The implantation of anchors into the anterior mandibular teeth is highly efficient, has a good corrective effect, is unlikely to damage important anatomical structures, and can well control the torque. Common oral deformities, such as deep overbites and bimaxillary protrusion, directly influence patients’ facial morphologies. Deep overbites resulting from mandibular anterior alveolar bone overdevelopment are normally corrected by lowering the anterior teeth. For patients with bimaxillary protrusion, adduction and reduction of the anterior teeth can effectively improve the facial shape. Strong anchoring is fundamental to ensure anterior teeth adduction. The use of effective anchors has become an important method for correcting bimaxillary protrusions after the extraction gap is closed[11, 12].
Orthodontic micro-screws have become popular research tools for scholars at home and abroad because of their small size, high comfort, simple implantation method, shortened treatment time and ability to meet the requirements of absolute anchoring. It is difficult to rely on imaging methods to evaluate the safety of sites for micro-screw implanting. Conventional X-ray film measurement can result in severe distortions and overlapping dental images, often resulting in failure to obtain cross-sections between roots, and cannot be used to measure bone thickness. In contrast, CBCT yields high-resolution images with a clear field of view and has high scanning efficiency, requires a low radiation dose, and results in few scanning artifacts. CBCT can be used for effective evaluation of the alveolar bone condition at the site of the implanted micro-screw and the positional relationship with adjacent anatomical structures, making it possible to avoid damage to important anatomical structures and provide a reference for clinical applications. In addition, CBCT scans can yield spatial information about the maxillofacial region, allowing three-dimensional images to be reconstructed by a computer to perform measurements in the maxillofacial region[14, 15]. Studies have shown that the stability of the micro-screw can be influenced by the choice of the implantation site, the bone condition at the implant site and the distance from the adjacent teeth. The application of CBCT will increase the accuracy of implantations and reduce the failure rate[16, 17].
Micro-screw implants are generally suitable for adult patients. Because the bone tissue of juvenile patients is in the active phase, the reconstruction of the bone tissue after loading is unstable, and bone resorption is enhanced. The bone tissue of adults is relatively stable, and bone reconstruction is gentle after implantation of micro-screws, thus improving the stability of the planting anchors. Conventional implantation sites for micro-screws are generally selected between two roots. For the anterior region, micro-screws are generally used to solve problems such as mandibular protrusion and deep lamination. Therefore, the conventional implantation site is between the two central incisors, between the central incisors and the lateral incisors, or between the lateral incisors and the canines. According to the data analysis of this experiment, the interradicular distance, buccolingual dimension and buccal lingual bone thickness between maxillary anterior teeth were positively correlated with distances below the alveolar ridge crest below 8 mm. Generally, between 2–3, the interradicular distance is the widest (approximately 2.77 mm) at 8 mm below the alveolar ridge crest. However, the choice of micro-implant diameter is often limited in the clinic. If the diameter is too large, cracks will occur in the cortical bone, which will affect stability. Therefore, when micro-screws are implanted in the mandible, it is recommended for implants generally selected in the clinic to have a diameter between 1.0 mm and 2 mm. Between 2–3, at both 4 mm and 8 mm below the alveolar ridge crest, the buccolingual dimensions are wide, with dimensions of approximately 7.04 mm and 6.82 mm, respectively. Therefore, the length of micro-screws should not exceed 6 mm in clinical practice; otherwise, due to its thickness and interradicular distance, the alveolar bone may be damaged by the micro-screw. Furthermore, if the micro-screws are too close to the root, they will cause loosening and shedding, resulting in failure of the implantation surgery and damage to other important anatomical structures due to incorrect positioning. Therefore, the micro-screw should be positioned in the central area between the two roots to ensure the continuity and stability of the anchor[21–23]. In clinical practice, it is necessary to consider the proper safety range of both the alveolar bone and the soft tissue. The closer the implant anchor is to the edge of the lip mucosa, the more friction is encountered, and the more difficult it is to maintain oral hygiene, causing the planting anchor to fall off due to inflammation[24, 25]. At 2 mm below the alveolar ridge crest, the interradicular distance and buccolingual dimension between the two central incisors are 2.700 mm and 4.99 mm, respectively, and those between the central incisors and lateral incisors are 1.38 mm and 5.88 mm, respectively. Avoiding this area during clinical operation is recommended.
The mechanism of implant anchorage correction for a deep overbite is mainly achieved by the depression of the lower anterior teeth. Studies have confirmed that desired results can be achieved in patients with deep overbite implants with anchors implanted between the two roots of the mandible, using a 100 g force for 3 to 6 months. Therefore, when implants are inserted into the mandible, the results are influenced by important factors such as the values of depression, depression time, and traction direction .
The implantation angle of micro-screws can significantly affect the stress of the cortical bone. As the implantation angle increases, the thickness of the cortical bone gradually decreases, the torque of the micro-screws increases gradually, and the shedding rate increases. Some studies have shown that the distance between adjacent roots is large, requiring an implantation angle of 90°; when the spacing between adjacent roots is small, the initial stability of the micro-screws can be significantly improved when the implantation angle is 60°-70°. In addition, the thickness of the cortical bone is positively correlated with the mechanical fitting force at the implanted screw-bone joint surface. In principle, the success rate of surgeries is significantly improved when the thickness of the cortical bone at the implant site is greater than 1 mm. Therefore, the patient's cortical bone should be measured before surgery. If the patient's cortical bone is thin, the appropriate implantation angle and implantation direction should be selected to improve the stability of the micro-screw. The experimental data from this study showed that between 2–3, the labial and lingual bone thicknesses were thickest 8 mm from the alveolar crest, and the stability was the highest; between 1–1, the buccolingual dimension was short, but the interradicular distance was still acceptable; and between 1–2, the interradicular distance was acceptable, but the buccolingual dimension was short. Caution should be taken if implants are required in this area, and it is recommended to design a suitable implantation method to improve the stability of the anchor.
Due to anatomical structural limitations, the anterior region of the mandible is rich in blood vessels for the lingual nerve tissue, and the bone in that region is weak. It is difficult to implant an anchor in this location, and an implant can easily fall off. Additionally, the buccal cortex of the mandible is thick, and the depression is more difficult to manage than is the depression of the maxillary anterior teeth. After depression, the mandibular anterior teeth are more likely to undergo root resorption. Changes in alveolar bone 8 mm from the alveolar crest were noted in this study, and the effects of the labial mucosa and ligaments on the experimental data were not considered. Because the implant supports the lower anterior teeth, the angle of traction may have affected the results of the study, causing the anterior labia to tilt. In addition, the low-pressure value and the depression time are also important factors affecting the results. In future studies, the effects of the condition of the patient’s labial soft tissue and the direction of the traction force on anchor placement will be analyzed.