Abstract: PURPOSE: This study analysed the mechanical features of the extraction space closure in clear aligners through mechanical research. Methods: A patient with four first premolars extraction was chosen as sample, and four plans with clear aligners for space closure were designed, including anterior retraction entirely, anterior retraction dispersedly, posterior movement mesially and posterior movement dispersedly. Finite element analysis was performed separately. Results: In anterior retraction, when there was no anterior space, the force acted on them more evenly, and the posteriors needed to provide more anchorage; when anterior space existed, the force acted on the central incisors first and largely, and the posteriors needed to provide less anchorage. In posterior anchorage, when the posteriors moved entirely with mesial movement, it might help to protect second premolar's anchorage while molars would receive more mesial force; when the posteriors moved separately without designed movement, the second premolar would undertake more anchorage and more tendency to incline mesially; when the posteriors moved entirely without designed movement, the anchorage would distribute more evenly. Conclusions: Selection of the way of retracting anteriors should be based on the difference of stress distribution in different ways in addition to individual discrepancy and clinical facts. Selection of the way of moving posteriors should be based on the distribution of anchorage as well as avoiding posterior open bite. Less amount of correction in each step has better axial control.

Key words: Clear aligners, Extractive cases, Anteriors retraction, Posterior anchorage, Finite element analysis