Evaluation of color stability and material surface stability of different types of clear aligners under curcumin staining in vitro

Aesthetics play a crucial role in patients' choice of orthodontic clear aligners, as poor color stability can compromise their appearance during wear. This study aimed to evaluate the color and surface stability of different types of orthodontic clear aligners exposed to various concentrations of curcumin solutions in vitro. Four types of clear aligners (Invisalign, Angelalign, Spark, and Dreve), along with non-thermoformed Dreve film, were cut into 5 mm × 5 mm squares and immersed in curcumin solutions of different concentrations and a control solution. The concentrations were determined based on the curcumin content in curry. Anhydrous ethanol was used as the solvent, which may influence surface properties and color stability and partially simulating damage caused by alcohol consumption or bacterial metabolites. After immersion for 12 h and 7 days, the squares were ultrasonically cleaned, and color changes were measured using a colorimeter, then converted to National Bureau of Standards (NBS) units. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to observe surface characteristics before and after staining. After 12 h of immersion in 10 µg/ml solutions, all samples except the Dreve film exhibited at least an "Appreciable: marked change," with Angelalign showing a more pronounced color alteration. After 7 days of immersion, most materials displayed color changes ranging from "Much: extremely marked change" to "Very much: change to other colors." Across all concentrations and time points, Invisalign aligners exhibited significantly higher ΔE* values than Dreve (P < 0.05). Although SEM and AFM results were not entirely consistent, both indicated that PU-based aligners were more susceptible to surface morphology changes than PET-G-based aligners. Enhancing the aesthetic color stability of orthodontic materials could contribute to improved clinical outcomes.