Orthodontic force distribution when retracting anterior teeth during aligner treatment

Clinical outcomes may differ from the simulation when the anterior teeth are retracted during aligner treatment. For example, the tooth axis of the anterior teeth may not be maintained, thereby resulting in excessive tipping on the lingual side and severe overbite and tooth root exposure. This study aimed to quantitatively evaluate the effects on orthodontic force magnitude and distribution of different movement methods when retracting anterior teeth during aligner treatment by performing two-dimensional birefringence evaluation （measurement using near-infrared birefringence and retardation）. Two types of aligners were prepared: one that tipped the anterior maxillary teeth by 0.63° to the lingual side and one that produced 0.25mm bodily movement of the tooth. Orthodontic forces exerted on a model by the aligners were compared, and the mechanical energy was calculated from the generated optical retardation using a two-dimensional birefringence measurement system （WPA-200-NIR）. Despite the insignificant difference observed in the amount of movement, significant differences were observed in orthodontic force magnitude and distribution when tipping and/or bodily movement occurred while retracting anterior teeth. Furthermore, loads were exerted in the other loci in addition to those in the anterior teeth. When retracting anterior teeth, orthodontic force magnitude and distribution varied on the basis of differences in the tooth axis tipping angle.