Biomechanical Evaluation of Clear Aligners and Quad-Helix in Maxillary Expansion: A Finite Element Study

Background: The quad-helix (QH), a conventional appliance, and clear aligners (CA), which have been increasingly used, are both used for maxillary expansion. However, their biomechanical behavior during dentoalveolar expansion has not been directly compared. This study evaluates their biomechanical responses using finite element analysis (FEA). Methods: Three-dimensional finite element models of a CA with attachments and buccal root torque and a QH appliance were constructed. Clinically relevant loading conditions were applied based on reported activation protocols. A 5 mm activation (1.4 N) was applied to QH, and a 0.4 mm deformation (0.7 N) to CA. Tooth displacement and stress distribution were evaluated at cusp tips, root apices, and bifurcation areas of canines, premolars and molars. Results: In both models, displacement was greater at the crown level and decreased toward the root, indicating a predominantly tipping pattern. Maximum buccal displacement was observed at the first molar in the quad-helix model (1.054 × 10−4 mm), whereas in the clear aligner model, the highest value was at the second molar (5.498 × 10−5 mm). The quad-helix demonstrated a more localized expansion pattern, primarily affecting the first molar and premolars, while clear aligners exhibited a more distributed posterior response. Stress concentrations were more localized in the QH model and more uniformly distributed in the CA model. Conclusions: Dentoalveolar expansion is influenced not only by movement magnitude but also by the pattern of force distribution. Differences in posterior force transmission between CA and QH were associated with distinct biomechanical response patterns under the simulated loading conditions evaluated in the present study.