Initial Biomechanical Response to Different Attachment Configurations and Class II Elastics During Clear Aligner–Based Canine Distalization Following Premolar Extraction: A Three-Dimensional Finite Element Analysis

<title>Abstract</title> <p>Objectives To compare the initial biomechanical responses generated by different attachment configurations and button-cut Class II intermaxillary elastic mechanics during a single 0.25-mm programmed activation for maxillary canine distalization with clear aligners following first premolar extraction. Methods A patient-specific three-dimensional finite element model comprising the maxillary dentition, periodontal ligament (PDL), alveolar bone, clear aligner, attachments, and orthodontic button was constructed from cone-beam computed tomography data. Six attachment configurations were analyzed, including one incorporating button-cut Class II intermaxillary elastic mechanics. A single 0.25-mm programmed distalization step was simulated through geometric mismatch between the activated aligner and pretreatment dentition, generating the initial force system through aligner deformation and elastic recovery. Initial three-dimensional crown and root displacements and principal and equivalent (von Mises) stress distributions within the aligner and PDL were evaluated. Results Attachment configuration influenced initial stress distribution, force transmission, and canine displacement patterns. The attachment-free model demonstrated the greatest overall initial canine displacement, extrusive component, and PDL stress. The optimized attachment produced a more balanced initial crown–root displacement pattern with a smaller extrusive component, although the initial response remained tipping-dominant across all configurations. Incorporation of Class II elastic mechanics into the angulated rectangular attachment configuration reduced initial canine displacement and slightly modified force transmission and PDL stress distribution without increasing distal crown displacement. Aligner stresses were concentrated around the activated canine and attachment–aligner interfaces, whereas cervical PDL stress concentrations accompanied the tipping-dominant initial displacement pattern. Conclusions Attachment configuration and button-cut Class II elastic mechanics produced configuration-dependent differences in the initial biomechanical response to a single programmed activation. The optimized attachment produced a more balanced initial crown–root displacement pattern, whereas Class II elastic mechanics modified initial displacement and PDL stress without increasing distal displacement. These findings characterize the immediate mechanical response at aligner engagement and should not be interpreted as evidence of cumulative attachment effectiveness or tooth-movement control over successive aligner stages. Clinical trial registration: Not applicable.</p>