Since the end of the twentieth century, the clinical application of mini-implant anchorage has become a research hotspot in the field of orthodontics. At present, the CAD/CAM guide for implants provides a new idea for the design of high-precision mini-implant templates:3D reconstruction and registration technology assist in the design of implantation positions, using the inverse technique to design the digital file of the template and the template made by rapid prototyping technology (RP).
To design a high-precision mini-implant template, an accurate digital model should be first established. There are direct and indirect ways of building a digital model. The direct method is the use of a small penetrating optical scanning probe to obtain the surface morphology of soft and hard tissues such as teeth and gingiva in the patient's mouth, omitting the operations of making impressions and plaster models. The indirect method needs to make a plaster model first and then apply related technologies to reconstruct the digital model after obtaining a three-dimensional model image by the scanning protocol.
The most commonly used three-dimensional modelling methods include optical scanning technology, CT scanning technology, and intraoral direct scanning technology. For dental crowns, optical scanning is more accurate. CBCT has obvious advantages in tooth root and jaw scanning reconstruction [8]. Therefore, we used the method of optical scanning combined with CBCT.
3D printing technology reconstructs a three-dimensional design model through computer aided design (CAD) software or reverse engineering. The 3D printer processes the layered materials according to the data read by the software to form a three-dimensional model. Currently, commonly used rapid prototyping technologies in the dental field include stereolithography appearance (SLA), fused deposition modelling (FDM), selective laser sintering (SLS), stereo inkjet printing (inkjet-based system, IBS), and low-temperature deposition modelling (LDM). This experiment used digital light processing (DLP) technology. The software Segma implant guide uses sequential point registration and global registration to establish a high-precision three-dimensional integrated model. The deviation analysis function of the software is used to detect the 3D deviation between the two models automatically. The chromatogram shows that the registration accuracy of the three-dimensional cone beam CT model and the three-dimensional optical scanning model is high, and the registration accuracy can reach 0.1 mm. The measurement accuracy can reach 0.001 mm.
Regarding the accuracy of the mini-implant template, Bae [11] reported that the angular deviation of the mini-implant implanted by the guide was a median of 3.14°, and the mesio-distal deviations in the coronal and apical areas were medians of 0.29 mm and 0.21 mm, respectively. Liu [19] reported that the position deviations in the mesio-distal, vertical, and buccopalatal directions were 0.42, 0.47, and 0.59 mm at the tip, respectively. The mini-implant deviations of the angles from the planned position in the distomesial, vertical, and buccopalatal directions were 1.2°, 1.3°, and 1.6°, respectively. The two mini-implant templates in this study are more accurate than the templates previously reported by Liu [19] in the vertical direction.
The application of templates can increase the accuracy of mini-implant insertion, thereby reducing damage to adjacent normal structures. The interval alveolar bone between teeth roots, especially between the maxillary first molar and the second premolar, is a frequently used mini-implant site in orthodontic clinics. However, the width of alveolar bone between tooth roots varies greatly, and it easily hurts the roots. Therefore, accurate implantation is necessary. Previous studies have also suggested the use of mini-implant guides when implanting between roots [8,9,10,11,12,13] to avoid damaging the roots in the mesial and distal direction.
In this study, two implant guides were designed to control the implant depth, which can avoid the influence of the zygomatic alveolar ridge when implant anchorage was implanted to a certain extent. We hope that the mini-implant can ensure sufficient bone retention without hurting the maxillary sinus.
The infrazygomatic crest has sufficient bone mass and a double layer of cortical bone, but it is adjacent to the maxillary sinus. Therefore, when implanting mini-implants in this area, the depth of insertion should be considered, avoiding injury to the maxillary sinus. Our previous research found that it is safe for mini-implants to penetrate the maxillary sinus within 1 mm [7]. The result provided guidance on the vertical limitation of mini-implants in the position of the infrazygomatic crest zone.
In this study, two implant templates were designed to control the implantation depth, which can avoid the influence of the maxillary sinus. We hope that the mini-implant template can ensure sufficient bone retention without hurting the maxillary sinus to ensure the safety of adjacent tissues and the stability of the mini-implant.