1.Craniomaxillofacial characteristics and new theories on DOD This region has the characteristics of directly affecting aesthetics; concentration of important organs; easily affecting the function of the oral and maxillofacial system; easy to be infected by communicating with the mouth and nose; rich blood transport; irregular bone morphology, and so on. And recent studies have shown that: due to the rich blood transport in the maxillofacial region, the delay period before bone traction can be shortened or even absent; maintaining a certain tension between the broken ends of the bone during traction can stimulate tissue regeneration; keeping the amount of lengthening unchanged every day, the higher the frequency of traction, the better the effect of osteogenesis, and so on. The characteristics of craniomaxillofacial and some new theories put forward high requirements for DOD: (1) try to adapt to the shape of the traction site, small and hidden, without damaging other tissues; (2) try to bury subcutaneous to reduce the chance of infection; (3) traction can be done immediately after surgery without a delayed period; (4) sustained traction under a certain force; (5) secure fixation, appropriate force application, precise force control; (6) assist occlusion recovery, etc., if necessary. (6) Assisting in occlusal recovery when necessary, etc. Accordingly, scholars have developed various types of DOD to meet different needs. Classification of DOD DOD can be divided into different types according to different classification methods. (1) According to the placement position, it can be divided into extraoral and intraoral DOD; (2) According to the traction site and use, it can be divided into cranial parietal, zygomatic, maxillary, palatal, and mandibular DOD, etc. In the case of mandibular DOD, for example, it can also be used to assist in occlusal recovery, and so on. The mandible, for example, can be subdivided into different subtypes (arthroplasty, ascending and lengthening of the mandibular body, increase in the height and width of the mandibular body, increase in the width of the mandibular arch, etc. DOD). (3) They can be categorized according to the type of traction (based on the focal principle): one-, two-, and three-focal DOD; they can also be categorized according to the material used to make the DOD, the frequency of force application, and so on. These classifications to a certain extent reflects the characteristics of a certain aspect of the DOD, but there are certain limitations, now according to the main body of the DOD components in the skin or mucous membrane inside and outside the DOD is divided into two major categories of external and built-in, will be the respective characteristics are described below. 3 External DOD The main traction component of external DOD is located outside the maxillofacial skin or oral mucosa. The earliest DODs used experimentally (1973) and clinically (1992) were external unidirectional, which could only form bone in the direction of the spiral traction bar. Molina et al. used an externally placed bi-directional DOD in 1995, which allowed for simultaneous lengthening of the mandible in both directions by making a bi-directional incision (ascending branch horizontally and body vertically). More recently, the ACE/Normed is an external multidirectional DOD that, with the opening of the hinge screws, allows multidirectional adjustment while retracting the bone in both directions. The external DOD used in the face has been improved and applied by many scholars because of its simple design, stable retention, easy removal, and especially long traction distance. For example, Antonio et al. in Mexico used external unidirectional or bidirectional DOD to lengthen the mandible of 167 patients by an average of 31 mm, but compared with the advantages, its shortcomings are also more prominent: large size, which brings a lot of inconvenience to the patients during the treatment; facial scars; easy to damage the facial nerve, etc. To solve these problems, the scholars put the DOD on the face and made it easy for the patients to adjust it in multiple directions at the same time. In order to solve these problems, scholars have improved the external DOD from extra-oral to intra-oral, and since the animal experiment in 1977, the intra-oral application has been continuously developing towards the direction of smaller size, more secure retention and orthodontic correction device. Intraoral external DOD is divided into tooth-retained and bone-retained types according to its retention method, and the latter can better synchronize the movement of teeth and bone. The traction structure can be attached to stainless steel crowns or miniature plates, and the retentive abutments are usually bilateral first cusps and first molars. It is suitable for older patients with crowding, small mandibular transverse diameter, and mandibular retraction without extraction correction, and can be retracted by 5-14 mm; it is also suitable for patients with low alveolar bone, such as the three-dimensional, tooth-retained DOD designed by Watzek et al, which increases both alveolar bone height and width.The tooth-retained DOD used by Dessner et al. looks like a partially removable prosthesis, and the one designed by Guerrero et al. designed a DOD that resembles an orthodontic arch expansion device. In fact, it is the development of traction osteogenesis that has led to changes in traditional orthodontic procedures, and the combination of the two requires that the DOD be further developed in the direction of compactness and three-dimensional controllability. 4 Built-in DOD The main components of the built-in DOD are buried under the maxillofacial skin or oral mucosa. For example, Steven et al. used an embedded subcutaneous DOD: a titanium nail is stabilized on the bone surface; it is small and flat, which reduces the subcutaneous dead space to reduce the chance of infection; and the force bar penetrates through the skin at the hidden part of the hairline. It is suitable for the traction of skull, midface bone and mandible, with the traction distance of 15-30mm, the disadvantage is that the external incision has a certain impact on the aesthetics. In contrast, the built-in DOD buried under the mucous membrane in the mouth is more acceptable to patients because of the absence of external incision, such as the DOD designed by McCarthy et al. Initially, the bone lengthening was not more than 20 mm, and scholars’ continuous improvement has made the built-in DOD in the mouth more and more perfect, and it has become one of the hot spots of the research on DOD in recent years. Domestic built-in DOD developed by Wang Xing and others, the maximum bone traction extension of an average of 36.5mm, and in the application of the fixed arm is located in the same side of the traction axis, traction is easy to produce displacement difference, then developed the fixed arm is located on both sides of the traction axis of the DOD, to improve the controllability. The micro-motor DOD for continuous traction used by Schmelzeisen (1996) and ploder et al. (1999) is also a built-in type. The controller drives the DOD once every certain period of time, generating a force of about 10 N. The daily traction is 1.01 mm, and the maximum traction is about 17.1 mm. The disadvantage of this type of DOD is that cartilage osteogenesis sometimes occurs due to the instability of the device, and there are also damages to the gears and cables of the force. Some scholars have studied the electric pump hydraulic device similar to the micro-motor DOD, except that the pump is outside the body, which further reduces the volume of the embedded object. DOD should make the patient feel comfortable, and in some cases it is better to assist the restoration of dentition and occlusion, so implant DOD is also one of the directions of the development of the built-in type. odo et al. designed a simple device is to screw the traction screw (implant) from the top of the alveolar ridge to the intersection of the osteotomy, where a small titanium plate is inserted as a support, and with the screw screwed in, the mobile bone section is gradually lifted up. The disadvantage is that the implant has to be replaced twice; Gaggl et al. designed a DOD that is a combination of a DOD and a dental implant, the implant portion of which is located in the basal and mobile bone segments respectively, and the two segments are gradually separated by rotating the screws inside the implant. Dental restoration can be performed by replacing only the internal structure of the implant without the need for a second implant. The disadvantages of this type of DOD are that when the implant sinks into the basal bone segment and does not provide adequate support for the mobile bone segment, traction may fail; the implant should be screwed in gently, which may result in unstable fixation of the bone segment and may not be conducive to the formation of new bone. The internal DOD is a great improvement over the external type, but the shortcomings are obvious: it is more traumatic to insert and remove; it is still slightly larger for children under 3 years of age; the maximum extension distance is shorter than that of the external type; it may be infected and lead to peri-central osteitis, which in turn leads to poor retention of the DOD; it does not accommodate the DOD well in special areas (e.g., atrophic alveolar bone); it is often uncomfortable when it is exposed to the mouth; and it affects aesthetics in the anterior region. The aesthetics of the anterior region is affected. This puts forward higher requirements for the built-in DOD: smaller size of the DOD; less invasive operation; no postoperative trauma with the outside world – completely buried; automatic force application; continuous traction, etc. So some scholars have made some new attempts: DOD made of degradable (absorbable) material, compared with metal, its main traction components gradually degrade some time after the end of traction, without the need for secondary surgery to remove; absorption does not affect facial development; retention body can be molded according to the anatomical site; there is no hot and cold sensitivity. fernando et al. [20] used this device to lengthen the jaw bone up to 40mm. The disadvantages of this DOD are that the retention plate made of degraded material is slightly thicker, with an edge of about 1.4 mm (titanium plate is about 0.5-1 mm); it also requires a force bar that is connected to the outside world and that can be removed only at the end of traction. Titanium-nickel alloy wire DOD. titanium-nickel alloy (TiNi-SMA) instruments that have undergone shape memory treatment can be automatically restored to their original shape after deformation under certain conditions. Domestic Hu Min et al [22, 23] utilized this property to achieve the purpose of distraction osteogenesis. It not only solves the problems of general built-in DOD, but also has the advantages of easy post-operative care; no foreign body sensation in the mouth; can be personalized; easy to process, cheap and so on. It can extend the length of the mandible as well as the vertical height. Its disadvantage is that the strength decreases with increasing distance, and traction is then only possible after surgery by virtue of the material’s own properties. The appropriate traction force, osteotomy and osteogenesis should be further explored. Magnetic DOD.Pittman [24] has used magnetic DOD for traction study of cranial vault, he fixed the magnet in the parietal bone of rabbits, and placed another magnet with opposite poles in the part of the percutaneous fixation frame facing the parietal bone magnet, and kept the distance between the two poles at 5mm, after a period of time, the osteogenesis of the cranial vault was satisfactory. However, there are many problems that need to be solved in magnetic DOD: for example, the magnet is easy to oxidize and rust; the magnitude of the magnetic force of the magnet is inversely proportional to the square of the distance, which makes it difficult to control the force in the application; the stability of the bone block where the magnet is located still needs to be strengthened, and the generated magnetic force is still slightly small, and so on. 5 Prospect DO technology promotes the development of minimally invasive and regenerative medicine technology, which requires the emergence of a new type of DOD that is more in line with the characteristics of the maxillofacial region. However, it should also be seen that the existing DODs are adapted to different needs by their respective advantages, and in some cases, they can only be used with a specific type of DOD, which cannot be used for all purposes in the short term. Therefore, the functions and characteristics of the original device will continue to be improved and strengthened, and then develop in the direction of more miniature, efficient, minimally invasive, comfortable, beautiful and individualized.