Effects of intraoral ageing on the mechanical and surface properties of clear aligner materials: A systematic review.

OBJECTIVE: The aim of this study was to systematically investigate the changes in the material properties and surface morphology of different aligner materials following intraoral use. MATERIAL- AND METHODS: A search was conducted on five databases and was based on PRISMA guidelines. Only clinical retrieval studies involving ex vivo analysis of aligners following intraoral use, published in English, were included. Risk of bias assessment was done using the modified Cochrane Risk of Bias Tool tailored for this systematic review, which had seven domains. RESULTS: This review included fourteen studies. Risk of bias assessment revealed four studies showed a moderate risk and ten studies showed a high risk. Among the selected articles, eight studied Invisalign® appliances, five studied thermoformed aligners, six studied directly printed aligners, and one studied ClearCorrect® aligners. Although statistical significance varied across studies, all the materials showed material degradation with intraoral use, with directly printed aligners exhibiting an increased relaxation index. Mixed results were obtained in surface characteristics with surface irregularities after use. Compositional analysis indicated hydrolysis of PETG aligners. CONCLUSION: The intraoral exposure contributed to the deterioration of the mechanical properties, dimensional stability, and surface characteristics of all the aligner systems. But the extent of these alterations varied based on the manufacturing process and material composition. None of the aligner systems could be defined as ideal, as their clinical performance is determined by fabrication procedure, intrinsic material characteristics, and intraoral factors. And thus, clinical findings should be interpreted with caution, and further high-quality standardised research is essential to obtain clinical significance.