The ‘Roller Coaster Effect’ in Clear Aligner Premolar Extraction Cases: Posterior Open Bite and Cephalometric Risk Factors

OBJECTIVES: This study aimed to investigate the occurrence of the roller coaster effect (RCE) in premolar extraction cases treated with clear aligner therapy (CAT), and to identify cephalometric risk factors. MATERIALS AND METHODS: Sixty five patients treated with CAT after four first premolar extractions were retrospectively analysed, including 34 with mild crowding (Mi) and 31 with moderate-severe crowding (Mo-Se). Posterior functional cusp distances (FCDs) were measured at the second premolar, mandibular first molar mesiobuccal (MB) and distobuccal (DB) cusps, and second molar on pre- and post-treatment digital models using a standardised 3D coordinate system. RESULTS: As mesial molar movement was minimal in this cohort, post-treatment FCDs increased significantly in the Mi group at all posterior sites, with the greatest changes at the first molars (MB: 1.64 ± 1.39 mm; DB: 1.48 ± 1.55 mm; both p < 0.001), while the Mo-Se group showed an increase only at the first molar MB cusp (p < 0.001). Comparisons indicated greater FCDs increases in the Mi group than in the Mo-Se group (all p < 0.05). Correlation analysis revealed positive associations between greater upper incisor proclination (U1-NA, U1-SN) and posterior FCDs increases. The regression analysis further identified a smaller ANB angle as an independent risk factor for RCE. CONCLUSIONS: Posterior FCDs served as a simple and clinically applicable indicator of RCE, most pronounced at the first molars and in mild crowding cases. Smaller ANB angles and increased maxillary incisor proclination were associated with higher RCE susceptibility, emphasising prudent case selection in extraction-based clear aligner therapy.