OAs have gained popularity among patients seeking aesthetic alternatives to conventional orthodontic appliances [34, 35]. In addition they facilitate oral hygiene [3] and offers greater comfort and freedom to patients during their daily activities [25, 36]. In the present study, with regard to information on the time of use of aligners, 55% of the participants reported using OAs ≥ 20 h/day. Regarding oral hygiene, 55% of the participants performed between two and three brushings; however, 45% brushed more than three times a day. From these data, it is possible to perceive that the participants were collaborators in relation to use and hygiene.
However, evaluating the diet, less than half (45%) of the participants followed the recommendation to remove the aligners to drink liquids, whereas the majority (55%) retained OAs. In vitro studies have shown that improper use of OAs during the feeding can lead to staining because the color stability of thermoplastic materials can be negatively influenced by the dyes in the drinks. Moreover, alteration of the polymer used to fabricate the OA can affect its transparency and compromise its main advantage [11–13, 19, 27, 29].
In the present study, ΔE* values ranged from 2.34 to 3.70. Of the six times evaluated, acceptable results (ΔE* < 3.3) were obtained in five, and at only one time, a color change visibly perceptible to laypeople was observed (ΔE* > 3.3) [37, 38]. In addition to the assessment of the color change using CIELAB, the ΔE* data were converted to the NBS equivalents to verify whether the observed color difference was clinically relevant [27, 31]. According to the NBS parameters, values ranging from 2.15 to 3.40 were obtained. Of the six times evaluated, a perceptible color change was observed at 5 (NBS = 1.5 to 3), and at 1 time point, a marked change in color was observed (NBS = 3 to 6) [27, 31]. Thus, the results of the present study indicated a perceptible but no exaggerated color change according to the two indices at most of the evaluated time points (ΔE* values ranged from 2.34 to 3.70, and NBS ranging from 2.15 to 3.40).
In contrast, in the in vitro study by Liu et al. [19], ΔE* values ranged from 0.30 to 27.81. This high variability between this laboratory study and the present clinical study may be due to the fact that in that study OAs were left in dye solutions for ≥ 12 hours, which differs from the patients' daily routine conditions, as observed in the present clinical study [19]. However, some patients may find even low levels of color variation in OAs unacceptable, as one of the main reasons for their choice of treatment is aesthetics [13, 27]. Thus, patients who are more demanding in this regard need to be aware of the possibility of a perceptible change in OA color during clinical use, and that this change is strongly associated with the characteristics of their diet, hygiene habits, and the removal of OAs during meals.
Regarding the change in color of OAs, most of the evaluated patients reported using OAs during fluid intake, which contributed to the observed color change. However, according to a previous study, ≥ 50% of American adults drink coffee every day, with an average consumption of 3.2 cups per day [39]. Thus, exposure of OA to coloring agents in the oral cavity is inevitable [40]. However, in the present study, this color variation was acceptable.
The data obtained through the questionnaire revealed that even after educating participants not to use OAs during food intake, 25% of them reported eating with the OAs during the evaluation period. Previous in vitro studies suggest that retaining OAs during meals causes a statistically significant difference in their color, and guidance regarding removal must be respected so as not to modify their transparency [12, 13, 19, 27].
In addition to the fact that patients retain OAs during food intake, the type of diet also influences the color change in the device. In the present study, patients who ingested acidic foods such as coffee, wine, soda, and juice showed a significant change in OA color. Furthermore, changes in the surface roughness of OAs were observed. The data showed that the greater the color change, the greater the roughness of the aligners. That is, the patient's diet not only influenced the color change, but also contributed to an increase in OA roughness [28]. This alteration is due to the characteristic of PU, a thermoplastic material used for manufacturing of OAs used in this study [19, 31, 41, 42].
PU is a thermoplastic polymer with excellent physical properties, and was the first material used for OAs. However, it is not an inert material and is affected by heat, humidity, and prolonged contact with salivary enzymes [42]. The surface of the PU-based materials tends to be rougher and may exhibit flaking, which favors porosity formation, increases surface roughness, and contributes to the accumulation of pigments [18, 19]. Surface porosity may result from material degradation when OAs are placed in the oral cavity [31, 41, 42]. This is because roughness increases due to microcracks and abrasions from speech, mastication, swallowing, and bruxism. This is turn can increase pigmentation and lead to a loss of OA transparency [25, 27].
Another factor related to the change in color and roughness is the accumulation of bacterial plaque and biofilms, which can be observed macroscopically as the formation of a calcified layer on the inner surface of the OA, impairing another advantage of OAs, hygiene [18, 28 12, 43]. Hygiene is also associated with the recommendations for OA use, as keeping the device in the mouth during meals can not only impair hygiene, but also modify the pH of saliva, especially with acidic and sugar-rich diets. Papadopoulo et al. [28] reported that the curved inner surface of OA can retain saliva and, if associated with poor oral hygiene, can have detrimental effects such as plaque retention, causing not only discoloration of the appliances, but also the appearance of dental problems such as sensitivity, erosion, and caries.
Wearing each OA for ≥ 2 weeks is not recommended. This is because PU tends to soften with prolonged use [28], which could alter the results of tooth movement planned at the beginning of treatment [13]. Thus, changes in the chemical and mechanical properties of OAs and their influence on the expected clinical performance should be considered in future studies.