Effects of graded palatal edge extension on tooth movement during maxillary arch expansion with clear aligners: a three-dimensional finite element study

To evaluate how palatal trimline shape and graded edge extension affect the initial biomechanical response of clear aligners during maxillary arch expansion. A symmetric full-arch finite element model was constructed from maxillary imaging data. Five aligners shared a scalloped buccal margin and a nominal thickness of 0.75 mm. Their palatal margins were scalloped without extension (Z0), straight without extension (Z1), or straight with 1-, 2-, or 3-mm extensions (Z2–Z4). A 0.20-mm buccal movement was programmed for each premolar and molar to generate the target aligner geometry. Each aligner was then assembled on the unmoved dentition to produce initial elastic loading. Tooth displacement, the signed crown-root displacement ratios (R/C), posterior tooth inclination angles, periodontal ligament (PDL) stresses, and aligner–tooth contact characteristics were evaluated. Z0 and Z1 showed similar maximum dentition displacement and peak PDL von Mises stress. Within the straight-margin series, maximum displacement reached 87.8 μm in Z3 and 91.9 μm in Z4, while peak von Mises stress reached 0.0664 and 0.0730 MPa, respectively. The first premolar showed the strongest initial expansion response. Its buccal crown displacement increased with extension, whereas opposing root displacement remained nearly stable. Its R/C became less negative, although crown and root displacements remained in opposite directions and the inclination angle increased slightly. Cervical PDL loading increased, with palatal tensile and buccal compressive regions. Compared with the unextended margin, extended margins produced larger contact areas at the first premolar and first molar and higher peak aligner–dentition contact pressure, with the first-premolar pressure maximum shifting from the palatal middle third to the cervical third with the 3-mm extension. The central incisor showed greater relative root participation and a reduced tipping tendency with longer extensions. The first molar response remained small, with a largely stable R/C pattern. Palatal edge extension increased initial tooth displacement, most clearly in the first premolar, and altered the PDL stress distribution. These findings support further investigation of palatal margin length as a design variable, though clinical evaluation remains necessary to determine the optimal extension length and patient tolerance.