Effects of root length on maxillary canine bodily movement in clear aligner treatment: a finite element analysis

Clear aligner (CA) therapy has become a widely used treatment option; however, the ability to predict tooth movement remains limited, particularly in extraction cases. Root length is a critical anatomical factor that determines the centre of resistance and the tendency for rotation under orthodontic loading. However, its influence on CA biomechanics has not been thoroughly investigated. This study evaluated the effects of variations in maxillary canine root length on tooth movement patterns and periodontal ligament (PDL) stress distribution during CA treatment using finite element analysis (FEA). Three experimental models were established according to maxillary canine root length: normal root length (Model 1), short root length (Model 2), and long root length (Model 3). Each model was analysed under both attachment and non-attachment conditions. The CA was designed to translate the canine distally by 0.25 mm in bodily movement. Crown displacement, root displacement, moment-to-force ratio (M/F), torque loss-to-crown displacement ratio (TL/CD), rotational movements, and PDL von Mises stress were evaluated. Root length substantially influenced tooth movement patterns. Compared with Models 1 and 2, Model 3 exhibited reduced crown-root displacement differences, higher M/F ratios, and lower TL/CD values. Model 2 exhibited the greatest inclination tendency from the initial stage, with the maximum torque loss and rotation angle. The application of attachments improved rotational control and stress distribution across all models, with the most pronounced effect observed in Model 2. The maximum PDL stress under non-attachment conditions was approximately 6.1-fold higher than that observed under attachment conditions. This FEA suggests that maxillary canine root length influences tooth movement patterns and rotational control in CA therapy. Shorter roots demonstrated increased inclination tendencies, suggesting potential benefits from attachment-based rotational control and conservative movement protocols. These findings may inform root length-specific treatment planning considerations in CA therapy, although clinical validation is warranted.