Do the mechanical and chemical properties of Invisalign™ appliances change after use? A retrieval analysis

Aim: To investigate the mechanical and chemical alterations of Invisalign appliances after intraoral aging. Materials and methods: Samples of Invisalign appliances (Align Technology, San Jose, California, USA) were collected following routine treatment for a mean period of 44 +/- 15 days (group INV), whereas unused aligners of the same brand were used as reference (group REF). A small sample from the central incisors region was cut from each appliance and the buccal surface was analysed by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy (n = 5). Then the appliances were cut (n = 25) and embedded in acrylic resin, ground/polished in a grinding polishing machine, and the prepared surfaces were subjected to Instrumented Indentation Testing under 4.9 N load. Force-indentation depth curves were recorded for each group and the following parameters were calculated according to ISO 14577-1; 2002 specification: indentation modulus (E-IT), elastic to total work ratio also known as elastic index (eta(IT)), Martens Hardness (HM), and indentation creep (C-IT) The mean values of the mechanical properties were statistically analysed by unpaired t-test (a = 0.05). Results: ATR-FTIR analysis confirmed the urethane based structure of the appliances, without important chemical differences attributed to the aging process. INV group showed significantly lower E-IT (REF: 2466 +/- 20, INV: 2216 +/- 168 MPa), HM (REF: 119 +/- 1, INV: 110 +/- 6 N mm(-2)) and higher eta(IT) (REF: 40.0 +/- 0.3, INV: 41.5 +/- 1.2%), and C-IT (REF: 3.7 +/- 0.2 INV: 4.0 +/- 0.1%). The increase in eta(IT) indicates that INV is a more brittle than REF, whereas the increase in C-IT, a decrease in creep resistance. Conclusion: Despite the lack of detectable chemical changes, intraoral aging adversely affected the mechanical properties of the Invisalign appliance.