The effects of anterior spaces on the intrusion of mandibular incisors with clear aligners in extraction cases: A finite-element analysis

Background: The intrusion of mandibular incisors presents significant challenges for orthodontists utilizing clear aligner therapy, primarily due to issues such as reduced accuracy in achieving intrusion and unexpected tipping movements. This study aims to investigate the biomechanical effects of varying anterior tooth spaces on the intrusion of mandibular incisors in extraction cases, offering insights that may enhance treatment outcomes. Methods: A finite element model was created, encompassing the mandibular dentition (excluding first premolars), mandible, periodontal ligaments, and clear aligners. Vertical rectangular attachments were strategically placed on the canines, second premolars, and first molars. Five model groups were constructed based on the size of the spaces between anterior teeth: G0 (no space), G1 (0.20 mm), G2 (0.40 mm), G3 (0.60 mm), and G4 (0.80 mm). The iterative method was employed to simulate orthodontic tooth movement under a mechanical load applied by clear aligners with a 0.20 mm intrusion step, prior to space closure achieved through sequential distalization of the anterior teeth in extraction cases. Results: The application of clear aligner therapy resulted in minimal intrusion of incisors accompanied by significant labiolingual tipping when no anterior spaces were present. However, moderate spacing (0.60 mm) significantly enhanced the amount of intrusive displacement while reducing labiolingual displacements. With the introduction of space, the Von Mises stress within the periodontal ligament was distributed more evenly across the incisors. Notably, moderate spacing optimized the distribution of Von Mises stress from the clear aligner in the anterior region, facilitating a more bodily movement pattern during intrusion rather than excessive tipping. Conclusion: Inefficient tooth intrusion and unintended tipping movements frequently arise in extraction cases treatedwith clear aligners. Our findings indicate that moderate spacing between anterior teeth is more effective in promoting bodily intrusion of mandibular incisors. It is recommended that appropriate spacing be incorporated into treatment planning to enhancethe efficiency of intrusion, thereby minimizing labiolingual tipping movements. Trial registration: Not applicable.