Effect of the Periodontal Ligament on Clear Aligner Material Surface Strain when Measured In-Vivo

Abstract

Introduction: Clear aligner therapy has become increasingly popular as an alternative to traditional braces. Despite advances in technology, the efficacy and efficiency of clear aligner treatment remains challenged. Numerous in-vitro studies into the force systems of clear aligners have been developed. Currently, there are no guidelines for evaluating orthodontic force systems intraorally with clear aligners. The effect of the periodontal ligament and intraoral environment on orthodontic tooth movement with clear aligners remains unknown. Aim: To determine if surface strain can be measured in-vivo with digital image correlation (DIC). And to evaluate the effect of the periodontal ligament and intraoral environment on the surface strain delivered by a clear aligner to a maxillary central incisor during lingual tipping. Methods: Clear aligners were designed with 2.0 degrees of lingual crown tipping about an estimated center of rotation on a maxillary central incisor for 33 subjects. Clear aligners were fabricated using two materials, Essix ACE and Zendura A, in 0.030 mil (0.75 mm) thickness. A dual-camera, three-dimensional DIC system was used to record the transverse strain on the facial surfaces on the subject’s model, in the subject’s mouth, and on the subject’s model after submersion in artificial saliva. The relationships between transverse strain and the predictors, aligner material and environment, were examined using linear mixed effects models, with a random intercept for subject. A statistical model was also fit to test associations between transverse strains and predictors aligner material, gender, and presence of artificial saliva. Estimates are reported as a means with 95% confidence intervals. The passive and active models and aligners were assembled and scanned by micro-CT to measure the gap size between the aligner and model. Displacement at the incisal half of the target tooth was measured with GeoMagic software. Results: Initial forces delivered by clear aligners produced a strain threshold high enough to be detected and captured intraorally by DIC. The strain distribution and contour differed between in-vivo and in-vitro, as well as between material types, but followed the same pattern and peaks. Mean transverse strain increased as the point of focus narrowed to the incisal half of the target tooth. The effects of the environment were statistically significant at the incisal half of the target tooth, with increased transverse strain in-vitro compared to in-vivo. Zendura A expressed significantly less transverse strain than Essix ACE within both environments. The effect of the material was more pronounced in-vitro for the entire dentition, but not at the incisal half of the target tooth. No differences in transverse strain were noted with the presence or absence of artificial saliva, as well as between genders. Strong reproducibility existed between trials, with more similar strain profiles in-vitro compared to in-vivo. Micro-CT images demonstrated a lack of gap between the aligner and passive model on the labial surface of the target tooth, while gap sizes increased at the interproximal contact points and on adjacent teeth. Based on inciso-gingival crown length of tooth, there was no difference in incisor displacement between subjects. Conclusions: This is the first study in which the strain distribution on a clear aligner has been studied in-vivo with DIC. Initial force delivered by clear aligners can be detected and evaluated intraorally. The difference in surface deformation between environments is only significant at the point of greatest tooth displacement, with increased transverse strain in-vitro. Material type influences clear aligner strain, with a more pronounced effect in-vitro. Clear aligners distribute strain to multiple teeth despite efforts to isolate movement to a single target tooth. And the presence of artificial saliva and influence of gender do not affect strain level or surface deformation.