The mechanical properties of dental thermoplastic materials in a simulated intraoral environment

The present study assessed the mechanical properties of dental thermoplastic materials in a simulated intraoral environment. Eight dental thermoplastic products - EVA (Bioplast), PE (Copyplast), PETG (Duran), PP (Hardcast), PC (Imprelon"S"), A+ (Essix A+), C+ ( Essix C+), and PUR (Invisalign) - were examined using the following tests: ( 1) 2-week water absorption test, (2) thickness-change tests with thermoforming and water absorption, (3) tensile tests under room temperature (23 degrees C) and in a simulated intraoral environment (37 degrees C). The results were as follows: (1) water absorption increased with time and ranking was (from highest to lowest): PUR, PETG, A+, PC, EVA, PP, C+, and PE. (2) The changes in thickness ranged from 74.9 to 92.6% in comparison with the specimens before thermoforming. Linear expansion with water absorption ranged from 100.3 to 119.9%. (3) The elastic moduli of PC, PETG, and A+ in the simulated intraoral environment showed significant increases in original sheets; in contrast PP, C+, PE, and EVA were significantly reduced. No significant changes were observed in PUR. Tensile yield stress of the specimens in the simulated intraoral environment decreased in comparison with original sheets. The present result suggests that the mechanical properties of dental thermoplastic materials varied due to environmental factors. Moreover, behavior change is influenced by molecular structure and orientation. The application of thermoplastic materials for orthodontic tooth movement requires a sufficient understanding of the material characteristics, optimal material selection, and design. (C) 2006 Elsevier Ltd and the Japanese Orthodontic Society. All rights reserved.