What are the basic principles of dentin preparation for piles? How to form an effective dentin shoulder collar? What are the specific requirements for the length of the pile? And finally, what are the key points for making impressions of piles? (a) Basic principles of pile preparation 1. Biological principles: To preserve the dental tissues, including the dental tissues of the crown and the dental tissues of the root. (1) Coronal tooth preparation: The important principle is to form a dentin shoulder collar. The dentin shoulder collar can provide sufficient retention for the pile core on the one hand and for the crown on the other hand; moreover, it can ensure that the crown will not fall off after the pile core crown is restored when it is subjected to a large fracture during use. Secondly, from the point of view of force resistance, sharp edges should be avoided as much as possible during the preparation of the tooth. These sharp edges are a source of local stress, which can lead to the breakage of the cement and the rupture of the restoration. (2) Root preparation: Under the premise of ensuring the strength and retention of the pile, we have to grind the root canal wall as little as possible so that the root canal wall has sufficient resistance. As we know from the previous descriptions, the diameter of the pile should not exceed one-third of the diameter of the root as much as possible, and certainly not too fine. A pile that is too thick will result in a thin root canal wall, which will eventually fracture the root canal wall after the force is applied; a pile that is too thin will most likely bend when the force is applied, which will result in the breakage of the adhesive and eventually lead to the fall of the pile or the fall of the crown. Therefore, to ensure the strength of the pile for retention, the root canal wall should be abraded as little as possible, so that the root canal wall can obtain sufficient strength. This is also the most important basic principle for preserving dental tissues. The longer the length of the pile, the better the retention force without destroying the apical closure and ensuring the integrity and resistance of the remaining root. This includes the following aspects: First: the apical closure area is greater than or equal to 4 mm. After completion of root canal treatment, the entire root canal is usually filled with dental cement. In the apical third, there is often a large number of fine branching root canals, which are the channels through which the root canals communicate with each other and the outside world. During root filling treatment, it is often difficult to completely fill these fine root canals, so it is necessary to use the dental cement inside the root canal to isolate the root from the outside tissue to avoid inflammation. Second: The length of the pile is greater than or equal to the height of the clinical crown. In terms of leverage, the longer the pile, the stronger the ability to resist external forces when the crown is subjected to them, and the less likely it is to cause root fracture or crown loss. Third: The length of the pile in the bone should be greater than one-half of the length of the root in the bone. This point is easily overlooked in clinical practice, especially in patients with periodontal disease, which causes the horizontal resorption of the alveolar bone and shortens the length of the root in the alveolar bone. This is because the length of the pile in the bone is too short, especially when the end of the pile is flush with the top of the alveolar ridge, and the top of the alveolar ridge may be used as a fulcrum to cause fracture or longitudinal fracture of the root after the force is applied. Therefore, the length of the pile in the bone should be greater than one-half of the length of the root in the bone. Only by ensuring this premise can we effectively resist the effect of external forces on the root and the pile after the force is applied, and achieve an effective restoration effect. Fourth: The length of the pile should be two-thirds or three-fourths of the root length. Example: In clinical practice, we encounter teeth in various positions that require pile crown restoration, and each tooth has more or less curved roots from the anatomical point of view. Therefore, in the root canal preparation, the effective root canal can only be the section above the bend. If this is neglected, there is a high risk that the root canal will be too long and too deep in the root canal preparation process, resulting in too thin a root canal wall and eventually lateral penetration of the root canal or fracture of the root canal during the stressing process after the final restoration is completed. Therefore, for curved root canals, we should first determine the effective length of the root canal by referring to the x-ray to determine the location of the curved area, so that the final restoration can be successful. Example: For posterior teeth, when three to four millimeters of coronal tissue remains, especially if the margins are still relatively intact, the pulp chamber can be used directly for retention without the use of a pile; however, when the coronal tissue loss is large and the margins are partially or completely missing, a pile is required for retention. In posterior teeth, the length of the pile should not be too long; excessively long piles tend to make the apical closure zone too short, which can easily lead to apical fracture or apical inflammation after restoration. In addition, posterior teeth are often multi-canal, so it is recommended to make multiple canals and use separate piles. Example: For maxillary posterior teeth there are usually three roots, palatal root, proximal mesiobuccal root and distal mesiobuccal root. The palatal root is the thickest one and is the preferred one. We use the palatal root as the primary root and can optionally use the proximal mesiobuccal root or the distal mesiobuccal root as a secondary root for split pontics. When there is relatively little tissue loss, an effective restoration can be achieved by using only one root. For mandibular molars, we can use a distal mesial root. The distal mesial root is usually straighter and thicker, so we can optionally use a proximal mesial lingual root or a proximal mesial buccal root as a secondary root. （This patient is a young woman who completed root canal treatment due to deep caries resulting in pulpal necrosis. As can be seen in Figure 4, her tooth was twisted toward the proximal center, defective, and had a poor local color. The patient requested a restoration to improve the aesthetics of the affected tooth. For a tooth with pulpal necrosis, a pile restoration can be performed after a week of completion of root filling treatment with no symptoms; if there is a large apical lesion, it is important to wait until the apical lesion has shrunk or disappeared completely after completion of root filling treatment before performing the pile restoration, otherwise it will lead to restoration failure. For this patient, a specific explanation of the entire restorative treatment process must be given before restoration. Before proceeding with the dental preparation of the pile, the first step is to prepare the tooth according to the full crown, because the pile crown will eventually be restored with a crown, and a portion of the tooth tissue should be left to provide retention. 1. Crown preparation: (1) When preparing the tooth according to the requirements of a full crown, it is not necessary to make the edge of the sulcus. This allows the gums to obtain an effective support without damaging the health of the periodontal tissue; on the other hand, a clearer impression can be obtained. (2) All the old fillings and some decayed tissues on the crown should be removed. These old fillings are often sealed temporarily with zinc oxide, and these materials, especially zinc oxide containing clove oil, will affect the bonding of resin, so these old fillings must be completely removed, so that we can see the amount of remaining dental tissues and will not affect the final bonding; in addition, the carious tissues must be removed, if these carious tissues are not removed If these carious tissues are not removed, it is very likely that secondary caries will occur in the future, which is also the main reason for the failure of the pile core restoration, therefore, removing the decayed material is a very important point in the dental preparation. (3) In the process of tooth preparation, some sharp edges will be produced through high speed drilling and cutting, and some too thin dental tissues will be produced, these thin-walled weak tips must be removed with drilling needle, that is, these unsupported dental tissues, sometimes we think they may provide certain retention and resistance. But in fact, it is very likely to fracture after the stress; then we have to trim the remaining root surface to remove these sharp edges. Of course, if we cannot obtain an effective dentin shoulder collar from the retained dentin, we can use orthodontic traction or periodontal crown lengthening to obtain an effective dentin shoulder collar. (4) Use a periodontal probe to measure the height of the dentin shoulder collar to ensure that it is 1.5 to 2 mm. 2. Root canal preparation: (1) Before root canal preparation, a suitable drilling needle needs to be selected. The selection should be made according to the length, shape, and diameter of the root canal. First, use the gdb to remove the residual cementum and some old fillings and temporary sealing materials, and then use the psrm to grind and cut the root canal, the tip of which has no cutting function, but its lateral edges can effectively remove the cementum and other weak tissues from the canal wall. (2) Before the root canal preparation, there is another point that is most important. Because root canal preparation is often invisible, the length of the root canal, the shape of the root bend, and the effective length of the root canal that can be utilized can only be observed through x-rays. It is very important to first determine the length of the root canal that can be prepared on the radiographs; it is also important to look at the direction of the root canal preparation and make sure not to deviate from it, which can lead to lateral root canal penetration, a very serious complication that can make the restoration of the pile impossible to continue. (3) The direction of the root canal must be drilled at low speed. Nowadays, some doctors may use high-speed drills to grind and cut the root canal, which may inadvertently remove some tooth tissue; in addition, once deviation occurs, the root canal wall may be too thin. The preparation is carried out slowly by lifting and pulling until the desired working length is reached. X-rays can be taken to determine the final length of the preparation to avoid over-preparation or under-preparation. (4) After the preparation of the root canal is basically completed, the shape of the root canal should be trimmed. Remove sharp edges and thin-walled weak tips, remove unsupported tooth tissue, remove the recesses in the root canal wall, and remove residual cement. Recesses in the root canal wall are very harmful for the subsequent operation and the completion of the restoration. If there is residual cement in the root canal wall, it is difficult to obtain an effective bond between the cement and the cement during the bonding of the pile, which often leads to insufficient bonding force, resulting in the loss of the pile. Therefore, we must remove the residual cement from the root canal wall so that the root canal wall can reach a smooth surface, which can eliminate the recess and obtain an effective bond. Example: Maxillary first molar, using a palatal root as a primary root. When the defect is large, a proximal mesiobuccal or distal mesiobuccal root can be used as a secondary root, both to provide auxiliary retention and to prevent the loss of the primary root. The direction indicated by the arrow is the palatal root used as the primary root. Example: For mandibular first molars, the distal mesial root is used as the primary root, which is sufficient for cases with relatively small defects. When the defect is large, the proximal mesiolingual root or the proximal mesiobuccal root should be used as a secondary root. The direction of the arrow is the distal mesial root, and you can see that the entire root canal is rounded and the sharp edges are effectively removed. This is the clinical preparation we need to achieve. In this case, in addition to the main root, the distal root, the lingual root or the buccal root should be used to improve the retention of the pile. There are two methods of pile impressions, the indirect method and the direct method. The direct method is less used at present, so we mainly talk about the indirect method of pile impression. (a) the ideal impression material 1, safe and non-toxic, no irritation; 2, color and odor taste; 3, easy to use, cost-effective; 4, can quickly wet the mouth tissue surface; 5, solidification time, viscosity texture to appropriate; 6, good elasticity, high strength, accurate size stability; 7, able to withstand changes in temperature and humidity in the working environment; 8, good compatibility with the model material, easy to 9. No gas is generated during the solidification of itself or the model to avoid the formation of small bubbles locally. (B) instrument preparation 1, tray: the instruments needed to take the impression, can be finished trays, but also to make individual trays. 2, slow elbow machine 3, screw conveyor: impression material transported to the root canal. 4.Enhanced wire: It can effectively prevent the impression of the pile from deformation or bending during the impression taking process. (C) the characteristics of the additive silicone rubber is currently very suitable for the pile impression material, it is very accurate, stability is also very good, but also can be repeated to fill the impression. At present, many companies can provide such additive silicone rubber on the market. For example, JMG in Germany and KANGTA in Switzerland can provide such additive silicone rubber. This kind of rubber is widely used in clinical applications, and a better impression making effect can be obtained. (D) the specific operation process of additive silicone rubber Additive silicone rubber can be used double one-step method. The impression material is placed on the tray, the impression material is also injected into the root canal, and the operation is performed in the mouth in one step, which is called the double one-step method. After the preparation of the tooth, the impression is made. The first step is to use a screw conveyor to slowly inject the impression material into the root canal opening. The speed of the screw conveyor must be slow, as too fast will often cause the impression material to fly off, and too slow will not effectively deliver the impression material into the root canal. The impression material must be injected slowly, as too fast an injection of impression material often makes the surrounding field of view unclear and makes it difficult to place the subsequent reinforcing wires into the root canal effectively. In addition, too much impression material can flow and interfere with our work. After slowly injecting the impression material into the root canal, the reinforcing wire is placed and then the impression material is injected around it, and finally the tray is placed in position to complete the entire impression. Figure 17 shows the impression made. It can be seen that the surface of the impression is smooth and the edges are very clear. The pile is relatively thick, which can obtain an effective fixation and resistance. In addition, it is important to avoid the use of violence during the entire impression making process that may cause the impression to release or partial tearing of the impression. These may result in inaccurate models, making subsequent restorations difficult or difficult to perform. Case: Mandibular second molar with a large defect requiring a pile crown restoration. After the preparation of the tooth and the root canal, we slowly injected the impression material into the root canal using a screw conveyor. Since the view of the posterior teeth is not as clear as that of the anterior teeth, it is important to ensure a clear view when injecting and to adjust the light. After the injection is completed, the reinforcement wire is placed. Since the posterior teeth are difficult to manipulate, the augmentation wire should be placed slowly, and it is important to try the augmentation wire before the impression is made to get a feel for its direction of operation. Otherwise, too much time is wasted on the placement of the reinforcement wire, which may cause the initial impression material to solidify, resulting in inaccurate impressions. Finally, we beat the remaining impression material to the reinforcing wire and the surrounding of the preparatory body, and finally get a good impression. (E) the characteristics of polyether rubber: 1, this material is suitable for a single impression method. The single impression method means that the material punched into the tray is of the same consistency as the material injected into the root canal. 2, there is a certain degree of hydrophilicity, first of all, hydrophilicity can well wet the hard and soft tissues of our mouth, but it may lead to water absorption and deformation, so in the clinical operation must ensure that the surrounding tissue is dry, blow dry the root, root canal. In addition, after taking out the impression, the impression should be infused as soon as possible to prevent it from water absorption and deformation. 3.The taste is bitter and the taste is not good. 4, thixotropy is good. Thixotropy means that it is possible to deform or move when it is touched. The polyether impression can obtain a fine surface, and the operation is also very easy. 5. The working time is relatively short, and the hardness is very high after polymerization. Therefore, we have to remove the impression in time to avoid damage to the oral mucosa. Also it is not easy to take off the impression.