1. Accidental pulp preparation: Problems occurring during pulp preparation are easy to detect and deal with. The main problems include inadequate pulp preparation resulting in imperfect exposure or non-exposure of the root canal opening, resulting in missed root canals; destruction of original restorations such as porcelain crowns during pulp opening; lateral penetration of the pulp chamber wall and perforation of the pulp chamber floor; splitting of the crown during pulp opening, which mostly occurs in teeth with original hidden cracks. 2. Loss of working length: Loss of working length is a common problem in root canal preparation and the result of a series of problems in the root canal preparation process. The main causes are root canal blockage, shoulder formation, instrument fracture and dentin debris accumulation in the apical area. In addition, displacement of the stop piece, change of the reference point, inaccurate x-ray projection angle and improper use of instruments must also be noted. Huang Yanjun, Department of Stomatology, Daqing Oilfield General Hospital Preventive principles: constant reference point, fixed stop piece position, constant X-ray projection angle, pre-bend all root canal files, pay attention to the bending direction of the root canal files should be consistent with the root canal bending, keep the original shape of the root canal as much as possible during the preparation process, keep enough flushing and repeatedly use small files to smooth the root canal, prepare number by number, do not skip number, when the small number one file adequately prepare the root canal, then change to the next number one file. When the small one file has prepared the root canal sufficiently, then change the next one file. Once the working length is lost, it should be handled accordingly according to different reasons. When the root canal is prepared by the step-back method, the working length is determined before the preparation, and after the preparation, due to the full expansion of the crown 2/3 of the root canal and the moderate straightening of the bend, a small part of the working length will be lost; for the medium and heavy bent root canals, the working length should be redetermined for every 3 expansions during the root canal preparation, so as not to cause excessive root canal preparation. 3. Root canal blockage: When the root canal file cannot enter the full length of the root canal and reach the apical narrowing, it is called root canal blockage. There are many reasons for root canal blockage. It mainly includes the accumulation of dentin debris and tissue debris in the apical area, blockage of filling material, blockage of cotton twist, paper twist, fractured instruments and adhesives. In order to avoid the occurrence of root canal blockage, it is better to follow the following prevention principles: the carious tissue, no base enamel and loose filler must be removed before pulp opening; the pulp opening hole should be adequately prepared, especially when there is full crown present, it should be fully extended; the wall of the pulp opening hole should be able to form a straight line with the middle and lower 1/3 of the root canal to avoid the crown obstruction when the instruments enter the root canal; the large filler or full crown should be sprayed with water when pulp opening to avoid the metal or The pulpal canal should be cleaned when the pulp is removed, the root canal should be explored and the root canal should be prepared with a lot of rinsing; the root canal file should be clean when it enters the root canal again; the root canal file should not be skipped; the small size file should be used repeatedly to open the root canal; the root canal file should not be rotated or used excessively; the preparation of the root canal should be done under wet conditions; the temporary seal should be perfected during the appointment to avoid foreign bodies entering the root canal. If a root canal that was previously open cannot be prepared to a determined working length, if the x-ray shows a certain distance between the diagnostic wire and the apical stenosis, and if the root canal length measuring instrument also shows that the root canal file does not reach the apical stenosis, it is generally possible to determine that the root canal is obstructed. Depending on the blockage, the appropriate treatment is applied, but in most cases, it is difficult for the operator to know the nature of the blockage. First, the root canal above the blockage is adequately prepared so that the instruments can reach the blockage without resistance, and then the following treatment methods are used: ① Use pre-bent 10# and 15# K files or expanders to pass the blockage: bend the tips of 10# and 15# K files by 3~4 mm at an angle of 45 degrees and rotate them along the circumference of the blockage to find the feeling of jamming. Once stuck, use the action of rotating and small amount of lifting toward the root tip, repeated many times, gradually pass through the blockage to reach the working length and take x-ray to determine. The root canal file must be pre-curved; a straight file may push the blockage deeper into the root canal or apical foramen. Dipping the tip of the file in a root canal lubricant containing EDTA will help significantly. ②Ultrasonic treatment method: Under the microscope, the tips of 15# and 20#K ultrasonic files can be pre-bent appropriately and ultrasonically shocked to remove or pass the blockage. Without microscopic conditions, the ultrasonic method can also be used to try to deal with, for mucoadhesive and dentin debris, the effect is good. For metal blockages and fractured instruments it is best to operate under a microscope. ③If the blockage site cannot be passed, it should be prepared to the blockage site and root filling, regular observation, and apical surgery if necessary. 4. Shoulder table formation and shoulder table passage surgery: shoulder table refers to the irregularity of the root canal wall caused artificially during root canal preparation, resulting in the root canal file not being able to reach the root tip through the original open root canal. The main causes of the shoulder: insufficient pulp chamber preparation and root canal crown preparation, not forming a straight channel with the apical 1/3; inadequate lubrication of root canal swabbing, accumulation of dentin debris in the apical area; root canal file without pre-bending, excessive force to the apical area before reaching the working length; in bending the root canal, the root canal file is changed too quickly, skipping the number or the root canal file is excessively thick; excessive rotation of the root canal file after reaching the working length; excessive The greater the curvature of the root canal, the more likely it is that a shoulder will occur. Prevention of shoulder formation: for the smooth root canal, 15#K file can reach the apical area, the tip 1/3 of the file should be pre-curved and gradually enter the apical area along the root canal curvature, do not over force to the apical area. After reaching the working length, the file should be lifted up and down for a short distance (1~3mm) until the root canal file can reach the apical area without resistance. Do not apply excessive force and rotation to the root of the instrument when preparing the apical area. Once the instrument is stuck and cannot reach the apical area, the root canal should be flushed immediately and the small size files such as 08#, 10#, 15# should be changed to scrape the irregular part of the root canal wall to prevent the formation of shoulder. Adequate swabbing and the use of root canal lubricant are essential. Small, curved, long root canals are more likely to form a shoulder. For small curved and calcified root canals, the preparatory steps to prevent shoulder formation are as follows: after adequate preparation of the pulp chamber and root canal opening, the pulp chamber is filled with swabbing solution and the working length is gradually achieved by rotating the pre-bent 06#, 08# and 10# K files 1/8 to 1/4 turns in combination with root canal lubricant. Once the working length is reached, the walls of the root canal are fully prepared by using a file action with a short lift of 1~3mm until there is no resistance; the walls of the root canal can be further expanded with an H file of the same size to obtain the exact working length. When the 15# file finishes the preparation and can reach the working length, the root canal should be prepared by the gradual depth method. It is vital to keep the root canal open with a 15# file every time the instrumentation is changed. Once a shoulder is formed, it is quite difficult to remove. It is possible to remove or pass a shoulder caused by a fine root canal file, while it is more difficult to pass a shoulder caused by a 25# or 30# file. The method of shoulder passage is consistent with the method of passage of root canal obstructions. Once the abutment is formed, even if the initial file is able to pass the abutment, further root canal preparation procedures and root canal filling still have the possibility of entering the abutment. If the shoulder cannot be passed and the original root canal cannot be accessed, the working length to the shoulder should be redefined and appropriate root canal preparation and filling should be performed. When the root canal preparation above the shoulder table is completed, a good access to the apical region is formed, which helps the shoulder table to pass, and a 10# pre-bent stainless steel K file should be used again to try to pass the root canal, and a certain percentage of the shoulder table can be passed at this time. After the formation of the shoulder, perfect root canal preparation and filling cannot be carried out, and its prognosis is related to the cleanliness of the unprepared and unfilled root canal below the shoulder. The prognosis is generally considered to be good for shoulder abutments that are close to the apical region and well cleaned. In cases that produce a shoulder abutment, the patient should be informed of the prognosis, followed up regularly, and promptly undergo apical surgery or retreatment when clinical symptoms or apical lesions appear on radiographs. 5. Apical area offset: (1) Artificial root canal: Artificial root canal is a kind of root canal offset in which the alignment and centerline of the prepared root canal are not consistent with the original root canal. The various causes of shoulder canals are related to the formation of artificial root canals, therefore, avoiding the formation of shoulder canals also prevents the occurrence of artificial root canals. Once the shoulder is created and the working length is lost, excessive force is applied to the apical area in order to regain the working length, resulting in an artificial root canal. Further preparation of the artificial canal will eventually result in perforation of the root canal wall. After the formation of an oversized shoulder or artificial canal, it is difficult to find the original canal and more difficult to prepare and fill it. The operator should first determine the presence or absence of perforation based on root canal length gauges, paper twist examination and diagnostic radiographs, and the perforation should be repaired either intradially or surgically. For cases with artificial root canals, if there is no perforation and the original root canal can be found and perfect preparation and treatment are obtained, the prognosis is the same as normal cases; if the original root canal is not found and more unprepared and root-filled root canals remain, the prognosis is poor and should be observed regularly and apical surgery should be performed if necessary. (2) Open apical foramen: Due to improper root canal preparation, the apical foramen is enlarged and teardrop-shaped, which destroys the anatomical structure of the normal apical foramen and leads to poor root canal filling. The main reason is that the root canal file is not pre-bent when the apical bend is not prepared, the instrument is rotated excessively, the root canal file is selected too large or excessive force is applied to the apical area. Due to excessive rotation of the root canal file in the root canal bend, a narrow spot, called elbow structure, is often formed above the open apical foramen. In order to prevent apical foramen openings, when preparing the apical area of 3-4 mm, the instrument must be pre-bent and pulled up and down in the direction of the root canal bend in small increments; prepare adequately with a small, flexible root canal file; and use the preparation technique in the opposite direction of the bend. If an open apical foramen has been formed, various filling methods can be used, with filling with a calcium hydroxide-containing root-filling paste and hot press adhesive technique being preferred; in cases of concurrent lateral penetration, MTA or a calcium hydroxide-containing paste with hot press adhesive technique should be used for filling. In cases that produce elbow-like structures, root filling can only be done up to the elbow and should be followed up regularly with apical surgery if necessary. 6. Lateral perforation of the root canal wall: The process of root canal preparation and shaping can cause perforation of different parts of the root canal. The site and size of the perforation, as well as the period of occurrence, have different effects on the prognosis of root canal treatment. The earlier the perforation occurs during root canal preparation and the larger the diameter, the more obvious the effect on the prognosis. (1) Perforation of the apical area: Perforation of the apical area includes excessive preparation of the apical foramen and lateral perforation of the root canal wall in the apical area. Lateral perforation of the root canal wall in the apical region mainly occurs in the curved root canal, shoulder formation or root canal blockage; the main reason is that the root canal file is not pre-curved or too large during the preparation process. Once the patient suddenly experiences pain, significant bleeding in the root canal, more blood at the tip of the paper twist and loss of sensation in the apical stenosis during the preparation process, lateral penetration should be highly suspected. Diagnostic radiographs should be used to confirm the diagnosis before further preparation. After perforation occurs, every effort should be made to find the original root canal, and the perforation should be closed as a new apical foramen together with the original apical foramen, preferably using vertical pressure technique, and MTA can be used to repair or fill the apical area when available. Root tip surgery should be performed after apical lesion formation. (2) Lateral perforation in the middle of the root: the formation of a shoulder in the middle of the curved root canal or the perforation of the root canal wall preparation after over-seeding are the main causes of lateral perforation in the middle of the root. It occurs in the medial wall or depression of the curved root canal and is manifested by sudden bleeding in the root canal or sudden discomfort of the patient in the preparation, fresh blood in the middle of the paper twist, and bright red perforated area visible under the microscope. Mid-root perforations can be repaired microscopically from within the root canal or surgically after root filling, and MTA is a good material for repair. Prevention of mid-root perforation should follow the principle of preparation of curved root canals, careful use of mechanical rotary instruments, and the use of preparation methods in the opposite direction of curvature. (3) Root canal crown perforation: Root canal perforation mostly occurs when the root canal opening is searched and enlarged and when the GG drill is used improperly. Characterized by sudden bleeding during preparation, the presence of perforation can be seen directly with the naked eye or microscope, and diagnostic radiographs and root canal length measuring instruments can help confirm the diagnosis. A variety of materials can be used for repair, such as silver-mercury and glass ions, etc. The most certain material for all types of perforation repair is MTA, which can be used for endodontic repair or endodontic plus surgical repair. 7. Improper root canal preparation: (1) root canal preparation beyond the apical hole: due to inaccurate working length or changes in the working length during preparation, resulting in instruments beyond the apical area preparation and pulling apart of the apical stenosis. This is manifested by fresh blood in the root canal or on the root canal file, increased pain during preparation of the apical area, and loss of feel at the apical stenosis, etc. X-ray diagnostic radiographs show that the coarse root canal file exceeds the apical hole, and the root canal length measuring instrument also has an indication. The working length decreases during the preparation of the bent root canal (usually 1~2 mm), and the change in working length should be monitored at all times during the preparation. After the apical stenosis is found to be lost, the working length should be redefined (1~2mm shorter than the original working length), a new apical stop should be established, 2~3 sizes larger than the original stop, and the root should be prepared and filled again. MTA can be used to close the apical zone when available. The destruction of the apical stenosis area can easily cause overfilling, and the apical closure effect is poor. The prognosis is related to the size and shape of the stenosis destruction, and apical surgery is required if necessary. (2) Excessive root canal preparation: This refers to excessive removal of buccolingual and proximal and distal mesial tooth tissue from the root canal wall. The preparation of the apical area should follow the principle that the size of the primary file is 2~3 sizes larger than the primary file, and the more curved the root canal the smaller the primary file should be. For the preparation of the middle and upper 1/3 of the root canal, especially when using the GG drill or other mechanical rotary instruments, unnecessary over-cutting of the tooth tissue should be prevented, resulting in weakness of the root canal wall and even root canal perforation or longitudinal fracture. (3) Inadequate root canal preparation: It means that the pulp tissue, dentin debris and microorganisms in the root canal are not completely removed, and the shape of the root canal after preparation does not form a continuous taper, making it difficult to obtain a tight three-dimensional filling. This is manifested by difficulties in reaching the working length of the primary file, the corresponding lateral pressure device or the primary adhesive tip; the lateral pressure device cannot enter or does not have sufficient lateral pressure space after the primary adhesive tip enters the root canal. If the root canal preparation is insufficient, the root canal preparation principle should be followed and the lateral pressurizer or vertical pressurizer should be pre-tested before root filling, and radiographs of the main adhesive tip should be taken when available.