Traction of a labially horizontal impacted maxillary central incisor with severe crown rotation: a finite element study

Background This model-specific biomechanical study investigated the effects of attachment-force configuration on the three-dimensional displacement, rotational behavior, and root surface stress of a horizontally impacted maxillary central incisor with approximately 90° of distal crown rotation. Methods Three finite element models with varying crown rotation (90°, 95°, 100°) were constructed from a single patient's cone-beam computed tomography dataset. Models B and C were parametric variants of Model A. Traction forces of 100 g were applied virtually via a button bonded on either the mesial or distal surface, with force directions tipped distally (0°, 5°, 10°, 20°) or mesially (0°, 10°, 20°, 30°). Displacement of specific landmarks, total tooth movement, rotation angles, and root surface stress were analyzed. Results The impacted incisor primarily exhibited inferior displacement, which generally decreased as the initial crown rotation increased or as the force tipping angle increased. Compared to mesial-button traction protocols, distal-button traction protocols produced greater total displacement and more pronounced crown rotation. In this specific model, initial coronal rotation gradually shifted from clockwise to counterclockwise (a favorable rotational tendency) within the tested tipping ranges. For distal-button traction protocols, coronal rotation shifted from clockwise to counterclockwise at approximately 10°-20° of distal tipping in Model A and 5°-10° in Models B and C. For mesial-button traction protocols, the transition occurred at approximately 20°-30° of mesial tipping in Models A and B and 10°-20° in Model C. Root surface stress was concentrated in the superior cervical region. Stress values were higher in the distal attachment-force configuration (0.60–0.64 MPa) relative to the mesial attachment-force configuration (0.44–0.52 MPa). Conclusions Among the tested protocols, the distal-button traction system showed a more favorable initial counterclockwise rotational tendency. The transition from clockwise to counterclockwise rotation was model-dependent: approximately 10°-20° in Model A and 5°-10° in Models B and C for distal-button traction configuration, and approximately 20°-30° in Models A and B and 10°-20° in Model C for mesial-button traction configuration. These are model-specific biomechanical observations and clinical applicability requires further validation.