The Optimal Initial Displacement in Rotated Maxillary Incisor Teeth with Clear Aligner in Different Periodontal Conditions: A Finite Element Analysis

Background: Clear aligner therapy (CAT) is widely used, yet safe per-stage rotation in periodontally compromised incisors remains uncertain. This study aims to define tooth position and support specific rotation limits by quantifying periodontal ligament (PDL) stress using finite element analysis (FEA). Methods: Four 3D FEA models (healthy; Stage I–III periodontitis) of maxillary central and lateral incisors were built in ANSYS 2024 R2. Mesial rotations of 1.25–3.0° were imposed on single teeth with a 0.5 mm PET-G aligner and attachments; the posterior segment was fixed. The PDL was modeled as nonlinear. Primary outcomes were peak PDL von Mises stress and total deformation; the mesh convergence was <5%. Results: At 3.0°, the healthy model produced 270.87 kPa (central) and 641.73 kPa (lateral). Stage I plateaued beyond ≈1.75° at ≈221.53 kPa (central) and ≈406.71 kPa (lateral). Stage II showed low central stress (86.20 kPa) but high lateral stress (2763.1 kPa) with greater deformation. Stage III yielded 825.39 kPa (central) and 1321.6 kPa (lateral). Deformation increased from <0.005 µm to ≈8.37 µm for centrals and from <0.005 µm to ≈11.139 µm for laterals with diminishing periodontal support. Conclusions: Safe rotational staging depends on periodontal support and tooth type. The recommended per stage angles are as follows: centrals ≤2.5° in healthy, 1.75° in Stage I, ≤1.0° in Stages II and III; laterals ≤1.75°, ≤1.25°, and ≤1.0°, respectively.