The 'roller coaster effect' in premolar extraction cases: clear aligners vs. straight-wire appliance

This study aimed to quantitatively investigate the intractable 'roller coaster effect' (RCE) that occurs in premolar extraction cases treated with clear aligner therapy (CAT) or straight-wire appliance (SWA). Protrusion cases treated with extraction of bilateral first premolars were included. Pre- and post-treatment cephalograms were obtained to measure the bending angle of occlusal plane (BAOP), namely the occlusal intersection angle between the anterior occlusal plane (AOP) and posterior occlusal plane (POP). BAOP is proposed as the indicator for quantifying RCE in this study. In the maxillary dentition, BAOP significantly decreased from 177.50 ± 5.57° to 171.10 ± 3.32° in the SWA group (n = 30), and from 178.00 ± 4.66° to 168.10 ± 5.63° in the CAT group (n = 36). In the mandibular dentition, BAOP had no significant change (from 164.90 ± 5.00° to 164.30 ± 6.40°) in the SWA group (n = 29), while significantly decreased from 163.40 ± 6.36° to 155.90 ± 7.48° in the CAT group (n = 37). In the both dentitions, the post-treatment BAOP was significantly smaller in the CAT compared to SWA group. Decrease of BAOP in the CAT group resulted from bending of the AOP rather than POP. Multiple linear regression analysis revealed that the mandibular canine crown length had a positive correlation with the mandibular post-treatment BAOP. Only two-dimensional cephalometric measurements were conducted. In premolar extraction cases, CAT undergoes more severe RCE at completion of its first-phase treatment compared to SWA at the end of treatment. Longer mandibular canine crown may mitigate mandibular RCE in CAT.