The effect of coffee and whitening systems on surface roughness and gloss of CAD/CAM lithium disilicate glass cermaics

Abstract

Objectives:

To investigate the effects of a coffee beverage and two whitening systems on the surface roughness and gloss of glazed Lithium Disilicate Glass-Ceramics (LDGC) for computer-aided design/computer-aided manufacturing (CAD/CAM) systems.

Methods:

Sixty-eight LDGC disks (12 × 10 × 2 mm) were prepared from blocks of CAD/CAM systems (IPS e.max CAD ceramic). Baseline measurements for surface roughness (Ra) and gloss (GU) were taken using a 3-D optical profilometer and a glossmeter, respectively; then specimens were randomized into four groups (n = 17). All specimens were immersed in a coffee solution (24 h × 12 days) then subjected to two whitening systems. G1-negative control (kept moist × 7 days); G2-positive control (brushed with distilled water, 200 g/load, 2 min twice daily × 7 days); G3-whitening toothpaste (Colgate optic white; relative dentin abrasivity (RDA) = 100, 200 g/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 outcomes were measured at baseline and after the treatments. Data were analyzed using paired T-test and one-way ANOVA (α = 0.05).

Results:

The mean surface roughness significantly increased (p ⩽ 0.002) for all groups after the designated treatment protocols. Among groups, the mean surface roughness of G2 and G3 were significantly higher (p ⩽ 0.001) (Ra: 0.51 and 0.57 μm, respectively) compared to the control group (Ra: 0.23 μm), and were not significantly different from G4 (Ra: 0.46 μm). Surface gloss decreased with no significant change within or among groups after treatment.

Conclusion:

All glazed LDGC had a significant increase in surface roughness after being subjected to simulated 1 year of coffee drinking and whitening systems (15% CP and whitening toothpaste), and the greatest change was associated with brushing (simulating 8 months). However, coffee beverages and whitening systems had no significant effect on the surface gloss.

Keywords

Bleaching CAD/CAM lithium disilicate surface roughness surface gloss whitening

Introduction

Esthetic dentistry has evolved in recent years with the introduction of new dental materials and modern clinical technologies.¹ It has also been driven by patients’ demand for treatment with optimum function, health, and esthetics.¹ The launch of Computer-Aided Design and Computer-Aided Manufacturing (CAD/CAM) technology allowed for the use of different materials such as; Lithium Disilicate Glass Ceramic (LDGC), Zirconia ceramics, and Resin-Ceramic Composites, to produce chair-side prostheses that are precise, esthetic, and durable in a time-efficient manner.²

The restoration’s surface texture (surface roughness, gloss, and luster) significantly impacts the esthetic outcome and longevity of the dental prostheses, especially in challenging clinical cases of a single crown in an esthetic zone.³ LDGC has proven to be biocompatible with excellent physical and mechanical properties such as; high esthetics, color stability, surface gloss, luster, low thermal conductivity, and wear resistance.⁴ The application of a glaze layer during the final step of the ceramic fabrication minimizes several physical external factors affecting the restoration’s surface smoothness, gloss, and luster.⁵ However, patients’ faulty oral habits or exposure to various dental clinical procedures might negatively affect their physical and mechanical properties.⁴ These include; patients’ daily intake of acidic/citric diet (coffee, carbonated beverages, lemon, etc.), sudden change in the oral temperature (hot/cold beverage or food), and tooth brushing with or without whitening toothpaste.^6,7 Moreover, clinical procedures include dental bleaching, prophylaxis, crown adjustment, and scaling and root planing.⁸ Hence, studies have found that these procedures may cause possible surface degradations related to the mechanical removal of the glazed layer and the chemical dissolution of the ceramic glass networks (silica particles), resulting in a rougher and less glossed surface.^4,9,10 Consequently, the affected surface will enhance stain adsorption, change its optical properties, wear the antagonist’s teeth, reduce its fracture strength, increase plaque adhesion, and the possibility of periodontal diseases and recurrent caries.^11,12 Therefore, these clinical procedures should be further researched to predict their esthetic effect on the surface properties of LDGC restorations. Although there are studies about the acidic effect of coffee beverages or whitening of CAD/CAM LDGC, limited information is available on the combined effect of coffee and different whitening systems on the surface texture of CAD/CAM LDGC. Therefore, this paper aimed to investigate the effect of a coffee beverage and two whitening systems (dental bleaching gels and whitening toothpaste) on the surface roughness and gloss of glazed LDGC for CAD/CAM systems.

Materials and methods

Experimental design

In this study, 68 glazed Lithium Disilicate Glass-Ceramic (LDGC) disks were subjected to a coffee beverage then treated using two different whitening systems to measure the surface roughness and gloss. The study investigated whitening treatment at four levels; negative control (no treatment), positive control (brushed with distilled water), whitening toothpaste, and at-home bleaching protocol. The study outcomes were surface roughness (Ra), and surface gloss (GU) measured at two-time points; at baseline and after the treatment. Surface roughness was measured using a non-contact 3D optical profilometry, while the surface gloss was determined by a glossmeter.

Specimen preparation

A total of 68 disks (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 high precision sectioning saw (Isomet 1000; Buehler Illinois, USA) under water cooling. Then each specimen was carefully examined, and any sharp edges were trimmed using a handpiece with a straight tungsten carbide bur (No. H129UK.HP.023, 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 and 30 s based on the manufacturer’s instructions. A glaze layer (IPS e.max Ceram Glaze paste, Ivoclar Vivadent, Schaan AG, Liechtenstein) was applied to all specimens on the experimental side before the final heat treatment in the furnace for the complete crystallization. One side of the ceramic surface was numbered with a marker, followed by the application of a thin layer of clear nail polish to maintain the mark, while the other side of the ceramic surface was used for the experiment.

Coffee immersion test

All specimens were immersed in a coffee solution (pH 5.1; Dunkin’ Donuts Original Blend Ground Coffee, Medium Roast, Shahia Food Limited Company, Saudi Arabia) done per the manufacturer’s instructions. The immersion period was 12 consecutive days, simulating 1 year of drinking coffee.^13,14 During the immersion cycles, specimens were kept stirred in the coffee solution in an incubator at 37°C. The coffee was used immediately after preparation and was replaced daily. At the end of each day, specimens were rinsed under running distilled water and gently dried to remove any coffee residue.¹³

Whitening treatment test

Specimens were randomized according to the designated treatment protocol into four groups (n = 17) for a testing period of 7 days. Group 1 (G1): no treatment (control group); was kept moist in a saline solution, changed daily. Group 2 (G2): was brushed with distilled water only. Group 3 (G3): was brushed with a suspension of whitening toothpaste and distilled water. Group 4 (G4): was bleached using a simulated at-home bleaching protocol. Throughout the testing period, specimens were stored in an incubator at 37°C. Moreover, after each treatment, specimens were rinsed with running distilled water for 1 min to remove any whitening systems’ residue, blot dried, and stored in a moist environment until the next treatment.

Simulated toothbrushing test

Group 2 and 3 were kept stable on a custom-made toothbrushing-holding device. Specimens were brushed using a soft, straight electric toothbrush (Oral B Pro-Expert, Procter and Gamble, Ohio, USA) using an oscillatory-rotating motion (continuous mode) at a rate of 8800 strokes/min. Each brushing cycle was carried out for 2 min/twice daily for a total of 28 min, under a force of 200 g load.¹⁵ This protocol corresponded to brushing twice daily (2 min/twice), as recommended by the American Dental Association. Our calculations were based on dividing the daily brushing time by 28 teeth while taking into consideration the several surfaces on each tooth, which has been reported to be 5 s/tooth.¹⁵ Therefore, 28 min of brushing one surface represented a period of around 8 months.¹⁵

Group 2 was brushed with only distilled water (negative control), whereas G3 was brushed using a whitening toothpaste (Colgate optic white sparkling white, Colgate-Palmolive Arabia Ltd., Saudi Arabia) with a relative dentin abrasivity (RDA) equivalate to 100, which is considered medium abrasivity by the International Organization for Standardization ISO (11609).¹⁶ The slurry consisted of a paste to water weight ratio of 1:1, in which the slurry was replaced daily.¹⁷

Simulated at-home bleaching test

Group 4 was bleached using 15% Carbamide Peroxide (CP) (pH 6.5; Opalescence PF, Ultradent Products, Inc., Utah, USA). The top surface of each specimen was dried, then a layer of the bleaching agent (0.5–1.0 mm thick) was applied and kept for 6 h/day based on the manufacturer’s instruction.

Surface roughness test

Characterization and imaging were performed using a 3D non-contact surface metrology with interferometry (Bruker Contour GTK, Bruker Nano Surfaces Division, Tucson, AZ, USA). Samples were measured by vertical scan interferometry using 5× Michelson magnification lens with a field of view of 1.5 × 1.5 mm, Gaussian regression filter, a scan speed of 1×, and a thresholding of 4. The microscope has a Vision 64 (Bruker) software that controls the instrument settings, data analyses, and graphical output. Measurement was taken across the sample at two-time points, at baseline and after treatment. Each sample was scanned three times and averaged accordingly to determine the roughness (Ra) value.

Surface gloss test

Gloss measurements were recorded using a glossmeter (Novo-Curve, Rhopoint Instruments, East Sussex, UK) with a projection angle of 60° geometry, which is in compliance with the ISO 2813.¹⁸ The device was calibrated between materials, and a black opaque lid blocked all ambient light. A positioning fixture was made from impression material and used on the glossmeter to position each specimen and ensure repeatable measurements using the same specimen orientation at the two tested time points: baseline and after treatment. Two gloss unit (GU) values were recorded from each specimen and averaged to represent the mean gloss value.

Scanning Electron Microscopy analysis

Two specimens per treatment group were randomly selected, and two specimens from baseline (total of 10 specimens) were analyzed by a Scanning Electron Microscopy (SEM) to confirm the various treatment effects on the ceramic surface topography. Each selected specimen was irrigated with 5 mL of distilled water, sonicated in deionized water for 10 min, and desiccated for 48 h. Then, specimens were sputter-coated for 2 min with gold/palladium, and images were taken with an SEM (JEOL 6390 LV, Peabody, MA, USA) at 1000× magnification.

Statistical analysis

To determine the adequate specimen size for statistically significant results, power analysis calculation determined that 17 specimens per group are needed at a 95% confidence level, power of 80%, and a standard deviation of 0.4. Surface roughness and gloss data were analyzed using SPSS (SPSS statistics v.23, IBM, New York, USA). A paired t-test was used to compare within the groups, and an independent t-test was used to compare each pair of groups. ANOVA test was used to check the difference between all groups, and Tukey’s post-hoc test was performed to find which specific group is different at α = 0.05 significant level.

Results

Surface roughness (Ra)

In general, the mean surface roughness Ra (μm) in all groups increased significantly (p ⩽ 0.002) after the incubation in coffee solution and the designated whitening treatment protocols. Among the treatment groups, the mean change in roughness values significantly increased in G2 and G3 compared to G1 (p ⩽ 0.001). However, G2 and G3 were not statistically significant than G4. The numerical values and comparisons within and among treatments are in Table 1. The 3D images of Ra graphical output and the SEM images representing the roughness differences between groups are presented in Figures 1 and 2, respectively.

Table 1.

Surface roughness means (standard deviation) at baseline (Ra-Baseline), after staining and the designated treatment protocols (Ra-Treatment), and the mean change values.

Groups	Ra-Baseline	Ra-Treatment	Ra-Change
G1	1.18 ± 0.78^Aa	1.41 + 0.81^Ba	0.23
G2	1.14 ± 1.56^Aa	1.65 ± 1.49^Bb	0.51
G3	0.88 ± 1.15^Aa	1.45 ± 1.11^Bb	0.57
G4	0.41 ± 0.31^Aa	0.87 + 0.69^Bab	0.46

G1: Control; G2: Brushed with distilled water; G3: Brushed with whitening toothpaste; G4: Bleaching.

Different uppercase letters indicate significant change within treatment (row, p < 0.05); while different lowercase letters represent changes among treatments (column, p < 0.05).

Figure 1.

The 3D images of Ra graphical output of lithium disilicate glass-ceramics surface representing groups after the designated treatments: (a) control group (no treatment), (b) brushed with distilled water, (c) brushed with whitening toothpaste, and (d) bleaching (15% CP).

Figure 2.

Scanning electron microscopic images (magnification ×1000) of lithium disilicate glass-ceramics surface representing groups after the designated treatments: (a) control group (no treatment), (b) brushed with distilled water, (c) brushed with whitening toothpaste, and (d) bleaching (15% CP).

Surface gloss (GU)

The mean surface gloss measurement (GU) of all groups decreased but not to a significant level (p > 0.05), irrespective of the treatment protocol. Likewise, no significant differences (p > 0.05) were found among all groups after the treatment. The numerical values and comparisons within and among treatments are in Table 2.

Table 2.

Surface gloss means (standard deviation) at baseline (GU-Baseline) and after staining and the designated treatment protocols (GU-Treatment).

Groups	GU-Baseline	GU-Treatment
G1	76.87 ± 7.33^Aa	73.92 ± 7.14^Aa
G2	79.20 ± 6.35^Aa	77.42 ± 8.18^Aa
G3	77.39 ± 8.12^Aa	76.02 ± 7.96^Aa
G4	74.85 ± 8.17^Aa	73.70 ± 6.83^Aa

G1: Control; G2: Brushed with distilled water; G3: Brushed with whitening toothpaste; G4: Bleaching.

Different uppercase letters indicate significant change within treatment (row, p < 0.05); while different lowercase letters represent changes among treatments (column, p < 0.05).

SEM

The tested whitening protocols affected the surface morphology compared to the control. Images taken at 1000× magnification showed brushing strikes related to G2 (Figure 2(b)) and G3 (Figure 2(c)) and pitted surfaces related to G4 (Figure 2(d)) created after the treatment. Qualitatively, the specimens’ surface had less deep and shallower indentation and appeared to have a smoother surface in G1 (Figure 2(a)) compared to G4.

Discussion

In dental clinical practice, CAD/CAM LDGC has become one of the most used materials in fabricating indirect restorative materials due to its superior function, biocompatibility, and esthetics.^19–21 However, patients with these restorations are subjected to different oral conditions such as; daily brushing, temperature change, acid-base shift, bleaching, and clinical dental procedures. All of which may impact the ceramic surface properties (i.e. roughness and gloss) that would affect its long-term esthetic outcome.²² Therefore, this study aimed to investigate the combined effect of a coffee beverage and different whitening systems on CAD/CAM LDGC surface roughness and gloss.

All tested specimens were standardized through a surface glaze to minimize surface porosity, reduce surface roughness, and produce a surface gloss that closely simulates the clinical situation.^23,24 Coffee beverage was chosen due to its high popularity among patients and its intense effect on most dental materials.^13,25 It was used to investigate its acidic effect on the surface roughness and gloss of glazed LDGC’s. The coffee incubation period was 12 days (total of 288 h), which represented 1 year of coffee consumption, in which each day (24 h) of coffee immersion resembled 1 month.^14,19,26 The study was designed to represent a clinical situation of patients drinking coffee and seeking whitening treatment to improve the esthetic outcome of their stained natural teeth along with their ceramic prosthesis, since it is challenging for patients to achieve whitening treatments on natural teeth without including the dental prosthesis.

Surface roughness was tested in this study, as it is an essential esthetic characteristic in ceramic restorations, as smooth surfaces are less prone to stain and bacterial colonization, subsequently, less susceptible to recurrent caries and periodontal diseases.^6,27,28 The measurement of surface roughness was done quantitatively using profilometry, calculated by using the Ra parameter. Additionally, SEM images were taken to evaluate the surface topography and texture of the specimens qualitatively. All tested groups had a significant increase in the surface roughness values after subjecting them to the coffee solution and the whitening treatment protocols, which was confirmed by the SEM images. The least change in surface roughness was related to the control group (only immersed in coffee solution), which may be explained by factors other than mechanical, such as the acidity of the coffee (pH 5.1) and the solution elevated temperature, which can negatively affect the ceramic surface roughness.^6,9 The solution’s acidity could degrade the glaze layer by the loss of alkaline ions and dissolve the silica, resulting in surface corrosion represented in the increased roughness.⁹ It is also in line with previous studies which reported that the low pH of the acidic beverages combined with elevated temperatures, such as coffee, resulted in a significant increase in surface roughness of CAD-CAM lithium disilicate and feldspathic ceramics.^9,11,12,29

In the current study, the highest significant roughness values were related to the simulated brushing protocol with or without the whitening dentifrice, which is in line with the findings of previous studies.^10,30,31 This may be justified by the mechanical and chemical combined effect represented in the continuous brushing motion and the abrasive action of the silica particles within the dentifrice, and the coffee’s acidity and heat.^9,11,12,29 This procedure may result in partial removal of the glazed layer, which forms surface craters that produce high surface roughness.^{9,11,12,24,29} Some studies showed no significant changes in Ra after brushing alone or with whitening toothpaste.^9,30 This inconsistency and differences in these results are due to the various testing protocols and the materials used, such as; toothbrush bristle type, brushing time, force, and dentifrice abrasivity.

The effect of bleaching agents on ceramics is controversial, as it has been reported in some studies to increase the surface roughness, while others showed no significant difference in surface roughness.^12,32–34 Carbamide peroxide was chosen as it is the most commonly used at-home bleaching agent.³⁵ In the current study, bleaching had a significant perceptible increase in surface roughness after treatment. This is in line with previous studies that used a high and low concentration of at-home bleaching agents (10%, 15%, 16%, and 35% CP) on either pressable ceramics, feldspathic porcelain, or lithium disilicate and resulted in a significant increase in Ra values.^33,34,36 Furthermore, the higher roughness results of other studies might be related to the increase in CP concentration and the application period.³³ We speculate that surface roughness is related to the leach of the free radicals (H⁺ or H₃O⁺) produced by the bleaching agents (alkali ions) into the glazed porcelain matrix; hence, dissolution of the ceramic glass networks which resulted in the altered and etched ceramic surface.³⁵ Additionally, the SEM images confirmed the results found in the profilometer analysis. In general, the roughness change for all specimens exceeded the known threshold for dental plaque accumulation (Ra: 0.2 μm).^9,10,24 This value may affect the restoration’s strength, stability, and esthetics by initiating surface cracks,³⁷ enhance plaque accumulation, secondary caries, periodontal inflammation, and ceramic texture change.^35,37 However, this value has not been systematically investigated in clinical trials.³⁸

Gloss is an optical phenomenon related to the material’s surface, which includes specular reflection and is responsible for the material’s lustrous or mirror-like appearance.³⁹ The factors reported affecting the surface gloss include the specimen’s refractive index, the angle incident light, and the surface topography.⁴⁰ As the LDGC specimens were cut from a block, their refractive index was constant. Furthermore, the incident light angle (60°) was done based on the ISO 2813 for specimens of medium gloss.¹⁷ Hence, surface topography was the interchangeable factor, which showed a decrease in surface gloss after coffee and whitening treatments but did not reach a significant level. As the highest roughness values in our study were related to brushing, it was compared to a similar study that subjected composite and ceramic materials to simulated brushing for 10 h.³⁸ The study showed a significant increase in ceramic roughness, but the gloss remained constant, and no deterioration was observed.³⁸ In most dental materials, the Ra and gloss are usually, but not always, interrelated; our study showed that the Ra and surface gloss was not correlated for LDGC ceramics.

Unfortunately, there is no standardized protocol to mimic the oral physiological environment.⁶ As in any in vitro designed study, this study’s main limitation is that it does not reflect the exact oral environment. Consequently, if patients with ceramic restorations undergo home bleaching, they should avoid applying the gel directly on the restoration’s surface. This can be done during the fabrication of the bleaching trays, as the bleaching gel reservoir should include only natural teeth to avoid changes in the ceramic restoration’s surface. Bleaching can also be done at the office under the dentist’s supervision by protecting the restoration before applying the bleaching gel. Further studies should be implemented to evaluate different CAD/CAM materials, translucencies, and different concentration, and types of whitening systems.

Conclusion

Within the limitation of this study, it can be concluded that all glazed LDGC had a significant increase in surface roughness after being subjected to simulated 1 year of coffee drinking and whitening systems (15% CP and whitening toothpaste). The tooth brushing (simulating 8 months) with or without whitening toothpaste had the highest roughness effect on glazed LDGC’s, followed by 15% CP bleaching gels, then the simulation of 1 year of coffee drinking (acidic beverages). However, the coffee beverage and the designated whitening treatments had no significant effect on the glazed LDGC’s surface gloss.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Sarah S Al-Angari

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