Effects of different aligner thicknesses and expansion step sizes on maxillary arch expansion with Invisalign Palatal Expander: a three-dimensional finite element study

BACKGROUND: This study aimed to investigate the biomechanical effects of different aligner thicknesses and expansion step sizes with the Invisalign Palatal Expander (IPE) on the midpalatal suture, teeth, and periodontal tissues during maxillary arch expansion using the finite element method (FEM), thereby facilitating its more precise and effective clinical application. METHODS: Three-dimensional (3D) finite element models, including teeth, bilateral maxilla, midpalatal suture, periodontal ligament (PDL), aligner, and attachments, were constructed. Stress distribution and initial displacement of the midpalatal suture, teeth, and PDL were evaluated under three aligner thicknesses (0.5, 1.0, and 1.5 mm) and different step sizes (0.1, 0.2, and 0.3 mm). RESULTS: During expansion with the IPE, increased aligner thickness markedly enhanced the buccal displacement of posterior teeth, increased midpalatal suture initial displacement, and promoted bodily movement rather than tipping, albeit with increased PDL stress. A greater step size increased buccal displacement of the first molar but reduced that of the primary molars. Thicker aligners (≥ 1.0 mm) combined with a larger step size directed mechanical effects posteriorly to a greater degree, promoting greater suture initial displacement in the posterior region. CONCLUSION: The thickness of the IPE aligner is the dominant factor regulating tooth movement patterns and midpalatal suture initial displacement, while step size modulates the magnitude and distribution of the mechanical response in the posterior maxillary arch. To promote greater posterior midpalatal suture displacement, we recommend using aligners with a thickness of at least 1.0 mm in combination with an appropriately increased step size.