Background Sequential distalization of the maxillary dentition is widely used to gain space for crowding relief, molar relationship correction, and facial profile improvement. Using three-dimensional finite element analysis, we evaluated the biomechanical impact of different distal-end clear aligner designs during premolar distalization in maxillary sequential distalization, with the aim of reducing mesial relapse of teeth that had already reached their planned positions and improving overall distalization efficiency. Material and methods Two initial models were constructed: In Model A, both maxillary molars were distalized by 2 mm to reach the planned positions. In Model B, both maxillary molars and the maxillary second premolar were distalized by 2 mm to reach the planned positions. Four aligner configurations were defined in Model A: A0, control (conventional design); A1, distal coverage removed at U7; A2, distal coverage removed at U6; and A3, distal coverage removed at U7 and U6. Five aligner configurations were defined in Model B: B0, control (conventional design); B1, distal coverage removed at U7; B2, distal coverage removed at U5 and U6; B3, distal coverage removed only at U5; and B4, distal coverage removed at U7, U6, and U5. We quantified tooth displacement, space-closure component ratios, and periodontal ligament (PDL) equivalent stress under each condition. Results For Model A, the contribution of U5 distal displacement to U5–U6 space closure was 68.08%, 66.48%, 70.44%, and 92.01% in A0–A3, respectively, and was highest in A3. In Model B, the contribution of U4 distal displacement to U4–U5 space closure was 69.22%, 68.15%, 70.55%, 70.47%, and 81.19% in B0–B4, respectively, and was highest in B4. Conclusion During second premolar distalization in maxillary sequential distalization, removing distal-portion coverage at U7 and U6 effectively reduced mesial relapse of already distalized molars, thereby protecting posterior anchorage. Similarly, during first premolar distalization, to reduce relapse of distalized teeth, distal-portion coverage should be removed at U7, U6, and U5.
Yang et al. (Thu,) studied this question.