Date of Award

Summer 2021

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Dentistry

First Advisor

Nimeri, Ghada

Second Advisor

Berzins, David

Third Advisor

Nimeri, Ghada

Abstract

Objective: There are several different thermoplastic retainer materials and cleaning methods available to fabricate and clean clear retainers. Some studies have investigated the changes in properties of clear retainers after being exposed to different staining agents, varying levels of mechanical strain, and different cleaning agents. However, there are few studies that compare different thermoplastic materials to each other when being subjected to these different types of stresses. The aim of this study is to evaluate the changes in properties of two commonly used thermoplastic retainer materials, polyethylene terephthalate (PETG) and ethylene/propylene (EP), after exposure to different retainer cleaning agents over 28 cleaning cycles. Methods: Samples were prepared by thermoforming sheets of Clear Splint Biocryl (PETG) and Invisacryl C (EP) with a Biostar, then cut into flat rectangular pieces. These samples were tested for surface roughness, color change, and flexural modulus after they were cleaned with four different types of retainer cleaning materials. Measurements were taken at three different time points for surface roughness and color change, while the flexural modulus was measured at the end of the experiment. Statistical analysis was completed with analysis of variation (ANOVA), t-tests, and post-hoc analysis. P values less than 0.05 was considered statistically significant. Results: The color change of both materials increased over time. Most of the color change for Invisacryl occurred between baseline and cleaning cycle 16. Listerine affected the color change of Biocryl the most, whereas there was no difference in cleaning method for the Invisacryl group. There was no significant difference in surface roughness for either material when cleaned with any of the cleaning methods at any time point. However, Invisacryl had a significantly higher surface roughness than Biocryl. Finally, there was no difference in elastic modulus between any of the cleaning methods for each material. There was a significant difference between the two materials, with Biocryl being the stiffer material than Invisacryl. Conclusion: Any of the cleaning methods evaluated in this study are recommended for use on PETG and EP materials, except for Listerine due to its ability to significantly impact a color change on Biocryl. When comparing the two retainer materials, Invisacryl C retainers are not recommended for patients with poor hygiene due to their increased surface roughness. On the other hand, Biocryl is recommended to patients with severe pre-treatment rotations, spaces, and intruded teeth due to its increased elastic modulus.

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