Biomechanical effects of varying intrusion-to-retraction ratios on anterior tooth displacement during extraction space closure with clear aligners: an iterative finite element analysis.

Using finite element analysis, the present work examined the three-dimensional biomechanical responses of anterior and posterior teeth to a range of intrusion-to-retraction ratios during anterior space closure with clear aligners after extraction of a maxillary first premolar. A three-dimensional model of a maxillary first premolar extraction case was built, comprising the upper dentition, periodontal ligament, aligner, and attachments. The intrusion-to-retraction ratio was altered in a stepwise manner to define seven groups (C0-R6), and movement was reproduced through an iterative simulation of alveolar bone remodeling. For anterior and posterior teeth alike, three-dimensional displacement, change in tooth angulation, and the torque-loss-per-posterior-displacement (TL/PD) ratio were quantified. As the ratio rose, anterior crown displacement generally shifted from extrusion toward intrusion. Compared with C0, the 1:1 intrusion-to-retraction ratio reduced crown extrusion by 59.5% in the central incisors and 81.4% in the lateral incisors while maintaining distal displacement of all anterior teeth. The central incisors shifted to mesial displacement in the R3, R5, and R6 groups, indicating reduced retraction efficiency, and began to tip labially at R5. In contrast, distal displacement was maintained in the lateral incisors up to R5 and the canines across all groups. In the posterior segment, mesial crown movement shifted to distal movement from R2 onward, although the associated vertical responses differed among the posterior teeth. Variability in the TL/PD ratio was greatest for the central incisors, whereas the lateral incisors and canines remained comparatively stable. Under the constraints of this finite element study, overly high intrusion-to-retraction ratios lowered retraction efficiency and produced labial tipping, chiefly in the central incisors. Among the evaluated conditions, a 1:1 ratio (the R2 group) showed a comparatively balanced combination of anterior vertical control and space-closure efficiency while maintaining distal displacement of all anterior teeth. This ratio may serve as a biomechanical reference for digital treatment planning during extraction space closure with clear aligners, although clinical validation is required before routine application.