Comparison of mechanical properties of 3-dimensional printed and thermoformed orthodontic aligners

Introduction: Orthodontic treatment using clear aligners has experienced exponential growth since its introduc-tion in the late 1990s. Three-dimensional (3D) printing has also grown in popularity among orthodontists, and companies have begun to produce resins to directly print clear aligners. This study aimed to examine the mechanical properties of commercially available thermoformed aligners and direct 3D-printed aligners under laboratory conditions and a simulated oral environment. Methods: Samples were prepared (approximately 2.5 x 20 mm) from 2 thermoformed materials, EX30 and LD30 (Align Technology Inc, San Jose, Calif), as well as 2 direct 3D-printing resins, Material X (Envisiontec, Inc; Dearborn, Mich) and OD-Clear TF (3DResyns, Barcelona, Spain). Wet samples were subjected to phosphate-buffered saline at 37 degrees C for 7 days, whereas dry samples were stored at 25 degrees C. Tensile and stress relaxation tests were carried out on an RSA3 Dynamic Mechanical Analyzer (Texas Instruments, Dallas, Tex) and Instron Universal Testing System (Instron, Norwood, Mass) to calculate elastic modulus, ultimate tensile strength, and stress relaxation. Results: The elastic modulus of dry and wet samples was 103.2 +/- 17.3 MPa and 114.4 +/- 17.9 MPa (EX30), 61.3 +/- 9.18 MPa and 103.5 +/- 11.4 MPa (LD30), 431.2 +/- 16.0 MPa and 139.9 +/- 34.6 MPa (Material X), and 38.4 +/- 14.7 MPa and 38.3 +/- 8.4 MPa (OD-Clear TF), respectively. The ultimate tensile strength of dry and wet samples was 64.41 +/- 7.25 MPa and 61.43 +/- 7.41MPa (EX30), 40.04 +/- 5.00 MPa and 30.09 +/- 1.50 MPa (LD30), 28.11 +/- 3.75 MPa and 27.57 +/- 4.09 MPa (Material X), and 9.34 +/- 1.96 MPa and 8.27 +/- 0.93 MPa (OD-Clear TF), respectively. Residual stress of wet samples at 2% strain for 2 hours was 59.99 +/- 3.02% (EX30), 52.57 +/- 12.28% (LD30), 6.98 +/- 2.64% (Material X), and 4.39 +/- 0.84% (OD-Clear TF). Conclusions: There was a significant difference in elastic modulus, ultimate tensile strength, and stress relax-ation among the samples tested. Moisture, specifically a simulated oral environment, appears to have a greater effect on the mechanical properties of direct 3D-printed aligners when compared with thermoformed aligners. This is likely to impact the ability of 3D-printed aligners to generate and maintain adequate force levels for tooth movement. (Am J Orthod Dentofacial Orthop 2023;163:720-8)