Abstract: 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.

Key words: orthodontic treatment, periodontally accelerated osteogenic orthodontics, clear aligner, three-dimensional finite element method, biomechanics