Changes in surface morphology, chemical and mechanical properties of clear aligners during intraoral usage: A systematic review and meta-analysis

To assess the scientific evidence related to the alterations in surface morphology, chemical and mechanical properties of clear aligners during intraoral usage.An extensive electronic search, with no restrictions, was performed in six databases. Studies were eligible for inclusion if they involved the comparison of surface morphology, chemical and/or mechanical changes of clear aligners before and after intraoral usage. Methodological shortcomings and the quality of the studies were ranked using the Cochrane's tools (RoB2 and ROBINS-I).Four articles were included in this systematic review. Only studies investigating the changes in Smart Track (LD30) material, used for Invisalign™ products, met the inclusion criteria. Regarding changes in surface morphology and chemical properties, LD30 exhibited distortion of surface serrations and ions exchange during usage. With respect to the mechanical alterations, the pooled estimate, of data from 70 aligners, showed a significant decrease in the indentation modulus (SMD=-2.41; 95% CI(-3.39, -1.43); P<0.001) and Martens hardness (SMD=-3,38; 95% CI(-6.24, -0.52); P=0.02) after usage; reflecting a decrease in both force delivery capacity and wear resistance. The main risk of bias affecting the available studies were the confounding and selection of participant's bias.Despite Invisalign™ being the aligner system most commonly used, there is a strong need for studies investigating the in-vivo aging of other materials, in order to precisely determine the best material that could serve treatment needs. According to the current evidence, LD30 exhibited changes in surface morphology, chemical, and mechanical properties during usage, which could contribute to the differences between the predicted and final clinical results.