Thermoplastic Materials for Clear Orthodontic Aligners: Mechanical Properties, Intraoral Aging, and Clinical Implications

Aim This narrative literature review aimed to evaluate thermoplastic materials used in orthodontic aligners and their influence on mechanical properties and clinical performance. Materials and Methods A literature search was conducted in PubMed and Google Scholar between November 1, 2023, and January 31, 2024, including studies published from 2015 to 2023. The search combined keywords related to “aligners,” “plastic,” and “mechanical properties.” Study selection, eligibility assessment, and data extraction were performed independently by two reviewers. Supplemental materials include the full search strategy and a structured data extraction table. Results From 197 identified records, 22 studies were included. The primary materials analyzed were polyethylene terephthalate glycol, thermoplastic polyurethane, and polycarbonate. Variations in material composition, thickness, thermoforming processes, and intraoral aging significantly influenced mechanical behavior, including stress relaxation, force delivery, and dimensional stability. Conclusions Intraoral aging, material properties, and clinical use patterns significantly affect aligner performance. Materials with improved resistance to stress relaxation and degradation may enhance treatment predictability. However, due to limited high-quality clinical evidence, further studies are required to establish definitive clinical guidelines.