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

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.