Effects of proactive or synchronous anchorage preparation of posterior teeth during anterior en masse retraction with clear aligners: a three-dimensional finite element study

INTRODUCTION: The increasing use of clear aligners for anterior retraction following first premolar extraction has highlighted undesirable movements of posterior teeth, impeding efficacy and predictability. Although the anchorage preparation of posterior teeth has been proposed to counteract this effect, the optimal methods and extent remain undefined. Effective anchorage preparation strategies to mitigate undesirable movements of posterior teeth during anterior en masse retraction were investigated in this study. METHODOLOGY: A finite element study based on a maxillary first premolar extraction model was conducted to examine posterior tooth movement with different anchorage preparation methods: (1) without any anchorage preparation (control); (2) proactive anchorage preparation (PAP), where 1°, 2°, and 4° distal tipping of posterior teeth were set prior to anterior tooth retraction; and (3) synchronous anchorage preparation (SAP), where 1°, 2°, or 4° distal tipping of posterior teeth was processed with anterior tooth retraction. RESULTS: Compared with the control group, the PAP group exhibited a similar tooth displacement pattern (P > 0.1). SAP effectively controlled mesial tipping when applied at 1.245° for U5, 1.386° for U6, and 2.706° for U7, whereas the optimal resistance to mesial movement required 1.258° (U5), 2.148° (U6), and 2.976° (U7) of SAP, respectively. A 1.25° SAP for U6 successfully prevented lingual torque. However, mesial rotation of the posterior teeth, vertical displacement, torque, and transverse displacement of U5 and U7 remained difficult to control despite SAP adjustments. CONCLUSION: Compared with PAP, SAP demonstrated superior effectiveness in controlling mesial tipping and displacement, while PAP showed better performance in managing mesial rotation and transverse movements. For optimal anchorage control during SAP implementation, clinicians should pay particular attention to posterior tooth mesial rotation as well as U7 extrusion and buccal torque development.