Effect of Ph on the Mechanical Properties of Thermoplastic Polyurethanes Used to Manufacture Dental Aligners. In Vitro Study

Abstract

The objective of this study was to evaluate in vitro the effect of pH on the mechanical properties of three different acetates brands used to manufacture dental aligners. MATERIALS AND METHODS: 27 specimens of thermoformed acetates of the brands Zendura A., Dentsply Sirona and Valdi were incubated in artificial saliva at 37°C in a Shel-Lab 1510E incubator and immersed twice a day for 5 minutes in beverages with different pH levels to measure its elastic limit on days 0, 7 and 14. STATISTICAL PROCESS CONTROL: Data analysis was performed using the KolmorogovSmirnov and Levene tests, and later with analysis of variance (ANOVA), multiple comparisons post -hoc with the Sheffe test and the Friedman statistical test for paired samples; all with a significance level of 0.05 using the IBM SPSS Statistics 26 statistical package. RESULTS: The elastic limit measurements on days 7 and 14 indicate significant differences between the different pHs and are not consistent between the three brands of acetates. The Zendura A brand at neutral pH increased and then recovered to 99%, at acidic pH it decreased to 85% then 64%, and at alkaline pH it increased more than half, then returned to 105% of baseline. The Dentsply Sirona brand in neutral pH had a decrease and then recovered to 96%, in acidic pH it increased progressively to 113% and 118%, and in alkaline pH it decreased to 69% to later recover to 84%. The Valdi brand in all groups increased and at the end was recovered, in neutral pH to 92%, in acidic pH to 87%, and in alkaline pH to 80% with respect to the initial value. CONCLUSIONS: The mechanical properties of the aligners vary and may be affected depending on the pH to which they are subjected, having variability in their elastic limit. The specimens subjected to acidic and alkaline drinks had a greater decrease in their elastic limit when compared to those subjected to neutral pH. In general, it was possible to observe that the specimens increased their elastic limit on day 7 and then approached their initial value on day 14.