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To investigate the mechanical and chemical alterations of Invisalign appliances after intraoral aging. 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 ( EIT ), elastic to total work ratio also known as elastic index (η IT ), Martens Hardness (HM), and indentation creep ( CIT ) The mean values of the mechanical properties were statistically analysed by unpaired t -test ( a = 0.05). 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 EIT (REF: 2466±20, INV: 2216±168MPa), HM (REF: 119±1, INV: 110±6 N mm −2 ) and higher η IT (REF: 40.0±0.3, INV: 41.5±1.2%), and CIT (REF: 3.7±0.2 INV: 4.0±0.1%). The increase in η IT indicates that INV is a more brittle than REF, whereas the increase in CIT , a decrease in creep resistance. Despite the lack of detectable chemical changes, intraoral aging adversely affected the mechanical properties of the Invisalign appliance.
Bradley et al. (Tue,) studied this question.