Nadia S. Al-Angari,Shahad Quwayhis,Nouf Almayouf,Shahad Meaigel,Abdulelah Aldahash,Sarah S. Al-Angari
Effect of staining and whitening systems on color stability of computer aided design/computer aided manufacturing lithium disilicate glass ceramic
Bleaching,Ceramics,Discoloration,Lithium disilicate,Milled
Purpose To investigate the effect of coffee stain and whitening systems on the color stability of CAD/CAM glazed lithium disilicate glass-ceramics (LDGCs). Methods 68 glazed LDGC discs (12 × 10 × 2 mm) were fabricated using blocks of CAD/CAM systems (IPS e.max CAD ceramic). Baseline color was measured (CIE/L*a*b*), and specimens were randomized into four groups (n = 17). All specimens were stained (coffee solution; 24 h/d × 12) and then subjected to two whitening protocols. G1- (kept moist × 7 days); G2-positive control (brushed with distilled water, 200gm/load, 2 min twice daily × 7 days); G3- whitening toothpaste (Colgate optic white; relative dentin abrasivity = 100, 200gm/load, 2 min twice daily × 7 days) and G4-simulated at-home bleaching protocol (Opalescence,15% carbamide peroxide (CP), 6 h/day × 7 days). The study outcome was color change (ΔE) measured at baseline, after staining, and after whitening treatments. Data were analyzed using paired T-test and one-way ANOVA (α = 0.05). Results All groups stained equally (p > 0.05) but were not clinically significant (ΔE ≤ 1.05). Stains were significantly (p ≤ 0.01) improved in G2 and G3 (ΔE = 0.69 and 0.63), yet were not eliminated compared to bleaching, which had the highest color improvement and completely removed the stains (ΔE = 0.72). Conclusions Glazed LDGC was color stable after simulating one year of coffee staining. Bleaching, using 15% CP for a week, completely removed the stains and returned the LDGCs to their original shade. However, simulating eight months of brushing, regardless of the toothpaste content, improved the color outcome but didn’t remove it completely.
Effect of staining and whitening systems on color stability of computer aided design/computer aided manufacturing lithium disilicate glass ceramic

To investigate the effect of coffee stain and whitening systems on the color stability of CAD/CAM glazed lithium disilicate glass-ceramics (LDGCs).

68 glazed LDGC discs (12 × 10 × 2 mm) were fabricated using blocks of CAD/CAM systems (IPS e.max CAD ceramic). Baseline color was measured (CIE/L*a*b*), and specimens were randomized into four groups (n = 17). All specimens were stained (coffee solution; 24 h/d × 12) and then subjected to two whitening protocols. G1- (kept moist × 7 days); G2-positive control (brushed with distilled water, 200gm/load, 2 min twice daily × 7 days); G3- whitening toothpaste (Colgate optic white; relative dentin abrasivity = 100, 200gm/load, 2 min twice daily × 7 days) and G4-simulated at-home bleaching protocol (Opalescence,15% carbamide peroxide (CP), 6 h/day × 7 days). The study outcome was color change (ΔE) measured at baseline, after staining, and after whitening treatments. Data were analyzed using paired T-test and one-way ANOVA (α = 0.05).

All groups stained equally (p > 0.05) but were not clinically significant (ΔE ≤ 1.05). Stains were significantly (p ≤ 0.01) improved in G2 and G3 (ΔE = 0.69 and 0.63), yet were not eliminated compared to bleaching, which had the highest color improvement and completely removed the stains (ΔE = 0.72).

Glazed LDGC was color stable after simulating one year of coffee staining. Bleaching, using 15% CP for a week, completely removed the stains and returned the LDGCs to their original shade. However, simulating eight months of brushing, regardless of the toothpaste content, improved the color outcome but didn’t remove it completely.

With the demand in modern clinical practice for an artistic smile, restoration discoloration remains a challenge in esthetic dentistry (Kara et al., 2013). Staining is an optical property that may affect a perfectly fabricated restoration resulting in its possible replacement (Petridis et al., 2012). It has multiple causative factors and various treatment modalities, ranging from bleaching as a conservative approach to veneers or crowns as the most aggressive option (Kara et al., 2013, Rodrigues et al., 2019, Alencar-Silva et al., 2019). Ceramic restorations' color stability is critical in satisfying the patient's esthetic demands. It is achieved by carefully selecting the ceramic material type, composition, and surface treatment (Fu et al., 2020). Therefore, lithium disilicate glass ceramic (LDGC) is the most widely used glass–ceramic due to its biocompatibility, excellent mechanical properties, and various esthetic shade options and translucencies (Alencar-Silva et al., 2019, Fu et al., 2020). Whitening systems such as bleaching gels, strips, and whitening toothpaste are non-invasive, safe, and easy approaches to improving teeth color (Al-Angari et al., 2020a, de Andrade et al., 2021, Al-Angari, 2021a, Al-Angari and Eisa, 2020b). However, a concern arises when patients with ceramic restorations demand teeth whitening, as it may alter the physical and optical properties of adjacent restorative materials such as Computer-Aided Design & Computer-Aided Manufacturing (CAD/CAM) ceramic systems and other restorative materials (Rodrigues et al., 2019, Al-Angari et al., 2021b). Bleaching may affect ceramics' surface properties depending on the peroxide concentration, time, and delivery method (Rodrigues et al., 2019). Whereas, toothbrushing cycles, load and the toothpaste composition may impact the ceramic’s characteristics (Rodrigues et al., 2019). As staining, peroxides' chemical action, and the whitening toothpaste's mechanical motion may affect the shade of glazed ceramics (Kara et al., 2013, Garza et al., 2016, Al-Angari et al., 2021b), this in-vitro study aimed to investigate the efficacy of staining and two whitening systems (bleaching gel and whitening toothpaste) on the color stability of glazed LDGC.

Glazed LDGC discs were subjected to staining then treated by two whitening systems to evaluate the color stability. The study investigated two factors: staining and whitening treatments measured at four levels; negative control (no treatment), positive control (brushed with distilled water), brushing with whitening toothpaste, and at-home bleaching system (Fig. 1). The study outcome was color change (ΔE) measured at different time points by an optical spectrophotometry.

Sixty-eight LDGC discs specimens (12 × 10 × 2 mm) were cut from blocks of LDGC (IPS e.max CAD LT A2, Ivoclar Vivadent, Schaan AG, Liechtenstein) using a low-speed diamond disk (Series 15LC Diamond; Buehler, Illinois, USA) in a sectioning saw (Isomet 1000; Buehler, Illinois, USA) under water-cooling and lubricant. This was followed by finishing the specimens using a tungsten carbide bur on a straight handpiece (Komet, Rock Hill, USA). The specimens were placed in a furnace (Programat PS10, Ivoclar Vivadent, Schaan AG, Liechtenstein) for sintering at a temperature of 850° C, for 24 min. 30 sec., after which, all experimental surfaces were glazed (IPS e.max Ceram Glaze paste, Ivoclar Vivadent, Schaan AG, Liechtenstein) before the final heat treatment in the furnace. After complete crystallization, specimens were numbered on the untreated side with a marker then a clear nail polish to protect it throughout the experiment.

L*a*b* values (Commission Internationale de l'Eclairage) were taken for each specimen at baseline, after staining, and after the assigned whitening treatments using a spectrophotometer (VITA Easyshade, VITA Zahnfabrik, Bad säckingen, Germany) with a light beam focus of 0.3 mm. All measurements were performed and repeated three times by one examiner. The baseline L* value was used for stratified randomization of the specimens into four groups. The color difference (ΔE) was calculated after staining (ΔEstaining: staining-baseline) and after the assigned whitening treatments (ΔEtreatment: treatment-staining): (Karakaya and Cengiz-Yanardag, 2020)

All specimens were subjected to a coffee solution (Dunkin' Donuts Original Blend Ground Coffee, Medium Roast, Shahia Food Limited Company, Saudi Arabia) prepared according to the manufacturer's instructions and used immediately after preparation. Specimens were kept in the stirring staining solution in an incubator at 37 °C for a total period of 12 days, in which coffee was replaced daily. After the staining period, specimens were rinsed with running distilled water for one min. to remove any coffee residue.

Group 1 (G1); the control group, was kept in a saline solution, changed daily. Group 2 (G2); was brushed with distilled water. Group 3 (G3); was brushed with a slurry of distilled water and whitening toothpaste. Group 4 (G4) at-home bleaching agent, 15% carbamide peroxide (CP) (pH 6.5; Opalescence PF, Ultradent Products, Inc., Utah, USA). A 1.0 mm thick layer of CP gel was applied onto the surface and kept for 6 h/day, based on the manufacturer's instruction. During the testing period, specimens were returned and kept in an incubator at 37 °C for a total treatment of seven days. After each treatment, specimens were rinsed with running distilled water for 1 min. to remove the whitening systems' residue, blot dried, and stored in an incubator, followed by color measurements performed immediately afterward.

Group 2 and G3 were fixed on a custom-made holder and brushed in an automated brushing device using a straight, soft electric toothbrush (Oral B Pro-Expert, Procter and Gamble, Ohio, USA). Brushing was done under a standardized force of 2 N, with an oscillatory-rotating movement (continuous mode) at a rate of 8800 S/min. (Lee et al, 2019) twice daily for 2 min. as recommended by the American Dental Association (Creeth et al., 2009, Al-Angari et al., 2021b). The total brushing time was 28 min., representing around 8 months of toothbrushing. (Lee et al., 2019, Al-Angari et al., 2021b). Group 2 was brushed with distilled water, while G3 was brushed using a suspension of distilled water and a dentifrice with a medium relative dentin abrasivity (RDA) equivalent to 100 (American National Standards Institute/American Dental Association. Standard No. 119; 2015., https://www.ada.org/en/science-research/dental-standards/dental-products/products-standards-technical-specifications-and-technical-reports) (Colgate optic white sparkling white, Colgate-Palmolive Arabia Ltd., Saudi Arabia) with a paste to water weight ratio of 1:1, in which the slurry (80 g) was replaced daily (Al-Angari et al., 2014). After each brushing cycle, specimens were rinsed with water and gently air-dried.

Before the study, a specimen size calculation showed that 17/group was determined to show a difference of 0.4 for ΔE to be statistically significant at 95% confidence level, power of 80%, and a standard deviation of 0.4. Data were analyzed using SPSS (SPSS statistics v.23, IBM, New York, USA). Comparisons within each group were determined using a paired t-test. An independent t-test was used to check the presence of a significant difference between each pair of groups. Differences between the groups were checked using the ANOVA test followed by Tukey's post-hoc test to determine the significant difference among groups (α = 0.05).

Regarding staining, all groups were stained equally, with no significant difference (p > 0.05). However, the staining was minimum and did not reach the clinical perceptible range (ΔE < 3.3). After using the designated whitening treatment protocols, G4 showed the highest color improvement (eliminated stains), yet had no significant difference (p > 0.05) due to the slight difference in the mean color change (ΔETreatment 0.73 vs. ΔEStaining 0.72). Groups 2 and 3 significantly (p ≤ 0.01) improved the color outcome, yet did not eliminate the stains. Furthermore, they were significantly different than G1 and G4. Among the treatment groups, G3 significantly changed color lightness (p = 0.02) compared to G1. However, the color change did not significantly differ from G2 and G4 (Table 1).

As CAD/CAM technology has considerably developed, the color stability of CAD/CAM LDGC remains a crucial factor in maintaining a long-term esthetic outcome (Karakaya and Cengiz-Yanardag, 2020). Most patients with CAD/CAM LDGC restorations would not refrain from drinking coffee or colored beverage/food. Instead, they would use different whitening agents to improve their teeth color regardless of its effect on the surrounding CAD/CAM LDGC restorations. This is based on their understanding that ceramics are not affected by environmental factors. Therefore, this study investigated the effect of staining and whitening systems on the color stability of CAD/CAM LDGC. Coffee was used due to its popularity and intense staining effect (yellow chromophores) on teeth and restorative materials (Alencar-Silva et al., 2019, Al-Angari et al., 2020a). The staining period was 12 days (total of 288 hrs.), representing one year of coffee consumption, as each day corresponding to one month in clinical service (Al-Angari et al., 2021b). This calculation was based on a coffee manufacturer, which states that the average coffee intake per person is around 3.2 cups, with each cup consumed in 15 min (Alp et al, 2018). The color change (ΔE) was evaluated using the golden standard CIE L*a*b* color system, which measures the color objectively to eliminate subjective evaluation and inter/intra-rater shade matching variation, especially with small numerical color changes (Alp et al, 2018). In this system, ΔE is clinically perceptible when it equals or exceeds 3.3 (Lawson et al., 2016). A single ceramic shade was used to standardize the results. Furthermore, shade A2 was selected as it is the most frequently used clinically (Alsayed et al, 2022). Simulating one year of drinking coffee, stained the LDGC yet was not clinically perceptible (ΔE ≤ 1.05). The stains are attributed to the coffee’s elevated temperature and its acidity (pH 5.1), which may have partially dissolved the glazed layer by degrading the silica particles and the alkaline ions (Dos Santos et al, 2017). Consequently, formed surface craters which increased the roughness allowing for stain absorption and a lesser ability to reflect light (Palla et al., 2018, Al-Angari et al., 2021b). However, LDGCs were color stable as they did not reach a clinically visible staining level. This is justified by the dense crystalline structure of the LDGCs, and the protective surface glaze, which limited stains to its surface rather than the ceramic’s. (Gasparik et al., 2019, Al-Angari et al., 2021b) The glazing material infiltrates into the ceramic matrix to reduce the porosities, which results in a smooth surface that would minimize external stain retention. (Garza et al., 2016, Dos Santos et al., 2017, Palla et al., 2018). This relates to previous studies, that coffee had no clinical perceptible color change on LDGC (Garza et al., 2016, Palla et al., 2018, Gasparik et al., 2019). Toothbrushing is an efficient mechanical measure to eliminate or minimize dental plaque and extrinsic stains (Garza et al., 2016). Specimens subjected to toothbrushing (simulating 8 months), regardless of the paste content, showed a significant color improvement. This was represented in a significant increase in the lightness values (L* coordinate) and a decrease in the blueness (b* coordinate) and greenness (a* coordinate) values, which directed the ΔE toward the lighter color scale, indicating color improvement. However, the resulting color outcome (ΔE ≤ 0.69) didn't exceed the average stain discoloration (ΔE ≤ 1.05), indicating that stains were partially eliminated. The mechanical motion and the polishing efficacy of toothbrushing may have removed the superficial accumulated chromophores. However, it kept residual chromophores. This supports our staining findings, that coffee may have formed craters within the gloss layer where residual stains were deposited (Al-Angari et al., 2021b). Therefore, the mechanical motion of brushing could not reach those minute areas; hence color was partially removed. Interestingly, whitening toothpaste showed the highest color improvement compared to the control group, which is justified by the combined effect of the mechanical motion of brushing and the abrasive action of silica particles within the dentifrice (Al-Angari et al., 2021b). Although the whitening toothpaste had 1% HP, which may oxidize organic surface stains during brushing, its effect was neglectable. This is justified by the extremely low peroxide level and the short contact time (2 min.) during the brushing cycles. Previous studies indicated that longer bleaching time yielded better color improvement regardless of the bleaching agent's concentration (Al-Angari et al., 2020a, Tinastepe et al., 2021, Arif et al., 2019). It also correlates with other studies, which found that the efficacy of whitening toothpaste is related to the abrasiveness in their formula regardless of the presence of the bleaching agent (Alshara et al., 2014, St John and White, 2015, Vaz et al., 2019). In our study, 15% CP had the highest color improvement as they completely neutralized the pigmentation and returned the stained samples to their original shade; therefore, no significant change was reported. This agrees with previous studies that bleaching may improve extrinsic stains of glazed ceramics (e.g: LDGC) but had no clinical nor statistical differences in the color (Kara et al., 2013, Gasparik et al., 2019). This corresponds with our stain results that the original shade of LDGC is stable and that staining or whitening occur on a superficial (glaze layer) and transient level. Unlike brushing, which partially removed the stains, bleaching eliminated all discolorations. This relates to the mechanism of action of the peroxides, as they dissociate into small free radicles (low molecular weight), which can easily diffuse within the glaze surface and oxidize more stains (Al-Angari et al., 2019). It agrees with previous studies, that bleaching removed superficial stains from glazed LDGC, yet had no significant effect on its original shade (Gasparik et al., 2019). Furthermore, when the original shade of glazed feldspathic ceramic restorations,without staining, was subjected to 15% CP with or without whitening toothpaste, it had no significant color difference. However, it was used for a longer period (2 weeks) (Rodrigues et al., 2019). Another study reported a noticeable color change (ΔE = 1.48) in LDGC restorations when subjected to 10% CP for two weeks. However, the specimens were not glazed, which allowed for a pathway, in the ceramic matrix, for the peroxide to infiltrate easily (Kara et al., 2013). Despite the advantages of in-vitro studies, related to their ability to standardized and have complete control over different variables (type of ceramic, stain, whitening treatments), the major limitation is the difficulty of simulating the complex oral environment; thus, randomized clinical studies are needed to verify the whitening treatment effect on glazed LDGC. Moreover, as patients may have more than one type of ceramics and may drink coffee or use bleaching agents for extended periods, further studies should be conducted on the effect of whitening treatment on different ceramic types and more extended staining periods. In summary, LDGC subjected to staining and whitening protocols showed minimum staining, less than the clinically perceptible limits. Although bleached specimens returned to their original shade, and brushing had a significant color improvement, it may affect the LDGC’s surface characteristics (e.g: gloss and surface roughness), creating areas of stain stagnation that would affect its optical properties (Al-Angari et al., 2021b).

Within the limitations of this in-vitro study, glazed CAD-CAM LDGC subjected to a simulated one year of coffee staining, was color stable as it stained but not to a clinically perceptible level. Furthermore, simulating eight months of brushing regardless of the toothpaste content had a significant effect on removing the extrinsic stains. Whereas bleaching, using 15% CP for a week, completely removed the stains and returned them to their original shade.

Dr. Nadia Alangari is the main author, designed the study, prepared the methodology protocol, provided the material, and the write up review. Dr.Quwahis prepared the samples, performed the methodology to group1, and manuscript write up. Dr.Almayouf prepared the experimental groups, the coffee solution mixture, and performed the methodology to group 2. Dr.Meiagel performed the methodology to group 3 and preparing the data for statistical analysis. Dr.Aldahash performed the methodology to group 4 and performed the color test. Dr. Sarah Alangari reviewed the methodology, the manuscript write up, and the statistical analysis.

The authors of this study “Effect of Staining and Whitening Systems on Color Stability of CAD/CAM Lithium Disilicate Glass Ceramic” declare that they have no known conflict of interest, any competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This study gained IRB approval from King Abdullah International Medical Research Center (SP19/451/R) and King Saud University (FR 0521).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.