Abstract
Objective: To evaluate the biomechanical effects of en-masse versus substep extraction space closure on posterior teeth during clear aligner treatment and to provide guidelines for optimising anti-tipping designs.
Methods: Twenty-four maxillary finite element models incorporating first premolar extractions treated by clear aligners were constructed. The models were divided into two groups: en-masse closure (I0, en-masse retraction; II0, en-masse mesialisation), and substep closure (Ia, canine distalisation; Ib, incisor retraction; IIa, premolar mesialisation; IIb, molar mesialisation). Within each group, different anti-tipping designs (1°, 2°, 3°) were applied to the posterior teeth.
Results: Compared to I0, groups Ia, Ib, and Ia+b exhibited less crown mesial displacement, root distal displacement, and mesial tipping of the posterior teeth. In contrast, groups IIa, IIb, and IIa+b demonstrated more crown mesial displacement, root distal displacement, and mesial tipping. Of note, IIa increased the mesial movement of the premolars and generated a distal force on the molars, mitigating their mesial inclination. IIb had a similar effect. Consequently, Ia, Ib, and Ia+b required significantly less anti-tipping design to maintain the pretreatment mesiodistal angulation of the posterior teeth. Conversely, IIa, IIb, and IIa+b necessitated more anti-tipping design to achieve bodily movement of the posterior teeth.
Conclusion: Substep retraction may reduce the mesial tipping and anti-tipping design of the posterior teeth. Substep mesialisation, however, may increase crown displacement, posterior mesial tipping, and the anti-tipping design; nevertheless, the reaction forces from adjacent teeth can partially alleviate these effects. Further clinical validation is necessary to confirm these findings.