The important factor in facial beauty and satisfaction with dentofacial appearance based on literature review is tooth color. Current researches have revealed that 17% to 53% of people from different population are dissatisfied with their tooth color.¹,²

The increasing demand for shiny white teeth, tooth color improvement and an attractive smile has led to advances in tooth bleaching products and composite resins.³ Tooth bleaching is a highly popular esthetic dental procedure. However, some concerns exist with regard to the risks and complications of dental bleaching such as tooth hypersensitivity and gingival irritation as well as increased enamel surface roghness, tooth softening, and subsequently increased risk of demineralization, degradation of dental restorations and unacceptable color change or color mismatch of restorations with the adjacent tooth structure.⁴,⁵

Over the counter (OTC) bleaching products such as whitening mouthwashes and toothpastes are also available.⁶ OTC bleaching products are affordable and can be used by patients at home without requiring a supervision by dentists. OTC products are available in the form of gel, whitening strips, dental floss, chewing gums, mouthwashes, and toothpastes. However, toothpastes account for over 50% of OTC bleaching products.⁷,⁸ This tendency to whitening tooth pastes maybe due to their low cost and easy to use application.⁹

The whitening tooth paste provide the same anti-caries and anti-gingivitis therapeutic benefits of conventional tooth pastes with additional whitening active components such as abrasives, adsorbent particles, peroxides, enzymes, or optical effect agents.¹⁰

The mechanism of action of whitening toothpastes is mainly based on the presence of high amounts of abrasives.⁷,¹¹ Continues usage and the presence of high quantity of abrasive particles in these abrasive toothpastes can improve enamel brightness and reflectance but this may cause excess wear and removal of tooth structure.⁹,¹⁰ These features of abrasive whitening toothpaste have led to adding other materials (peroxide, activated charcoal) in whitening toothpastes that allow whitening without the risk of tooth damage.

The efficacy of peroxide in the composition of toothpastes is a matter of discussion due to its low concentration, the natural instability of it in an aqueous formula, the additional dilution by salivary flow and short duration of contact with the tooth structure. However, previous studies reported that a toothpaste containing 1% hydrogen peroxide significantly decreased the yellowness and increased the brightness of teeth compared with a peroxide-free conventional toothpaste containing silica.¹²,¹³,¹⁴

Recently, activated charcoal has gained the spotlight because of its claimed advantages. The whitening effect of this compound is due to its high potential to absorb stains, chromophores and stain spots. This is because the activated charcoal is highly porous and provides a large surface area (>1000 m²/g) for absorption of stains. Although this effect has not been scientifically proven, 96% of commercial products containing activated charcoal claim that they can effectively bleach the teeth.¹⁴,¹⁵

Composite resins are increasingly used for dental restorations due to their favorable color and minimal invasiveness. Color and surface roughness are important factors determining the durability of composite resin restorations.¹⁶ Over 80% of patients complain of color mismatch of their composite restorations and adjacent teeth.¹⁷ Considering the patient's tendency to have whiter teeth and overcome the color mismatch of composite restoration with low cost and easy to use treatment, it seems imperative to evaluate the effect of whitening tooth pastes on the aesthetic quality of composite resin such as color. On the other hand, the composition and physical properties of resin composite compare to enamel are different, so the behavior of these materials are likely affected by brushing with whitening toothpastes.

A few studies have investigated the effect of whitening tooth paste on the color of resin composites.²,¹⁵,¹⁸ These studies evaluated the color immediately after polymerization of composite restoration.

It should be considered that the color alteration and superficial deterioration of a restorative material may be caused by physical/chemical factors such as temperature, pH, humidity, ultraviolet irradiation, absorption, and adsorption of pigments and mechanical factors.¹⁹ Different methods have been introduced to reconstruct the proposed factors. Accelerated artificial aging (AAA) is a precise method for assessment of physical, chemical, and optical changes of nonmetal restorative materials such as composite resins due to aging.²⁰ AAA usually use extensive intervals of exposure to ultraviolet radiation, moisture, and changes in temperature.²¹ This condition can change reaction of composites in exposure to whitening toothpastes.

To the best of author's information, there is a gap information the influence of whitening tooth paste on the color of aged composites. Thus, this study aimed to assess effect of different whitening toothpastes on the color of aged micro hybrid composite.

This experimental study was performed on five groups including four groups of whitening tooth pastes and one control group (GC). The sample size for this study was calculated to compare the color change in five groups. If the ratio of mean difference between groups to standard deviation is two, with the test power of 90% and type 1 error rate of 5%, in each group of study nine samples would be required.²² Ethical approval code was (IR.TUMS.DENTISTRY.REC.1397.166) in Tehran university of medical sciences. Table 1 presents the materials used in this study and their composition.{Table 1}

Fabrication of samples

A stainless steel mold (2 mm × 7 mm) was used for this purpose. The A2 shade of Spectrum TPH 3 microhybrid composite was applied in the mold placed on a glass slab and a transparent Mylar strip. After applying the composite, another glass slab and Mylar strip were placed over it and it was compressed such that the excess composite leaked out. The sample was then light-cured using a light-curing unit (Guilin Woodpecker Medical Instrument Co,), guilin, China with 1000 mW/cm² light intensity for 20 s from both sides. The samples were polished with 1000 and 1200-grit abrasive papers with 10 strokes for each side. The testing surface was marked.

Measuring the color parameters before aging

The color parameters of the samples were measured based on the International Commission on Illumination L*a*b* color space using a spectrophotometer (Easyshade, VITA Zahnfabrik, BadSackingen, Germany). The spectrophotometer was used after calibration according to the manufacturer's instructions. The samples were placed against a white background and the three parameters of L*, a* and b* were measured. For the purpose of standardization, a jig with a suitable size for the discs was fabricated. The discs were mounted on the jig and their color parameters were measured three times at the center against a white background, then the mean was calculated.

Aging

The samples underwent AAA in a Xenontest Alpha LM (Heraeus Kulzer, Hanau, Germany) device for 300 h corresponding to 1 year of composite aging in the oral environment. This device had a filter to simulate daylight (within the range of visible light) by changing the spectral energy distribution of xenon lamp. The samples were held by clasps under similar conditions such that they were under full exposure of light. The device was adjusted at 37°C and 100% humidity according to ISO 7491. The color of composite discs was measured again after 300 h of aging by the spectrophotometer as explained earlier. Color change (ΔE_m) calculated based on this formula: ΔE_m= ([ΔL_m] ² + [Δa_m] ² + [Δb_m) ²])^½. The differences were determined with ΔE₁ (after aging − base line). Then, it was investigated whether composites showed any color mismatch (△E1 > 5.5) to be included in the study or not.²³

Grouping of samples

Composite resins were randomly divided into five groups (n = 9). One group served as the Gc and underwent brushing with distilled water while the remaining four groups were subjected to brushing with four toothpastes namely Bencer (Gb), Optic White (Go), Colgate Total Whitening (Gt), and Perfect White Black (Gp).

Brushing of samples

For the purpose of standardization, a device was specifically designed for this purpose. The samples were placed on a glass mold. A mixture of toothpaste and distilled water (50 w/50 w) was prepared in a beaker on a vibrator. The solution was poured in the container of device such that the entire composite surface was immersed in it. Tooth brushing was performed in back-and-forth motion within 5 mm range. The speed of toothbrushing was adjusted at 60 rpm. Each sample was tooth-brushed for one hour corresponding to twice toothbrushing per day, each time for 2 min for a total period of 15 days.¹⁸ Finally, all samples were rinsed and dried.

The CIE values (L_m, a_m, b_m) were determined using a spectrophotometer again. The color changes (△E_m) were calculated based on the formula that mentioned earlier. The differences were defined by ΔE₂ (after brushing-after aging) and ΔE₃ (after brushing-base line).

Statistical analysis

Data were analyzed using SPSS version 24 (IBM Corporation North Castle Drive, MD-NC119 Armonk, NY 10504-1785 US). The Q-Q plot and Shapiro–Wilk test were applied to assess the distribution of data. The results showed that data were not normally distributed. Thus, changes in each of the a*,b* and L* parameters after tooth brushing and after aging in each group and the color parameter changes (ΔL_2, Δa₂, Δb_2, ΔL_3, Δa₃, Δb₃) and ΔE₂ and ΔE₃ were calculated for each toothpaste group and compared using the Wilcoxon test and independent sample median test. P < 0.05 was considered statistically significant.

Table 2 presents the a*,b* and L* parameter in the five groups after aging and before tooth brushing. The b* parameter significantly decreased after tooth brushing in Gb (P = 0.008) and Go (P = 0.015) groups, compared with after aging. This reduction was borderline significant for Gp (P = 0.05) while it was not significant in Gt (P = 0.066). The change in b parameter was not significant in the Gc (P = 0.72) according to Wilcoxon test.{Table 2}

The a* parameter significantly decreased after tooth brushing in Gb (P = 0.015) and Gp (P = 0.038) groups. This change was not significant in other groups (P > 0.05). The L* parameter increased in all groups after tooth brushing but this increase was only significant in Gb, Go, and Gp (P = 0.008).

Table 3 shows color change parameters in specimens. The color parameter changes, ΔE₃ and △E₂ were not significantly different among the five groups (P > 0.05).{Table 3}

The minimum △E₃ was noted in Gb and Gp (△E < 3.3), which is the critical threshold for clinically acceptable color change).²⁴ The maximum △E₂ was noted in Gp (4.1) and the minimum △E₂ was noted in Gt (2.02).

This study assessed the effect of different whitening toothpastes on the color of aged micro hybrid composite. The results revealed no significant difference in color change of composite following tooth brushing with whitening toothpastes with different mechanisms of action. Thus, the null hypothesis of the study was confirmed. However, the maximum △E₂ was noted in Gp while the minimum △E₂ was noted in Gt. The highest and the lowest △E₃ was seen in Gc and Gb, respectively. It should be noted that △E₃ reached to <3.3 only in Gp and Gb (charcoal whitening toothpastes), which indicates that the color change caused by aging would not be perceivable by the human eye after tooth brushing with charcoal whitening toothpastes after 15 days' application.

Nano crystalline structure of activated charcoal with excessive surface zone (>1000 m²/g) and a high number of prose produce effective capacity of this component to cleaning of dentition and absorption of chromophores in oral cavity.¹⁰,¹⁴

Greater space for the water molecules to diffuse into the polymeric network by degradation after AAA may contribute to lower color stability of resin composite.²⁵ We believe that high potential of adsorbent of activated charcoal may absorbed these water and degradation by products and influenced the optical properties of aged dental composite.

The abrasiveness of these toothpastes depends on their manufacturing process and the amount of carbon.¹⁵,²⁶ In addition, Gb have some other abrasive components such as dicalcium phosphate dehydrate, thickener silica and abrasive silica. Dicalcum phosphate is a cleaning agent for natural teeth and dental composite.²⁷ Another charcoal tooth paste also contains hydrated silica abrasive factor. Gp contains two kinds of surfactants such as sodium lauryl sulfate and cocamidopropyl betaine, which may make the hydrophobic agents available and distribute the toothpaste particles in the oral cavity. Therefore, presence of these two surfactants in the composition of Gp may increase the efficacy of its active components. These explanations may be contributing to discriminate the effect of these two charcoal toothpaste on color of aged composite. However, the abrasive particles in Go dentifrice and Gt are silica, calcium pyrophosphate and hydrated silica, respectively.

Since anti-calculus abrasive products containing phosphate do not have a favorable taste, higher amounts of flavor should be added to toothpastes with higher amounts of abrasives. Therefore, Limonene has been added to the composition of Gp. The authors believe that Limonene may have greater effect on surface properties of aged composites due to its acidic nature, resulting in higher △E₂.

Gt only contains hydrated silica and TiO₂ pigments, and sodium hydroxide to adjust its pH. As a result due to its higher pH and lower amounts of abrasives, the lowest color change of aged composite after brushing has been seen in this group (△E₂).

Colgate contains hydrogen peroxide and due to its synergistic effects with silica and tetra-sodium pyrophosphate, it ranked second in terms of color change of aged composite.

De Moraes Rego Roselino et al.,²⁸ in an in situ study assessed the effect of whitening toothpastes on different composites in clinical setting and reported that different abrasive toothpastes had insignificant effect on color stability of composites.

Based on author's literature review, only three experimental studies were assessed the effect of whitening tooth pastes on the color of resin composite which were without staining and aging.²,¹⁵,¹⁸ Al-Shalan¹⁸ showed that the color change is related to the kind of restorative material and whitening tooth paste. The result of Roopa's research¹⁵ showed that a greatly significant color change was seen with whitening toothpaste after 2 weeks' usage in compomer and composites. While Hashemikamangar et al.² showed in their research that toothpastes had no effect on the color of the composites, We cannot compare the results of these studies with our study, because they used composite samples without aging.

Changes in b* parameter is associated with patient's satisfaction, in comparison to a* and L* parameters.¹⁰,¹⁶

In charcoal toothpastes, the a* and b* parameters significantly decreased after toothbrushing while the L* parameter significantly increased. In other words, samples with a shift to blue and green and their lightness increased after toothbrushing with these whitening toothpastes. However, the △b₂ and △a₂ in these groups had no significant difference with the corresponding values in other groups, which was in agreement with the results of de Moraes Rego Roselino et al.²⁸ Limonene present in the composition of Gp has optical isomerism and may change the lightness (L* parameter) to some extent.²⁸ In the control and Gt groups, the a*, b* and L* parameters slightly changed after toothbrushing. This finding may be due to the fact that the only bleaching agents in the composition of Gt are hydrated silica and titanium oxide pigments, which do not significantly affect the color and lightness of composites discoloration after aging. However, in Go group, due to the presence of hydrogen peroxide with chemical mechanism of action, the b* parameter significantly decreased while the L* parameter significantly increased. It means that the yellowness of composites decreased and their lightness increased.

The efficacy of toothpastes depends on the distribution of particles in the toothpaste, their formulation, geometry of toothbrushes, filaments of toothbrushes, toothbrushing technique adopted by the operator and saliva secretion rate.¹¹ In this study, a toothbrushing machine was designed to standardize the technique of tooth brushing and the concentration of toothpaste used. This was a strength of this study.

This study had some limitations that limited the generalization of results to the clinical setting. The composite samples were flat while the restoration surfaces follow the anatomical contour of the teeth in the clinical setting. Moreover, we diluted the toothpastes with distilled water which is different from the clinical setting (presence of saliva, enzymes, proteins, and ions).¹⁸ Furthermore, we did not have adequate knowledge about the size of particles and percentage of each toothpaste ingredient since the manufacturers do not clearly disclose the composition of their products.¹⁸ Last but not least, we only assessed one type of composite (bis-GMA based, microhybrid). Similar studies on other composite types and in presence of extrinsic stains in addition to AAA are required.

Within the limitations of this in vitro study, the results showed that there is no significant difference in the change of the color parameters of spectrum composite following tooth brushing with Gp, Gb, Gt and Go for two weeks. However, based on the compression of L*, b* and a* parameters before and after tooth brushing, it became significantly lighter and showed a shift to blue and green after tooth brushing with charcoal toothpastes (Gp and Gb). In addition, it became significantly lighter and its yellowness decreased after the application of Colgate Go. The color change caused by aging would not be perceivable by the human eye after tooth brushing with charcoal whitening toothpastes (△E < 3.3).

Financial support and sponsorship

This Project was supported and funded in part by Dental School, Tehran University of Medical Sciences (Number: 6404).

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.