Comparative evaluation of color stability of three composite resins in mouthrinse: An in vitro study.

AIM: The aim of this study was to compare the effect of chlorhexidine mouthrinse on the color stability between three different types of composites.
MATERIALS AND METHODS: A total of 30 samples of size 10-mm length, 1-mm width, and 1-mm thickness were prepared on a customized microglass slide from each of the composite materials and immersed in 20 ml of distilled water followed by incubation at 37°C for 24 h. The samples were divided into three groups (n = 10) - Group I: A nanofilled composite, Filtek Z350XT (3M ESPE, St. Paul, USA); Group II: A microhybrid composite, Polofil Supra (Voco GmbH, Germany); and Group III: A nanoceramic composite, Ceram.x Sphere TEC (Dentsply, Konstanz, Germany). Baseline color values were recorded using a spectrophotometer (V-770 UV-Visible/NIR Spectrophotometer, Easton, Maryland, USA) according to the laboratory scale. After baseline color measurements, ten randomly selected specimens from each group were immersed in 20 ml of 0.2% chlorhexidine mouthrinse (Rexidin Plus, Aurangabad, India) for 24 h. The postimmersion color values of the samples were then recorded, respectively, using the same spectrophotometer. STATISTICAL ANALYSIS USED: The statistical analysis was done using one-way ANOVA followed by Tukey's post hoc test.
RESULTS: Statistically significant difference was observed between the mean color change values in the three groups (P < 0.05) with the highest color change (delta E [ΔE]) in Group III (Nanoceramic composite). The ΔE for Group I (Nanofilled composite) was 3.16, Group II (Microhybrid composite) was 3.32, and Group III (Nanoceramic composite) was 3.51.
CONCLUSION: All the three types of composites displayed color changes after immersion in mouthrinse, but the color shift depended on the material used, and the nanofilled composites (Filtek Z350XT, 3M ESPE, St. Paul, MN, USA) had higher color stability.

INTRODUCTION

Resin-based restorative materials have evolved over the past decades and are widely used to meet esthetic demands in dental practice. The color stability of these restorative materials in a dynamic oral environment is an important criterion influencing its clinical longevity which still continues as an inherent challenge to the material. Although the quality of composite resin restorations has improved with the advent of new technologies in material science in recent years, discoloration of the composite resin materials remains to be the major long-term clinical problem. Furthermore, the assessment of color stability and discoloration is one of the commonly used outcome measurement tools that rate the success and failure of composite resin restorations in the clinical practice.[1]

Discoloration of composite resin may occur due to several extrinsic and intrinsic factors. Extrinsic factors include adsorption and absorption of colorants from exogenous sources. The degree of color change due to exogenous sources can vary based on the presence of surface roughness, oral hygiene status, nutritional habits, smoking habits, and consumed beverages. Intrinsic discoloration may occur due to change in color of resin material itself such as change in resin matrix or at the matrix/filler interface, whereas chemical discoloration associated with alteration/oxidation of amine catalyst, polymeric matrix structure, or unreacted methacrylate group may occur due to incomplete polymerization.

A recent study suggested that the effects of intrinsic discoloration were less in completely polymerized composite resin materials as there were no perceptible color changes observed after water storage.[2] Hence, the extrinsic discoloration is the most significant factor affecting the color stability and long-term success of composite resin restorations, which focuses the need for dental researchers and material scientists to improve the resistance to discoloration of new resin-based materials for esthetic restorations.[3]

The etiology for discoloration of composites is multifactorial, of which proprietary mouthrinse is one of the causative factors.[4] The low pH of the mouthrinse may influence the hardness, wear, and color stability of composites. Furthermore, the alcoholic ingredients are not the only factor causing softening of resin composites, but studies have revealed that color stability can be reduced by both alcohol-containing and alcohol-free mouthrinses.[5] The emulsifiers and organic acids present in the mouthrinse may lead to surface degradation of the resin composite materials.[6] Since color stability of composites is of major concern that could even be affected by nonalcohol-containing mouthrinse, this study was carried out to compare the effect of a nonalcohol-containing mouthrinse on the color stability of three different types of composites.

MATERIALS AND METHODS

Materials used in this study are listed in Table 1 and Figure 1.

Table 1

Materials used in this study

Materials	Shade	Manufacturer	Resin matrix	Filler
Group I - Filtek Z350 XT	A2	3M ESPE, St. Paul, MN, USA	TEGDMA, UDMA, BisGMA	Nonaggregated 20-nm silicaNonaggregated 4-11-nm zirconiaSilica cluster filler
GROUP II - Polofil Supra	A2	Voco, GmbH, Germany	TEGDMA, UDMA, BisGMA	Sintraglass multifillersMicrofillers - 0.05 µm Macrofillers - 0.5 and 2 µm
Group III - Ceram.x Sphere TEC	A2	Dentsply, Konstaz, Germany	Reduced resin matrix content	The Sphere TEC fillers (≈15 µm)Nonagglomerated barium glass fillers (≈0.6 µm), Ytterbium fluoride (≈0.6 µm)Methacrylic polysiloxane nanoparticles
Mouthrinse used in this study
Mouthrinse		Composition		Manufacturer
Rexidin Plus (nonalcohol based)		Chlorhexidine gluconate solution IP diluted to chlorhexidine gluconate 0.2% w/v Triclosan USP 0.05% w/vSodium monofluorophosphate USP 0.07% w/vIn a pleasantly flavored aqueous base Color-Brilliant Blue		Indoco Remedies LTD., Aurangabad, India

TEGDMA: Triethylene glycol dimethacrylate, BisGMA: Bisphenol A diglycidyl ether dimethacrylate, UDMA: Urethane dimethacrylate, USP: United states pharmacopeia, IP: Indian pharmacopeia

Figure 1

Materials used in this study. (1) A nanofilled composite; Filtek K Z350XT (3M ESPE, St. Paul, MN, USA). (2) A microhybrid composite; Polofil Supra (Voco, GmbH, Germany). (3) A nanoceramic composite; Ceram.x Sphere TEC (Dentsply, Konstaz, Germany). (4) Chlorhexidine mouthrinse (Rexidin Plus, Aurangabad, India). (5) LEDition Polymerization Unit (Ivoclar Vivadent AG, Liechtenstein). (6) Composite filling instrument (GDC Composite Filling Instrument). (7) Blaze Aesthetic Polishing Kit (Medicept UK LTD). (8) Microglass slides and specimen preparation

Specimen preparation

Thirty specimens of size 10-mm length, 1-mm width, and 1-mm thickness were prepared on a customized microglass slide (12 mm × 1 mm) from each of the composite material using composite filling instrument (GDC Composite Filling Instrument). Wax index of 1-mm thickness and 1-mm width was made on either side of the microglass slide for standardization. The thickness was confirmed using a vernier caliper. Another glass slide was placed on the top and gently pressed for 30 s to extrude the excess material and for achieving a smooth surface [Figure 1]. Each specimen was cured using LED light-curing unit (LEDition Polymerization Unit, Ivoclar Vivadent AG, Liechtenstein) for 20 s for two cycles with a light intensity of 600 mW/cm2 from the upper and lower surfaces of the specimens and then polished with Blaze Aesthetic Polishing Kit (Medicept UK Ltd) in three sequences from prepolishers to high-gloss polishers following the manufacturer's instruction.

The specimens were divided into three groups (n = 10).

Group I – A nanofilled composite; Filtek Z350XT (3M ESPE, St. Paul, MN, USA)

Group II – A microhybrid composite; Polofil Supra (Voco, GmbH, Germany)

Group III – A nanoceramic composite; Ceram.x Sphere TEC (Dentsply, Konstaz, Germany).

All the prepared samples were immersed in 20 ml of distilled water in separate containers according to the group, respectively, followed by incubation at 37°C for 24 h. After 24 h, baseline color values of each sample were recorded using a spectrophotometer (V-770 UV-Visible/NIR Spectrophotometer, Easton, Maryland, USA) [Figure 2]. Colorimetric values of the specimens were determined using the L*a*b* system of the Commission Internationale de l’Eclairage (CIE L*a*b* Color Scale) [Table 2].

Figure 2

V-770 UV-Visible/NIR spectrophotometer

Table 2

Baseline and postimmersion measurements

Material (shade - A2)	Mean			Delta E
	L*	a*	b*
Group I
Nanofilled composite (baseline measurements [before immersion])	67.62	−0.33	14.34	3.16
Nanofilled composite (postimmersion measurements)	66.45	−0.26	11.41
Group II
Microhybrid composite (baseline measurements [before immersion])	68.59	−1.14	12.40	3.32
Microhybrid composite (postimmersion measurements)	66.72	−1.26	15.15
Group III
Nanoceramic composite (baseline measurements [before immersion])	69.02	−1.82	11.60	3.51
Nanoceramic composite (postimmersion measurements)	66.95	−1.02	14.35

*The mean difference is significant at the 0.05 level

After baseline color measurements, ten randomly selected specimens from each group were immersed in 20 ml of 0.2% chlorhexidine mouthrinse (Rexidin Plus, Aurangabad, India) for 24 h. After 24 h, the specimens were washed with distilled water and then blot-dried before subjecting to spectrophotometric analysis. The postimmersion values were recorded for each sample using the same UV spectrophotometer. The measurement of the discoloration of composite was analyzed by the UV spectrophotometer. The formula used for calculating color differences in this system is as follows:[2]

ΔE*ab = ([ΔL*]²+ [Δa*]²+ [Δb*]²)½

The data were collected, tabulated, and subjected to statistical analysis.

The main principle of spectrophotometry is that every substance absorbs or transmits certain wavelengths of radiant energy, but not other wavelengths. It measures the light absorption peak of the material subjected to the analysis. The wavelength of measurement spectrum used in this study was between 400 and 720 nm.

Statistical analysis

Color change between the groups was compared by performing one-way ANOVA. Multiple comparisons were made by Tukey's post hoc test [Table 3]. The data were analyzed using statistical software SPSS Version 16 (IBM Corp, Chicago, IL, USA).

Table 3

Descriptives

	n	Mean	SD	SE	95% CI for mean		Minimum	Maximum
					Lower bound	Upper bound
Group I	10	3.1690	0.08006	0.02532	3.1117	3.2263	3.05	3.33
Group II	10	3.3210	0.09882	0.03125	3.2503	3.3917	3.19	3.47
Group III	10	3.5130	0.11908	0.03765	3.4278	3.5982	3.37	3.72
Total	30	3.3343	0.17296	0.03158	3.2697	3.3989	3.05	3.72
ANOVA Color stability
		Sum of Squares		df	Mean Square		F	Significance
Between the groups		0.594		2	0.297		29.370	0.000
Within the groups		0.273		27	0.010
Total		0.868		29
Multiple comparisons Color stability Tukey’s HSD
Groups (I)	Groups (J)	Mean difference (I−J)		SE	Significance		95% CI
							Lower bound	Upper bound
Group I	Group II	−0.15200*		0.04498	0.006		−0.2635	−0.0405
	Group III	−0.34400*		0.04498	0.000		−0.4555	−0.2325
Group II	Group I	0.15200*		0.04498	0.006		0.0405	0.2635
	Group III	−0.19200*		0.04498	0.001		−0.3035	−0.0805
Group III	Group I	0.34400*		0.04498	0.000		0.2325	0.4555
	Group II	0.19200*		0.04498	0.001		0.0805	0.3035

*The mean difference is significant at the 0.05 level. SD: Standard deviation, SE: Standard error, CI: Confidence interval, HSD: Highly significant difference

RESULTS

All samples displayed color changes after immersion, and there was a statistically significant difference observed between the mean color change values in three groups. Clinically acceptable value for color change in dental materials was presumed to be ΔE ≤3.3. Group III (Nanoceramic composite) had the highest color change (ΔE: 3.51 – clinically unacceptable) than the Group I (Nanofilled composite, ΔE: 3.16 – clinically acceptable) and Group II (Microhybrid composite, ΔE: 3.32 – clinically acceptable).

DISCUSSION

The frequent reasons for replacement of tooth-colored restorative materials are staining and discoloration.[7] Discoloration can occur due to intrinsic factors or extrinsic factors. Intrinsic discoloration may occur due to aging of material itself, such as alteration of the resin matrix and changes in the interface of matrix and fillers.[8] Extrinsic discoloration may occur due to adsorption or absorption of colorants from exogenous sources such as coffee, tea, nicotine, beverages, and mouthrinses.[9] This study evaluated the effect of mouthrinse on the color stability of three different types of composites as it is one of the most important factors, which threatens the color stability of esthetic restorations.

The color changes can be assessed using various instruments including spectrophotometer and colorimeter. In this study, spectrophotometers are used because they are considered to be more accurate in measuring color change than colorimeter.[10] Spectrophotometers contain monochromators and photodiodes that measure the reflectance curve every 10 nm or less.[11] The spectrophotometer used in this study (V-770 UV-Visible/NIR Spectrophotometer, Easton, Maryland, USA) has wavelength ranged from UV to NIR (190–2700 – option 3200 nm). The gratings and detectors’ switchover can be set to change automatically between 800 and 900 nm. An advantage of the UV spectrophotometer is the fact that it can identify the exact levels of compounds within a particular spectrum sample. Each color and the saturation of the color are identifiable.

In this study, the color stabilities of three different composites immersed in chlorhexidine mouthrinse for 24 h were evaluated which are equivalent to 2 min of use daily for 2 years.[12] Studies have revealed that chlorhexidine-containing mouthrinse having 0.2% of chlorhexidine gluconate could cause perceptible color change in composites (clinically acceptable value assumed to be ≤3.3).[13]

The ability of chlorhexidine-containing mouthrinse solutions to change the color of restorative materials depends on the type of restorative materials, capability of resin matrix to absorb water, and type of filler and filler content in resin composites. The staining potential of composites can also be attributed to its surface structure. The rougher the surface, the higher the susceptibility of the material to extrinsic staining.

Resin composites absorb fluids that may cause discoloration. The amount of water absorption is related to the resin ingredient of composites and the quality of interaction between the resin and the filler. Excessive water absorption plasticizes the resin ingredients by hydrolyzation and by microcrack formation. Consequently, the interface between the matrix and fillers allows discoloration.[14]

In the present study, Group I (Nanofilled composites) had the least color change (mean value: 3.16) than the other two groups. Nanofilled composites contain nanoparticles that can fill the gaps between large particles which result in lesser and smaller voids in resin composites. Due to small particle size of the filler, the surface area of the fillers has increased dramatically, and the interaction between the matrix and filler surface is also increased. Nanofilled composites also have higher resistance to water absorption and lesser filler matrix debonding.[15] This could partly explain higher color stability nanofilled composites than other two types of composites tested in this study.

When the samples of Group II (Microhybrid composite) were immersed in chlorhexidine mouthrinse, the mean color change value was 3.32. Microhybrid composites contain filler particles with an average size of 0.01–0.04 μm which are larger than nanofillers (20 nm). Large filler particles are more susceptible to discoloration and water absorption than smaller filler particle, and the effect of coloring agents on the quality of the bond between the matrix and filler can cause discoloration. Water absorption may cause microcracks by expanding and laminating the resin component and hydrolyzing the silane and decrease the functional life of composite resins; thus, microcrack formation and interfacial gaps between the filler and matrix allow soluble coloring agents of mouthrinses to penetrate and cause discoloration.[16] This reason could be attributed to the lesser color stability of microhybrid composites than nanofilled composites. These results were in accordance with those of Gürdal et al.[17]

In the present study, of the three types of composites, Nanoceramic composite (Group III) had the highest color change (mean value: 3.51). The nanoceramic composites comprise organically modified ceramic nanoparticles and glass fillers and these nanoceramic particles and nanofillers have methacrylate group available for polymerization.[18] The highest color change of nanoceramic composites can be related to structural difference. A study demonstrated that Ceram.x did not yield better surface quality than other nanofilled composites. This was explained by low volumetric filler content of material and porosities detected on Ceram.x specimens. Rough surface has been shown to mechanically retain stains more than smooth surface.[19] Our findings are in accordance with those of Celik et al.[18] and Jung et al.[19]

However, in clinical conditions, saliva, salivary pellicle, foods, and beverages consumed may have additive/mitigating effects on the physical and esthetic properties of these groups of restorative materials.[17] Further studies are, therefore, necessary to evaluate these parameters in vivo conditions, but routine in vitro testing of esthetic restoratives is recommended for new products. Instead of premature replacement of the restorations, routine use of whitening tooth paste, repolishing techniques, and bleaching procedures are viable ways to remove superficial stains of composite resin.[2021] Despite the findings about the role of various extrinsic and intrinsic substances on the discoloration of composite resins, studies on methods of stain removal have been limited.

CONCLUSION

With the limitations of the present experimental study and the parameters used, it can be concluded that:

All the three types of composites tested in this in vitro study displayed color changes after immersion in mouthrinse, but the color shift depended on the material used

Nanofilled composites had higher color stability than other two types of composites

Resin formulation of composites including the filler has a direct impact on its susceptibility to stain by external agents, and the chlorhexidine mouthrinse can be considered as stainable solutions.

Clinical significance

Use of mouthrinse by patients should be subjected to dental supervision to control their adverse effects on the esthetic quality of restoration

Patients should be warned to expect composite restorations to discolor if they regularly use mouthrinses which contain high concentration of chlorhexidine gluconate

Knowing the composition of restorative material is important to select appropriate material for each clinical application and to use it in an effective way to promote its best properties.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.