Effect of in vitro aging by water immersion and thermocycling on the mechanical properties of PETG aligner material

  • Benjamin A. Ihssen
  • Jan H. WillmannEmail author
  • Amr Nimer
  • Dieter Drescher
Original Article



The mechanical properties of orthodontic aligners made from thermoplastic polymers decrease over time in the intraoral milieu. However, there is a lack of information on this topic in the literature. Thus, the elastic properties of polyethylene terephthalate glycol (PETG) aligner films were investigated in vitro under extreme temperature changes simulated by thermocycling, environmental temperature and water absorption.

Materials and methods

A total of 60 specimens made from PETG aligner films (CA Clear Aligner, Scheu Dental, Iserlohn, Germany) were divided into three groups (immersed in distilled water, subjected to accelerated ageing by thermocycling, control). These groups were again divided and tensile testing was performed for all groups at 22 and at 37 °C. Young’s modulus (E), 0.2% offset yield strength (Rp02) and ultimate tensile strength (UTS) were evaluated. Water absorption was determined using an analytical scale.


All treated specimens showed water absorption, whereby specimens that were thermocycled absorbed 48% more water than the immersed ones. Young’s modulus and UTS were significantly lower for all three groups at 37 °C compared to the corresponding groups tested at 22 °C. Thermocycled and immersed groups showed a significantly lower Young’s modulus compared to the control group tested at the same temperature. The mean Rp02 was statistically different when comparing the control group tested at 22 °C to the one tested at 37 °C.


The results of this study add to the understanding of the clinically well-known degradation of orthodontic aligners during wear time. Extreme alternating temperatures along with warming up to intraoral temperature and water absorption can reduce the material’s Young’s modulus and may therefore promote a decrease of resulting orthodontic forces.


Orthodontic tooth movement Elastic modulus Artificial ageing Tensile strength Mechanical phenomena 

Der Effekt von In-vitro-Alterung durch Thermozyklierung und Wasserimmersion auf die mechanischen Eigenschaften von PETG-Alignermaterial



Die mechanischen Eigenschaften von kieferorthopädischen Alignern aus thermoplastischen Polymeren unterliegen einer zeitabhängigen Degradation im intraoralen Milieu. Diesbezüglich bietet die aktuelle Literatur nur unzureichende und kontroverse Informationen. Insbesondere betrifft dies die Frage, inwieweit die Materialdegradation von extremen Temperaturwechseln und Wasseraufnahme abhängt. Die elastischen Eigenschaften von Polyethyleneterephthalate-Glycol(PETG)-Alignerfolien wurde in einem In-vitro-Setup untersucht, unter dem Einfluss von extremen intraoralen Temperaturwechseln, simuliert durch Thermozyklierung, Umgebungstemperatur und Wasserimmersion.

Material und Methoden

Insgesamt 60 Probekörper aus PETG-Alignerfolie (CA Clear Aligner, Scheu Dental, Iserlohn, Deutschland) wurden in 3 Gruppen geteilt (Immersion in destilliertem Wasser, beschleunigte Alterung durch Thermozyklieren, Kontrolle). Diese Gruppen wurden wieder unterteilt und alle Gruppen wurden Zugtests unterzogen, bei 22 und bei 37 °C. Für jede Gruppe wurden der Elastizitätsmodul, die 0,2 %-Dehngrenze sowie die nominelle Zugfestigkeit ermittelt. Die Wasserabsorption wurde mittels Analysewaage ermittelt.


Während alle behandelten Gruppen eine Wasseraufnahme zeigten, nahmen die thermozyklierten Probekörper im Mittel 48 % mehr Wasser auf als die in Wasser eingelegten. Alle 3 bei 37 °C getesteten Gruppen zeigten signifikant kleinere Elastizitätsmoduli und nominelle Zugfestigkeiten als ihre korrespondierenden bei 22 °C getesteten Gruppen. Die thermozyklierten und Immersionsgruppen hatten statistisch signifikant geringere Elastizitätsmoduli als die Kontrollgruppen, die bei gleicher Temperatur getestet wurden. Statistisch signifikant unterschiedlich war die mittlere 0,2 %-Dehngrenze nur beim Vergleich der bei 22 °C getesteten Kontrollgruppe mit der bei 37 °C getesteten Kontrollgruppe.


Die Ergebnisse dieser Studie tragen zum Verständnis der klinisch wohlbekannten Degradation von kieferorthopädischen Alignern während ihrer Tragezeit bei. Extreme wechselnde Temperaturen gemeinsam mit der Erwärmung auf Mundhöhlentemperatur und Wasserabsorption können den Elastizitätsmodul von PETG-Alignermaterial erniedrigen und deshalb einen Abfall orthodontischer Kräfte begünstigen.


Kieferorthopädische Zahnbewegung Elastizitätsmodul Künstliche Alterung Zugfestigkeit Mechanische Phänomene 



This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors. Aligner raw material for specimen fabrication was provided by Scheu Dental, Iserlohn, Germany.

Compliance with ethical guidelines

Conflict of interest

B.A. Ihssen, J.H. Willmann, A. Nimer and D. Drescher declare that they have no competing interests.

Ethical standards

This article does not contain any studies with human participants or animals by any of the authors.


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Copyright information

© Springer Medizin Verlag GmbH, ein Teil von Springer Nature 2019

Authors and Affiliations

  1. 1.Department of OrthodonticsUniversity of DuesseldorfDuesseldorfGermany
  2. 2.Division of SurgeryImperial College LondonLondonUK

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