Distribution of orthodontic force during canine retraction in clear aligner treatment

Orthodontic clear aligners have rapidly gained popularity. In cases of tooth extraction during clear aligner treatment, a bowing effect occurs, where the tooth tilts toward the extraction socket, in contrast to the simulation. Quantitative evaluation of orthodontic force is indispensable for elucidating the cause; however, many points regarding orthodontic force remain unclear. Therefore, we quantitatively evaluated the magnitude and distribution of the orthodontic force （as measured by the near-infrared birefringence and retardation） applied to the entire tooth and dentition by the clear aligner during canine tooth retraction using a two-dimensional birefringence measurement method. Additionally, we examined the effects of the amount of retraction of the canine teeth and the shape of the edge of the clear aligner on the orthodontic force. Eight clear aligner types were prepared by changing the amount of retraction of the canine teeth and the shape of the clear aligner margin, and the orthodontic forces applied by the clear aligners to the model were compared. A two-dimensional birefringence measurement system was used for measurement, and the orthodontic force was calculated from the generated optical retardation. A mechanical load was recognized in the canine tooth to which the orthodontic force was applied and in the other teeth. Additionally, differences in marginal morphology affected the magnitude and distribution of orthodontic force. Our results suggest that paying attention to the moving distance and the shape of the margin of the clear aligner is necessary when distally moving the canine teeth.