Biomechanical analysis of clear aligners for mandibular anterior teeth intrusion and its application in the design of new aligner attachment

Background: During the process of intruding the mandibular anterior teeth with clear aligners, the teeth are susceptible to unintended displacement, leading to complications such as excessive alveolar bone resorption and root exposure, which can significantly compromise the treatment outcome. Therefore, it is imperative to investigate the underlying causes and develop effective coping strategies. Methods: CBCT data from 30 patients were collected before and after the intrusion treatment of mandibular anterior teeth. Changes in the labial-lingual inclination of the mandibular anterior teeth and the distance between the root apex and alveolar bone wall were statistically analyzed. Finite element models of mandibular anterior teeth undergoing vertical intrusion with standard clear aligners were created with eight incisor mandibular plane angles (IMPA) to assess displacement trends, labial and lingual moments, and crown contact forces. Additionally, six aligner attachments were designed to simulate and analyze their biomechanical mechanisms. Results: Significant differences were observed in changes before and after treatment. When the IMPA was 90°, the crown experienced a labial moment. The labial root control ridge increased the labial moment of the crown, while the lingual root control ridge and labial attachments increased the lingual moment. The lingual fossa excavation group also increased the labial moment. The lingual root control attachment enhanced the lingual movement of the crown, whereas the lingual excavated attachment promoted labial movement. During the intrusion of mandibular anterior teeth, a comprehensive design incorporating labial intrusive attachments, labial root control ridges, lingual root control ridges, and lingual fossa excavation can be employed to ensure true vertical intrusion of the lower anterior teeth. Conclusion: This study performed finite element analysis under different inclination angles and various working conditions of the mandibular anterior teeth to reveal their biomechanical changes during intrusion, and innovatively designed the lingual fossa excavation, contributes to the development of novel orthodontic techniques and improves clinical treatment outcomes.