Abstract
Aligner-type orthodontic appliances （“aligners”）, which are more esthetically pleasing than fixed orthodontic appliances, have become widely used in recent years. However, several aspects remain unclear concerning the effects of tooth movement variables, including direction and distance on the orthodontic force. Therefore, we evaluated the magnitude and distribution of the orthodontic force exerted by aligners during arch expansion through two-dimensional birefringence measurement （e.g., near-infrared measurements of optical retardation）. Additionally, we investigated how differences in the number of teeth activated in parallel and the amount of activation affect the orthodontic force of the aligner. We prepared two types of aligner that either moved only one tooth （first molar） or three teeth （first premolar, second premolar, and first molar）, each with two activation types on one side, 0.25 versus 0.50mm, for a total of four types. We subsequently compared the orthodontic force applied by the aligner to the bite model. We used a two-dimensional birefringence evaluation system for measuring the optical retardation generated by birefringence due to orthodontic forces and calculated the mechanical energy from these data. The lateral expansion of only one tooth produced a greater load than the lateral expansion of three teeth when the same amount of activation was set. The orthodontic force for single-tooth expansion was reduced when the amount of activation was doubled. The reaction force on adjacent teeth was strong regardless of the number of expansion teeth. The results suggest that the number of tooth activations and the movement distance during dentition activation affect the magnitude and distribution of the orthodontic force.
