Dynamic biomechanical changes of clear aligners during extraction space closure: Finite element analysis.

INTRODUCTION: Clear aligners (CAs) have recently become popular and widely used orthodontic appliances. Research on CA biomechanics has become a focal point in orthodontics to improve the efficiency of CA treatment and address challenging issues, such as extraction. The biomechanical characteristics of CAs in space closure have been reported. However, previous studies have mainly focused on static biomechanical analysis that cannot demonstrate the dynamic biomechanical changes in CAs during space-closing. Given that these biomechanical changes can be significant and have considerable clinical value, this study aimed to investigate these characteristics. METHODS: Sequential extraction space-closing models were derived from included patient data and refined using modeling and CA design software. A finite element analysis was performed to obtain biomechanical raw data. This study introduced a dual coordinate system and space geometry analysis to demonstrate the biomechanical properties accurately. RESULTS: As space closure progressed, the instantaneous tooth displacements increased, indicating an enhanced space closure force because of the increased strain in the CA extraction area. Meanwhile, the central axis of rotation of the anterior teeth continuously moved toward the labial-apical direction, showing a gradually enhanced vertical and torque control effect. CONCLUSIONS: During space closure, CAs undergo specific biomechanical changes, including increased contraction and control forces on both sides of the gap. These biomechanical effects are beneficial to alleviate the roller coaster effect gradually. Meanwhile, more reasonable staging design strategies can be proposed on the basis of this biomechanical mechanism.