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 Table of Contents  
Year : 2020  |  Volume : 17  |  Issue : 3  |  Page : 90-96

Color changes induced by different staining solutions and different light curing units in dental resin composites

1 Department of Restorative Dental Sciences, College of Dentistry, Taibah University, Al Madinah Al Munawwarah, Saudi Arabia
2 Department of Operative Dentistry, Faculty of Dentistry, King Abdulaziz University, Jeddah, Saudi Arabia

Date of Submission07-Dec-2019
Date of Acceptance01-Jun-2020
Date of Web Publication30-Oct-2020

Correspondence Address:
Rania Z Mubarak
Department of Restorative Dental Sciences, College of Dentistry, Taibah University, Al Madinah Al Munawwarah 2904-42312
Saudi Arabia
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/tdj.tdj_56_19

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To evaluate color changes induced by three different staining solutions and three different light curing devices in two different resin composites.
Materials and methods
Two different resin composites (ESTELITE Σ QUICK and ESTHET.X HD) were tested in this study. Hundred discs (5 mm diameter × 2 mm thick) of each tested material were made. Ten specimens were used as control, while other 90 specimens subdivided into three subgroups (n = 30); (a) cured with halogen-light curing-unit, (b) cured with plasma-arc unit, and (c) cured with argon laser of 488 nm. Each subgroup was further subdivided into three classes (n = 10): (a) immersed in tea for 10 min three times daily for 3 months, (b) immersed in coffee for 10 min three times daily for 3 months, and (c) immersed in Arabic coffee for 10 min three times daily for 3 months. Between staining challenges, specimens were immersed in artificial saliva. Specimens were tested for quantitative color changes using Quanta Environmental Scanning Electron Microscope. Data was statistically analyzed using three-way analysis of variance and Tukey's post-hoc test (P ≤ 0.05).
The mean color change of coffee (126.53 pixel) showed the highest value. This was followed by tea (117.96 pixel) and Arabic coffee (113.5 pixel) with no statistically significant difference between the two solutions. Meanwhile, laser showed the highest mean value (122.55 pixel) whereas; there was no statistically significant difference between QHL (117.9 pixel) and plasma arc curing (117.54 pixel). No significant difference was found between the two tested materials ESTELITE Σ QUICK and ESTHET.X HD.
The two tested composites performed similarly under the test conditions. Curing with argon laser along with the coffee as staining solution affected the color stability of the tested materials.

Keywords: light curing units, color assessment, color change, common beverage, dental composites

How to cite this article:
Mubarak RZ, Abo El Naga AI. Color changes induced by different staining solutions and different light curing units in dental resin composites. Tanta Dent J 2020;17:90-6

How to cite this URL:
Mubarak RZ, Abo El Naga AI. Color changes induced by different staining solutions and different light curing units in dental resin composites. Tanta Dent J [serial online] 2020 [cited 2020 Nov 27];17:90-6. Available from: http://www.tmj.eg.net/text.asp?2020/17/3/90/299632

  Introduction Top

The application of resin composites for dental restorations currently occupies a large part of the routine work of dentists not only due to patients' demands for esthetic restorations, but also due to favorable handling properties, adhesive properties, and capacity of mimicking the dental structure as well[1]. Recently, patients seek better color-matching restorations and composite resins are used to satisfy this need[2]. Hence, the proper color match to the adjacent tooth is important not only in the first period of service, but also over a longer period of time. Color stability of composite resin is an important property influencing its clinical longevity, which continues as a challenge inherent to material[3],[4].

Consequently, assessment of color stability and discoloration has been included in commonly used outcome measurement tools that rate the success and failure of composite resin restorations in dental clinical practice[5].

Several extrinsic and/or intrinsic factors can affect the color stability[6]. External discoloration can be the result of dietary and smoking habits, bad oral hygiene and adsorption or absorption of water soluble stains throughout the resin matrix[7],[8]. The composition and size of the filler particles affect surface smoothness and susceptibility to extrinsic staining[9]. Several studies have been conducted to address the influence of exogenous factors such as dyes from beverages on the color stability of resin-based materials. Most of these surveys include the influence of tea and coffee on the color stability of the dental restorative materials. Some of these experiments show that changes in color and shade are greatest in tea[10], whereas, some indicated coffee had the most effect in color change in the samples[11].

On the other hand, intrinsic factors, such as the resin matrix of composites[12] and incomplete polymerization[13],[14], have a considerable influence on color stability. This is usually attributable to chemical degeneration of the filler-resin bond and solubility of the resin matrix[12]. Adequate polymerization of resin composites is important to ensure optimum physico-mechanical properties. The effectiveness of polymerization is not only dependent upon the chemistry of the material and the filler particle size, but also on the light curing units, including spectral distribution, exposure time and intensity[15].

Today, there are a range of light-curing units, such as conventional quartz-tungsten-halogen, light-emitting diode (LED), plasma arc curing (PAC) and laser used in dentistry. Halogen curing light is among the most frequently used LCUs in dental offices[16]. In spite of the popularity of this curing light, it possesses some drawbacks including limited lifetime and high temperature generated by the filament that can damage the curing light bulb and cause pulp injuries as well, the large difference between electrical input and light output with time[17],[18]. A decrease in light output will cause a low degree of monomer conversion, which will affect the color stability of the composite[16]. The extent of polymerization depends on the conversion percent or the ratio of double bonds converted to single bonds. Composites with a higher percent conversion have greater mechanical properties, greater wear resistance and better color stability[19].

The LED curing units were developed to overcome the problems inherent with halogen units and can polymerize light-cure resin-based composites generating much less heat[20]. The efficacy of this device is due to the coordination of the light-emitting spectrum with absorption spectrum of camphorquinone[21],[22]. Infra-red spectrum is not emitted by the LED in contrast to the halogen LCUs. Longer lifetime and more stable output are among the advantages of LED curing lights. High powered LED is also available with an output equal to that of halogen[23].

Moreover, aiming at clinical time reduction, high-power units, xenon PAC units, were introduced in the market. However, PAC units were found to produce significant heat as the energy passes through the electrodes. Argon laser is another option for composite resin polymerization. It emits blue and green lights. Blue light can polymerize dental composite resins that have camphorquinone as the photoinitiator[24]. Laser light has some characteristic features like being monochromatic and collimated and having selective absorption[25].

Hence coffee and tea, the two most common beverages, are consumed many times daily throughout the world, participating in the staining of oral tissues, teeth, and prostheses. Therefore, it is necessary to investigate the potential risk that the consumption of these beverages poses to composite resins as they age, along with the effect of different light-curing units on the properties of these composite resins, as these factors work collectively to cause the net resultant staining and discoloration.

  Materials and Methods Top

Materials used in this study

Two types of resin composites

ESTELITE Σ QUICK (EQ) (resin-based dental restorative material) (Tokuyama Dental, Tokyo, Japan).

ESTHET.X HD (EX) (high definition micro matrix restorative) (Dentsply Caulk, USA).

Three types of common beverages:

Coffee (Nescafe) (Nescafe, Nestle, Montes Claros, Brazil).

Lipton tea sachet (Unilever Golf FZE, Dubai, United Arab Emirates).

Arabic Coffee Royal Quick Preparation (Mochachino, Jeddah, Saudi Arabia).

Preparation and grouping of the specimens

The two composites (EQ and EX) were tested in this study, where 100 discs (5 mm diameter × 2mm thick) of each tested material were prepared. Ten specimens were used as control, while other 90 specimens subdivided into three subgroups (n = 30): (a) cured with halogen-light curing-unit (PRO-DEN Systems Inc., North Lombard Street, Portland, USA), (b) cured with plasma-arc unit (Apollo95E, California, USA), and (c) cured with argon laser of 488 nm.

Dietary colorants used in this study were common beverages with natural and artificial colors, which may cause staining of composite resin surfaces. Three different beverages were used in this experiment: tea, coffee, and Arabic coffee.

Thus, each subgroup was further subdivided into three classes (n = 10): (a) immersed in tea (prepared by placing tea sachet together with 10 g of white sugar in boiling water for 5 min and then the sachet was removed) for 10 min three times daily for 3 months, (b) immersed in coffee (prepared by mixing 12 g of natural coffee powder and 10 g of white sugar with 200 ml of boiling water) for 10 min three times daily for 3 months, and (c) immersed in Arabic coffee (prepared by mixing the coffee powder in the packet together with 200 ml of boiling water for 5 min) for 10 min three times daily for 3 months. Between staining challenges, specimens were immersed in artificial saliva.

Color measurement

Specimens were tested for quantitative color changes using Quanta Environmental Scanning Electron Microscope and specific computer software. By using XT Document program, the scanning photomicrograph were taken by the QESEM [Figure 1]a and converted to have Quantitative computerized image analysis using digital scanner with a special computer program. This program divides the surface of all specimens' image on computer monitor into points [Figure 1]b. Each point has a pixel value at the two coordinates (x, y) as shown. From the data of the two coordinates, the gray value for each point was calculated.
Figure 1: (a) Scanning photomicrograph taken by the QESEM. (b) Specimen's image divided into points. Each point has a pixel value at the two coordinates (x,y).

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Statistical analysis

Data were presented as mean and SD values. Regression model using three-way analysis of variance (ANOVA) was used in testing significance for the effect of material, staining solution, curing technique and their interactions on color. Tukey's post-hoc test was used for pair-wise comparison between the mean values when ANOVA test is significant. The significance level was set at P value less than or equal to 0.05. Statistical analysis was performed with IBM SPSS Statistics, version 20.

  Results Top

Three-way ANOVA results showed that curing unit, staining solution and the interaction between the three variables had a statistically significant effect on mean color. Material had no statistically significant effect on mean color value [Table 1].
Table 1: Descriptive statistics for color values

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Effect of staining solutions

Coffee showed the statistically significantly highest mean color value. There was no statistically significant difference between tea and Arabic coffee; both showed lower mean values. Control showed the statistically significantly lowest mean color value [Table 2].
Table 2: Comparison between color values of different staining solutions

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Effect of material

There was no statistically significant difference between mean color values of the two materials [Table 3].
Table 3: Comparison between color values of the two materials

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Effect of curing units

Laser showed the statistically significantly highest mean color value. There was no statistically significant difference between PAC and HQL; both showed lower mean values. Control showed the statistically significantly lowest mean color value [Table 4].
Table 4: Comparison between color values of the different curing units

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  Discussion Top

Composite resins are widely adopted for esthetic procedures due to their excellent properties and the power of bonding to the tooth structure. However, one of the dramatic drawbacks is the color change with time, which is a major cause for restorations replacement[26].

There are many extrinsic and intrinsic factors that influence color stability of composite resins[27]. In this study, the immersion medium and light curing sources were analyzed as prominent elements to optical alterations of resin composites. Since color plays an important role in obtaining natural appearance, which is a goal in the recent esthetic era, color stability of dental restorations is one of the most important features disturbing its longevity. Thus, it is mandatory to estimate and understand the color changes that occur after the light-activation of resin composites[28].

In general, it was approved that the color stability of the resin composites is related to the resin matrix, dimensions of filler particles, degree of polymerization, depth of cure, photo initiators and staining agents[29],[30].

It was shown that different beverages are contributing factors to composite color instability. Our study showed observable results of visible discoloration in the tested composite material when exposed to all immersion media coffee, tea, and Arabic coffee. According to Lepri and Palma-Dibb[31], different beverages caused discoloration of composite resins. That was in accordance with other investigations[12],[32],[33].

As exhibited by Domingos et al.[30], and Poggio et al.[34], coffee caused the highest color change among all groups which was in accordance with the current study.

This finding disagrees with Ertas et al.[11], who evaluated the discoloration of different types of composite resin upon exposure to tea, cola, coffee, red wine, and water. They claimed that there was no significant difference in color changes for composites stained in coffee or tea solutions. Also the finding of the present study disagrees with Yazici et al.[35], who claimed that the effect of coffee on color change of composite was similar to tea after storage in both solutions.

As tea and coffee are the most frequently consumed drinks, it was chosen to observe their effects on long-term usage. Many studies reported that coffee causes more discoloration than tea which was also in a line with the current study[11],[36].

It was evidenced that coffee discoloration is associated with both adsorption and absorption mechanisms. Adsorption of colorant on the surface and absorption in the subsurface layer. It was claimed that coffee includes yellow color causing materials that have low polarity, which are released and penetrate to the organic part of the materials. This adsorption and penetration of colorants into the organic phase of the materials were explained due to the compatibility of the polymer phase with the yellow colorants of coffee[37]. This finding was in confirmation with the findings of Gupta et al.[3], while the effect of the Arabic coffee was similar to tea in causing discoloration in this study, this might be because of the concentration of the caffeine in it.

On account of denaturized materials in tea and a compound with tannins, these materials caused chemical interactions that led to discolorations both intrinsic and extrinsic. The first may be due to penetration of yellow pigments in to them through micro-cracks or interfacial gaps at the interface between filler and matrix, while extrinsic one may be due to adsorption of polar colorants and yellow pigments existing in tea, onto the surface of resin composite materials[8],[11],[35].

It was reported by several authors that the tannic acid existing in tea may cause discoloration of resin composites[38].

Accordingly, it was conveyed that discoloration in resin composites exposed to tea solution are a result of adsorption of tea stains, while coffee stains are due to both adsorption and absorption. It was advocated that both tea and coffee contained yellow colorants with different polarities[32].

According to this study, the artificial saliva also promoted a slight color change in the specimens. The same result was obtained by Omata et al.[39], when comparing the color change of composite resin specimens immersed in distilled water and artificial saliva, which observed that the distilled water group did not undergo any color change, while the artificial saliva did[39].

Saliva contains no pigment, so its susceptibility to causing discoloration may be due to their degree of water absorption and the hydrophilic/hydrophobic nature of the resin matrix and departure of soluble materials from the structure. Water absorption occurs mainly as direct absorption by the resin matrix[3].

It was shown that glass filler particles cannot absorb water, yet they can contribute to water adsorption at the surface of the material. The level of water sorption is a function of the resin content of the material and the strength of the resin-filler interface. The water absorption causes filler–matrix debonding or hydrolytic degradation of the fillers. In addition, it makes micro-crack or interfacial gaps between filler and matrix, and allows intrinsic discoloration.

If the resin matrix is capable of absorbing water, it is also capable of absorbing any other fluid, which ultimately leads to discoloration. In addition, discoloration might be due to the differences in the refractive index of filler and matrix which might increase after water absorption[11],[38]. That may be considered an additional reason for the discoloration in all groups.

When evaluating the effects of staining solutions on the color stability of EX EQ, utilized in the current study, there was no statistically significant difference between mean color values of the two tested materials. The composite color changes were related to their compositions. Since both EX EQ, are a light activated composite, which contains Bisphenol A diglycidyl ether dimethacrylate (BIS-GMA) and tri ethylene glycol dimethacrylate (TEG DMA) as the resin matrix and 60% (volume) Silanized fluoro-aluminum borosilicate glass, 71% (volume) silica/zirconia, respectively. The filler average particle size is 0.6 and0.2 μm, respectively.

It was revealed that the presence of TEG DMA in materials cause a high amount of hydrophilic capacity and more sensation of BIS-GMA to tonality and water absorption[8]. Moreover, it has been noted that a composite with large filler particles are more prone to water aging discoloration than a composite with small filler particles, which is in line with the hydrolytic degradation matrix filler interfaces. Thus, a composite with large filler particles has more color permeability than a composite with small filler particles. Accordingly, we can conclude that the composite, in the presence of small-to-large filler particles, with a BIS-GMA and TEG DMA resin base, is more prone to color discoloration and water sorption[40],[41].

The resin composite should be adequately polymerized to achieve optimal mechanical and optical properties. Since the higher degree of conversion, the smaller the amount of residual monomers available to form colored degraded products[42]. Incomplete polymerization of the resin composite may cause undesirable properties, such as water absorption and solubility of the unreacted monomers, making it more susceptible to staining[35]. Observing light sources as factor, it was noted that this might cause color alteration to the studied composite resin. Yet, any light source presented significant color changes[27]. This probably occurred because the light source interfered with the effectiveness of the polymeric conversion, because the quality of the polymerization reaction associated with the composition of the resin matrix and the degree of conversion of the photoinitiator are the elements responsible for the esthetic restorative materials' color stability[14].

Several studies reported that the curing lights caused a significant difference in color change[30],[34], which was in accordance with the current study, where laser showed the statistically significantly highest mean color value. Although there are numerous reports of enhanced physical properties of laser-polymerized composite resins, there are conflicting reports about argon laser curing where increased shrinkage and brittleness of some small-particle resins has been reported. One author suggests that the increased penetration and heat of the laser might result in both a greater degree of polymerization and a higher degree of polymerization shrinkage compared to conventional light curing[43]. Recent research on the influence of light intensity has suggested that the more rapid rate of cure obtained by high-intensity curing does not allow enough time for stress relaxation by flow of partially cured material[44].

If these results can be extrapolated to the argon laser, perhaps the greater depth of penetration and intensity of the laser beam result in a rapid polymerization with no chance for stress relaxation.

As regards PRO-DEN systems which is a high-power halogen-light device, it can promote the formation of polymer chains with lower molecular weight and residual monomers and, consequently, partial polymerization of the material, with part of the photoinitiator remaining idle.

Camphorquinone, which is the most frequently utilized photoinitiator, has a yellow coloring and its color changes when photoactivated, thus becoming 'transparent.' However, when the irradiation is not sufficient, a small amount of camphorquinone remains inactive, causing a residual yellow in the final color of the composite resin, which may give a darker color to the material and this could be one of the factors that might have interfered in the composite resin light when photoactivated by the unit[14].

Therefore, the light source is an important factor to be taken into account when an esthetic restoration is performed with composite resin.

On the other hand, the PAC unit possesses the inherent property of rapid curing. This may result in the formation of short polymer chains, as the faster rate of cure might not allow sufficient time for the pre-gel phase of the material to absorb the polymerization/contraction stresses[45]. A high degree of conversion not only gives hardness and strength to a material, but also responsible for color stability. Thus, a reduction in the remaining double bonds to the lowest possible level is normally considered a desirable feature of a polymerization system[46].

Another explanation for the undesirable color change of PAC specimens could be the high susceptibility to water sorption at the resin-filler interface. Color perception is directly connected to light scattering by particles in the composite resin and interfaces among them. This is one of the well documented weak points of composite materials[47].

Nevertheless, a research[35] showed no statistically significant difference in the color change results when the different types of light sources were compared. Hence, the light source is an important factor to be taken into account when an esthetic restoration is performed with composite resin.

  Conclusion Top

Based on the employed methodology and the obtained results, it may be concluded that:

When working in esthetics area, upon the selection of composite resin material, color stability is an important variable to be considered. Despite advancement in resin and filler technologies, composites still remain vulnerable to staining by daily consumed drinks. Where coffee was the tested immersion medium that had the most influence on color stability of the composite resin, followed by tea; and Arabic coffee. These stains limit the esthetic lifespan of composite restorations. Moreover, regarding the light sources argon laser promoted the greatest color changes in the tested resin composites. Further research to understand and limit stain absorption within composites is recommended.

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Conflicts of interest

There are no conflicts of interest.

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  [Figure 1]

  [Table 1], [Table 2], [Table 3], [Table 4]


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