Comparative evaluation of the effect of bleaching agent on color change of single-shade vs. bulk-fill composite resin: an in vitro study

Introduction

Aesthetic dentistry has always been very rewarding and challenging for a clinician. Obtaining color harmony between natural tooth and restoration is of critical importance, a parameter used by patients to judge the competence and quality of work of a clinician [1]. Tooth shade matching is a complex phenomenon, and factors, such as chroma, hue, value, translucency, texture, opacity, and gloss, are all considered in a shade selection [2]. Amidst the demands of modern clinical workflows, shade selection with multi-shade composi­tes can be very time-consuming with a high possibility of errors. This has led to the introduction of single-shade composites in restorative dentistry, which use smart chromatic technology, enhancing the predictability of color matching and reducing the chairside time [3].
Color adjustment potential is based on the principle of scattering of light to facilitate blending the shade of a surrounding tooth color with the resin, known as the chameleon effect [4, 5]. This characteristic feature makes these resin composites highly promising, as they simulate the natural tooth shade using only a single-shade [6].
Exposure to oral fluids and acidic environments throughout time may result in discoloration of the resin restoration, impacting clinical longevity of the resin [7]. Discoloration of restoration are due to extrinsic or intrinsic factors; the intrinsic factors include the properties of the resin material, while the extrinsic factors involve the oral environment [8]. Changes in color of the restoration and compromised aesthetics are the major reasons for restoration replacement [9].
Bleaching is a minimally invasive treatment moda­lity for discolored teeth. However, interaction between the bleaching agent and restorative material is unavoidable, and may result in alterations in color, surface, and mecha­nical properties of the resin. Natural teeth bleaching with existing restoration can result in a color mismatch between the restoration and natural surrounding teeth, which pre­viously were aesthetically matching [10, 11].

Objectives and research hypothesis

This study evaluated the effect of bleaching on color stability and the potential of a single-shade composite to demonstrate color change after bleaching procedure. Also, it compared the ability to adapt to the newer, lighter shade of the surrounding tooth post-bleaching of a single-shade composite with that of a multi-shade composite.
The research hypothesis for the present study was that no statistically significant difference in color change (ΔE) between the single-shade composite and the bulk-fill composite after bleaching procedure can be observed.

Material and methods

No biological materials were used in the laboratory study using composite resins, therefore, ethics committee approval was not required. All procedures were carried out in accordance with the ethical rules and principles.

Study design and sample size calculation

This study analyzed two types of composite resins: Vitara APS Unique single-shade composite and nano-hybrid Tetric N-Ceram Bulk-Fill composite, as presented in Table 1. All samples were subjected to thermocycling and bleaching. Color measurements were obtained at baseline (t0) after thermocycling, and after bleaching process (t1), as shown in Figure 1. Sample size was calculated using G*Power statistical software, with a confidence level of 95%, at a power of 90%, and an effect size of 1.68. The calculated sample size was (n = 15) per experimental group.

Specimen preparation

Fifteen composite pellets were made for each group, namely group 1: Vittra APS Unique single-shade composite, and group 2: nano-hybrid Tetric N-Ceram Bulk-Fill composite, utilizing silicon molds (4 × 4 × 3 mm). The silicon molds were filled with the respective composite, and a mylar strip was placed on the upper surface of material, followed by 15 seconds polymerization with a curing light [1]. All the composite pellets were polymerized with the same curing light (SmartLite Focus, Dentsply Sirona, USA). A radiometer (Woodpecker LED-F, Woodpecker Medical Instrument Co., China) was periodically used to monitor the output of curing light, ensuring an intensity of at least 1,000 mW/cm2 throughout the preparation of specimens. All the prepared specimens were then polished using a resin poli­shing kit (Shofu Super-Snap Rainbow Technique Kit, Shofu Dental Corp., Japan), with a sequence from extra-coarse to extra-fine for 10 seconds each, at a speed of 10,000 rpm. A new polishing disc was applied to polish each specimen, which was then rinsed with water for 10 seconds to remove debris from the surface. To ensure consistency and uniformity, a single operator performed all specimen preparation steps. Samples were kept in artificial saliva (Table 2) throughout the next phases of the study [12, 13]. All samples were subjected to 1,000 cycles of thermocycling by alternating temperatures between 5°C and 55°C using a refrigerator and an incubator, with individual dwell time of 30 seconds and transfer time of 10 seconds [14].

Color measurement procedure

Samples’ baseline color (t0) was measured after thermocycling using Vita Easy Shade V spectrometer, cali­brated according to the manufacturer’s instructions. The device tip was placed at the center of restoration flush to the surface for accurate measurements, and then the color parameters of composite samples were recorded [15, 16]. The color of the samples was measured in terms of lightness (L), chroma (C), and hue (H), represented by absolute color magnitude (E) of the samples. Mean values of E were calculated for each group, both pre- and post-bleaching procedures. To evaluate the color change in samples before and after bleaching, ΔE was calculated as follows:

Bleaching protocol

Composite samples of both groups were bleached using 40% hydrogen peroxide (Opalescence Boost). Three cycles of bleaching were performed, each lasting for 15 minutes, with an interval of 5 minutes between each cycle. Bleaching agent was applied to the sample’s surface according to the manufacturer’s instructions. After bleaching, the samples were stored in artificial saliva for 1 week to rehydrate the bleached samples [17, 18]. Post-bleaching color assessment of the composite samples was done, following the previously mentioned procedure, and the values (t1) were documented for each sample.

Statistical analysis

Data were collected, tabulated, and statistically analyzed using the Statistical Package software (SPSS version 23.0). Since all data did not follow a normal distribution and the sample size in each group was relatively small (n = 15), non-parametric tests were employed. Paired data were analyzed using the Wilcoxon signed-rank test, while comparisons between independent groups were conducted using the Mann-Whitney test.

Results

Descriptive statistics regarding the recorded mean values ± standard deviation (SD) were used to illustrate the color (E) of composite specimens in terms of lightness (L), chroma (C), and hue (H), for both group 1 and group 2. The Wilcoxon signed-ranks test was applied for intra-group statistical comparison of the color parameters of composite specimens of Vittra APS Unique single-shade composite and nano-hybrid Tetric N-Ceram Bulk-Fill composite before and after the exposure to bleaching agent (Table 3).
Before bleaching, higher values of mean E (6.2714 and 5.6429) were observed for both groups compared with the values of mean E (4.6000 and 4.12860) recorded after bleaching, which were reduced. It was observed that the value L (lightness) increased and moved closer to the whiter/lighter color spectrum, while the value of C (chroma) decreased slightly after bleaching, indicating a decrease in color saturation and intensity. However, H (hue) remained relatively stable, as the composite samples were bleached and assessed for color change. No statistically significant difference (p = 0.063 and p = 0.310) was observed in the values of E of the single-shade composite group and the bulk-fill composite specimen group in intra-group analyses. After bleaching, a statistically significant difference in the mean ΔE values (p = 0.002) between the single-shade and bulk-fill composite groups was recorded, as presented in Table 4. The Mann-Whitney test was carried out to compare the difference in the composite’s color between the two composite groups before and after bleaching. A statistically significant value indicated that an aesthe­tically appreciable difference was noted after bleaching when comparing the two composite groups.

Discussion

Resin-based composites have become an integral part of restorative dentistry, with patients gradually insisting on having an aesthetic smile. However, these resin-based composites show many variations in their chromatic properties with aging, i.e., extrinsic and intrinsic factors, such as colored beverages, diet, saliva, and oral environment [19, 20].
Thermocycling between 5°C and 55°C simulates the thermal fluctuations encountered in the oral cavity, including consuming hot and cold beverages, thereby providing a realistic model for assessing the color stability of dental materials [14]. Artificial saliva serves as a medium that mimics the ionic composition and pH of natural saliva, providing a more realistic simulation of oral conditions than distilled water. This approach allows evaluation of how composite materials interact with salivary components over time, influencing their color stability and susceptibility to staining [21].
A bleaching procedure is a non-invasive, conservative treatment modality for removing stains from dental hard tissues and enhancing smile aesthetics [22]. Bleaching can be performed by the dentist, in-office application, or by the patient, at home application. It helps removing the stains, achieving a lighter shade of the teeth by releasing reactive oxygen species [23]. The contact of bleaching agent with the surface of existing composite restoration is unavoidable, and does affect the mechanical, chemical, and aesthetic properties of resin-based composites [24]. For this study, Opalescence Boost 40% hydrogen pero­xide, a chemically activated bleaching agent, was selected to prevent excessive dehydration of the teeth surface during the bleaching procedure, which could lead to misleading results in terms of color change [25].
The CEILAB system (Commission Internationale de I’Eclairage; L*, a*, b*) measures color based on the CIELAB color space, which quantifies color using three parameters: lightness (L*), red-green axis (a*), and yellow-blue axis (b*). This system provides objective and reproducible color measurements, essential for evaluating subtle color changes [15, 16]. In the current study, Vittra APS Unique single-shade composite was employed. This composite uses smart chromatic technology because of the incorporation of supra-nanohybrid particles and its patented technology of advanced particle system (APS), which optimizes particle distribution and interaction between fillers and resin to exhibit the chameleon or the blending effect [4, 5, 26, 27].
Bulk-fill composites were chosen in this study for comparison, as they have a greater curing depth of 4-6 mm in contrast to a 2 mm curing depth of conventional composites. This property helps reduce polyme­rization shrinkage, is responsible for greater translucency, and thus, enhanced aesthetic properties [28, 29].
The results of the present study reveal that group 1 (Vittra APS Unique single-shade composite) demonstrated a greater change in the CIELAB metric. A greater variation was observed in pre- and post-bleaching shade of the single-shade composite compared with the bulk-fill composite, highlighting its better ability to undergo color change and potentially matching the newer, lighter shade of the surrounding bleached natural tooth. A statistically significant difference was observed in the ΔE values between the two groups, indicating that the group 1 (single-shade composite) may yield better results in compatibility with the surrounding teeth after exposure to the bleaching agent (Table 3).
Santana et al. [30] stated in their work that the ability of a single-shade composite to blend with the adjacent tooth shade depend heavily on its increased translucency, which helps it mirror both the surroundings and substrate present beneath. In line with our study, Nouraha Talt et al. [1] reported excellent ability of a single-shade composite (Omnichroma) to blend with natural tooth shade after bleaching in comparison with a nano-hybrid composite. Single-shade composites are less time-consuming and show better results in shade matching after in-office bleaching treatment, as shown in a study conducted by Forabosco et al., where the ability of a single-shade resin was tested after in-office application of Opalescence Boost 40% [2]. In contrast, Chen et al. [8] observed that single-shade composites are more susceptible to discoloration, which affects their long-term success rate when subjected to aging and bleaching. Moreover, bulk-fill was reported to show more visible color change, leading to a mismatch post-bleaching and staining process, as stated by Erturk-Avunduk. In their study, bulk-fill resin composites, i.e., Bis-GMA and TEGDMA, demonstrated the highest color change after immersion in beverages, causing staining [28].
Nano-hybrid bulk-fill composites exhibit greater staining, and tend to become dark due to their higher filler content and smaller particle size, which increase surface roughness and water sorption. These properties allow more penetration, leading to enhanced discoloration. This response underscores the significance of material selection in achieving long-term aesthetic outcomes in restorative dentistry [1, 28, 31].
Based on the present study’s results, the initial research hypothesis was rejected. A significant color difference was observed between the single-shade and the bulk-fill composite after bleaching procedures.
A limitation of the current study is that a comparison between a single-shade composite and a multi-shade composite would have been more justified for color assessment and aesthetics than a bulk-fill composite.
For better analysis and clinical significance of the findings, further clinical studies should be conducted to evaluate the ability of composites of both the groups investigated, to blend and adapt to the newer, lighter shade of adjacent teeth post-bleaching exposure.

Conclusions

Single-shade composite showed better ability to undergo color change and become lighter, indicating its better ability to match the shade of the natural tooth shade after bleaching treatment than the bulk-fill composite. A delay in replacing existing composite restorations when subjected to a bleaching procedure ensures an aesthetic and more conservative treatment modality.

Disclosures

1. Institutional review board statement: Not applicable.
2. Assistance with the article: None.
3. Financial support and sponsorship: None.
4. Conflicts of interest: None.

References