
The recent increased attention to tooth function and aesthetics has led to the widespread implementation of various dental treatments1,2). Prominent aesthetic treatments include aesthetic restorative materials that match tooth color for restoration and prosthetics, as well as in-office tooth bleaching procedures for discolored teeth3,4).
Aesthetic restoration is a method that simultaneously satisfies both functionality and aesthetics. It uses materials that blend with the natural tooth color to restore the teeth. The materials commonly used for aesthetic restorations include light-cured composite resin (CR), light-cured bulk-fill resin (BF), compomer (CP), resin-modified glass ionomer cement (RMGI), and glass ionomer cement (GI)5,6).
Aesthetic restorative materials should meet several requirements, including excellent marginal sealing to prevent pulp irritation and secondary caries due to the infiltration of external substances. Additionally, they should maintain proper contact with the adjacent teeth and ensure functional occlusion of the opposing teeth7). Furthermore, the material should maintain its gloss to provide an aesthetic appeal and a smooth finish. They should possess adequate hardness to resist wear from occlusal forces and avoid flexural fracture8). The material should also resist chemical degradation from saliva, food, erosion, and corrosion due to temperature changes9). Additionally, it should exhibit excellent color stability without discoloration from extrinsic factors such as coffee, sodas, and black tea6,10).
CR exhibits a high strength and elastic modulus, reducing deformation and fracture under occlusal forces. They are easy to handle and offer excellent aesthetics11). However, owing to polymerization shrinkage during the curing process, marginal leakage may occur, necessitating a layering technique for restoration12). BF contains lower inorganic filler content than hybrid or microfilled composites. However, their larger filler particles allow bulk restoration of up to 4 mm in a single layer and reduced polymerization shrinkage13). However, owing to their low filler content, their strength and wear resistance are inferior to those of hybrid and microfilled CR14,15). GI offers excellent biocompatibility and releases fluoride after polymerization, providing a cariostatic effect on the teeth16). It also has the advantage of minimal polymerization shrinkage17). However, compared with CR, they have lower fracture toughness and wear resistance, and their initial solubility is high, requiring approximately one day to achieve the final setting. Additionally, it is less aesthetic because of its opacity, relative to the natural tooth color18). CP has been designed to address the shortcomings of CR and GI by combining their advantages. After initial light-curing, they absorb moisture from the oral environment and release fluoride, offering durability similar to that of CR7). However, they release less fluoride than glass ionomer materials and are more complex to handle19). RMGI is a hybrid material that combines glass ionomers and CR components to overcome the limitations of GI materials. It exhibits compressive strength, tensile strength, and fluoride, comparable with those of GI, while exhibiting lower solubility in acidic solutions20). However, they have limited shade options and poorer physical properties and tooth adhesion than CR21). Therefore, in aesthetic restorative treatment, it is essential to carefully consider the characteristics and limitations of various materials to select and utilize the most appropriate material.
In addition to restorations using aesthetic restorative materials, in-office tooth bleaching is a prominent aesthetic treatment in dentistry. In-office tooth bleaching is conducted under the guidance of a dentist and is used to restore tooth color or enhance the natural whiteness of teeth within a short period22). Commonly used tooth-bleaching agents, such as 30∼35% hydrogen peroxide, penetrate the enamel and dentin, releasing free radicals that oxidize internal pigments and ultimately create a whitening effect5,23). The duration and frequency of tooth-whitening procedures vary depending on the individual’s oral condition24). As professional tooth whitening does not guarantee permanent results, it is advisable to perform regular treatments to ensure long-term effectiveness. The potential side effects of tooth bleaching include increased tooth sensitivity. If the tooth bleaching agent comes in contact with the gingiva, it can lead to gingivitis or ulcers25,26).
Restoration and tooth bleaching using aesthetic materials are common treatments used to improve aesthetics. However, when these two treatments are combined, it is unclear whether the surface characteristics of the materials can be maintained without compromising their aesthetic effects. According to previous studies, combining tooth bleaching with aesthetic restorative materials may result in decreased surface hardness and increased roughness of the materials due to chemical reactions between the aesthetic restorative materials and tooth bleaching agents27). Additionally, it has been reported that the adhesive strength may be compromised28). Moreover, aesthetic restorative materials, when continuously exposed to saliva and food in the oral cavity, may undergo discoloration due to the physical penetration of extrinsic factors, indicating that the whitening effect is not permanent29,30). Therefore, research on the surface characteristics and color stability of various aesthetic restorative materials used for in-office tooth bleaching is crucial. Comprehensive studies are needed to address changes in surface characteristics, including gloss, Vickers hardness, surface roughness, and surface morphology, before and after tooth bleaching treatment, and changes in the color stability of various aesthetic restorative materials following post-bleaching staining.
The purpose of this study was to provide useful information for selecting appropriate materials in clinical situations by comparing and analyzing the surface characteristics and color stability of various aesthetic restorative materials after in-office tooth bleaching. To this end, two null hypotheses were established: First, there will be no significant differences in gloss, Vickers hardness, surface roughness, and surface morphology among various aesthetic restorative materials subjected to in-office tooth bleaching. Second, there will be no significant differences in color stability among the various aesthetic restorative materials subjected to in-office tooth bleaching.
The aesthetic restorative materials used in this study included CR, BF, CP, RMGI, and GI. For each material type, a product with a distinct composition and delivery form that is currently used in dental clinics was selected to form the experimental groups. Shade A3 was prepared for all experimental groups (Table 1).
Information on the Experimental Materials according to the Manufacturer’s Instructions
Code | Type | Product | Composition |
---|---|---|---|
CR | Light-cured composite resin | FiltekTM Z250 Universal Restorative (3M, St. Paul, MN, USA) | Silane-treated ceramic, bisphenol A diglycidyl ether dimethacrylate, bisphenol A polyethylene glycol diether dimethacrylate, diurethane dimethacrylate, triethylene glycol dimethacrylate, aluminum oxide, N, N-dimethyl benzocaine |
BF | Light-cured bulk-fill resin | FiltekTM One Bulk Fill Restorative (3M, St. Paul, MN, USA) | Silane-treated ceramic, aromatic urethane dimethacrylate, diurethane dimethacrylate, silane-treated silica, ytterbium fluoride, water, silane-treated zirconia, 1,12-dodecane dimethacrylate, ethyl 4-dimethyl aminobenzoate |
CP | Compomer | Dyract XP (Dentsply Sirona, York, PA, USA) | Urethane dimethacrylate, carboxylic acid-modified dimethacrylate, triethyleneglycol dimethacrylate, trimethacrylate resin, dimethacrylate resins, camphor quinone, ethyl-4 (dimethylamino) benzoate, butylated hydroxytoluene, UV stabilizer, strontium-alumino-sodium-fluoro-phosphor-silicate glass, silicon dioxide, strontium fluoride, iron oxide pigments, and titanium oxide pigments |
RMGI | Resin-modified glass ionomer cement | Fuji Filling LC (GC, Tokyo, Japan) | 2-hydroxyethyl methacrylate, urethane dimethacrylate, iron (III) oxide, photoinitiator, butylated hydroxytoluene |
GI | Glass ionomer cement | Fuji IX (GC, Tokyo, Japan) | Powder: fluoro-alumino-silicate glass, polyacrylic acid, pigment Liquid: distilled water, polyacrylic acid |
Stainless steel molds were used to prepare the disc-shaped specimens for each experimental group. The polyester film was placed on a glass slide and the mold was positioned on top of the film. Each material was packed into a mold using a plastic spatula for the CR and BF groups. The polyester film and glass slide were then placed on top and pressure was applied. The specimens were light-cured for 20 seconds on both the front and back surfaces using an LED curing light (DCL-30 Cordless Curing Light; Dentall, Bucheon, Korea) according to the manufacturer’s instructions.
In the CP group, a Dyract XP capsule was loaded into the Dyract gun and injected into the mold. The polyester film and glass slide were then placed on top and pressure was applied. Following the manufacturer’s instructions, the specimens were subjected to light curing for 20 seconds on both sides using an LED curing light.
In the RMGI group, the material was applied to a mixing pad using a dispenser with a cartridge and mixed using a plastic spatula for 10 seconds until the color was uniform. The mixed material was then packed into a mold, and pressure was applied using a polyester film and a glass slide. Subsequently, the specimens were light-cured for 20 seconds on both the front and back surfaces using an LED curing light, according to the manufacturer’s instructions.
In the GI group, the powder and liquid were measured on a mixing pad according to the manufacturer’s instructions and mixed using a plastic spatula for 30 seconds. The mixed material was then filled into a mold, and pressure was applied using a polyester film and a glass slide. The material was allowed to set for 4 minutes and 15 seconds according to the manufacturer’s guidelines. After the curing time for each group, the specimens were removed from the molds and prepared as disc-shaped specimens with a diameter of 10.0±1.0 mm and a thickness of 1.0±0.1 mm.
To perform bleaching treatments on the specimens, each specimen was fully immersed in 2 ml of 30% hydrogen peroxide (Junsei, Tokyo, Japan) in a 24-well plate (SPL Life Science, Pocheon, Korea) and maintained at 37±1°C for 20 minutes. The specimens were then rinsed with distilled water for 10 seconds and dried thoroughly in compressed air. This bleaching procedure was repeated four times to replicate the in-office tooth bleaching procedure.
A gloss meter (Novo-curve; Rhopoint Instrumentation, St. Leonards-on-Sea, UK) was used to measure the gloss of the specimens before and after bleaching. The measurement angle was set to 60°, and calibration was performed. Subsequently, each specimen was positioned in the measurement window of the gloss meter and three random locations were measured. The average value of these measurements was calculated and used as the representative value for each specimen.
A Vickers hardness tester (MMT-X; Matuzawa, Akita, Japan) was used to measure the surface microhardness of the specimens before and after bleaching. A diamond indenter was applied to the surface of each specimen under a load of 200 g for 10 seconds. This procedure was repeated thrice at random locations, and the average value was calculated and used as the representative value. The Vickers hardness number (VHN) was determined as follows:
(P=indentation load, d=diagonal length impression in micrometers).
The average roughness (Ra) was assessed using an optical 3-dimensional surface profilometer (Contour GT-X3 Base; Brucker, Bremen, Germany) to measure the surface roughness of the specimens before and after bleaching. Measurements were conducted using a 3D non-contact method, and three random locations on each specimen were measured at a magnification of 50×. The average of these measurements was calculated and used as the representative value.
To observe changes in the surface morphology of the specimens before and after bleaching, they were coated with platinum under vacuum using a sputter coater. Surface observations were performed using a scanning electron microscope (SEM; S-3000N, Hitachi, Nagoya, Japan) at an accelerating voltage of 10 kV and magnifications of 500× and 10,000×.
To evaluate the color stability of the bleached specimens, they were immersed in various staining solutions. The stained beverages included coffee (KANU mini mild Americano, Dongsuh Food, Incheon, Korea), Pepsi Cola (Pepsi Cola, Lotte Chilsung Beverage Co., Ltd., Seoul, Korea), and black tea (Twinings Pure Ceylon Tea, R. Twining and Company, Andover, UK), with distilled water used as the control. Solutions that required mixing of the powders and liquids were prepared according to the manufacturer’s recommendations. Coffee was prepared by dissolving 0.9 g of powder in 120 ml of water. Black tea was prepared by steeping 2 g of tea bags in 200 ml water for 3 minutes.
To simulate in-office tooth bleaching, various aesthetic restorative material specimens that had undergone bleaching were fully immersed in 2 ml of staining beverages in 24-well plates. The specimens immersed in each beverage were stored in a thermostatic water bath at 37±1°C. After 1 day of immersion, the stained specimens were rinsed with distilled water, dried thoroughly, and color measurements were performed. The staining solution was replaced with a fresh solution for subsequent immersion. This process was repeated after 3 and 7 days of immersion to measure the changes in the specimen color.
A spectrophotometer (CM-5; Konica Minolta, Tokyo, Japan) was used to evaluate the color stability of the specimens. The surface of each specimen was measured at three random locations, and the average value was considered the representative value for the specimen. The color change (ΔE*) was calculated using the formula below, and this was used to assess color stability:
L*=Lightness value, degree of whiteness and blackness
a*=Saturation value, degree of redness and greenness
b*=Saturation value, degree of yellowness and blueness
The results of the assessments of surface characteristics and color stability for the various aesthetic restorative materials treated with bleaching were analyzed using SPSS software (IBM SPSS Statistics 25.0; IBM Co., Armonk, NY, USA). One-way analysis of variance and paired t-tests were conducted, and Tukey’s post-hoc test was used to compare differences between groups. The statistical significance level was set at 0.05 (p=0.05).
The results of the surface gloss measurements before bleaching treatment indicated that the CP group (123.62±2.86) exhibited the highest gloss, followed by the BF group (119.38±1.01), the CR group (118.86±2.02), the RMGI group (88.09±2.21), and the GI group (45.88±4.63) (Fig. 1). The gloss of the BF group did not differ significantly from that of the CR and CP groups (p>0.05), but significant differences were observed among the other experimental groups (p<0.05).
The results of the surface gloss measurements after bleaching showed that the CP group (120.40±1.04) maintained the highest gloss, followed by the BF group (117.81±1.27), with no significant difference in gloss between CP and BF (p>0.05). The gloss values for the other groups were 112.03±1.49, 66.23±3.80, and 42.83±5.72 (p<0.05) for CR, RMGI, and GI, respectively.
Comparing the changes in gloss before and after bleaching, we found that the gloss of the BF and RMGI groups significantly decreased (p<0.05). In contrast, the CR, CP, and GI groups did not show significant differences in gloss before and after bleaching (p>0.05).
Before bleaching treatment, the Vickers hardness of the GI group was the highest (98.41±6.10), followed by the BF group (78.54±3.20), the RMGI group (65.68±4.02), the CP group (62.21±2.76), and the CR group (98.41±4.67) (Fig. 2). There were no significant differences between the CP and RMGI groups (p>0.05), whereas significant differences were observed among the other experimental groups (p<0.05).
After bleaching treatment, the Vickers hardness was highest in the GI group (89.33±6.38), followed by the BF group (78.56±2.21), the CP group (62.96±1.59), the RMGI group (56.48±3.08), and the CR group (50.12±3.50). Significant differences were observed among all experimental groups (p<0.05).
The Vickers hardness values of the RMGI and GI groups significantly decreased after bleaching (p<0.05), while the CR, BF, and CP groups did not show significant differences (p>0.05).
Before bleaching, the GI group showed the highest surface roughness (0.26±0.02), followed by the RMGI group (0.21±0.01), the CP group (0.19±0.04), the BF group (0.17±0.03), and the CR group (0.02±0.01) (Fig. 3). There were no significant differences between the BF and CP groups, or between the CP and RMGI groups (p>0.05). However, significant differences were observed among the other experimental groups (p<0.05).
After bleaching, the surface roughness values were highest for the RMGI group (0.27±0.04), followed by the GI group (0.27±0.03), the CP group (0.19±0.02), the CR group (0.17±0.02), and the BF group (0.17±0.01). There were no significant differences in the roughness values among the CR, BF, and CP groups, or between the RMGI and GI groups (p>0.05). Significant differences were observed among all the other experimental groups (p<0.05).
When comparing the surface roughness values before and after bleaching, the roughness values of the CR and RMGI groups significantly increased after bleaching (p<0.05), whereas those of the BF, CP, and GI groups were not significantly different (p>0.05).
Observation of the specimens using scanning electron microscopy before and after bleaching revealed that the surface morphologies of the CP, CR, and BF groups were similar, with no significant changes in the overall surface structure (Fig. 4). In contrast, the RMGI group exhibits a densely packed topography with noticeable deep cracks after bleaching. In addition, the GI group exhibited a significant increase in deep and wide cracks after treatment, with severe surface chipping overall.
Among the specimens immersed in distilled water for 1 day, the GI group exhibited the highest ΔE* value, with a measurement of 7.51±1.22 (Fig. 5, Table 2). The RMGI group showed the second highest ΔE* value at 6.75±1.37, while the values for BF, CR, and CP groups were 0.48±0.40, 0.46±0.10, and 0.35±0.11, respectively. There were no significant differences between the CR, BF, and CP groups or between the RMGI and GI groups (p>0.05). Among the specimens immersed in distilled water for 3 days, the GI group showed the highest ΔE* value with a measurement of 8.16±1.08, followed by RMGI, CR, BF, and CP in descending order. There were no significant differences among the CR, BF, and CP groups (p>0.05). However, significant differences were observed between groups (p<0.05). Among the specimens immersed in distilled water for 7 days, the GI group showed the highest ΔE* value with a measurement of 7.48±1.23, followed by RMGI, CR, BF, and CP in descending order. There were no significant differences between the CR, BF, and CP groups (p>0.05) or between the RMGI and GI groups (p>0.05). Therefore, color stability was the highest in CP, followed by BF, CR, RMGI, and GI across all immersion periods. Comparing the ΔE* values over different immersion periods in distilled water for each experimental group, no significant differences in color change were observed regardless of the duration of immersion (p>0.05).
Color Change (ΔE*) Values of Five Experimental Materials Immersed in Distilled Water, Coffee, Pepsi Cola, and Black Tea for 1, 3, and 7 Days (n=6)
Immersion media | Group | Before-1st (ΔE*) | Before-3rd (ΔE*) | Before-7th (ΔE*) |
---|---|---|---|---|
Water | CR | 0.46±0.10Cb | 0.91±0.18Bc | 1.27±0.12Ab |
BF | 0.48±0.40Ab | 0.36±0.10Ac | 0.71±0.24Ab | |
CP | 0.35±0.11Ab | 0.28±0.10Ac | 0.50±0.24Ab | |
RMGI | 6.75±1.37Aa | 6.67±0.64Ab | 6.87±0.90Aa | |
GI | 7.51±1.22Aa | 8.16±1.08Aa | 7.48±1.23Aa | |
Coffee | CR | 2.96±0.52Bd | 4.69±1.27Ad | 6.16±1.37Ad |
BF | 2.83±1.07Bd | 4.20±1.30ABd | 5.06±0.86Ad | |
CP | 5.29±0.82Bc | 7.44±1.17Ac | 8.85±0.68Ac | |
RMGI | 11.03±0.45Ca | 13.91±1.08Ba | 16.40±1.15Aa | |
GI | 9.00±1.33Bb | 11.62±1.13Ab | 13.22±0.65Ab | |
Pepsi Cola | CR | 2.63±0.39Cc | 6.28±1.17Bc | 9.09±0.24Ab |
BF | 0.31±0.18Ad | 0.30±0.11Ad | 0.34±0.19Ac | |
CP | 0.50±0.13Ad | 0.55±0.21Ad | 0.58±0.21Ac | |
RMGI | 10.13±1.15Ca | 16.77±1.24Ba | 19.59±1.32Aa | |
GI | 6.81±0.79Bb | 8.42±0.33ABb | 9.08±1.46Ab | |
Black tea | CR | 1.29±0.30Cb | 2.00±0.43Bc | 2.72±0.49Ac |
BF | 0.89±0.37Bb | 1.42±0.43ABc | 1.93±0.57Ac | |
CP | 1.02±0.32Bb | 1.52±0.41ABc | 2.15±0.56Ac | |
RMGI | 9.81±1.19Ca | 12.64±1.46Ba | 17.51±0.87Aa | |
GI | 9.52±1.12Aa | 9.94±1.78Ab | 10.50±1.58Ab |
Uppercase letters indicate significant differences between days within the same immersion media and experimental materials (p<0.05). Lowercase letters indicate significant differences between experimental materials at the same time point within the same immersion media (p<0.05). Calculated using one-way analysis of variance and Tukey’s post hoc test.
CR: light-cured composite resin, BF: light-cured bulk-fill resin, CP: compomer, RMGI: resin-modified glass ionomer cement, GI: glass ionomer cement.
Among the specimens immersed in the coffee for 1 day, the RMGI group exhibited the highest ΔE* value of 11.03±0.45. This was followed by the GI (9.00±1.33), CP (5.29±0.82), CR (2.96±0.52), and BF (2.83±1.07) groups in descending order. After 3 days of immersion, the RMGI group showed the highest ΔE* value of 13.91±1.08, followed by GI, CP, CR, and BF in that order. After 7 days of immersion, the RMGI group again recorded the highest ΔE* value of 16.40±1.15, followed by GI, CP, CR, and BF. There were no significant differences between the CR and BF groups for the immersion period of 1, 3, and 7 days (p>0.05). In contrast, significant differences were observed among the other groups for each immersion period (p<0.05). Consequently, color stability was ranked as follows: BF, CR, CP, GI, and RMGI for all immersion periods. Comparing the ΔE* values of each group based on immersion duration in the coffee, all groups showed a significant increase in color change as the immersion period lengthened (p<0.05).
Among the specimens immersed in Pepsi Cola for 1 day, the RMGI group exhibited the highest ΔE* value with a measurement of 10.13±1.15. This was followed by GI (6.81±0.79), CR (2.63±0.39), CP (0.50±0.13), and BF (0.31±0.18). There was no significant difference between the BF and CP groups (p>0.05); however, significant differences were observed among the other groups (p<0.05). Among the specimens immersed for 3 days, the RMGI group exhibited the highest ΔE* value of 16.77±1.24, followed by GI, CR, CP, and BF. There was no significant difference between the BF and CP groups (p>0.05); however, significant differences were observed among the other groups (p<0.05). After 7 days of immersion, the RMGI group also recorded the highest ΔE* value with a measurement of 19.59±1.32, followed by CR, GI, CP, and BF. There were no significant differences between the BF and CP groups (p>0.05) or between the CR and GI groups (p>0.05). Therefore, the color stability was the highest in the order of BF, CP, CR, GI, and RMGI across all immersion periods. Comparing the ΔE* values of each group immersed in Pepsi Cola over different immersion periods, significant differences were observed in the color change with longer immersion periods for all groups except BF and CP (p<0.05).
Among the specimens immersed in the black tea for 1 day, the highest ΔE* value was observed in the RMGI group (9.81±1.19). This was followed by GI (9.52±1.12), CR (1.29±0.30), CP (1.02±0.32), and BF (0.89±0.37). There were no significant differences between the CR, BF, and CP groups (p>0.05) or between the RMGI and GI groups (p>0.05). For the specimens immersed for 3 days, the highest ΔE* value was observed in the RMGI group at 12.64±1.46, followed by GI, CR, CP, and BF. No significant differences were observed among the CR, BF, and CP groups (p>0.05); however, significant differences were observed among the other groups (p<0.05). For the specimens immersed for 7 days, the RMGI group again exhibited the highest ΔE* value at 17.51±0.87, followed by GI, CR, CP, and BF. No significant differences were observed among the CR, BF, and CP groups (p>0.05); however, significant differences were observed among the other groups (p<0.05).
Therefore, the color stability was the highest in the order of BF, CP, CR, GI, and RMGI across all immersion periods. When the ΔE* values of each group immersed in the black tea were compared, CR, BF, CP, and RMGI showed a significant increase in color change with longer immersion times (p<0.05). In contrast, the GI group did not exhibit significant color changes after longer immersion periods (p>0.05).
In-office tooth bleaching primarily uses high concentrations of hydrogen peroxide to generate reactive oxygen species, which oxidize the organic components of the tooth structure, leading to rapid whitening. Owing to these characteristics, in-office tooth bleaching has become a popular aesthetic treatment method for effectively improving discolored teeth31). However, excessive chemical reactions associated with high concentrations can lead to the separation of the matrix and fillers in aesthetic restoratives, potentially resulting in increased surface roughness, sensitivity to staining, bacterial adhesion, and biofilm formation32).
The use of aesthetic restoratives to address issues such as cervical abrasion and dental caries, has increased, and the growing interest in aesthetics has driven a higher demand for these treatments33). Technological advancements have led to various aesthetic restorative materials that exhibit differences in mechanical, physicochemical, and biological properties depending on their components and composition34,35). Notably, the discoloration of restoratives can be a major reason for their replacement, leading to significant time and cost implications36). Therefore, excellent surface properties and color stability are essential for the long-term use of aesthetic restoratives.
This study aimed to provide foundational information for selecting materials that optimize aesthetic effects and patient satisfaction by comparing and analyzing the surface properties and color stability of various aesthetic restorative materials subjected to in-office tooth bleaching. The goal was to minimize the impact of material surface characteristics, while maximizing the aesthetic benefits and patient satisfaction.
Gloss is a surface property that indicates the amount of light reflected from a surface and represents the optical phenomenon observed on the material’s surface37). It can be easily perceived by patients and thus serves as a criterion for evaluating successful aesthetic treatment38). Gloss can vary depending on the angle of incidence and degree of surface finish39). Therefore, in this study, the gloss was objectively measured by setting the measurement angle of the gloss meter to 60°, based on previous research9). After bleaching, a general decrease in gloss was observed across all experimental groups, with a significant reduction observed specifically in the BF and RMGI groups. The CP, BF, and CR groups maintained high glossiness before and after bleaching, whereas the RMGI and GI groups exhibited lower glossiness. Consequently, the null hypothesis was partially rejected, indicating no significant difference in gloss unit (GU) among the various aesthetic restoratives after in-office bleaching treatment. Gloss may be related to microstructural characteristics, and scanning electron microscopy revealed that the RMGI and GI groups exhibited pronounced deep cracks after bleaching. Because gloss meters measure the amount of light reflected based on surface characteristics, these surface features are likely to influence gloss. Therefore, the use of materials from the CR, BF, or CP groups is recommended when gloss is an important criterion in the selection of aesthetic restoratives.
Surface hardness is a critical property that determines the resistance or sensitivity of a surface to deformation, fracturing, or scratching when subjected to external forces40). For long-term use, aesthetic restoratives must possess robust hardness and should not exhibit wear or fracture due to external pressures or stimuli, such as tooth brushing. Therefore, in this study, the Vickers hardness was measured and analyzed to observe the changes in the mechanical properties of aesthetic restoratives before and after bleaching. The GI and RMGI groups exhibited a significant decrease in Vickers hardness after bleaching, whereas no significant differences were observed in the CR, BF, and CP groups. Consequently, the null hypothesis was partially rejected, stating that there were no significant differences in Vickers hardness among the various aesthetic restoratives after in-office bleaching. Although the GI group exhibited the highest Vickers hardness values before and after bleaching, the decrease observed only in the RMGI and GI groups may be associated with cracks in the surface microstructure. Therefore, the GI or BF type is recommended when high mechanical resistance is required.
Surface roughness refers to the extent of fine irregularities on a surface, and the smoothness of the surface is related to the accumulation of bacteria or plaque, which affects the longevity of aesthetic restoratives32). In this study, the surface roughness before and after the bleaching treatment was measured using a 3D noncontact method at 50× magnification. The CR and BF groups exhibited low surface roughness before and after bleaching, whereas the RMGI and GI groups exhibited higher surface roughness. There were no significant differences in the surface roughness values before and after bleaching between the BF, CP, and GI groups. In contrast, the CR and RMGI groups showed a significant increase in surface roughness. Notably, the BF group maintained low Ra values before and after bleaching treatment, showing no significant increase. The CR group exhibited the largest difference in surface roughness values before and after bleaching; however, it demonstrated lower surface roughness values after treatment, compared with the other groups. This is similar to a previous study where applying a 35% hydrogen peroxide solution to aesthetic restorative materials resulted in increased post-bleaching surface roughness values compared with pre-bleaching41). However, the significant differences in the surface roughness values of CP and GI, as well as the lack of significant differences in the surface roughness values of CR, differed from the results of this study. This discrepancy may be attributed to the differences in the concentration of the hydrogen peroxide solution. Therefore, the null hypothesis was partially rejected, indicating no significant difference in surface roughness among the various aesthetic restoratives after in-office bleaching treatment. According to a previous study, an average surface roughness greater than 0.3 µm can lead to functional or microbiological issues, indicating a high likelihood of restoration replacement. Additionally, an average surface roughness greater than 0.2 µm can facilitate bacterial adhesion and plaque accumulation40). In this study, the RMGI and GI groups exhibited values greater than 0.2 µm after bleaching, indicating a higher potential for bacterial adhesion and plaque accumulation than other groups. Therefore, when surface roughness is critical for selecting aesthetic restoratives, the use of materials from the BF and CR groups is recommended.
Scanning electron microscopy was used to analyze changes in the surface morphology of various aesthetic restorative materials after the bleaching treatment. After bleaching, shallow and small cracks were observed in the CR, BF, and CP groups, and deep and wide cracks were distinctly observed in the RMGI and GI groups. Notably, the GI group exhibited severe collapse and cracking. These findings are consistent with an earlier study that reported visible cracks or porosities42). Owing to the uneven distribution of glass particles within the matrix, the highly porous GI may react with the acidic components of tooth-bleaching agents; this could lead to dissolution or structural weakening, thus generating cracks or pores43). Despite these surface changes, the GI group exhibited higher Vickers hardness than the other groups. This discrepancy suggests that factors such as viscosity may influence hardness because no direct correlation between porosity and microhardness has been reported43). These results indicate that changes in surface morphology can affect surface properties such as gloss, surface roughness, and color stability.
With increasing interest in aesthetic restorations and the emergence of new aesthetic restorative materials, color stability has become a crucial factor5). Substances such as coffee, Pepsi Cola, and black tea can markedly contribute to extrinsic staining44); hence, these three beverages were selected to comprehensively evaluate color stability. While a previous study assessed staining after 3 and 7 days of immersion, this study also included observations of the initial staining on the first day to facilitate the evaluation of the effect of bleaching. Additionally, as prior research has indicated that significant color changes are not observed beyond 7 days of immersion5), this study focused on evaluations after 1, 3, and 7 days of immersion.
After immersing the experimental specimens in staining solutions to simulate discoloration, the ΔE* values were evaluated using a spectrophotometer after 1, 3, and 7 days. The results showed that in distilled water, there were no significant differences in color change among all experimental groups, except for the CR group, at all-time points. For coffee, significant differences in color change were observed among all the experimental groups after 1, 3, and 7 days. The BF group demonstrated the highest color stability, and the RMGI group showed the lowest color stability. In Pepsi Cola, after immersion for 1, 3, and 7 days, the ΔE* values of the BF and CP groups did not show significant differences, indicating the highest color stability. In contrast, significant differences in color change were observed among the other experimental groups, with the RMGI group exhibiting the lowest color stability. After immersion in black tea for 1, 3, and 7 days, the GI group demonstrated no significant difference in color change, indicating the highest color stability. In contrast, significant differences in color change were observed among the other experimental groups, with the RMGI group showing the lowest color stability. According to previous research, GI-based aesthetic restorative materials exhibited significant color changes, with ΔE* values exceeding 3.3, indicating ΔE* values beyond clinically acceptable limits45,46). Materials containing GI tend to absorb water, which may contribute to increased discoloration47). Therefore, the null hypothesis that there would be no significant differences in color stability among various aesthetic restorative materials subjected to in-office bleaching treatment was partially rejected. Based on these results, it is recommended to use materials from the BF group for patients who prioritize aesthetics or who need to restore the anterior teeth with aesthetic restorative materials.
This study analyzed the changes in various surface properties of different aesthetic restorative materials, including gloss, Vickers hardness, surface roughness, and surface morphology, before and after bleaching treatment. To assess color stability more comprehensively, a range of commonly consumed beverages were used as staining solutions, addressing the limitation that the results from a single staining solution may not be generalizable. Using this approach, this study provides a comprehensive evaluation of the surface properties and color stability changes of various aesthetic restorative materials, establishing a foundation for selecting and applying suitable materials under different oral conditions.
This study did not accurately reproduce the oral environment with continuous salivary flow and other factors. Unlike the in-office tooth bleaching treatments performed in clinical settings, all tooth bleaching treatments were completed within a single day. Additionally, although the sample size for most experiments was set to six based on prior research, a large sample size is necessary to obtain more reliable results48). Owing to these limitations, generalizing the results of this study may be challenging. Future research should address these limitations to obtain more reliable results.
This study investigated the changes in the surface properties and color stability of various aesthetic restorative materials, including CR, BF, CP, RMGI, and GI. The evaluation of surface properties included gloss measurements, Vickers hardness, surface roughness, and observation of surface morphology following in-office tooth bleaching.
The glosses of the specimens before and after bleaching decreased in the following order: CP, BF, CR, RMGI, and GI. When comparing gloss before and after bleaching, significant reductions were noted in the BF and RMGI groups (p<0.05). No significant differences were observed among the other groups (p>0.05). Before bleaching, the Vickers hardness values of the specimens were ranked as follows: GI, BF, RMGI, CP, and CR. After bleaching, the rankings were GI, BF, CP, RMGI, and CR. When the Vickers hardness values before and after bleaching were compared, significant reductions were observed in the RMGI and GI groups (p<0.05). No significant differences were observed among the other groups (p>0.05). Before bleaching, the surface roughness decreased in the following order: GI, RMGI, CP, BF, and CR. After bleaching, the order was changed to RMGI, GI, CP, CR, and BF. A comparison of the surface roughness before and after bleaching revealed significant increases in the CR and RMGI groups (p<0.05). In contrast, no significant differences were observed among the remaining groups (BF, CP, and GI; p>0.05).
After 7 days of immersion in distilled water, CP exhibited the highest color stability after bleaching, followed by BF, CR, RMGI, and GI. There were no significant differences among the groups, except for CR (p>0.05). After immersion in coffee for 1, 3, and 7 days, the order of color stability was BF, CR, CP, GI, and RMGI. Significant differences were observed among all the experimental groups (p<0.05). After 7 days of immersion in Pepsi Cola, the order of color stability was BF, CP, GI, CR, and RMGI. No significant differences were observed between the BF and CP groups (p>0.05); however, significant differences were found among the CR, RMGI, and GI groups (p<0.05). After 1, 3, and 7 days of immersion in black tea, the order of color stability was BF, CP, CR, GI, and RMGI. No significant differences were observed in the GI group (p>0.05), whereas significant differences were observed in the other experimental groups (p<0.05).
Based on these results, this study is anticipated to provide useful foundational data for clinicians in selecting aesthetic materials with minimal impact on surface properties, while offering optimal aesthetic outcomes and patient satisfaction.
None.
Conflict of interest
No potential conflict of interest relevant to this article was reported.
Ethical approval
Not applicable.
Author contributions
Conceptualization: Song-Yi Yang. Data curation: Ji-Won Choi. Formal analysis: Ji-Won Choi. Funding acquisition: Song-Yi Yang. Investigation: Ji-Won Choi. Methodology: Song-Yi Yang. Project administration: Song-Yi Yang. Resources: Song-Yi Yang. Software: Song-Yi Yang. Supervision: Song-Yi Yang. Validation: Song-Yi Yang. Visualization: Ji-Won Choi. Writing-original draft: Ji-Won Choi. Writing-review & editing: Song-Yi Yang.
Funding
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (RS-2023-00211180).
Data availability
Authors may provide raw data upon request.
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