The effect of different thickness of power ridge on torque control during anterior retraction using clear aligners in patients with periodontal disease: a finite element analysis

BACKGROUND: Periodontal disease is a common disease and leads to the loss of supporting structures and pathologic tooth migration. In order to achieve a functionally sound and aesthetically satisfactory treatment outcome, combined orthodontic-periodontal therapy may be necessary. In this study, we focus on the anterior tooth movement when designing different thickness of power ridge for patients with periodontal disease and investigate its biomechanical effects during anterior retraction using clear aligners (CAs). METHODS: Two models of maxillary with different heights of alveolar bone were constructed, and three models of CAs were created with different power ridge designs. This study includes a total of six finite element models, which included alveolar bone, the first premolars extracted maxillary dentition, periodontal ligaments (PDL), attachments, and CAs. And the biomechanical effects were analyzed and compared in each model during anterior retraction using CAs. RESULTS: Compared to the models with normal height of alveolar bone, the patients with alveolar bone loss had more tooth displacement when the retraction distance was the same. And the center of the rotation of the anterior tooth moved towards the root direction. The total displacement of the central incisor increased in models with power ridges, and 0.5 mm power ridges caused the maximum displacement in both normal alveolar bone and reduced alveolar bone height models. In normal alveolar bone models, the crown and root moved in the same direction in 0.7 mm power ridges, but in reduced alveolar bone models, it was 0.5 mm. The von Mises stress values and maximum principal stress values for PDL increased as the depth of power ridges increased, and 0.7 mm power ridges caused concentration of stress in both models with normal alveolar bone or reduced alveolar bone. CONCLUSIONS: In patients with periodontal disease, the displacement of the entire maxillary dentition increased in comparison to those with normal alveolar bone. And an appropriate thickness of power ridge is beneficial to achieve torque control of anterior teeth. However, excessive thickness of power ridges is not suitable for anterior tooth retraction because of the stress concentration on the PDL.