Comparative Analysis of Clinically Relevant Mechanical Properties of Frequently used Thermoformed Materials for the Fabrication of Orthodontic Clear Aligners: A Literature Review

Clear aligners offer advantages that make them attractive to patients; however, they have not achieved the same efficacy as conventional orthodontic treatment. These aligners are traditionally fabricated using thermoformed polymeric materials, including polyethylene terephthalate glycol (PETG) and polyurethane (PU). Understanding the mechanical properties of these materials- specifically elastic limit, elastic modulus, and stress relaxation is clinically relevant for predicting their behavior and performance during orthodontic treatment. To compare the mechanical properties (elastic limit, elastic modulus, and stress relaxation) of the most commonly used thermoformed materials in orthodontics according to current literature. A comprehensive literature search was conducted in PubMed, Scopus, Embase, and Web of Science databases for articles published between 2019 and October 2024 that reported mechanical properties of PETG and PU materials used in orthodontic applications. Ten articles were selected for inclusion in this literature review. Most studies focused on PETG material and employed in vitro methodologies. Young's modulus was the most frequently analyzed parameter, followed by elastic limit (yield strength), while stress relaxation was evaluated in fewer studies. The majority of investigations demonstrated decreased mechanical properties under thermoforming, humidity, and clinical usage conditions; however, some studies reported contradictory findings. Based on current evidence, it is not possible to establish the superiority of one material over another regarding mechanical properties. The diversity of methodologies used for measuring mechanical properties limits comprehensive understanding of these materials' behavior. Development of standardized testing protocols is recommended for future research.