Biomechanical Effect of Unexpected Minor Aligner Circumference Changes on Lower Incisors During Intrusion: A Finite Element Analysis.

Objective: Unplanned changes in aligner circumference may occur during anterior teeth intrusion with clear aligner treatment (CAT), potentially affecting root movement, an important yet often overlooked factor. This study investigated the biomechanical effects of minor aligner circumference changes on lower incisors during intrusion.

Materials And Methods: Nine finite element analysis models of CAT were created based on the original model. These models were divided into the following groups: the 2 groups of incisors intrusion (0.25 and 0.5 mm), 3 groups of 0.25 mm intrusion combined with aligner circumference changes (-0.1, -0.2 and + 0.1 mm), and 4 groups of 0.25 mm intrusion combined with labial crown torque (0.5°, 1°, 1.5° and 2°).

Results: Compared to the control group (0.25 mm intrusion), the experimental group (0.5 mm intrusion) showed that lower incisors exhibited greater vertical intrusion (0.016 mm vs. 0.012 mm), more lingual crown movement (0.10 mm vs. 0.04 mm), and increased labial root movement (0.021 mm vs. 0.006 mm) sagittally. The 0.25 mm intrusion combined with a -0.1 mm aligner circumference increased labial root movement of lower incisors. As the aligner circumference shortened, this effect became more pronounced. Conversely, the group with a +0.1 mm aligner circumference reduced labial root movement. With additional labial crown torque applied to the lower incisors, the roots shifted from labial to lingual. The central incisor achieved pure intrusion at a labial crown torque of 1.06° per 0.25 mm intrusion.

Conclusions: Minor changes in the aligner circumference might significantly induce sagittal root movement of lower incisors during intrusion. Orthodontists should carefully monitor unplanned variations in aligner circumference during incisor intrusion to minimise unintended root movement.