# Do the mechanical and chemical properties of Invisalign$^{TM}$ appliances change after use? A retrieval analysis

Gerard Bradley, T; Teske, Lauren; Eliades, George; Zinelis, Spiros; Eliades, Theodore (2016). Do the mechanical and chemical properties of Invisalign$^{TM}$ appliances change after use? A retrieval analysis. European Journal of Orthodontics, 38(1):27-31.

## Abstract

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 (ηIT), 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±168MPa), HM (REF: 119±1, INV: 110±6 N mm(-2)) and higher ηIT (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 ηIT 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.

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 (ηIT), 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±168MPa), HM (REF: 119±1, INV: 110±6 N mm(-2)) and higher ηIT (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 ηIT 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.

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Item Type: Journal Article, refereed, original work 04 Faculty of Medicine > Center for Dental Medicine > Clinic for Orthodontics and Pediatric Dentistry 610 Medicine & health English 3 March 2016 13 Jan 2016 10:54 05 Apr 2016 19:51 Oxford University Press 0141-5387 This is a pre-copy-editing, author-produced PDF of an article accepted for publication in The European Journal of Orthodontics following peer review. The definitive publisher-authenticated version European Journal of Orthodontics, 2016, 27–31 doi:10.1093/ejo/cjv003 is available online at: http://doi.org/10.1093/ejo/cjv003 Publisher DOI. An embargo period may apply. https://doi.org/10.1093/ejo/cjv003 25740599
Permanent URL: https://doi.org/10.5167/uzh-118550