As humans evolve and food becomes more refined, the burden on the jawbone decreases, resulting in a decrease in bone volume in the jawbone, but not in tooth volume, which ultimately leaves the teeth in an insufficient position to erupt, resulting in an obstructed third molar. Third molar obstruction often causes pericoronitis, damage to the second molar and other diseases, and often requires extraction. Due to the special location of the blocked tooth, especially the mandibular third molar, which is adjacent to important anatomical structures and closely related to the adjacent teeth, it is difficult to perform the surgery. In this paper, we review the progress of research on the traditional surgical extraction method of mandibular obstructed third molar, new methods to prevent postoperative complications, and transplantation of obstructed molar by domestic and foreign scholars. 1. Advantages and disadvantages of traditional surgical extraction methods for mandibular obstructive third molars (1) Surgical approach The buccal or lingual approach is chosen according to the position of the tooth in the mandibular alveolar bone and the thickness of the buccolingual bone plate. Two types of incisions are usually used for the extraction of obstructed teeth by the buccal approach, angular incision and simple longitudinal incision. For obstructed teeth that have partially erupted, high, buccal, vertical or near-medial oblique obstruction, it is still convenient to remove the bone from the buccal side, but the bone plate on the buccal side is thicker, and the buccal debridement is often larger, and the postoperative reaction is heavier. For lingual obstruction, especially horizontal lingual obstruction, it is more convenient and less traumatic to use lingual debridement method. (2) The choice of extraction instruments for extraction of obstructed teeth Domestic extraction of obstructed teeth is customary to use the hammer and chisel method to remove bone and split teeth, the advantage of which is that it is simple and less time consuming to extract teeth, and it generally takes only 5-10 min for a skilled physician to extract ambiguous obstructed teeth, and the trauma is not great. The disadvantage is that the hammering is very vibrating and individual patients may experience pain in the temporomandibular joint area, which brings psychological and physical pain to the patients. Foreign countries have been applying high-speed turbo drill for bone removal and tooth division since 1958, which has obvious superiority and has been widely used in clinical practice. However, it also has certain defects, such as taking longer time, poor vision, easy to obscure part of the vision by putting the head into the mouth, blurring the vision by spraying water, and easy to roll the drill needle around the soft tissues causing damage and gas entering the surrounding tissues causing emphysema by improper operation. Traditional extraction of obstructed teeth, especially low level ambulant obstruction, usually results in large amount of bone removal, more damage, and more postoperative complications, including postoperative swelling, pain, bleeding, and damage to the mandibular nerve and fracture of the mandible. In particular, damage to the inferior alveolar nerve causes numbness of the patient’s lower lip and fractures of the mandible. The incidence of inferior alveolar nerve (IAN) injury after extraction of mandibular interrupted third molars has been reported in the literature to range from 0.5% to 8%. The incidence of permanent inferior alveolar nerve injury is less than 1%. The incidence of lingual nerve injury ranged from 0.6% to 22%. 2.New method of extraction of low ambulatory obstructive third molars As low ambulatory obstructive teeth are extracted by traditional surgical methods, many complications may arise, especially damage to the inferior alveolar nerve and fracture of the mandible. In response to these problems, some scholars have started to explore new methods to reduce surgical trauma in recent years. The development of imaging technology has made it possible to accurately evaluate the relationship between the blocked tooth and the surrounding important anatomical structures, such as the relationship between the blocked tooth and the inferior alveolar nerve and the mandibular alveolar bone, before surgery. Apical radiographs usually do not show the entirety of the buried blocked tooth, and in clinical practice, panoramic radiographs are most commonly used to evaluate the morphology of the blocked tooth and the relationship of the root to the mandibular nerve canal. CT is the best imaging method to show the relationship between the mandibular nerve canal and the buccolingual direction and the upper and lower alveolar bone. In particular, cone beam CT (CBCT) has advantages over conventional CT in the field of dentistry, as it has low radiation dose, so that the patient is exposed to less radiation, can reconstruct the mandibular molar and mandibular nerve canal in three dimensions, can reconstruct the maxillary and mandibular bone in three dimensions, measure the height and thickness of the bone, measure bone density, etc. CBCT has been widely used in orthodontics, oral and maxillofacial surgery Such as oral implant and alveolar surgery. Based on the panoramic film and the full range of imaging information provided by CT, oral and maxillofacial surgeons can choose the least risky surgical approach and the least amount of debridement. For the low obstructed teeth that have been diagnosed by CT to contact or intersect with the mandibular nerve canal foreign scholars have started to use non-traditional surgical methods and have achieved better results, avoiding damage to the inferior alveolar nerve and fracture of the mandible. (1) Preoperative orthodontic traction In cases where the roots of the third molars of low mandibular ambulatory obstructed teeth intersect with the mandibular nerve canal, orthodontic traction is first used to pull out the obstructed teeth and extract them after the roots are free from the mandibular nerve canal. Advantages: Less invasive, avoiding the risk of surgical injury to the inferior alveolar nerve and fracture of the mandibular angle. Disadvantages: The whole treatment process is long, requiring surgical opening to expose the crown of the blocked tooth, orthodontic bonding of brackets, 3-6 months of orthodontic traction and secondary extraction surgery. Bonetti et al. used orthodontic retraction to successfully retract various types of embedded blocked teeth and then extracted them to achieve better results. There are two types of orthodontic brackets that can be cemented to the dentinal surfaces of the blocked teeth, while orthodontic brackets for proximal or horizontal blocked teeth can only be cemented to the enamel portion of the distal mesial teeth. For vertical or distal orthodontic brackets bonded to the symphyseal surface of the blocked teeth, the resistance of the blocked teeth is in the center of the molar teeth during orthodontic traction, and the teeth generally do not rotate and erupt along the path of physiological eruption, and the traction time is shorter. In addition to axial movement, rotary movement is also generated during the traction process, which makes the blocked tooth free from contact with the distal middle of the second molar and avoids affecting the second molar, but prolongs the traction time, and it generally takes 6 to 12 months for the root of the blocked tooth to be free from the mandibular nerve canal. (2) Crown removal technique (coronectomy technique) For cases where the root of the mandibular blocked third molar intersects with the mandibular nerve canal, in order to avoid damage to the inferior alveolar nerve during surgical extraction, the crown of the blocked tooth is first removed during the first surgery without extracting the root, and the root is slowly moved out of the mandibular nerve canal before the second stage surgical extraction, which can avoid damage to the inferior alveolar nerve. et al. through 47 mandibular low obstructed third molars intersecting the mandibular nerve canal in 43 patients. The average follow-up date was 9.3 months (1 to 48 months) using surgical removal of the crown first. The average total root movement was 3.4 mm at 6 months, 3.8 mm at 12 months, and 4.0 mm at 24 months, with all roots detached from the mandibular nerve canal. The advantage of this method is that it is less invasive and avoids damage to the inferior alveolar nerve and the risk of damaging the lingual nerve and causing mandibular fractures. The disadvantages are the need for secondary surgery, the lack of a large sample of cases and long-term results, and the limitations of this technique, which is not applicable to very low level mandibular obstructive molars. (3) Orthognathic surgical mandibular ascending sagittal split ramus osteotomy (SSRO) extraction. Indications: For very low level or buried group of teeth located below the mandibular nerve canal. For IMT with very low position or located below the mandibular nerve canal, it is difficult to protect the inferior alveolar nerve from damage and may cause fracture of the mandibular angle by the traditional intraoral flap debridement method. Therefore, some scholars have explored the application of orthognathic surgery with sagittal splitting of the mandibular ascending branch to extract low level ambulatory obstructive teeth and achieved better results. Features: ITM can be extracted under direct vision IAN, short operation time and low postoperative complications. 3, autologous obstructive molar transplantation Hale first reported his autologous tooth transplantation technique in 1956, and in the same year, Miller described his molar transplantation technique. Other authors have successively reported their experience with molar transplantation, and autologous tooth transplantation has been undergoing many years with variable success rate reports. reich et al. transplanted 44 autologous molars in 34 patients aged 11 to 25 years. Incompletely developed molar teeth were transplanted into the bone pocket of the missing tooth area. The root development was required to be more than 1/3, and the ideal root length was 2/3 development. 19 months of follow-up, 42 cases were successful except for 2 cases of local infection failure, with a success rate of 95.5%. Success criteria: no clinical symptoms, no infection, no loss, no root resorption and no appearance of pulpal symptoms, etc. High success rate of autologous tooth transplantation, few complications, good functional and aesthetic results. He proposed five most basic procedural requirements: (1) non-invasive extraction to avoid damage to the root sheath and root bud; (2) maxillary molar graft area sometimes requires maxillary sinus lift; (3) the graft area must be a four-walled bone pocket; (4) avoid premature occlusal contact; and (5) stabilize the grafted tooth with mesh suture. Regarding the developmental aspects of the roots after grafting, Bauss et al. reported a comparison of the effect of freshly extracted sockets and surgically prepared bone pockets on the root development of grafted teeth. 62 patients aged 15.8-20.3 years; 64 teeth, 22 of which were surgically prepared bone pockets and 42 of which were freshly extracted sockets that had been reconditioned. Observations ranged from 1 year to 7.9 years. The mean observation was 4.2 years, and there was no significant difference between the two for root development stage III (root development less than 1/2), and for root development stage IV (root development between 1/2 and 3/4), the root development of surgically prepared bone pockets was shorter than that of freshly extracted sockets. It is believed that the implantation of teeth in surgically prepared bone pockets during the root development stage impaired postoperative root development, and a possible explanation is that the Hertwig’s epithelial root sheath (HERS) was damaged during the grafting process Recent studies have been performed on tooth-derived interstitial bone marrow stem cells. The aim is to explore the study of dental embryos that can regenerate teeth. Some scholars have also transplanted tooth embryos from rats and added BMP-4 to promote tooth growth and development. 4. Treatment of distal mesial alveolar bone defect of the second molar Most of the mandibular third molars with low level obstruction or proximal mesial obstruction after surgical extraction are accompanied by bone defect of the distal mesial alveolar bone of the second molar, which is prone to periodontal disease of the distal mesial root of the second molar. (1) Guided bone regeneration (GBR) There are two types of biofilms for guided bone regeneration: non-resorbable (biodegradable) and resorbable (biodegradable). Aimetti et al. used their own control after extraction of 11 bilateral low lying obstructive third molars, with resorbable biofilm placed on one side as the experimental group and the other side as a blank control. At 12 months postoperatively, a significant difference was found between the experimental and control groups, with the experimental group almost returning to the preoperative alveolar ridge height, which was significantly higher than the control group. The results showed that the GBR technique was effective in treating the distal mesial bone defect of the second molar, and Hoffmann et al. also achieved good results in repairing the distal mesial bone defect of the second molar using a non-resorbable film of high-density polytetrafuoroethylene (dPTFE). (2) Platelet-rich plasma (PRP) PRP is a bone promoter, which is a platelet concentrate obtained by gradient centrifugation of autologous venous blood, and its addition of coagulant (commonly 10% calcium chloride solution and thrombin) can form a gel, which can be used alone or in combination with autologous bone, allogeneic bone, allogeneic bone and other biological materials to inject into the tissue. Defects are induced to grow tissue. Sammartino et al. performed a self-control study on 18 young patients with bilateral third molar obstruction. After extraction of the obstructed third molar, autologous PRP was placed in the alveolar socket on one side as the experimental group and the other side as a blank control group. At 12 weeks after the operation, it was found that the height of the alveolar ridge in the experimental group was significantly greater than that in the control group. It is believed that PRP has the effect of accelerating the regeneration of the bone defect in the distal middle of the second molar after the extraction of the obstructed third molar. 5. Interrupted teeth and mandibular angle fracture and condylar fracture Subhashraj et al. By retrospectively analyzing 2033 mandibular fracture cases, traffic injuries (64%) and bruxism (19%) resulted in 532 cases (26%) of mandibular angle fracture, and it was found through case review that the occurrence of mandibular angle fracture was 2.62 times higher in cases with the presence of mandibular third molars than in those without mandibular third molars; meanwhile The number of unerupted third molars was higher than that of erupted third molars. This suggests that the presence of an interrupted third molar in the ambulatory mandible increases the risk of mandibular angle fracture, while Inaoka et al. found that the absence of an interrupted third molar increased the chance of condylar fracture and decreased the chance of mandibular angle fracture. 6. Conclusion Clinical research on mandibular third molar obstruction is becoming more and more advanced, and the indications for various surgical methods are being explored in order to achieve a reduction in operative time and complications. With the development of medical equipment and biomaterials, the advancement of medical technology, the improvement of surgical instruments and surgical methods, the complications of extraction of obstructed molars are bound to become lower and lower; the clinical application of grafting of obstructed molars deserves to be re-understood.