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BACKGROUND: The thin alveolar bone in the lower anterior region increases the risk of labial bone resorption when intruding the teeth with clear aligners. The effect of sagittal overcorrection design on the labiolingual control of mandibular anterior teeth intrusion has not been fully investigated. OBJECTIVE: To explore the effect of overcorrection on the changes in the displacement and stress of the mandibular anterior teeth, especially the cervical and apical regions. METHODS: Through a male volunteer cone-beam CT data, the three-dimensional reconstruction of the mandible and teeth was conducted in the MIMICS and GEOMAGIC software. Moreover, the models of periodontal ligaments, attachments, and appliances were created in the SOLIDWORKS software. First, the study was divided into canine intrusion group and incisor intrusion group. Then, the overcorrection (0°, 1°, 2°) was designed on the bilateral mandibular central and lateral incisors. A total of six models were established. The models were assembled and imported into the ANSYS software to analyze and calculate the displacement and stress level. RESULTS AND CONCLUSION: (1) In the canine intrusion group, canines intruded and tipped lingually while incisors extruded and tipped lingually. In the incisor intrusion group, canines extruded and tipped lingually while incisors intruded and tipped lingually. (2) Without overcorrection, the incisors necks moved lingually while apexes moved labially. With overcorrection, the incisors tended to be upright, followed by labial tilt. The least cervical and apical displacements were detected under 1° overcorrection. (3) With overcorrection, the incisal cervical stress concentration area shifted from labial to lingual in the canine intrusion group, whereas the stress concentration area shifted from lingual to labial in the incisor intrusion group. (4) The incisors tended to tilt lingually when intruding the mandibular anterior teeth with clear aligners. The sagittal overcorrection design was conductive to maintain the stable position of incisors. However, the amount of overcorrection should be moderate. Excessive overcorrection might increase the labial inclination tendencies of incisors. [ABSTRACT FROM AUTHOR]

BACKGROUND: The reciprocal force generated by the molar distalization with clear aligners can lead to anchorage loss. The effect of arch shapes and missing second premolars on anchorage has not been reported. OBJECTIVE: To analyze the effect of arch shapes and missing second premolars on anchorage during molar distalization with clear aligners using the finite element method. METHODS: Cone-beam CT data from an adult male were acquired from the database to establish the maxilla-upper dentition-periodontium-rectangular attachment-clear aligner model. The distal movement amount designed on the bilateral second molars was set to 0.25 mm. First, there were two groups in the study: second premolar bilateral presence and absence groups. Then, four subgroups in each group were created: tapered arch, ovoid arch, square Class II Division 1 arch, and Class II Division 2 arch groups. The Ansys software was used to calculate the displacement of the anchorage tooth and the stress of the periodontal ligament. RESULTS AND CONCLUSION: Mesial tipping and extrusion of first molars and premolars, labial inclination and intrusion of anterior teeth occurred during the upper second molar distalization with clear aligners. When the bilateral second premolars were missing, the mesial displacement of first molars increased significantly while that of first premolars and anterior teeth decreased in all groups. The square Class II Division 1 arch group showed the least anterior labial inclination, while the tapered arch group showed the most. There was no significant difference between the ovoid arch group and the tapered arch group. Moreover, the magnitude of tipping in the square Class II Division 2 arch group was slightly higher than that in the Class II Division 1 arch group. The stress of the periodontal ligament of the anchorage teeth was concentrated on the cervical and apical regions of the teeth. And the lowest stress level was detected in the square arch group. Compared with the other groups, the stress on the labial cervical area of the periodontal ligaments was also significantly relieved in the square arch group. To conclude, the square arch is more favorable in terms of anterior anchorage control and periodontal ligament stress distribution. Anterior labial inclination efficiency can be increased in cases of Class II Division 2 by designing the anterior labial inclination in conjunction with molar distalization. If the second premolar is missing during molar distalization, it is not conducive to opening up the space in the area of the missing tooth. [ABSTRACT FROM AUTHOR]

BACKGROUND: There is an increasing demand for orthodontic treatment, and periodontally accelerated osteogenic orthodontics (PAOO) technique can make it possible to move orthodontic teeth that are limited by thin alveolar bone. OBJECTIVE: To investigate the biomechanics of orthodontic tooth movement before and after periodontally accelerated osteogenic orthodontics (PAOO) surgery to increase alveolar bone volume using the three-dimensional finite element method. METHODS: A patient undergoing PAOO surgery before orthodontic treatment to increase bone volume on the labial side of the mandibular anterior region was selected. The patient was under invisible orthodontics. Two three-dimensional finite element models were constructed based on the patient’s preoperative and 6-month postoperative cone beam CT data. Both models simulated the movement of tooth 33: experiment I: distal-central movement of 0.25 mm; experiment II: lingual movement of 0.25 mm; and experiment III: intrusion movement of 0.10 mm. The stress distribution and initial displacement trend of tooth 33, periodontal ligament and surrounding alveolar bone under the action of the invisible aligner were analyzed before and after the PAOO procedure. RESULTS AND CONCLUSION: Dental stress analysis: In the same orthodontic tooth movement, the maximum Von-Mises stress and overall stress values of tooth 33 were all higher before surgery than after surgery; there were similar distribution areas of maximum equivalent stress and overall distribution trends of Von-Mises stress before and after surgery. Periodontal ligament stress analysis: In the same orthodontic tooth movement, the maximum Von-Mises stress and overall stress values of the periodontal ligament were higher before surgery than after surgery, and there were similar distribution areas of the maximum equivalent stress and overall distribution trends of Von-Mises stress before and after surgery. Alveolar bone stress analysis: In the same orthodontic tooth movement, the maximum Von-Mises stress values of the alveolar bone around tooth 33 were higher before surgery than after surgery, while the equivalent stress distribution showed a gradual decrease from the top of the alveolar ridge to the root. Initial displacement analysis: In the same orthodontic tooth movement, the initial displacements in the main displacement direction for all six observation points of tooth 33 were smaller before surgery than after surgery, and showed a tendency to gradually decrease from the tooth tip to the apex. Therefore, there were differences in the biomechanical characteristics of orthodontic tooth movement before and after the PAOO surgery. With the clear aligner, the postoperative equivalent stress values on the dentition, periodontal ligament, and surrounding alveolar bone were lower than before the surgery, and the initial displacements of the orthodontic teeth after the surgery are larger than before. These findings suggest that PAOO can release the restriction of thin alveolar bone on the movement of orthodontic tooth by increasing alveolar bone thickness, effectively improving the force on the roots, periodontal ligament, and alveolar bone, avoiding the stress concentration on orthodontic tooth in the thin alveolar bone area that can cause complications when moving, and improving the efficiency of tooth movement. [ABSTRACT FROM AUTHOR]

BACKGROUND: Intrusion of overerupted posterior teeth with clear aligner for adult patients with different bone densities, orthodontists mostly choose the diaphragm thickness by subjective experience, and finite element analysis is expected to provide a theoretical basis for choosing the appropriate thickness of diaphragms for patients with different bone densities. OBJECTIVE: To analyze the effects of diaphragm thickness and bone density on the intrusion of overerupted maxillary first molar in adult patients using clear aligners using finite element analysis. METHODS: Four groups of finite element models were created based on four bone densities (cancellous bone types I, II, III, and IV) and assembled with three clear aligner models with diaphragm thicknesses of 0.5, 0.75, and 1 mm, respectively. Finite element analysis was then performed using Ansys 2021 software. Clear aligners applied a 0.2 mm intrusion displacement to the maxillary first molar was simulated in each group. The von Mises stress and tooth displacement trends of root, periodontal ligament, alveolar bone of first molar and its adjacent teeth were compared between groups. RESULTS AND CONCLUSION: (1) The von Mises stress distribution of the first molar was the same in all groups, mainly at the root furcation; the second premolar was distributed in the proximal mid-cervical 1/3; the second molar was distributed at the buccal root furcation. (2) The displacement of the first molar and adjacent teeth was close to mesial displacement; the first molar displayed a buccal displacement trend, whereas adjacent teeth displayed a palatal displacement trend. At class II density of alveolar bone, the first molar X- and Y-axis displacement was the smallest, and the Z-axis displacement was the greatest; displacement of adjacent teeth increased with decreasing bone density. In the Z-axis direction, the displacement of the first molar and adjacent teeth was proportional to the thickness of the diaphragm. The smallest displacement was observed in the second premolar 1 mm and second molar 0.5 mm groups in the X- and Y-axis directions. (3) These results concluded that the use of clear aligners to lower the overerupted molars is less likely to cause pericemental fibrosis and root resorption. Decreased bone density does not increase the risk of root resorption. Class II bone density is optimal for molar intrusion. The maximum equivalent stress of the periodontal ligament and alveolar bone is proportional to the thickness of the diaphragm. [ABSTRACT FROM AUTHOR]

BACKGROUND: The rotation is unpredictable tooth movement in clear aligner treatment. Tooth shape can affect the efficiency of tooth de-rotation, but there is no relevant biomechanical study. OBJECTIVE: To investigate aligner activation influencing on biomechanics of clear aligner orthodontic therapy of rotated mandibular teeth with different shapes. METHODS: A three-dimensional finite element model of mandibular dentition-periodontium-alveolar bone-appliance with highly simulated crown shape was established based on cone-beam computed tomography and intraoral optical scanning data of an adult who had rotated lower central incisor and second premolar. The initial displacement, movement trend and von Mises stress distribution of periodontium of target teeth and adjacent teeth were analyzed when the loading of three different aligner activation of 1.0°, 1.5° and 2.0° was applied to rotated the left lower central incisor (group A) and second premolar (group B). RESULTS AND CONCLUSION: (1) The displacement and periodontium stress of target teeth were significantly higher in the group A than those in the group B under different rotation loads. Increasing the aligner activation could improve the efficiency of tooth de-rotation to a certain degree, but the risk of periodontal tissue injury was also increased. It was suggested that aligner activation should not exceed 1.5° for the lower incisor and 2.0° for the lower premolars. (2) In group A, the displacement of meso-adjacent tooth was 65%-105% as much as target tooth. For incisor de-rotation, we should pay attention to the adverse effect on adjacent teeth. (3) During the rotation movement, mesial tipping and slightly intrusive movement of target teeth were observed, and adjacent teeth showed the trend of tipping movement, which required additional measures to strengthen the grip on three-dimensional movement of teeth. [ABSTRACT FROM AUTHOR]

BACKGROUND: When invisible appliances are used in tooth extraction, canine distal movement is a retraction pattern that is beneficial to save the anchorage of posterior teeth and resist the arch effect of braces. However, canine distal movement changes the wrapping of braces around the teeth and the vacuole elasticity due to tooth extraction using the appliance, and relieves the constraint to the movement of adjacent teeth, which may have an impact on anterior tooth intrusion. OBJECTIVE: To study the effect of canine distal movement on anterior tooth intrusion in invisible orthodontic treatment by using finite element method METHODS: The mandibular finite element model of the subtractive first premolar was established, including teeth, periodontal ligament, alveolar bone, invisible appliance, and accessories. Step-by-step intrusion of the anterior teeth was conducted through simulation of clinical canine distal movement. In control group, the canines did not move. In experimental group 1, the canines moved 1 mm away, while in experimental group 2, the canines moved 1/3 of the extraction space. The effect of canine distal movement on anterior tooth intrusion was analyzed. RESULTS AND CONCLUSION: The maximum anterior tooth intrusion occurred in the experimental group 1, with the vertical displacement of the canine incisal edge by 0.017 867 0 mm, the vertical displacement of the central incisor by 0.009 647 0 mm, and the vertical displacement of the lateral incisor by 0.007 777 5 mm. The minimum anterior tooth intrusion occurred in the control group, with the vertical displacement of the canine incisal edge by 0.008 195 3 mm, the vertical displacement of the central incisor by 0.005 238 0 mm, and the vertical displacement of the lateral incisor by 0.005 689 2 mm. In the experimental group 2, the canine incisal edge moved 0.014 463 0 mm vertically, the central incisor moved 0.009 294 7 mm vertically, and the lateral incisor moved 0.007 013 9 mm vertically. In all cases, the von Mises stress of anchorage molars was smallest, the displacement of the incisor edge was greater than that of the root apex, and the stress appeared differently between the labial and lingual sides of the periodontal ligament. To conclude, canine distal movement can improve intrusion in the anterior teeth by appliance. However, different distal displacement has different effects on anterior tooth movement. The closer the anchor tooth is to the target tooth, the greater the force is, and vice versa. Invisible orthodontic treatment is mainly for inclined and intruded anterior teeth. [ABSTRACT FROM AUTHOR]