Date of Award
Master of Science (MS)
Background/Objectives: The purpose of this study was to investigate the thermal properties, particularly glass transition temperature, of the polymers that are used to fabricate three different types of modern orthodontic aligners. Invisalign, (Align Technology, Inc, Santa Clara, CA, USA), Simpli5 (Allesee Orthodontic Appliances, Sturtevant, WI, USA), and ClearCorrect (ClearCorrect, Round Rock, TX, USA) were examined both as-received and after clinical use to determine if any differences were present both between and within aligners. Materials/Methods: Orthodontic aligners were collected from three different patients using the systems under investigation after two weeks of intraoral use. Duplicate, un-used samples were obtained from the manufacturers for direct comparison. The aligners were then sectioned into sizes that were compatible with the instrumentation being used to analyze them. Differential Scanning Calorimetry (DSC) was used to individually analyze the thermal properties of each sample. The resulting thermograms were then compared to investigate potential differences between brands and conditions. Of particular interest was the temperature at which each polymer went through the glass transition phase. Enthalpy relaxation, recrystallization temperature, and melting point were also analyzed. Results: There was no statistical difference in glass transition temperature between as-received and after use Invisalign, ClearCorrect, or Simpli5 aligners (p>0.05). In addition, there was no significant difference in recrystallization peak and recrystallization enthalpy between as-received and after use Simpli5 aligners (p>0.05). There was a significant decrease in melting peak and melting enthalpy between as-received and after use Simpli5 aligners (p<0.05). A lack of recrystallization and melting peaks indicates that Invisalign and ClearCorrect are a thermoset material while the presence of these peaks indicates that Simpli5 is thermoplastic. All materials possessed a glass transition temperature above the maximum temperature that is found intraorally. Conclusions: Glass transition temperature did not significantly change after clinical use in the tested orthodontic aligners, indicating the stability of this property throughout normal treatment. All three types have a glass transition temperature above the maximum temperature that is found intraorally, which has been shown to be a benefit to an aligner’s mechanical properties. Melting peak and melting enthalpy showed a small decrease after use in Simpli5, indicating some structural aging intraorally.