that connects the dental bone or alveolar bone. The sudden tearing sensation that is evident when a tooth is dislocated by the doctor during extraction is actually a fracture of the calcified portion of the periodontal ligament, not a pulling off of the non-calcified portion of the fibers. When the periodontal ligament is torqued, it is this part of the calcified fibers that is also injured. The tooth sits in the alveolar socket and is held by the periodontal ligament, creating its own physiological degree of looseness. In fact, it can be seen as a micro-movement joint in the body. When a tooth is suddenly subjected to an abnormal force, if the soft tissue of the periodontal ligament is partially strained, it will basically heal within two weeks and the symptoms will disappear, which is often referred to as “dental oscillation”. However, if the tooth is dislocated by an external force and then fixed to heal, it will take more than two months, which means it needs to heal by calcification. This is similar to the healing process of a fracture or the refixation of a large ligament in the body. When periodontal inflammation attacks and destroys the bonded epithelium, the periodontal ligament is in a state of congestion, and at this time, coupled with the dental trauma force, the tooth is under regular abnormal activity, which causes the periodontal ligament in the inflammation area to be damaged and ruptured, and at the same time, due to the inflammation and dental trauma, the periodontal ligament is unable to contact with the dental bone or the fixed position of the alveolar bone, forming calcification healing, resulting in the loss of functional stimulation of the alveolar bone, making the alveolar bone Resorption, resulting in loss of attachment and periodontal pockets. 1.3 Alveolar bone resorption It should be noted that if inflammation and trauma to the dentition can induce tooth loosening, extensive alveolar bone resorption is the direct cause of tooth loosening. Inflammation and trauma can be adjusted and controlled. However, once resorption of the alveolar bone has occurred, it cannot be restored. To what extent does the tooth become loose due to bone resorption? With the elimination of the inflammation and the removal of the coaptation trauma, it has been observed clinically that even when the alveolar bone is resorbed to 1/3 of the root tip, the tooth does not loosen and can function normally due to the presence of periodontal membrane potential (Figure 9). Of course at less than 1/3, the tooth starts to loosen. If accompanied by inflammation and dental trauma, the tooth may also loosen more than II degree if the alveolar bone resorbs less than 1/3 of the root length. Therefore, tooth loosening is the result of the combined effect of the three causes mentioned above. If one factor is reduced through treatment, the looseness of teeth is clinically observed to be decreasing. By removing or remedying all the factors that cause tooth loosening, the tooth looseness can be restored to normal. With periodontal systemic treatment, inflammation can be well controlled. The removal of the trauma of the dentition and the compensation of the effects of the resorption of the alveolar bone require the use of restorative means to achieve the goal. This is the loose tooth fixation. 2, loose teeth fixed is a very important part of the periodontal treatment system of periodontal treatment include: In the whole system of periodontal treatment, gum disease generally with supragingival cleaning + etiology treatment can be cured. Individual cases with very deep pseudogingival pockets require subgingival scaling. Gum surgery is mainly a reshaping of the gums. Most cases of periodontal disease can be well controlled by subgingival scaling. Only a small percentage of cases are suitable for periodontal surgery. As for loose teeth, as long as they meet the indications for loose tooth fixation, they can be considered for loose tooth fixation after basic periodontal treatment or surgical treatment. In terms of the sequence of periodontal systemic treatment, loose tooth fixation belongs to the last step or the last two steps of periodontal disease treatment. This must be taken into account in the periodontal treatment plan development. 3, the significance, principle and indications of loose teeth fixed due to loose teeth, according to the patient’s requirements and the actual situation, as well as the clinical treatment conditions, we do loose teeth fixed significance: 1) restore the function of the affected tooth. 2) To improve the service life of the affected tooth. 3) To reduce the rate of tooth extraction. 4) Contribute to periodontal maintenance and treatment. 3.1 There is a theoretical basis for fixation of loose teeth 3.1.1 The remaining periodontium still has a certain degree of support As long as the periodontium is present, it has a certain degree of support for the tooth. The remaining periodontium may not be sufficient to resist the dental forces on the affected tooth, but it is still relevant to reduce the burden on the abutment teeth. If the remaining periodontium is able to resist 30% of the cementation force, the requirement and number of abutment teeth can be reduced accordingly. 3.1.2 Change single tooth movement to overall movement Single tooth loosening of II degree or more is usually more than two directions of movement beyond the normal physiological looseness. If two or more teeth are fixed as a whole, all the teeth that are fixed can only move uniformly according to the direction of the force after the force is applied. Then only the tooth with the least mobility can move in accordance with the range of motion. If the loose teeth are fixed with a reasonable design, the mobility of the loose teeth will be significantly reduced without damaging the abutment teeth, and the purpose and effect of fixation will be achieved [3]. 3.1.3 The sum of the periodontal membrane support of each single tooth is less than the overall support formed by these periodontal membranes According to the results of the study, the sum of the individual resistance to occlusal forces of the periodontal membranes remaining in the same number of each loose tooth is less than the resistance formed by the same number of all loose teeth fixed together [5] (Figure 10). The expression is: A+B+C+D+E+F Indications for periodontal fixation: any treatment has its indications, beyond which the treatment is prone to failure. 3.2 Indications for anterior loose tooth fixation 3.2.1 Requirements for abutment teeth The anterior loose tooth fixation is generally based on two cuspids as abutment teeth. Posterior teeth are generally used at least one molar as abutment teeth. Therefore, the requirements for abutment teeth include: 1) Looseness <Ⅰ degree. 2)Alveolar bone resorption <1/2 root length. 3)No significant displacement or relocation. 4)Periodontal disease is well controlled. 5)Endodontic disease was treated. 3.2.2 Requirements for loose teeth: (Figure 11, 12) 1) Periodontal disease has been effectively controlled. 2) Tooth looseness is less than 3 degrees. 3) Periodontal membrane is still present at the root tip. 4) The cemental trauma has been eliminated or can be eliminated after fixation. 5)Endodontic disease is treated. 4, loose teeth fixed type and advantages and disadvantages of loose teeth fixed method many, the scope of application is not the same. At present, according to the literature, loose tooth fixation is mainly classified according to the length of fixation time, different fixation principles and different materials used. 4.1 Classification according to fixation time Temporary fixation of loose teeth (Short-term Splint): This type of fixation is mainly applied to loose teeth caused by dental trauma or acute inflammation. The duration of fixation is usually several months, and the longest is not more than one year. The purpose of fixation is to temporarily stabilize the tooth and facilitate periodontal tissue healing. This method is not applicable if it is predicted that the looseness of the tooth will not disappear after healing of periodontal inflammation and trauma. This method uses mainly waxed wires, wires and composite resins as fixation materials. The Provisional Splint: The concept of "provisional" here is up to five years. This is usually done at the patient's request in conjunction with systematic periodontal treatment. Temporary fixation usually uses composite resin as the matrix, plus wire or high-strength fibers, and our clinical experience is that it usually lasts no more than five years. This is mainly due to the composite resin properties. The aging problem of composite resin in the oral environment is not well solved yet, so as the performance of composite resin decreases in the oral environment for a long time, the fixation splint will break and fall off (Figure 18). On the other hand, purely rely on resin protrusion for adhesive fixation, its strength also can not withstand long-term occlusal shear force (mainly horizontal force), usually the fiber is constantly, but the resin has been aging loose. Therefore, there is a view that any periodontal splint fixation with composite resin as the bonding interface is temporary fixation. Therefore, clinically, if resin is designed to be used for temporary fixation of loosened teeth, it is important to consider that: 1) Fixation is time-limited. If the tooth looseness is predicted to be relieved within the time limit, it can be used. If the tooth is irreversibly loose, it is important to explain this to the patient, while leaving room for repair and redesign if the splint falls off and breaks. 2) Resin splint fixation has aesthetic advantages. For the fixation of irreversible loosening of anterior teeth, considering the patient's strong request, affordability and work-life schedule, resin splints can be designed for fixation. However, it must be made clear to the patient about its timeliness, and even an informed consent form can be signed to avoid unnecessary medical disputes. Permanent fixation of loose teeth (Definitive Splints): According to the English expression, it should be understood as "final splinting" fixation. It is understood as "lifelong splinting" fixation. As the name implies: permanent fixation means that both technically and materially, it should be able to function in the oral environment for a long time. In terms of current restorative materials, the only materials that have stood the test of history and time are alloy materials and porcelain materials. Therefore, to achieve the purpose of permanent fixation of loose teeth, only these two types of materials and restorative expertise can be used to achieve. Here there is a crossover of disciplines. The lack of communication and understanding between periodontal and restorative specialties greatly limits the development of permanent fixed splints for loose teeth. In some cases, it is easy to carry out in the primary general practice instead. 4.2 Permanent fixation of loose teeth is generally divided into two categories 4.2.1 Permanent fixation of non-removable loose teeth Steps: 1) Root canal treatment: Subject to the indications for permanent fixation of loose teeth, root canal treatment should be performed on the relevant loose teeth and abutment teeth. This is mainly for two reasons: first, the pulp condition of the loose teeth with periodontal disease are in an abnormal state; second, the use of restorative means to do loose teeth fixed must be dental preparation, the amount of tooth grinding is large, easy to hurt the pulp. 2) Loose teeth fixed design and implementation: non-removable loose teeth permanently fixed design basically follows the restoration of professional fixed bridge design principles, more than the use of joint crown design. Various alloy materials are recommended for posterior teeth; porcelain-like materials are used for anterior teeth for aesthetic requirements (Figures 22, 23). A. The requirements for abutment teeth are relatively low because the periodontium is still present in the loose tooth and has some support. B. The design of permanent fixation of non-removable loose teeth should take into account not only the fixation of loose teeth, but also the elimination of trauma to the dentition. Therefore, the design of anatomical profile and adjacent surfaces requires restoring the standard profile or reducing the diameter appropriately, and lining up the central sulcus or incisive ridge into the arch line to achieve the balance of the dental forces. C. When designing crown retainers, space should be left for subsequent periodontal maintenance. So try to design for inlays, half crowns or supragingival full crowns. It is not advocated to do subgingival crown margin, and it is absolutely forbidden to have subgingival crown margin overhang, otherwise it is a great damage to periodontal tissue. 4.2.2 Permanent fixation of removable loose teeth is usually done in combination with restoration of missing teeth and fixation of teeth that have become loose but do not need to be extracted. Sleeve crowns or articulated clips are often used to achieve the fixation of loose teeth. Sleeve crowns are used in cases where multiple teeth are missing and many remaining teeth are loose. The most important feature is that the teeth involved (usually all remaining teeth in a single jaw) are combined into a single unit, reducing the looseness. The main disadvantages are: 1) All the teeth involved in the denture have to undergo root canal treatment. 2) A lot of grinding of the involved teeth is required to obtain a common seating channel. 3)The cost is too high, and its price and cost are currently unaffordable for most of the country. For these reasons, it is not widely carried out in our country now. In addition, foreign countries are actively developing removable permanent fixation of loose teeth for periodontal fixation in advanced periodontal disease. An innovative periodontal splint with joints to fix loose teeth has been developed in Japan (Figure 24 ). 4.3 Clinical considerations The following points should be taken into account when preparing the design of a plan for the fixation of loose teeth for a patient: 4.3.1 Consideration should be given to the fixation of loose teeth when designing the periodontal systemic treatment for the patient. 4.3.2 The timing of loose tooth fixation is usually performed after systemic periodontal treatment. However, in individual cases (e.g. when fixation is more conducive to forceful scaling on the tooth to prevent dislodgement during treatment), it may be considered before systemic treatment. 4.3.3 The option of fixation of loose teeth must be negotiated with the patient. The price, number of visits, treatment process, and prognosis should be explained to the patient in detail, and only after the patient agrees and signs the informed consent form. Otherwise, it is easy to cause medical disputes. 4.3.4 The permanent fixation of loose teeth should be designed taking into account the factors of fixation, removal of coincidental trauma, and favorable periodontal maintenance. 4.3.5 Temporary fixation of loose teeth is generally used in anterior teeth, mainly considering the low-cost aesthetic requirements. However, it is important to communicate with the patient in place especially on the duration of use and to obtain the patient's understanding. Once dislodged, fixation can be considered by repairing, redoing or replacing porcelain continuous crowns or casting lingual with nail splints.