ATT, the best restorative way of biocompatibility, has absolute advantages over dental implants in terms of the formation of periodontal membrane healing and even pulp healing [10, 11]. Their aesthetics and usability are also superior to other restorative methods . However, the operation process of ATT is difficult and complicated, therefore, many dentists are willing to choose a simple method, such as dental implant or fixed partial denture [1, 13].
An important factor for the success of ATT is to maximize the integrity and vitality of the periodontal membrane on the root surface of the transplanted teeth . A short extra-alveolar time of the donor tooth decreases the likelihood of damage to periodontal membrane cells on the root surface and increases the likelihood of forming a normal periodontal membrane after transplantation [6, 14]. Thus, the transplanted teeth can perform normal occlusal and masticatory functions [11, 15]. With increased extra-alveolar time of donor tooth, the vitality of periodontal membrane cells decreases, and the proportion of postoperative cemental healing increases, resulting in decreased success rate . At present, no uniform standard exists for the safe time of the extra-alveolar time of the donor tooth. Hammarström et al. used two different extra-alveolar times for transplanted teeth. After initial ankylosis, the ankylotic area does not increase in a 15 min extra-alveolar period group, whereas progressive ankylosis is observed in the 60-min extra-alveolar period group . Andreasen and his colleagues observed normal PDL healing in more than 80% of cases after an extra-alveolar time of 18 min . In our retrospective study, 15 min was set as the test value of the independent t-test.
Another important factor affecting the prognosis of ATT is the proper distance between the alveolar socket and the donor tooth  which can improve the blood and nutrition supply of periodontal membrane cells and avoid the physical extrusion to the periodontal membrane for improved success rate of ATT [5, 17]. Therefore, the preparation of the recipient site accurately and reduction in the number of positioning trials with the donor tooth are also our goals .
A digital technology assisted ATT treatment approach has been developed, including CBCT analysis, simulation, and preparation of 3D model and guide plate, to reduce the extra-alveolar time of donor tooth, prevent potential damage to the PDL, and prepare the alveolar socket accurately . We used CBCT to analyze the morphology of the donor tooth and recipient site to simulate the transplantation process on the Mimics software, which can assess the feasibility of the surgery intuitively and estimate the possibility of invasion of anatomical structures, such as the alveolar neural tube or the maxillary sinus cavity . It helps increase the predictability of treatment outcomes and reduces the difficulty of doctor–patient communication . The preoperative 3D donor tooth model of 1:1 size is designed and printed . Thus, in the process of preparing alveolar socket, the 3D model tooth is used to replace the donor tooth for trial implantation, shorten the extra-alveolar time, reduce the number of positioning trials with the donor tooth, and avoid the periodontal membrane injury [7, 12, 19,20,21]. The 3D model can improve the efficiency in preparing the alveolar socket and prevent excessive preparation. The digital guide plate can be consistent with the occlusal surface of the 3D model to guide the direction and depth of the preparation of the alveolar socket to achieve the accurate preparation of the alveolar socket and ensure that the position after the preparation is exactly the position we designed before operation. Results showed that the CAD combined with 3D model teeth and guide plate was helpful in shortening the preparation time of alveolar socket and extra-alveolar time of donor tooth and reducing the number of positioning trials with the donor tooth. The mean extraction time of donor tooth (5.56 ± 3.11) was far less than 15 min (P < 0.05). A previous study showed that the probability of root resorption is remarkably reduced when the donor tooth is isolated for less than 15 min .
The application of 3D printing technology in ATT has achieved good results, but this technology still has some limitations. For example, it takes about 1.5–2 h in each case at the segmentation, simulation and production stage. A shorter time is expected by improving the accuracy of automatic identification of the software and increasing the printing speed. The model teeth of some cases are not completely consistent with the shape of the donor tooth [5, 20]. We found the same problem during the surgery. In this study, 21 cases with some differences in the shape of the 3D model with the actual donor teeth were included. Thus, the second preparation of the alveolar socket was needed, which led to increased preparation time of the alveolar socket, extra-alveolar time of donor tooth, and number of positioning trials with the donor tooth. Compared with 20 cases, which could successfully place the donor tooth after preparation, cases which could not successfully place spent more preparation time of alveolar socket, extra-alveolar time of the donor tooth and had more positioning trials with the donor tooth (P < 0.05). The mean extra-alveolar time of the donor tooth in the group wherein the donor tooth cannot be placed successfully (7.29 ± 2.57) was still less than 15 min (P < 0.05). And even if there were differences in the models, the survival rate of this group was not affected, which meaned that the model differences only extended the time of donor tooth removal and alveolar socket preparation within a certain range, but would not reduce the survival rate of tooth.
From the acquisition of CT data to the implementation of the surgery, a number of potential sources of error is present at each stage of the process . We speculated that the following factors might explain the difference between 3D models and donor tooth.
First, the accuracy of CT may affect the accuracy of 3D modeling. The CT scanning layer thickness and voxel affect the image resolution . A high resolution results in high accuracy of observation, measurement, and outline of the structure. The gray value of the image affects the doctor’s judgment of the tissue structure. The position of the patient’s jaw, whether the mouth is kept in the correct opening position, and whether the jaw is moved will affect the final accuracy of the CT [17, 23]. Second, when the Mimics software is used to simulate the tooth transplantation operation, the shape of the donor tooth should be manually outlined, separated, and reconstructed [5, 15], which may lead to errors . After 3D reconstruction, the root of the donor tooth model is rough and needs to be smoothed , which may result in a deviation to the root morphology of the model tooth. Although model teeth are made 1:1 with the donor tooth, the periodontal membrane has a certain thickness, which may result in the unsuccessful placement of the donor tooth. Third, the oxidation polycondensation is a common problem of photocurable resin materials . A certain time interval is observed between the production of 3D model teeth and the use of 3D model teeth. The aggravation of the deformation of resin is also a problem that should be considered.
In addition, the computer-aided rapid prototyping technology produces complex 3D physical models by selective modification. Print layer thickness is one of the key parameters, It will affect the accuracy of the replica [26, 27].
In addition, the horizontal layer at a time with stepwise submergence along the vertical axis, during this process, the precision of the printer may also lead to the deviation of the model .