Effects of Sodium Hypochlorite and Hydrochloric Acid on Hardness and Surface Roughness of Orthodontic Thermoplastic Retainers

Abstract

Background and Aim: No consensus has been reached on a safe method for disinfection of orthodontic retainers without altering their physical properties. This study investigated the effects of 5.25 sodium hypochlorite (NaOCl) and 10 hydrochloric acid on hardness and surface roughness of thermoplastic orthodontic retainers. Materials and Methods: In this in vitro study, 40 samples measuring 10 x 8 x 2 mm were prepared using 1-mm-thick polyethylene terephthalate glycol (PETG) Essix sheets, and randomly divided into 4 groups (n=10). In group A, the samples were immersed in 5.25 NaOCl for 15 minutes followed by immersion in 10 hydrochloric acid for 15 minutes. This process was repeated twice in group B and thrice in group C. Group D served as the control group. Surface roughness was measured by a profilometer, and hardness was measured by a Vickers hardness tester. Data were analyzed by the Kruskal-Wallis, Mann-Whitney, and Bonferroni tests (alpha=0.05). Results: Immersion frequency had no significant effect on surface roughness (P=0.096). A significant difference was found in hardness among the groups (P=0.008). The mean hardness of group B was significantly lower than that of group C (P=0.004). The three test groups had no significant difference with the control group in this regard (P>0.05). Conclusion: According to the results, 15 minutes of immersion in 5.25 NaOCl followed by 15 minutes of immersion in hydrochloric acid for 3 times had no significant clinical effect on the hardness and surface roughness of PETG sheets used for the fabrication of thermoplastic orthodontic retainers. © 2025 Faculty of Dentistry, Tehran Medical Sciences, Islamic Azad University.