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

Background: During the process of intruding the mandibular anterior teeth (MAT) with clear aligners (CA), the teeth are susceptible to undesigned buccal and lingual inclinations, leading to complications such as excessive alveolar bone resorption and root exposure that significantly compromise the treatment outcome. Therefore, it is imperative to investigate the underlying causes and develop effective coping strategies.

Methods: We first statistically analyzed the clinical issues, then used FEA to explore their underlying mechanisms to guide the design of attachments in clinical practice. Specifically, CBCT data before and after the intrusion treatment of MAT were collected to analyze the labial-lingual inclination of the MAT and the distance between the root apex and alveolar bone wall. Finite element analysis (FEA) models of MAT undergoing vertical intrusion with standard CA 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 (RCR) increased the labial moment of the crown, while the lingual RCR and labial attachment (LA) increased the lingual moment. The lingual fossa excavating holes (LFEH) group also increased the labial moment. The lingual RCR enhanced the lingual movement of the crown, whereas the LFEH promoted labial movement. During the intrusion of MAT, a comprehensive design incorporating labial intrusive attachments, labial RCR, lingual RCR, and LFEH can be employed to ensure true vertical intrusion of the lower anterior teeth.

Conclusion: This study revealed the biomechanical changes during intrusion, and innovatively designed the LFEH, thereby promoting the development of novel orthodontic techniques and improving clinical treatment outcomes.