Color stability of resin composites for orthodontic attachments: an in vitro study

ABSTRACT Objective: The aim of the present study was to evaluate the color stability of Filtek Z350 XT, Filtek Z250 XT, Z100 resin composites and Transbond XT orthodontic resin, all used in orthodontic attachments, when immersed in popular beverages. Methods: Thirty disk-shaped specimens of each resin composite (2 x 5mm) were manufactured and randomly divided into six groups according to immersion solutions: coffee, red wine, white wine, regular beer, dark beer and deionized water (control). The specimens were fully immersed in each of the solutions for six days at 37°C, representing approximately six months of consumption. The color measurements were evaluated by a reflection spectrophotometer, at baseline (before immersion) and after staining. L*a*b* coordinates were measured and the color change (ΔE00) was calculated using the CIEDE2000 formula. The data were analyzed by ANOVA/Tukey tests at a significance level of 0.05. Results: The resin composites immersed in white wine and regular beer showed either imperceptible or clinically acceptable ΔE00, and no difference from the control group (p= 0.4449 and p= 0.467 respectively). Immersion in coffee and red wine were considered clinically unacceptable and were significantly different from the control group (p= 0.0028 and p= 0.0475 respectively). Conclusion: Based on the results of the present study, the consumption of coffee and red wine may cause color change of the resin composite attachments above the visual acceptability threshold, and impair aesthetics during treatment.


INTRODUCTION
The quest for improved aesthetics during orthodontic treatment has increased the number of treatments performed with clear aligners. This technology emerged the late 20 th century as an alternative to conventional orthodontic treatment, and has been widely used worldwide, especially in the last ten years. In addition to aesthetics, their main advantages are removal during eating and cleaning of teeth, this way, clear aligners are better for periodontal health than fixed appliances. 1 Furthermore, patients treated with clear aligners appear to feel lower levels of pain during the first days of treatment. 2 Attachments are one of the components often required to increase the predictability and efficiency of tooth movement during treatment with clear aligners. 3 These are additions of resin composite bonded to the tooth enamel, to create an anchor point for the aligner. Their design is predefined during the planning phase, and must not be modified after bonding.
In this case, the resin composite should not undergo the traditional polishing process. 4 Resin composites are porous materials that may absorb coloring agents from food. 5  Patients who seek treatment with clear aligners have greater aesthetic demands; 7 thus, the staining of attachments could lead to dissatisfaction during orthodontic correction. Orthodontic attachments should not be polished, and consequently, may be more susceptible to staining. Therefore, the objective of this study was to determine the color stability of Filtek Z350 XT (color: A1E), Filtek Z250 XT (color: A1), and Z100 (color: A1) resin composites, and Transbond XT orthodontic resin (single color), used for orthodontic attachments, when fully immersed in coffee, red wine, white wine, regular beer, dark beer, compared to deionized water (control). These beverages were selected because they are commonly found in patients' diets. The null hypothesis was that the tested resin composites would not present visually unacceptable staining after immersion in different beverages.

MATERIAL AND METHODS
A total of 120 disk-shaped specimens 5-mm in diameter and 2-mm thick were made using three restorative resin composites and an orthodontic resin composite, as shown in Table 1.
The specimens were manufactured with a metallic matrix in were washed with abundant water and dried with absorbent paper, to avoid dehydration, then the final color measurement of the resin composites was performed, using the same spectrophotometer and parameters.  Table 1: Material specifications, manufacturers' information, color, particle size, composition and batch number. The CIEDE2000 color-difference formula is based on the CIE L * a * b * color space. 9,10 The color change (ΔE 00 ) between baseline and after six days of immersion in the beverages was calculated using the CIEDE2000 formula:

Resin
where: ΔL', ΔC' and ΔH' refer to lightness, chroma, and hue differences between color measurements, respectively. K L , K C and K H are the parametric factors for the conditions and illumination influence. R T (rotation function) is responsible for the interaction of hue and chroma differences in the blue region.
S L , S C and S H are the weighting functions for the color difference adjustment, considering the location variation of the L*a*b* coordinates. 9,10 The visual acceptability and perceptibility thresholds described

RESULTS
The color change averages (ΔE 00 ) and the standard deviations of all the specimens are shown in  Transbond XT resin group presented the lowest ΔE 00 among the resin composites when immersed in red wine and coffee, and was statistically significant (p = 0.0475, p = 0.0028, respectively).
Filtek Z250 XT and Transbond XT resin groups (p = 0.0022) presented the lowest ΔE 00 when immersed in black beer. No statistical differences were found among the four different resins among the other beverages. Although the specimens did not present coloring particles, this color change may be related to sorption and hydrolysis, which are characteristics of the monomers used in the resin composites. 17 The pigmentation ability of the resin composite is directly connected to its hydrophilic properties. The more hydrophilic the resin, the greater its ability to absorb not only water, but also water-soluble coloring agents 18 . Water may conduct the staining agent towards the material, resulting in more intense staining of resin composites. 19 Clear aligner technology is increasingly specialized and the attachments should be added to the tooth surface not only to enhance aligners retention, but also to promote difficult tooth movements. 20 The forces and movements required for maloc- The ability of resin composites used in attachments to prevent shape and surface alteration during six months of treatment was evaluated by Barreda et al. 4 , demonstrating that different resin composites could affect the surface, but not the shape of the attachments. In the presen study, Filtek Z350 XT resin composite revealed less surface wear, in agreement with Feinberg et al. 22 Other study demonstrated that the use of resin composites with different viscosities does not influence the shape and volume of attachments. 23 According to a recent systematic review, treatment with clear aligners is still deficient in controlling anterior extrusion, anterior buccolingual inclination, and is not effective in controlling rotation of rounded teeth, in particular. 24 However, the attachments are continually being modified to increase the range of tooth movements that can be achieved, 25 therefore, the importance of studying the resin composites used for this purpose.
As a strength of this study, the evaluation of the color change was conducted with CIEDE2000, since it has shown better adjustment potential, compared with the previous method, CIE L*a*b*. These two methods showed a statistically significant difference in measuring the perceptibility and the acceptability of the color of restorative materials. Additionally, the results of the present study addressed the evaluation of Transbond XT, an orthodontic resin, available in the majority of orthodontic offices, which presented reduced staining when compared to the other resin composites.
Furthermore, color stability and other factors, such as physical