Three-dimensional finite element analysis of bracketless clear aligner expansion with different buccal bone plate...

The thickness of the buccal cortical plate of maxillary posterior teeth limits the safe range of transverse expansion with bracketless clear aligners.

This retrospective study analyzed cone-beam CT data from 100 adults with normal occlusion. Buccal bone plate thickness was measured at four sites (F1-F4: 3, 6, 8 mm apical to cemento-enamel junction and root apex) for maxillary first/second premolars (PM1, PM2) and first/second molars (M1, M2). Based on PM1 thickness at F1 (mean ± SD), three maxillary models with thin (0.42 mm), medium (0.72 mm), and thick (1.02 mm) buccal plates were reconstructed. For each, aligner-dentition-periodontal ligament-bone systems with four buccal root torque angles (0°, 0.5°, 1.0°, 1.5°) were built, yielding 12 finite element models. Periodontal ligament (PDL) stress, tooth displacement, and root control (R/C ratio) of PM1 were analyzed.

PM1 exhibited the thinnest buccal plate at F1 (0.72±0.30 mm), significantly thinner than other posterior sites (P<0.001). Thinner plates showed greater crown tipping and higher cervical PDL stress at low torque. Increasing torque to 1.0° promoted bodily movement, reduced peak PDL stress (57.1% reduction in thin-plate model from 0° to 1.0°), and improved root control. Torque of 1.5° showed similar effects.

The maxillary first premolar region is highest-risk for buccal dehiscence during clear aligner expansion. Individualizing buccal root torque (≈1.0°-1.5°) based on bone thickness enhances bodily movement, reduces PDL stress, and may improve safety in thin-bone patients.