Gradient thickness in clear aligner gingival for incisor torque control in extraction cases simulated by finite element analysis

Objectives: The objective of this study was to analyze the impact of gradient thickness of clear aligners(CAs) on incisors during retraction in extraction cases. Materials and methods: A three dimensional(3D) finite element model was constructed including maxilla, dentition, periodontal ligaments(PDLs) and CAs. The gingival-side thickness of the CAs was set as 0.8, 0.9, 1.0, 1.1, and 1.2 mm respectively. The occlusion-side thickness was adjusted to maintain the average thickness at 0.75 mm. Incisor retraction was simulated, after which, the total and segmental displacement as well as stress distribution were calculated. Results: In all groups, the incisors exhibited lingual tipping and extrusion, while the canines and premolars demonstrated mesial tipping. With the increase in the gingival-side thickness of the CAs, the maximum displacement and apical reverse movement of the incisors were reduced, accompanied by the labial and apical movement of the rotation center. Concurrently, the retraction and extrusion of incisors decreased. The stress distribution in PDLs of incisors became more uniform. Conclusions: The gradient thickness CA can reduce the extrusion and apical reverse movement, produce uniform stress distribution in PDLs, and shift the rotation center labially and apically. This proposal can be a promising strategy for torque control of incisors and prevention of root resorption.