Three-dimensional assessment of tooth movements in clear aligner treatment of moderate and severe crowding

OBJECTIVES: To evaluate angular and linear tooth movements following orthodontic treatment for correction of moderate to severe crowding using clear aligner therapy (CAT). MATERIALS AND METHODS: This retrospective study included 46 patients treated with clear aligners in permanent dentition for correction of moderate to severe crowding. Twenty-three patients (6 males, 17 females; mean age 20.0 years) were treated with premolar extractions and 23 patients (8 males, 15 females; mean age 17.3 years) were treated without premolar extractions. Three-dimensional tooth linear and angular displacements were quantified in the pre- and post-treatment digital dental models superimposed using the Slicer Automated Dental Tools module of 3D Slicer software ( www.slicer.org ). Generalized estimation equations were used for group comparisons (p < 0.05). RESULTS: CAT correction of moderate to severe crowding with premolar extraction led to significant retroclination of upper and lower incisors (upper: -6.71° ±7.79, P < 0.001 and lower: -5.11° ±8.49, P < 0.001). Anchorage control in extraction cases was limited, with approximately equal molar mesialization and incisor lingualization. In non-extraction cases, crowding was corrected through arch expansion with flaring of incisors. Vertical changes were similar between groups. CONCLUSIONS: CAT of moderate to severe crowding without extraction led to arch expansion and proclination of incisors. In contrast, premolar extraction cases often presented incisor retroclination and molar mesial tipping, requiring careful biomechanical management. CLINICAL RELEVANCE: These findings help clinicians plan treatments, manage patient expectations, and anticipate side effects, enabling the use of proper strategies and resources to optimize outcomes in both extraction and non-extraction cases.