Fig. 3
The movement tendency of the maxillary dentition. The coordinate system was based on the entire dentition (global coordinate system). Set I, initial distalization of the second molar; Set II, initial distalization of the first molar. Model A, control model without anchorage reinforcement; Model B, Class II elastics attached to the tooth by buttons; Model C, Class II elastics attached to the aligner by precision cutting. As shown by the arrows, with the distalization of the upper molars, the anterior and posterior teeth move in opposite directions due to the exerted reciprocal force. The x-axis represented the coronal plane (+ left, −right), the y-axis represented the sagittal plane (+ posterior, −anterior), and the z-axis represented the vertical plane (+ superior, −inferior)