Biomechanics of Thermoplastic Clear Aligners for Orthodontics

Abstract

Clear thermoplastic teeth aligners with incremental misfits have become popular in orthodontics, but the biomechanics of these devices is not well understood. Neither is the tooth movement induced by such devices. The aim of this study is to determine the forces and moments exerted by the aligners on the underlying teeth using computer models validated with state-of-the-art optical strain measurements. The models provide a powerful design tool that can improve the safety, reliability and efficiency of orthodontic treatments using such devices. The results also provide a better understanding of tooth movement under mechanical forces.Finite element (FE) models are created from Micro-CT scans of an aligner and a model arch of teeth. The models are uniformly meshed with 0.3-mm long elements. Linear-elastic mechanical properties provided by the material manufacturers are used. Simulations are performed using Abaqus, a commercially available finite element software program. Fitting of the two components is simulated using interference fit, followed by frictional surface-to-surface interaction. The assembled FE model is validated by comparing the predictions of the teeth-aligner gaps and aligner surface strains with experimental data. The experimental teeth-aligner gaps are obtained from the Micro-CT scans whereas the aligner surface strains are measured using a 2-camera digital image correlation (DIC) system. The long-time (time dependent) behavior of the aligner is validated by comparing finite element predictions of force and moments with experimental data collected from a custom apparatus. The validated FE model is extended to provide a complete model of the teeth, periodontal ligament (PDL) and the bone structure around the tooth roots. Three different cases, incisor tipping, premolar derotation and molar distalization, are simulated to determine the tooth movement and orthodontic loading imposed by the aligner. Good agreement between model predictions and measured data are obtained for both the teeth-aligner gaps and aligner surface strains. The linear regression between prediction and measurement for teeth-aligner gaps sampled at different positions has a R2 value of 0.99. The mean difference between prediction and measurement for the aligner surface strains (von Mises) over 1544 nodes on the labial side and 1929 nodes on the lingual side is 0.07% and 0.01%, respectively, both are lower than the mean background noise. A comparison between the FE and experimental results for long term behavior also shows good agreement. The long-duration orthodontic force decreases by 24% to 34% due to the creep behavior of the aligner. An FE case study is performed to simulate the effectiveness of the aligners for different tooth movements. For incisor tipping, premolar derotation and molar distalization, the effectiveness is 33.9%, 24.2% and 16.2% respectively. For premolar derotation with attachment between the tooth and aligner, the effectiveness increases to 50.0%. A FE model for clear thermoplastic teeth aligners has been successfully developed and validated. The model can therefore be used with confidence to predict the forces and moments applied to teeth by the aligners, thus improving our understanding of the biomechanics of such devices and the tooth movement they induce.