INTRODUCTION

Tooth bleaching, which is a noninvasive procedure, is among the most popular treatments to improve tooth color and appearance.¹ Numerous products and methods for tooth bleaching have been described in the literature.^2,3 The original concept of at-home bleaching (HB) used 10% carbamide peroxide gel in a customized tray with supervision and guidance of a dental clinician.¹ Nowadays, there are many vital tooth bleaching procedures, including HB, in-office bleaching (OB) and over-the-counter bleaching systems.^2-4 Different bleaching agents, concentrations, times of application, product format, and application have been reported.²

Bleaching agents (carbamide peroxide and hydrogen peroxide) used in tooth bleaching are able to provide perceptible color changes. At the same time, these products may produce color and surface alterations of resin-based composite (RBC) restorations.⁵ Such changes are influenced by the type and concentration of bleaching agents and bleaching time,^5-7 the type of the RBC (composition and percentage of organic and inorganic phases),^5,6 and various abrasive/erosive procedures related to diet and oral hygiene.⁸

CIELAB color space and its associated CIELAB () and CIEDE2000 (ΔE₀₀) total color difference formulas are extensively used for color research in dentistry. In this sense, whiteness variations are commonly evaluated by means of total color differences or differences among one of the three axes that describe the CIELAB color space (ΔL*: differences in lightness; Δa*: differences in red-green axis; and Δb*: differences in yellow-blue axis).^9,10

However, whiteness of a material is adequately portrayed with a whiteness index. Some whiteness indexes based on the CIE 1931 XYZ color notation system have been used in dental research: the CIE whiteness index (WIC),¹¹ the whiteness index (WI),¹² and the optimized whiteness index (WIO).¹³ The most recent published whiteness index for dentistry (WI_D) is based on CIELAB space and correlates to perception of tooth whiteness.¹⁴

According to the latest guidance on color measurements published by the International Organization for Standardization,¹⁵ color stability after aging and staining (or after bleaching procedures) should be assessed on the basis of 50:50% acceptability (AT: =2.7 and ΔE₀₀=1.8) and 50:50% perceptibility (PT: =1.2 and ΔE₀₀=0.8) thresholds.¹⁶ In this sense, if the total color difference measured before and after aging or staining is at or below PT, it represents an excellent match; if the difference is between PT and AT, it represents an acceptable match; and if the difference is above AT, it represents an unacceptable match. In addition, WI_D variations (ΔWI_D) can be assessed through comparison with 50:50% acceptability and 50:50% perceptibility thresholds for whiteness (WPT=0.61; and WAT=2.9, respectively) obtained in a cohort study with laypersons.¹⁷

Therefore, the purpose of this study was to evaluate the color stability and whiteness of two RBCs subjected to dental bleaching procedures (at-home bleaching and in-office bleaching) and artificial accelerated aging (450 KJ/m² and 900 KJ/m²), testing the hypothesis that both bleaching and aging procedures produce color changes and whiteness variations of RBCs greater than the 50:50% acceptability thresholds (AT and WAT).

METHODS AND MATERIALS

Specimen Preparation

A total of 180 disc-shaped specimens (10 mm in diameter and 1 mm thick) of shades A2 and A3 from two dental RBCs (Filtek Z250 [Z2] and Filtek Z350 XT [Z3]; Table 1) were fabricated using polytetrafluoroethylene molds. Thus, 45 specimens (n=5) were fabricated from each RBC-shade combination and divided into nine experimental treatments (Table 2).

Table 1 Description of the Materials used in this Study

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Table 2 Experimental Treatments Used in the Study

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RBCs were packed into the mold and pressed between two glass slides lined with polyester film (Mylar, DuPont, Wilmington, DE, USA). The specimens were light activated (40 seconds; 20 s/side) using a light curing unit (Radii; SDI, Bayswater, Victoria, Australia) at 1200 mW/cm². Specimens were kept in a dark environment at 37°C before testing. Combinations of experimental treatments, according to different bleaching and accelerated aging procedures, are described in Table 2.

Color Differences

Computations for the CIEDE2000 color difference (ΔE₀₀) metric were done according to the following equation^29,30:

where ΔL′, ΔC′, and ΔH′ are the differences in each parameter (lightness, chroma, and hue, respectively) for a pair of specimens using CIEDE2000. The weighting functions (S_L, S_C, and S_H) adjust the total color difference for variation in the location of the color difference pair in L*, a*, b* coordinates. The parametric factors (K_L, K_C, and K_H) are correction terms for experimental conditions. Finally, a rotation function (R_T) accounts for the interaction between chroma and hue differences in the blue region.^29,30 To calculate the CIEDE2000 color difference formula, discontinuities due to mean hue computation and hue-difference computation were taken into account.³¹

Color differences were finally evaluated through comparisons with the 50:50% color difference thresholds (PT=0.81 ΔE₀₀ units and AT=1.77 ΔE₀₀ units) for tooth-colored restorative materials, established in a prospective multicenter research project.¹⁶

Whiteness Index for Dentistry

The Whiteness Index for Dentistry (WI_D) is a CIELAB-based index with a linear formulation. WI_D values were obtained according to the following equation:¹⁴

WI_D variations (ΔWI_D) can be assessed through comparison with recently published data on acceptability and perceptibility thresholds for whiteness obtained in a cohort study with laypersons.¹⁷ The 50:50% perceptibility level was determined at 0.61 ΔWI units (WPT=0.61) while 50:50% acceptability level was determined at 2.90 ΔWI_D units (WAT=2.90).¹⁷

RESULTS

CIELAB color coordinates (L*, a* and b*) and WI_D values for Z2.2, Z2.3, Z3.2, and Z3.3, before and after bleaching and with different accelerated aging procedures are shown in Tables 3 through 6. As the time of aging increased, the mean values of L* and WI_D decreased (p≤0.05) (Tables 3 through 6).

Table 3 Mean and Standard Deviation Values of CIELAB Color Coordinates and Whiteness Index (WI_D) Followed by the Statistical Groupings for Specimens of Filtek Z250 Shade A2 (Z2.2) ^a

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Table 4 Mean and Standard Deviation Values of CIELAB Color Coordinates and Whiteness Index (WI_D) Followed by the Statistical Groupings for Specimens of Filtek Z250 Shade A3 (Z2.3) ^a

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Table 5 Mean and Standard Deviation Values of CIELAB Color Coordinates and Whiteness Index (WI_D) Followed by the Statistical Groupings for Specimens of Filtek Z350 Shade A2 (Z3.2) ^a

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Table 6 Mean and Standard Deviation Values of CIELAB Color Coordinates and Whiteness Index (WI_D) Followed by the Statistical Groupings for Specimens of Filtek Z350 Shade A3 (Z3.3) ^a

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CIEDE2000 total color difference (ΔE₀₀) values between two different bleaching treatments on the same composite/shade group for each aging protocol are shown in Figure 1. Comparing different bleaching treatments at T0, T1, and T2, all values of color difference (ΔE₀₀) were below AT (acceptable match). Color differences below PT (excellent match) were shown by the following comparisons: Z2.2 (T2), Z2.3 (T0 and T2), Z3.2 (T0 and T2), and Z3.3 (T0 and T1) (Figure 1).

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ΔE₀₀ values between two different aging procedures on the same composite/shade group for each bleaching treatment are shown in Figure 2. Considering different aging procedures, the comparison between T1 and T2 showed CIEDE color differences below AT (acceptable match) for Z2.2, Z2.3, and Z3.3 within C and OB groups (Figure 2A,B) and for Z2.3 within HB group (Figure 2C). For all other groups, mean values of ΔE₀₀ were above AT (unacceptable match) (Figure 2).

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Figures 3 and 4 show mean and standard deviation values of WI_D after different bleaching treatments and accelerated aging procedures on Z2 and Z3, respectively. Comparing different aging procedures always showed significant differences (p≤0.05) (Tables 3 through 6). All values of ΔWI_D between bleaching treatments and control group for the same aging procedure were below WAT. Only some values of ΔWI_D were below WPT (Table 7).

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Table 7 Mean and Standard Deviation Values of Differences in Whiteness Index (ΔWI_D) for Specimens of Filtek Z250 (Z2.2 = Shade A2 and Z2.3 = Shade A3) and Filtek Z350 (Z3.2 = Shade A2 and Z3.3 = Shade A3) Between each Bleaching Procedure (OB or HB) and Control Group for the Same Aging Procedure (T0, T1, and T2) ^a

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DISCUSSION

The present study was designed to respond to relevant questions regarding color changes in RBC after bleaching and aging procedures. Although the CIELAB color difference metric is the most commonly used in dentistry, it has been demonstrated that the CIELAB color space assumes equal influence or weight for all color coordinates.³² However, some studies^33,34 have suggested a discrepancy on sensitivity to the different color coordinates within the dental color space. In recent years, CIEDE2000²⁹ color difference was increasingly implemented in color research. Studies suggested that ΔE₀₀ shows a better correlation with visual perception than ,^25,32,35,36 which is the reason for using the CIEDE2000 color difference metric in the present study. Nevertheless, there is only one study on color changes after dental bleaching using this metric.⁸

The use of total color differences, ^6,7,37-39 and ΔE_00,⁸ is very popular for evaluating color changes after dental bleaching. Yet, understanding and considering color differences within the visual perceptibility and acceptability thresholds is very relevant in clinical dentistry. Thus, studies on dental color should qualify their statistical analysis of the data associating them to PT and AT values. The most popular value for acceptable color difference used to be 3.3 units.^6,7,37-39 Recently, an ISO standard¹⁵ defined PT and AT values for tooth colored restorative materials. Such values were established in a prospective multicenter study¹⁶ that used CIELAB () and CIEDE2000 (ΔE₀₀) total color difference metrics. The present study used PT and AT values reported in that work.¹⁶

Although it is common to use color difference formulas to evaluate color changes after bleaching procedures, a sole evaluation of color differences does not offer enough information on how color coordinates change. Thus, it is not adequate to compare whiteness values using only color difference metrics ( or ΔE₀₀). Although previous studies used other indexes for evaluating whiteness,^13,26,40,41 the present study used a recently published whiteness index (WI_D), which is based on CIELAB color space and was specifically designed for dentistry and dental applications.¹⁴ WI_D was compared with other whiteness indexes and showed a strong correlation to visual perception of tooth whiteness.¹⁴

The ΔWI_D values between different bleaching treatments (OB, HB, and control) for the same aging procedure (Table 7) were analyzed using values of whiteness thresholds (WPT and WAT) from a previous study¹⁷ that found different WPT and WAT values for different observer groups (WPT = 0.61 and WAT = 2.90 for laypersons and WPT = 0.44 and WAT = 2.15 for dentists). Since patients are the evaluators (observers) of dental bleaching treatments, and the influence of observer population on color difference thresholds (PT and AT) has been previously reported,¹⁶ the present study used WPT = 0.61 and WAT = 2.90.

Considering color differences and whiteness variations after bleaching of all experimental groups, the mean values of ΔE₀₀ and ΔWI_D were below their corresponding acceptability value (AT=1.77 and WAT=2.90, respectively). Therefore, despite the variations, the color changes and whiteness variations should be acceptable to observers (patients). Thus, the first part of the study hypothesis was not confirmed because bleaching procedures did not produce color changes and whiteness variations of RBCs greater than the 50:50% acceptability thresholds (AT and WAT).

The term “color stability” has been widely used in dental color research. A physical magnitude is stable when its variation along a process (in this case, bleaching or aging) is lower than the instrumental error. However, color is a psychophysical property; therefore, observer's interpretations (evaluations) should be considered. In this sense, color variations below the perceptibility threshold will not be detectable by a standard observer. Thus, color stability of a dental material, from an observer point of view, can be defined as a color variation (described by a color difference) lower than the PT. In most cases, the bleached nanocomposite showed color differences below PT values (Z3.3 after T0 and T1 and Z3.2 at time of aging) (Figure 1), suggesting this material presents appropriate color stability for the aging protocols evaluated in the present study.

Previous studies showed that bleaching treatments may affect the elution of monomers and other substances from RBCs.^42-44 The three-dimensional polymer network of RBCs, which consists of carbon-carbon (C-C)-single or (C-C)-double bonds, may react with oxidants like hydroxide peroxide, increasing unpolymerized monomers, additives and unspecific oxidative products release.^43,44 Complete polymerization of RBCs is still a challenge to overcome. Thus, the lower the conversion rate of an RBC, the more residual monomers can be eluted.⁴⁵ Both RBCs used in the present study were light activated under the same conditions (eg, activation time, light energy and distance), therefore they should not be considered as study variables. Yet, though the manufacturer did not provide the exact percentage of monomers, the three-dimensional polymer network is very similar for both RBCs. The main difference between the two RBCs is the amount of inorganic phase, which can influence the degree of conversion of an RBC. As is known, as the filler loading increases, the degree of conversion increases and the water sorption decreases. Thus, the greater color stability of the nanocomposite may be associated to its higher filler content (Table 1).

Previous studies showed that total bleaching time is more important than the concentration of the bleaching agent.^2,8 Although it was not an objective of the present study, the results did not show a significant difference between the effect of the different bleaching treatments on color of RBCs (Figure 1A).

Color stability of RBCs has been evaluated using an artificial aging chamber, exposing the specimens to ultraviolet light and elevated temperatures^20,21,46 or by immersing the RBC in various staining beverages.^47,48 The present study used an artificial aging procedure to simulate clinical service because restorations are exposed to different variables, such as temperature changes and constant humidity. This procedure reproduces, in a short period of time, the effects of long-term exposure of RBCs in an oral environment. However, as with most in vitro experiments, there are limitations to the accelerated aging process that do not consider clinical variables, such as the influence of the saliva components, pH levels, and brushing. Further, and as mentioned previously, bleaching treatments may affect the elution of monomers and other substances from RBCs, which may also affect the surface roughness of these materials and, as a consequence, changing color perception. A previous study showed changes in the value of L* coordinate with different surface roughness values.⁴⁹

After the accelerated aging process, L* and WI_D values decreased (specimens became darker), while b* values increased (specimens became more chromatic) for both composites (Table 3 through 6). In addition, a* values generally increased. In all cases color changes and whiteness variations after aging procedures, which estimated 1 year and 2 years of clinical service,²¹ were above acceptability thresholds: AT = 1.77 (Figure 2) and WAT = 2.90 (Figures 3 and 4). Therefore, the second part of the study hypothesis was accepted, meaning aging procedures produced color changes and whiteness variations of RBCs greater than the 50:50% acceptability thresholds (AT and WAT).

Previous studies also showed a decrease of L* values and an increase of b* values after artificial aging procedures of RBCs.^20,21,46-48 However, the assessment of color and/or whiteness variations in these studies was based solely on a quantitative evaluation without taking into account the limits of visual perception and, therefore, its clinical impact.

CONCLUSIONS

Within the limitations of this study, aging produced unacceptable color matches of RBCs while bleaching treatments produced, in most cases, imperceptible color changes of nanocomposites.