Key Points: Avoid "Roller Coaster Effect" In Clear Aligner Cases

To explore the effect of the auxillary devices on “the roller coaster effect”occuring in extraction cases with clear aligners. According to the inclusion criteria, 32 extraction cases with clear aligners were selected. The first premolars were extracted and the reciprocal anchorage during anterior retration was designed. The pre-operation and post-operation digital models were obtained and superimposed by the stable palate ridge, to measure the three-dimensional displacement of maxillary incisors, canines and molars. the mean extrusion amount of maxillary incisors was 2.8±1.2mm, resulting in the deepening of overbite. Personalized bite ramp could effectively prevent that (p<0.05). There was no difference on the canine tipping movement between the horizontal attachment group and optimized attachment group (p>0.05). The mean amount of molars mesial movement was 3.4±1.2mm. The molar mesial tipping movement in the horizontal attachment group was lower than that in the optimized attachment group, with statistical significance (p<0.05). in extraction cases with clear aligners, personalized bite ramp on the maxillary incisors and canines was necessary to prevent “the roller coaster effect”. Meanwhile, a larger attachment on canines could make clear aligners wrap canines tightly, facilitating canine bodily movement. A horizontal rectangle attachment with the length not exceeding 4mm should be placed on molars, preventing mesial tipping movement. The position of attachments on the canines and posterior teeth should be in line, just like the postion of barckets in pre-adjusted edgewise technique.