Initial biomechanical response to different attachment configurations and class II elastics during clear aligner–based canine distalisation following premolar extraction: a three-dimensional finite element analysis

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 distalisation with clear aligners following first premolar extraction. 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 experimental configurations were analysed, including one incorporating button-cut Class II intermaxillary elastic mechanics. A single 0.25-mm programmed distalisation 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. Attachment configuration influenced initial stress distribution, force transmission, and canine displacement patterns. The attachment-free model demonstrated the largest palatal and extrusive crown-displacement components and the greatest PDL stress values. The paired optimised root-control attachment configuration exhibited a smaller extrusive crown-displacement component and lower absolute root-displacement components, 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. Attachment configuration and button-cut Class II elastic mechanics produced configuration-dependent differences in the initial biomechanical response to a single programmed activation. The paired optimised root-control attachment configuration exhibited a smaller extrusive crown-displacement component and lower absolute root-displacement components, whereas Class II elastic mechanics modified initial displacement and PDL stress without increasing distal displacement. These findings characterise 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.