Upper incisal torque movement with clear aligners: A three‑dimensional finite element analysis

Torque movement is less effective in clear aligner therapy (CAT). This study aimed to analyze the torque movement behavior of incisors under different morphological modifications of aligner trimming lines by using three-dimensional finite element analysis to provide insights for clinical improvement.

A finite element model of the maxillary dentition was constructed from cone beam computed tomography data of an adult. CAT models were generated with different trimming line designs (scalloped, straight, and extended). A 0.5° palatal root torque movement was designed for incisors in each setup model with different center of rotation (COR) positions of 0, 1, 3, and 5 mm from the incisal edge.

The incisors exhibited uncontrolled palatal tipping when COR was set at the incisal edge with all three trimming lines. Little compressive stress was observed on the palatal-incisal area of the incisors. Aligners with extended trimming line generated more compressive stress in the labial-cervical area of the incisors than those with straight and scalloped trimming lines, and the incisors showed less labial root displacement. As the programmed COR moved away from the incisal edge by 1, 3, and 5 mm, more compressive stress was found in the palatal-incisal area of the incisors. The incisor movement type shifted from palatal tipping, palatal root torque to labial tipping.

Extended trimming line is favorable but insufficient to achieve palatal root torque of upper incisors in CAT. Setting the COR some distance from the incisal edge in setup improves the predictability of such torque movement.