Evaluation of the color stability of two different posterior tooth colored restorative materials [version 1; peer review: 2 approved with reservations]

Background: The aim of this in vitro study was to evaluate the color stability of esthetic restorative materials (Cention N, Solare Sculpt) after exposure to different staining solutions (coffee, green tea and Diet Coke). Methods: Cylindrical specimens of both materials (n=40/material) were prepared using 4x8 mm metal molds. They were further divided (n=10) based on the beverages in which they are immersed. The color of each sample was recorded immediately after sample preparation and at 60 days after the staining procedure. Color changes were then analyzed statistically. Results: Color differences (ΔE) were statistically significant between Cention N and Solare sculpt in all beverages with Cention N showing highest staining after 60 days. Among all the beverages, coffee showed the highest level of staining. Conclusions: Staining beverages caused significant discolorations for both test materials. Cention N showed greater color variations with all beverages compared to Solare Sculpt. Coffee showed the highest staining with both materials, followed by Diet Coke then green tea. This is a well and scientifically- written article. It compares color stability of two different types of composites when immersed in three routine drinks (coffee, Kola, green tea). They found that coffee induced more staining compared to other drinks, and difference between the two composites were statistically significant. The are many similar articles in literature. Please justify the importance of your research.


Introduction
The use of tooth-colored restorative materials has been on the rise in the recent times. Patient awareness and improvement in the properties of composite materials have led the clinicians to use these materials, even in the posterior regions. But strength and other properties like staining have remained as a challenge with the newer materials that are being introduced 1 .
Recently developed esthetic materials like Cention N and Solare Sculpt have been reported to have improved properties for use as posterior restorative material. One important aspect of esthetic restorative materials is color. Interactions between the components of a restorative material play a key role in their color stability. Composite resin materials are mainly discolored due to the intrinsic or extrinsic factors. The intrinsic discoloration of the composites can be determined by the photo initiator, quality of resin matrix and by the inorganic filler, in the same way the extrinsic discoloration is caused especially by the colorants present in beverages and foods through adsorption and absorption 2 . In the present contemporary society there is a high prevalence of consumption of coffee, green tea and soft drinks, causing discoloration of the tooth-colored restorations 3 .
Many studies 4-6 have focused on comparing the physical properties of Cention N with composite materials and others. To our knowledge, no study has compared the staining ability of Cention N with Solare Sculpt.
Thus, the present study aimed to evaluate the staining susceptibility of a recently developed nanohybrid composite (Solare Sculpt) and Cention N after immersion in artificial saliva, coffee, green tea and Diet Coke using a spectrophotometer. The null hypothesis tested in this study was that there would be no difference in staining susceptibility among the materials tested and no difference in staining ability of all the beverages used.

Methods
Solare Sculpt and Cention N (Shade A2) were the materials used in the study. Detailed descriptions of the products used are presented in Table 1. A total of 80 metal molds 4 mm thick and 8 mm in diameter were prepared for each specimen. Each restorative material was prepared as per manufacturer instructions and placed in the metal rings to prepare the 80 samples (n=40 for each material). The ring was then placed between glass slabs to remove the excess and to obtain a flat surface. All the samples were light-cured with a blue phase light-curing unit (Ivoclar Vivadent, Schaan, Liechtenstein, UK) for 20 seconds on both sides. Cured specimens were then removed from the mould and were finished and polished.
Finishing was done with fine-grit Sof-Lex flexible disks (3M ESPE, MN, USA), and then polished using rubber cups (Shofu dental products, San Marcos, USA) at low speed. All samples were stored in distilled water for 24 h after curing. All the samples of both materials were then divided according to the beverage in which they are immersed (n=10): Coffee. The coffee was prepared by dissolving 10 grams of coffee powder (Bru coffee, Hindustan Unilever Limited Mumbai, Maharashtra) in 500 ml of boiling distilled water. After 10 min of stirring, the solution was filtered through a filter paper. The filtered solution was then dispensed into the containers, at 10 ml per container that were labeled as coffee group.
Green tea. The green tea was prepared by using two tea bags (Lipton Green Tea, Hindustan Unilever Ltd., Mumbai, India) according to the manufacturer's instructions. First, 500 ml of water was boiled then allowed to cool down to 80°C as measured by the Lab Thermometer (Ludwig Schneider, Wertheim, Germany). The green tea bags were then steeped in the 80°C water for 1 minute, and the bags were then removed from the water. This prepared green tea was then dispensed into the containers, at 10 ml per container that were labeled as green tea group.

Diet Coke. A 300 ml Diet Coke can (Hindustan Coca-Cola
Bevarages Pvt. Ltd., Pune, India) was used for dispensing into the containers, at 10 ml per container that were labeled as the Diet Coke group.

Staining procedure
After the baseline analysis, the test group specimens were placed in bottles with their respective staining solutions for one hour every day up to 60 days, while the control group specimens were placed in the artificial saliva for 24 hours every day up to 60 days. After each day's one-hour staining period, the stained test specimens were removed from their respective staining solution, rinsed in distilled water and transferred back to the bottles with artificial saliva (E SALIVA, Yash Pharma Laboratories, Uttarakhand, India) for the remaining time.
After the 60 days of immersion period the specimens were rinsed with distilled water for five minutes and blotted dry with absorbent paper and the final color of all specimens were measured as described above. The CIE 1976 L*a*b* color system 4 was used to calculate the color differences. The following formula was used to calculate the color differences (ΔE) at various time intervals: Where ΔE represents the mean color change, ΔL the mean of baseline L values, Δa the mean of baseline a values and Δb the mean of baseline b values.

Statistical analysis
Unpaired t-test was used to compare the color change values between the two main groups and analysis of variance (ANOVA) was used to compare the color change values between their sub-groups. Data was analyzed using SPSS version 20 software. All statistical analysis was performed at 95% level of confidence, with the significance level established at P < .05.

Results
The color change values (ΔE) observed between the two groups and subgroups can be seen in Table 2 and Table 3. The p-value obtained between the groups shows statistically significant difference between the two materials and between the sub-groups of each group.
Both the groups stained in artificial saliva showed the least color change after 60 days, while those stained in the coffee showed the highest color change, followed by Diet Coke and green tea. Among the two materials, Cention N had exhibited more staining than Solare Sculpt in all immersions (Table 2 and  Table 3).

Discussion
Success or failure of an esthetic restoration primarily depends on the color stability of the material 7 . The extent of discoloration in vitro depends on various factors, including the Incomplete polymerization of the resin composites leaves unreacted monomers, which leads to discoloration of composites by aging and subsequent reactions with other substances 3,4 .
To avoid this, all samples were stored in distilled water for 24 h after curing 8 .
Discoloration can also occur through water sorption and food intake. Since the foods and beverages consumed contain a variety of coloring agents, they can alter the color of the resin composites through absorption and/or adsorption of colorants during the long period of exposure. The staining solutions used in this study are common in the diet, and some have the known potential to stain tooth-colored restorative materials. According to Guler and others, storage of samples for 15 days in coffee solution simulates the coffee consumption of an individual for at least 1 year 8 . Thus, in the present study 60 days of storage was done, equating to about 48 months of beverage consumption. The staining protocol (quality and quantity and concentration of solutions prepared, and also daily changing of staining solutions) were strictly followed to mimic the in vivo conditions. The surface roughness of the restorative surfaces can also affect the discoloration potential because the more the rougher the surface, the greater the surface area for contact with the coloring agents 7 .
That is the reason all the samples were finished and polished with fine-grit Sof-Lex flexible disks and rubber cups.
There are two categories for color evaluation, one is visual and other is through the use of instruments. Instrumental assessment can potentially eliminate subjective errors in color assessments which may be observed visually. Therefore, in the present study a digital spectrophotometer was used for assessing the color change 9 .
In the present study following immersion in the test beverages, all restorative materials exhibited a color change of ΔE > 1.5, and those immersed in tea and diet coke exhibited a color change of ΔE >3.3 10-12 . The results indicated that both the groups stained in artificial saliva had shown the least color change (ΔE < 1) while those stained with coffee showed the highest color change (ΔE > 5).
Of the two materials tested, Cention-N had exhibited greater staining than Solare Sculpt after 60 days. The final matrix of Cention N comprises of 12-40% monomers that may leach out; alternatively, fluoride release from the material may be the reason for color change over a period of time.
Among the subgroups, the coffee solution caused the highest degree of staining followed by Diet Coke and green tea. The least staining was seen with the artificial saliva solution. The higher discoloration caused by the coffee solution to the composites has been well established. Coffee contains yellow colorants with low polarity. The discoloration caused by coffee might be due to both adsorption and absorption of these colorants 13,14 . The low polarity of these colorants results in slower discoloration. The absorption and penetration of the colorants into organic phase of materials may be due to the compatibility of the polymer phase with the yellow colorants in the coffee 15,16 . Diet Coke did not produce as much as discoloration as coffee, potentially due to its lack of yellow colorants 16-18. .
The specimens immersed in green tea gave ΔE values of between 2 and 2.5 for all the specimens tested. Green tea has a thin green color. In most of the specimens, the green colorant had no greening effect on the specimens, regardless of the composite. This trend suggests that the tested green tea solution did not contain tannins and the concentration of green colorant must be too low to be effective 17 .
Specimens immersed in artificial saliva showed no visible color change. For ΔE values, the slight change observed might be due to the adsorbed mucin of the artificial saliva on to the composite surfaces.
Composite resins that can take up water are also competent to take in other fluids containing pigments, leading to discoloration, with water mainly acting as a medium for stain penetration into the resin matrix 17,18 . This is the reason discoloration is seen in both groups. To our knowledge this is the first study to compare Cention N with a composite (solare Sculpt) in terms of staining. As such, further studies are required for further substantiation.

Conclusion
Within the limitations of the present study, the null hypothesis, that there would be no difference in staining susceptibility among tested materials, was rejected. Staining beverages caused significant discoloration for both test materials. Cention N showed higher color variations with all beverages compared to Solare Sculpt. Coffee showed the highest degree of staining with both materials, followed by Diet Coke and then green tea.

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