Distraction osteogenesis is a specific form of clinical tissue engineering. It is a surgical procedure to lengthen bone by surgically severing the bone and then pulling it through special, easily maneuverable instruments at an appropriate rate to guide the formation of new bony structures. The process involves: cutting or severing of the bone to create a fracture, localized rest of the fracture for a certain period of time, distraction of the broken end to create a gap, fixed healing, and bone remodeling and maturation. It is a dynamic and continuous process, as well as the process of alternating bone damage and repair. The application in craniofacial surgery has been greatly developed in recent years. The development process of bone lengthening technology Bone lengthening technology was first applied by Ilizarov in 1951 in the treatment of human long bone fractures. Since then, a large number of experimental and clinical studies have been carried out in the treatment of bone defects of limb bones, but there have always been many complications.In 1973, Snyder firstly reported that the application of external lengthening device to repair the defects in experimental canine mandibular defects had achieved good results.In 1977, Michieli applied the intraoral lengthening device to lengthen the mandible of experimental canine mandibular defects by 15 mm.In 1992, McCarthy reported that the application of the lengthening device in experimental canine mandibular defects had achieved good results. In 1992, McCarthy reported the clinical application of lengtheners for the treatment of craniofacial deformities. In recent years, bone lengthening techniques have become a new hot spot in craniofacial surgery. The principle of bone lengthening is that the gradual application of traction to living biological tissues creates tension, which can stimulate and maintain the regeneration and growth of certain tissue structures. The principle of traction bone lengthening is also that under the action of traction, a continuous, slow force will be generated between the bone segments that truncate the cortex of the bone, and this force will encourage the regeneration of bone tissue and and periosteal soft tissue, which will result in the formation of new bone in the gaps between the distracted segments of the bone and lead to the simultaneous growth of periosteal soft tissue. Therefore, the application of this technique corrects the bone deformity as well as the accompanying soft tissue deformity. Classification of bone extenders and their characteristics According to the way of application, bone extenders can be divided into three main categories. The first to be used in craniofacial surgery was the external type of lengthener, in which the retractor device was fixed to the jawbone by means of a fixation pin passing through the facial skin. The movement of the retractor fixation pin during retraction lengthening, together with the conspicuous exposure of the cheeks to the outside of the mouth, inevitably left a permanent scar. In 1996, the intraoral lengthening device appeared. The intraoral lengthening device makes an incision inside the mouth and avoids the remaining facial scar, but it is more difficult to apply in patients who are too young to have a precise control of the traction vector and the total amount of traction is limited, and a second surgery is needed to remove the lengthening device. There is also a category of intraosseous lengthening devices, which are embedded in the bone. They are characterized by the ability to lengthen very small bone fragments. No fixation screws or plates are required. Due to their small size, they are more easily tolerated by the patient. Bone lengthening techniques in craniofacial surgery Progress in the treatment of traumatic and post-tumor resection maxillofacial deformities Traumatic and post-tumor resection maxillofacial deformities are often present with a large amount of bone tissue and soft tissue defects. In the past, bone grafts were often used for repair, but in cases where the defects were too large and the surrounding soft tissues were less elastic, forming scarred soft tissue sets, the grafted bone blocks were often insufficient to fully restore the mandibular morphology. The application of double-end or triple-end distraction osteogenesis technique, which lengthens the remaining mandibular bone block or free grafted bone block, expands the soft tissues, and provides a good foundation for dental reconstruction, has achieved good results in the clinic. The procedure is divided into three steps: the first step carefully separates the tissues at the mandible to provide as much soft tissue as possible and free grafting of the fibula flap is performed in parallel. In the second step, the graft is lengthened vertically and sagittally by applying a head-frame external retractor. The third step removes the retractor and further trims any unsatisfactory areas. The restoration of the face was of good shape in the presence of poor surrounding tissue. Occlusal deformities often occur after trauma due to conservative or incorrect surgical methods, complications of maxillofacial fractures, or delayed treatment of the occlusion due to severe intracranial and abdominal trauma. All of them can be treated with different lengthening devices, and the lengthening distance can be up to 13-16mm, and the occlusion and facial morphology of the patient are significantly improved compared to the preoperative period. Bone lengthening technique is an extremely effective way to treat trauma-induced maxillofacial deformities. Treatment of congenital craniofacial deformity: Cleft lip and palate is the most common congenital craniofacial deformity, and a considerable proportion of patients (25%-60%) are accompanied by more serious mid-face dysplasia, which is routinely treated by orthognathic surgery, but due to the limitations of orthognathic surgery, the deformity cannot be completely corrected, and there are often recurrences after surgery. The application of bone lengthening technique has greatly improved this situation. Currently, there are three main ways to correct maxillary and midface deformities. One is the intraoral lengthening technique, the second is the tooth-loaded mask-type lengthening technique, and the third is the tooth-loaded firm external type lengthening technique. Early surgical correction of the deformity by cleft lip and palate surgery in the patient’s early childhood can improve facial morphology, speech and swallowing function. Maxillary dysplasia can be corrected by applying Le Fort I osteotomy traction, which can be accompanied by bone grafting as appropriate. The application of a sturdy external lengthening device is a method that has gained worldwide attention. The main advantages of this method are that the direction of traction can be precisely adjusted, 10-15 mm of traction can be easily achieved, it is less affected by lip scarring, and it can be used for Le Fort I, II, and III osteotomies in both adults and children. Despite the many advantages of the external extender, the internal extender is more acceptable to the patient psychologically because it is embedded under the mucosa, small in size, more invisible, has relatively little effect on daily life, and is fixed and stable. The indications are similar to those of external extenders. However, since the jawbone needs to be strong enough and have enough area to fix the extender, patients with too weak jaws cannot apply this method. The incidence of hemifacial shortening in congenital malformations is second only to cleft lip and palate. It belongs to the first and second parotid arch syndrome. It can involve the skull, ears, masticatory muscles and nervous system.In 1969, Pruzansky described three types of hemifacial microsomia with severe hypoplasia of the mandible. The first type is dysplasia of the condyle and mandibular branch; the second is an anomaly of the temporomandibular joint, displaced medially and anteriorly. The condyle is flat and the articular fossa is absent; in the third category, the entire mandibular branch, articular fossa, and temporomandibular joint are absent. The main goal of applying bone lengthening techniques to treat hemifacial shortening is to lengthen the mandibular branch and try to make the occlusal plane as horizontal as possible. This approach, which replaces complex orthognathic surgery, has been used extensively in recent years and has dramatically changed the concept of reconstructive surgery for mandibular dysplasia and has become an efficacious and reliable method of treating this deformity. Kaneshige et al. treated 20 patients with premature closure of the cranial suture by applying a midface Le Fort III osteotomy and a built-in lengthener, and concluded that the traction vector is mainly determined by the shape of the craniofacial bones when the built-in type of lengthener is installed. Nowadays, there are two main types of extenders: a zygomatico-cranial type, which is installed with the zygomatic and temporal bones fixed; and a zygomatico-zygomatic type, which is fixed with the anterior and posterior zygomatic bones, respectively.Crouzon syndrome, which has a flattened temporal bone, is suitable for the former type of extender. The Apert syndrome, on the other hand, has a prominent temporal bone, which produces a force toward the midline during traction, and the latter type of lengthening device is applicable. Long-term results of bone lengthening techniques in the treatment of craniofacial deformities Studies on the long-term results of applying bone lengthening techniques in the treatment of craniofacial deformities have also made some progress in recent years.Byung Chae Cho et al. followed a group of nine patients with severe cleft lip combined with maxillary dysplasia for 1-6 years after applying a sturdy external type of extender. At the completion of traction, the average traction was 13.6 mm; after 6 months, 10.8 mm; and after 1-6 years, the average was 10.4 mm, with a response rate of 23.0%. This indicates that the maxillary growth rate after traction is lower than the mandibular. Therefore more overcorrection should be done in children than in adults to compensate for future recurrence and lower growth rate. In contrast, Cheng Ting Ho et al. treated 6 patients with cleft lip combined with maxillary dysplasia in children with a sturdy external type of extender and followed them for 3 years. The results were good facial morphology, occlusal relationship and arch morphology with stable results and no significant recurrence.Alvaro et al. also found that the SNA angle was reduced by 2.4 from 10.2 postoperatively in 17 patients with cleft lip and palate at 2 years follow-up, which is about 23.5% reduction.Pradip et al. reviewed the postoperative changes in 12 children with hemifacial shortening. After traction, the length of the mandibular branch increased by 13.04 mm, after one year, it decreased by 3.46 mm, and after 5-10 years, it increased by 3.83-4 mm. the rate of growth was 0.87 mm per year. during the same period, the normal side of the mandibular branch grew by 1.15 mm per year. it was demonstrated that bone distraction did not affect bone growth, and that although there was early recurrence, the growth was more stable in the later stages. Despite some reports, the long-term efficacy of the bone lengthening technique is still lacking and needs to be further strengthened in the future. In summary, bone lengthening technique, as a relatively new technique, has its unique advantages in the treatment of craniofacial deformities, and now, there are scholars who control the vector of the lengthening device through model simulation to make it more manipulative. In the future, intraoperative application of endoscopy and preoperative computer-aided design will certainly further reduce intraoperative injuries and enhance the precision of postoperative results, so that bone lengthening technique will be more and more widely used.