Comparison of physical, mechanical, and optical properties between thermoplastic materials and 3-dimensional printing resins for orthodontic clear retainers

Abstract

Introduction: This study investigated the physical, mechanical, and optical properties of 3-dimensional (3D) printing resins compared with thermoplastic materials to evaluate their suitability for the fabrication of orthodontic clear retainers. Methods: Samples were prepared from thermoplastic sheets (Duran [Scheu-Dental GmbH, Iserlohn, Germany] and Zendura [Bay Materials LLC, Fremont, Calif]) and biocompatible 3D-printing resins (Dental LT Clear V2 [Formlabs Inc, Somerville, Mass] and OrthoFlex [Nextdent BV, Soesterberg, The Netherlands]) according to the manufacturer's instructions. The materials were characterized by Fourier transform infrared spectroscopy, differential scanning calorimetry, and water sorption tests. Mechanical properties were assessed by tensile tests and hardness under 3 different conditions: dry, wet (24-hour water immersion), and aged (thermocyled for 10,000 cycles). Surface characteristics were qualitatively and quantitatively evaluated by scanning electron microscopy and 3D confocal imaging, respectively. Optical properties were assessed by ultraviolet-visible spectroscopy and color stability tests by immersion into various staining solutions. Results: The mechanical properties of the 3D-printing resins were more markedly altered in different testing conditions (dry, wet, and aged) than in thermoplastic materials. The surface roughness, transparency, and color stability of 3D-printing resins are significantly inferior, especially NextDent OrthoFlex. Conclusions: The evaluated 3D-printing resins are more brittle and less ductile compared with the thermoplastic materials. The 3D-printing resins also do not meet the clinical thresholds of surface roughness and optical properties for the fabrication of orthodontic clear retainers. Further postprocessing of the 3D-printing resins may be required to improve these properties.