Influence of clinical crown length and attachment configuration on premolar derotation with clear aligners: a three-dimensional finite element analysis

This study aimed to investigate the influence of clinical crown length and attachment configuration on the rotational control of mandibular second premolars during clear aligner therapy using finite element analysis. Six three-dimensional mandibular models, including a 45º mesially rotated second premolar, were designed with two different clinical crown heights (standard and long). Three attachment strategies were tested for each crown height: (1) a single buccal beveled vertical rectangular (BVR) attachment (Models 1 and 4), (2) dual bucco-lingual BVR attachments (Models 2 and 5), and (3) a single buccal attachment combined with auxiliary elastic forces simulated via lingual buttons (Models 3 and 6). A derotation degree of 1.2º was programmed. Tooth displacement, aligner deformation, and von Mises stresses in the periodontal ligament (PDL) were analyzed. The dual-attachment configuration on long clinical crown (Model 5) yielded the greatest tooth displacement (33.34 µm) and the lowest aligner deformation (56.52 µm). Conversely, the standard crown with a single buccal attachment exhibited the least movement (22.58 µm) and the highest deformation (63.08 µm). A consistent inverse pattern was identified between aligner deformation and rotational efficiency. Regarding PDL stress, the highest peak von Mises stress (0.187 MPa) was observed in Model 2, localized primarily in the cervical region. Conversely, Model 4 yielded the lowest stress (0.131 MPa) with the most favorable distribution pattern. Increased clinical crown length (long-crown models) directly contributed to greater overall tooth displacement and rotational movement compared to standard crown lengths. This effect was most pronounced in dual-attachment configuration (Model 5). The incorporation of dual bucco-lingual BVR attachments consistently improved rotational control by enhancing aligner engagement and decreasing aligner deformation, despite concentrating PDL stress at the cervical region of the root. While auxiliary elastics were intended to facilitate movement, they yielded slightly lower rotational efficiency compared to dual attachment configurations.