The effect of whitening toothpastes on the surface properties and color stability of different ceramic materials
Tooth brushing is one of the most common methods used to provide oral hygiene . However, people’s expectations of toothbrushing today are not only to improve oral health, but also to achieve whiter teeth. To meet this demand, toothpastes are formulated with various ingredients to remove stains and prevent plaque build-up. Abrasives are the major cleaning agents in toothpaste formulations, according to recent studies . Unfortunately, depending on the restorative material, abrasives that play an effective role in whitening and stain removal can create undesirable surface roughness on teeth or dentures. This can damage the restorative material and cause scratches that can lead to discolouration. The amount of wear is closely related to the particle size, density and arrangement of the abrasives in the paste, as well as the frequency and force of brushing . Rough surfaced restorations are more prone to plaque accumulation and staining. An increase in bacterial retention has been reported for roughness values greater than 0.2 μm . Oral care manufacturers are constantly improving and developing new approaches to teeth whitening to meet individual expectations. Therefore, there is a wide variety of products on the market today that address the problem of tooth discolouration. Pastes containing silica, calcium pyrophosphate, sodium bicarbonate, calcium carbonate act by mechanically removing the coloured biofilm and chromophores from the enamel surface . Oxidants such as hydrogen peroxide and calcium peroxide reduce the severity of colouration by chemically changing the pigments adhering to the tooth surface . Optical whitening toothpastes containing blue covarine, which can be considered a new technology, create a colour change through a light effect, instead of eliminating or changing the pigments on the tooth surface. This light effect is achieved by coating the tooth enamel with a fine blue layer . Another whitening group that has gained popularity in recent years is toothpastes and powders containing activated charcoal . It is believed that activated charcoal has an effect on extrinsic pigments in a similar way to the abrasion caused by toothpaste . However, there are concerns in the existing literature regarding its impact on the surface. Charcoal has been described as an abrasive mineral for teeth or gingival tissue. Increasing the size of charcoal particles also adversely affects surface smoothness and increased surface roughness can also lead to caries formation and discolouration . Modern dentistry has seen a shift from traditional manufacturing methods to Computer Aided Design-Computer Aided Manufacturing (CAD-CAM) technology . This technology offers a standardized and predictable approach by eliminating traditional measurement and model acquisition methods, thereby reducing the margin of error and shortening production time . The increasing demand for aesthetics has driven the development of ceramic materials used with CAD-CAM technology. The successful combination of mechanical properties, such as wear resistance and high rigidity, with aesthetic and biological compatibility features has resulted in a growing preference for ceramic materials as restoration materials . Resin ceramic materials developed for CAD-CAM workflow have enabled the increased use of polymer and ceramic combinations in dental restorations and are preferred in minimally invasive dentistry applications due to their excellent processability and superior aesthetic properties . These materials can be classified as resin nanoceramics (RNCs) and polymer infiltrated ceramics (PICN). Resin nanoceramics consist of nanometric sized ceramic fillers randomly distributed in a polymer matrix . Zirconia-reinforced lithium silicates, another material compatible with CAD-CAM systems, offer advantages in terms of both superior aesthetic properties and mechanical strength due to their high glassy content, making them frequently used in clinical practice . The purpose of this in vitro research was to investigate the effect of toothpastes with different chemical properties on the surface properties and colour of CAD-CAM resin nanoceramics and zirconia-reinforced lithium silicate. The null hypotheses of the study were: (a) whitening toothpastes will not effect the surface roughness of hybrid and glass ceramic materials, (b) whitening toothpastes will not cause discolouration of the tested materials. 

Preparation of samples A total of 96 rectangular samples (2 × 10 × 12 mm 3 ) were prepared by cutting CAD-CAM blocks (A2, HT) made of resin-based nanoceramic (Cerasmart, GC Corp., Tokyo, Japan) and zirconia-reinforced lithium silicate (Celtra Duo, Dentsply, Constance, Germany) with a water-cooled precision cutter (Micracut 201, Metkon, Bursa, Turkey) (Fig. ). Both sides of all samples were grounded with silicon carbide abrasive papers (Gripo 2 V, Metkon, Bursa, Turkey) with 400, 600, 800, 1000 and 1200 grit at 100 rpm respectively . The specimens were cleaned in an ultrasonic bath (Vitasonic-II, Vita Zahnfabrik) and dried for 60 min at room temperature. The Celtra Duo samples were then randomly separated into two groups. One group was treated with glaze paste (Celtra Duo Universal Glaze, Dentsply, Sirona) and subjected to firing (Programat P-310, Ivoclar Vivadent, Schaan, Liechtenstein) at a sintering temperature of 820℃ for 1 min with a sintering speed of 60 °C/min, while the other group remained untreated . A digital caliper (Alpha Tools, Mannheim, Germany) was used to check the final thickness of the samples. The specimens of each group were further divided into four subgroups ( n = 8) for brushing with toothpastes containing different whitening ingredients. Roughness measurement The initial surface roughness values of the samples were measured via a contact profilometer (Surtronic 25, Taylor Hobson, Leicester, UK) with a measuring length of 4 mm, cut-off length 0.8 mm and stylus speed of 1 mm/sec, then recorded in µm. The instrument was calibrated prior to each measurement. Measurements were carried out on three distinct areas of the sample surfaces and the initial surface roughness values (Ra0) were obtained by averaging. After the brushing process, the final surface roughness values (Ra1) were registered by applying the same principles. All measurements were performed by the same observer (Ş.E.G) . Color measurement Initial colour measurements of all specimens were made using the standard D65 light source with a dental spectrophotometer (Vita Easyshade Advance, Vita Zahnfabrik, Germany) in accordance with the CIE L*a*b* colour system. Measurements were taken from the center of the specimens on a grey background. The instrument was positioned perpendicular to the sample surface. Before each measurement, the spectrophotometer was calibrated in “single tooth” mode with a white calibration plate supplied by the manufacturer. In order to avoid possible variations in colour values, all measurements were taken three times and averaged. After brushing, the final colour values were measured by using the same method . The colour difference (ΔE) was calculated using the following equation ΔE= [(ΔL*) 2 + (Δa*) 2 + (Δb*) 2 ] 1/2 . Mechanical brushing A brushing simulator (DentArGe TB-6.1 Brushing Simulator, Analitik Medikal, Turkey) was used to simulate the brushing process (Fig. ). Each specimen was fixed in the six seperate plastic containers of the simulator with condensed silicone (Zetaplus, Zhermack SpA, Badia Polesine, Italy). FDA-certified toothbrushes of medium hardness (Dipadent, Difaş, İstanbul, Turkey) were screwed to the plastic toothbrush holder arms. For all study groups, 1:1 mixture of toothpaste and distilled water was prepared and placed in plastic containers to cover the samples . 1 g of toothpaste was mixed with 1 ml of distilled water. A digital analytical balance (Radwag AS220R2, Poland) and syringe were used to adjust the amounts. It was considered to ensure that the mixture was always present on the samples. The control group brushed with a conventional toothpaste (Colgate Max Fresh, Colgate-Palmolive, New York, USA). The other three groups brushed with three different whitening toothpastes. A silica-containing toothpaste (Opalescence, Ultradent Products Inc., Utah, USA) was used for the second group, an activated carbon-containing toothpaste (Curaprox Black in White, Curaden, Kriens, Switzerland) for the third and a blue covarine-containing toothpaste (Signal White Now, Unilever, France) for the fourth group (Table ). 30,000 brushing cycles, equivalent to 3 years, were performed on all samples. The speed of the brush was 250 per minute with a back-and-forth motion . To ensure standardization, a new toothbrush and toothpaste mixture were prepared for each sample. Brushing was conducted at room temperature (25 °C) with a vertical force of 350 g, a stroke length of 10 mm, and a reciprocating motion at a rotational speed of 40 mm/sec. The toothbrushes were changed every 5,000 cycles . After brushing, all samples were washed in an ultrasonic bath and dried for 24 h. Surface morphology The surface morphology of the samples was analyzed by using scanning electron microscopy (SEM) (Quanta FEG 450; Oxford Instruments, Uedem, Netherlands) at x2000 magnification, under low vacuum, at 20 kV with a working distance of 9.3–11.2 mm. SEM image of one sample for each group is presented. Obtained data were processed by SPSS V28 (IBM Corp. IBM SPSS statics for windows, Armonk, NY, USA). The normality of the data was analyzed with Kolmogorov-Smirnov test. Kruskal-Wallis test was performed to compare the data among the groups for both surface roughness and color stability. Mann-Whitney test is used for pairwise comparison of groups. For analysis of dependent quantitative data, Friedman and Wilcoxon tests were used. A p-value of ≤ to 0.05 was considered statistically significant. 

A total of 96 rectangular samples (2 × 10 × 12 mm 3 ) were prepared by cutting CAD-CAM blocks (A2, HT) made of resin-based nanoceramic (Cerasmart, GC Corp., Tokyo, Japan) and zirconia-reinforced lithium silicate (Celtra Duo, Dentsply, Constance, Germany) with a water-cooled precision cutter (Micracut 201, Metkon, Bursa, Turkey) (Fig. ). Both sides of all samples were grounded with silicon carbide abrasive papers (Gripo 2 V, Metkon, Bursa, Turkey) with 400, 600, 800, 1000 and 1200 grit at 100 rpm respectively . The specimens were cleaned in an ultrasonic bath (Vitasonic-II, Vita Zahnfabrik) and dried for 60 min at room temperature. The Celtra Duo samples were then randomly separated into two groups. One group was treated with glaze paste (Celtra Duo Universal Glaze, Dentsply, Sirona) and subjected to firing (Programat P-310, Ivoclar Vivadent, Schaan, Liechtenstein) at a sintering temperature of 820℃ for 1 min with a sintering speed of 60 °C/min, while the other group remained untreated . A digital caliper (Alpha Tools, Mannheim, Germany) was used to check the final thickness of the samples. The specimens of each group were further divided into four subgroups ( n = 8) for brushing with toothpastes containing different whitening ingredients. 

The initial surface roughness values of the samples were measured via a contact profilometer (Surtronic 25, Taylor Hobson, Leicester, UK) with a measuring length of 4 mm, cut-off length 0.8 mm and stylus speed of 1 mm/sec, then recorded in µm. The instrument was calibrated prior to each measurement. Measurements were carried out on three distinct areas of the sample surfaces and the initial surface roughness values (Ra0) were obtained by averaging. After the brushing process, the final surface roughness values (Ra1) were registered by applying the same principles. All measurements were performed by the same observer (Ş.E.G) . 

Initial colour measurements of all specimens were made using the standard D65 light source with a dental spectrophotometer (Vita Easyshade Advance, Vita Zahnfabrik, Germany) in accordance with the CIE L*a*b* colour system. Measurements were taken from the center of the specimens on a grey background. The instrument was positioned perpendicular to the sample surface. Before each measurement, the spectrophotometer was calibrated in “single tooth” mode with a white calibration plate supplied by the manufacturer. In order to avoid possible variations in colour values, all measurements were taken three times and averaged. After brushing, the final colour values were measured by using the same method . The colour difference (ΔE) was calculated using the following equation ΔE= [(ΔL*) 2 + (Δa*) 2 + (Δb*) 2 ] 1/2 . 

A brushing simulator (DentArGe TB-6.1 Brushing Simulator, Analitik Medikal, Turkey) was used to simulate the brushing process (Fig. ). Each specimen was fixed in the six seperate plastic containers of the simulator with condensed silicone (Zetaplus, Zhermack SpA, Badia Polesine, Italy). FDA-certified toothbrushes of medium hardness (Dipadent, Difaş, İstanbul, Turkey) were screwed to the plastic toothbrush holder arms. For all study groups, 1:1 mixture of toothpaste and distilled water was prepared and placed in plastic containers to cover the samples . 1 g of toothpaste was mixed with 1 ml of distilled water. A digital analytical balance (Radwag AS220R2, Poland) and syringe were used to adjust the amounts. It was considered to ensure that the mixture was always present on the samples. The control group brushed with a conventional toothpaste (Colgate Max Fresh, Colgate-Palmolive, New York, USA). The other three groups brushed with three different whitening toothpastes. A silica-containing toothpaste (Opalescence, Ultradent Products Inc., Utah, USA) was used for the second group, an activated carbon-containing toothpaste (Curaprox Black in White, Curaden, Kriens, Switzerland) for the third and a blue covarine-containing toothpaste (Signal White Now, Unilever, France) for the fourth group (Table ). 30,000 brushing cycles, equivalent to 3 years, were performed on all samples. The speed of the brush was 250 per minute with a back-and-forth motion . To ensure standardization, a new toothbrush and toothpaste mixture were prepared for each sample. Brushing was conducted at room temperature (25 °C) with a vertical force of 350 g, a stroke length of 10 mm, and a reciprocating motion at a rotational speed of 40 mm/sec. The toothbrushes were changed every 5,000 cycles . After brushing, all samples were washed in an ultrasonic bath and dried for 24 h. 

The surface morphology of the samples was analyzed by using scanning electron microscopy (SEM) (Quanta FEG 450; Oxford Instruments, Uedem, Netherlands) at x2000 magnification, under low vacuum, at 20 kV with a working distance of 9.3–11.2 mm. SEM image of one sample for each group is presented. Obtained data were processed by SPSS V28 (IBM Corp. IBM SPSS statics for windows, Armonk, NY, USA). The normality of the data was analyzed with Kolmogorov-Smirnov test. Kruskal-Wallis test was performed to compare the data among the groups for both surface roughness and color stability. Mann-Whitney test is used for pairwise comparison of groups. For analysis of dependent quantitative data, Friedman and Wilcoxon tests were used. A p-value of ≤ to 0.05 was considered statistically significant. 

Surface roughness Surface roughness values before brushing did not differ significantly between material groups ( p > 0.05) (Table ). However, brushing increased surface roughness for all materials. Notably, the roughness of the material surfaces brushed with Opalescence™ and Curaprox™ differed significantly from the other groups (Table ). The surface roughness values of CS samples brushed with both Curaprox™ and Opalescence™ were found to be higher than those of CD and CDG samples. When evaluating the effect of different toothpastes on the surface roughness of the materials, no significant difference was observed in any of the CS ( p = 0.426), CD ( p = 0.102), and CDG ( p = 0.129) samples (Table ). A significant difference was observed between the materials brushed with Curaprox™ when analysing the change in surface roughness values of the samples before and after brushing ( p = 0.008). Specifically, surface roughness change of CS samples were significantly higher than CD and CDG groups in the Curaprox™ brushed materials (Table ). Color change Between the toothpaste groups, CS showed a significant difference in colour change ( p = 0.01). The control group had the highest colour change (7.54), while Curaprox™ had the lowest (4.05) (Table ). No significant difference was found between the brushed CD samples ( p > 0.05) (Table ). However, a significant difference was found in CDG samples ( p = 0.018). Opalescence™ treated samples exhibited the highest ΔE value (8.0), while the control group exhibited the lowest (5.65) (Table ). For the samples brushed with Curaprox™, a statistical difference ( p = 0.034) was found between all material groups. CD and CDG colour changes were significantly higher than CS ( p < 0.05). Surface morphology Figure shows surface images of all groups and nonbrushed specimens. Following the brushing process, brush marks and varying degrees of deterioration were observed on all sample surfaces. The brush marks on CS samples were more noticeable than on CD and CDG samples. 

Surface roughness values before brushing did not differ significantly between material groups ( p > 0.05) (Table ). However, brushing increased surface roughness for all materials. Notably, the roughness of the material surfaces brushed with Opalescence™ and Curaprox™ differed significantly from the other groups (Table ). The surface roughness values of CS samples brushed with both Curaprox™ and Opalescence™ were found to be higher than those of CD and CDG samples. When evaluating the effect of different toothpastes on the surface roughness of the materials, no significant difference was observed in any of the CS ( p = 0.426), CD ( p = 0.102), and CDG ( p = 0.129) samples (Table ). A significant difference was observed between the materials brushed with Curaprox™ when analysing the change in surface roughness values of the samples before and after brushing ( p = 0.008). Specifically, surface roughness change of CS samples were significantly higher than CD and CDG groups in the Curaprox™ brushed materials (Table ). 

Between the toothpaste groups, CS showed a significant difference in colour change ( p = 0.01). The control group had the highest colour change (7.54), while Curaprox™ had the lowest (4.05) (Table ). No significant difference was found between the brushed CD samples ( p > 0.05) (Table ). However, a significant difference was found in CDG samples ( p = 0.018). Opalescence™ treated samples exhibited the highest ΔE value (8.0), while the control group exhibited the lowest (5.65) (Table ). For the samples brushed with Curaprox™, a statistical difference ( p = 0.034) was found between all material groups. CD and CDG colour changes were significantly higher than CS ( p < 0.05). 

Figure shows surface images of all groups and nonbrushed specimens. Following the brushing process, brush marks and varying degrees of deterioration were observed on all sample surfaces. The brush marks on CS samples were more noticeable than on CD and CDG samples. 

To maintain oral hygiene, toothbrushes and toothpastes are the most commonly used tools. Ideally, a toothpaste should effectively clean and remove external stains without damaging tooth enamel and restorations. However, various properties added to these tools can cause permanent changes to teeth and restorations . This research compared the effects of four different toothpastes, which containes silica, blue covarine, activated carbon, and one conventional, on the surface roughness and colour values of CAD-CAM materials. The null hypotheses were rejected as significant changes observed in roughness and colour of the materials. All samples were treated with the same surface finishing procedures to eliminate surface irregularities and ensure standardization. Studies have shown that surface roughness values exceeding 0.2 μm lead to biofilm formation on restoration surfaces, increased adsorption of colorant particles and material wear . A study reported that surface roughness values of 0.5 μm and above could be differentiated by the individual’s tongue . In line with this, our study obtained similar initial surface roughness values for all samples (CS = 0.28 ± 0.09, CD = 0.26 ± 0.07, CDG = 0.17 ± 0.04). Similarly, Siam et al.  conducted a study on zirconia-reinforced lithium silicate specimens (Celtra Duo) which were divided into two subgroups, glazed and polished. No statistically significant difference was found between the two surfaces ( p = 0.8204) when the surface roughness of the specimens was measured using an optical profilometer. Likewise, our study found no statistically significant difference in the surface roughness values of CD and CDG samples before brushing. Abrasions on the surface of restorations are a crucial factor affecting their clinical lifespan. Various ageing protocols are used to simulate long-term material evaluation, with thermal cycling, mastication cycles, and brushing simulations being the most common . This study, observed an increase in surface roughness values for all materials after the ageing process with brushing. Similarly, Kim et al. , reported increased surface roughness values for Cerasmart and Celtra Duo blocks after thermal ageing in their investigation of surface properties of different CAD-CAM blocks. In contrast, Picolo et al.  found no significant difference in surface roughness after brushing for lithium silicate glass ceramics reinforced with zirconia. This variation is believed to be caused by differences in toothpaste types and dilution ratios, as well as the use of a soft toothbrush for brushing. In a study evaluating the surface roughness of chairside CAD-CAM materials after brushing, the Cerasmart group showed the lowest surface roughness values before brushing and the highest roughness values after brushing when compared to leucite-based glass ceramic and polymer infiltrated glass ceramic blocks . Sugiyama et al.  investigated the surface roughness of CAD-CAM blocks with different contents after mechanical cleaning. They reported higher roughness values for composite blocks (Shofu Block) compared to lithium silicate blocks (Celtra Duo). In our study, the surface roughness values of the CS group specimens brushed with Curaprox™ and Opalescence™ were significantly different from CD and CDG samples ( p < 0.008). This variation is likely due to the lower hardness of resin compared to ceramic-based materials. When examining the post-brushing colour change values, a significant difference was found between the CS and CDG groups ( p = 0.010, p = 0.018, respectively), while no difference was observed in the CD group ( p = 0.669). Pouranfar et al.  investigated the colour change between ceramic polymer (Cerasmart, VITA Enamic) and lithium disilicate (IPS E-max CAD) after 12 years of brushing. Our findings are consistent with their finding that the color change of ceramic polymers was greater. The degree of staining depends on the stain resistance of the material and the bleaching agent content. The higher degree of colour change in the CS group, compared to the CD group, can be attributed to the resin content in the material. The CDG group’s higher color change compared to the CD group is believed to be due to the abrasion of the glaze material and the material’s increased susceptibility to external factors. In recent years, various teeth whitening products, including toothpaste and mouthwash, have been introduced to markets and pharmacies. However, the effectiveness and potential downsides of these non-prescripted products remain unclear. According to a study, toothpastes containing silica can erode the resin matrix, resulting in increased surface roughness. However, toothpastes with hydrated silica have lower abrasive properties . The toothpastes used in this research contained different types of silica. Opalescence™ toothpaste contains silica, while Signal™ White Now, Colgate™ Max Fresh, and Curaprox™ Black in White toothpastes contain hydrated silica. The brushing process resulted in a significant increase in the roughness values of CS samples brushed with Opalescence™, which may be attributed to its silica content. Activated charcoal is added to toothpastes to enhance their whitening properties by absorbing coloring substances. While some studies suggest that it can effectively clean teeth due to its porous and large surface area, the literature on this topic is limited . Palandi et al.  reported that charcoal-containing products did not have a whitening effect and might cause negative changes in enamel topography in their brushing study with bovine teeth. Thomas et al.  stated that charcoal-containing pastes showed lower whitening and higher abrasion than other alternatives. A study investigated the colour and surface properties of composite resin toothpastes containing whitening agents. The results showed that toothpastes containing activated carbon exhibited no statistical difference in colour change compared to conventional toothpastes . Our study found that charcoal-containing pastes had similarly low whitening efficiency, but were the only group that caused a significant difference in surface roughness change values compared to other paste types. The use of blue covarine in toothpastes aims to create a fine, translucent appearance on tooth enamel, reducing the yellowish color and making teeth appear whiter and brighter by shifting the shade on the scale towards white . However, while some studies in the literature contradict this claim, no research has been conducted specifically on CAD-CAM ceramics. A study was performed to examine the impact of whitening toothpastes and mouthwashes on bovine dentin. The study found that blue covarine content resulted in similar color change values to traditional fluoride toothpastes . Demir et al.  stained two different composite resins and then brushed them with toothpastes having different chemical ingredients. They reported that the toothpaste containing blue covarine provided a partial improvement, but this was not within clinically acceptable limits. Another study investigating the effect of whitening and conventional toothpastes on the discolouration of various composite resins reported that blue covarine-containing pastes had no different effect than conventional pastes . The results also displayed that Signal™ White Now produced similar color changes to conventional toothpaste in all three material groups. The scanning electron microscope (SEM) images confirmed the changes measured with the profilometer. The evaluation of these images revealed that brushing ceramics with abrasive toothpastes caused more significant changes in surface morphology compared to conventional toothpaste. The CS samples brushed with Opalescence™ and Curaprox™ toothpastes showed deeper and more prominent lines. Some of the limitations of this research include the inability to imitate the thermal and pH cycles of the mouth and the inability to reflect nutritional habits, saliva proteins and enzymes in the experiments. In addition, the samples have flat surfaces devoid of the anatomical pits and fissures found in natural teeth, hindering the complete simulation of polishing and brushing processes. Further studies conducted in conditions closer to the oral environment, utilizing a wider variety of toothpaste products and CAD-CAM materials, would increase the accuracy of the results. 

Irrespective of the toothpaste used, it was concluded that the roughness of all sample surfaces increased after brushing. Resin-containing nanoceramics exhibited greater wear compared to glazed and non-glazed zirconia-reinforced lithium silicates. It is important to note that toothpastes containing charcoal may impair surface smoothness. The impact of tooth whitening toothpastes on colour alteration varies depending on the properties of the material. 

