Effect of Three Commonly Consumed Beverages on Surface Roughness of Polished and Glazed Zirconia-Reinforced Lithium Silicate Glass Ceramics.

OBJECTIVES: This study aimed to assess the effect of three commonly consumed beverages on surface roughness of polished and glazed zirconia-reinforced lithium silicate (ZLS) glass ceramics.
MATERIALS AND METHODS: In this experimental study, 104 rectangular specimens were cut from Vita Suprinity blocks with 2 mm thickness. After ultrasonic cleaning and firing of the specimens, they were finished and polished in two groups. Specimens in the first group were polished using a 2-step polishing kit while the second group specimens were glazed and heated in a porcelain firing oven according to the manufacturer's instructions. Baseline surface roughness was measured using a profilometer. Specimens in each group were then randomly divided into 4 subgroups (n=13) for immersion in artificial saliva (control group), cola, orange juice and black tea. Surface roughness was measured again and data were analyzed using two-way ANOVA.
RESULTS: The highest and the lowest mean Ra were found in orange juice and saliva subgroups, respectively in both glazed and polished groups. The Ra values of both polished and glazed groups significantly increased after immersion in orange juice and cola (P<0.05). The polished surfaces showed insignificantly higher surface roughness compared with glazed surfaces (P>0.05).
CONCLUSION: Orange juice and cola significantly increased the surface roughness of both polished and glazed ZLS ceramics. Type of surface finishing (polishing versus glazing) had no significant effect on the surface roughness of specimens following immersion in different beverages. Copyright© Dental Research Center, Tehran University of Medical Sciences.

INTRODUCTION

Dental ceramics are increasingly used in esthetic and restorative dentistry [1,2]. They account for a high percentage of dental restorations because they highly resemble the texture and color of natural teeth. Biocompatibility and polishability (in order to minimize plaque accumulation and subsequently prevent gingival inflammation) are some other favorable characteristics of dental ceramics [3]. Furthermore, the brisk development of computer-aided design/computer-aided manufacturing (CAD/CAM) technology further increased the application of ceramics in dentistry [4]. At present, monolithic ceramic restorations have gained increasing popularity due to their enhanced fabrication process and lower frequency of complications such as chipping and delamination of the veneering ceramics [5,6]. Among the materials used for the fabrication of CAD/CAM monolithic restorations, zirconia-reinforced lithium silicate (ZLS) glass ceramics have both favorable mechanical properties of zirconia and esthetic properties of glass ceramics; therefore, they are used for the fabrication of a wide range of restorations such as inlays, onlays, partial crowns, veneers and anterior and posterior crowns [7,8]. Recently, VITA introduced a new ZLS glass ceramic (Vita Suprinity PC; Vita Zahnfabrik) enriched with approximately 10wt% zirconia [5].

Following the fabrication of different types of restorations, they often require chairside adjustment of occlusion and cervical contour with a fine-grain diamond rotary cutting instrument [9-13]. Although these adjustments are necessary for correction of contour and/or improvement of esthetics, this process can damage the glaze layer and create a rough and coarse surface, which can enhance dental plaque accumulation and consequent development of caries and periodontal disease [11,14-19]. A rough surface also adversely affects the color and light reflection of restoration, and the higher the roughness, the lower the optical reflection would be [20-22]. Roughness resulted from restoration adjustment may increase the surface flaws and consequently decrease the porcelain strength [23].

Higher roughness is not only a side effect of chairside adjustment, but also a consequence of porcelain degradation as the result of exposure to acidic agents [24-30]. This phenomenon can result in release of potentially cytotoxic components from the ceramic [27,31].

The effect of acidic agents on the surface of different types of dental porcelain has been documented in some studies but there is not enough evidence about the alterations caused by acidic agents on the surface of ZLS ceramics. The aim of this study was to evaluate the effect of commonly consumed beverages (cola, orange juice and black tea) on surface roughness of polished and glazed ZLS glass ceramics. The null hypothesis was that different beverages would not increase the surface roughness of polished and glazed ceramic specimens.

MATERIALS AND METHODS

In this in vitro experimental study, the CAD/CAM ZLS glass ceramic blocks (Vita Suprinity PC; Vita Zahnfabrik, Bad Säckingen, Germany) were used to fabricate the specimens. A total of 104 rectangular specimens were sectioned from Vita Suprinity blocks (18×14×12mm) with an average thickness of 2 mm using water-cooled low-speed diamond saw (Vari/cut VC-50; Series 15 LC diamond, Leco Corp., MI, USA). The specimens were then cleaned ultrasonically and were completely crystalized using a porcelain firing oven (Programat EP5000; Ivoclar Vivadent AG, NY, USA) according to the manufacturer’s instructions. After the crystallization cycle, a single operator polished the specimens with a low-speed hand piece and 600-grit silicon carbide papers and the specimens were then randomly allocated into two groups. Specimens in the first group were polished using a 2-step polishing kit (Vita Suprinity Polishing Set technical; VITA Zahnfabrik, Bad Säckingen, Germany) and a low-speed hand-piece. In the second group, a thin layer of glaze material (Vita Akzent Plus Glaze LT; Vita Zahnfabrik, Bad Säckingen, Germany) was applied on the specimens and they were heated in a porcelain firing oven, according to the manufacturer’s instructions. The specimens were immersed in distilled water at 37°C for 24 hours before and after measurement of baseline surface roughness (Ra) using a profilometer (TR200 Plus portable surface roughness tester; Testech NDT, Beijing, China). The test specimens in each group were randomly divided into 4 subgroups (n=13) for immersion in the following beverages:

Subgroup A: Immersed in artificial saliva (control group)

Subgroup B: Immersed in orange juice

Subgroup C: Immersed in cola

Subgroup D: Immersed in black tea

Table 1 presents the information of the beverages used in this study.

Table 1.

Characteristics of the materials used in this study

Immersion Solution	Manufacturer	Chemical Composition	pH	Immersion Temperature
Artificial saliva	Professional Dietetics, Italy	KCl (0.4 g L−1), NaCl (0.4 g L−1), CaCl2_2H2O (0.906 g L−1), NaH2PO4_2H2O (0.690 g L−1), Na2S_9H2O (0.005 g L−1), and urea (1 g L−1)	6.5	37ºC
Orange juice	Sunich, Iran	Orange juice, water, sugar, orange pulp, natural flavors, antioxidant ascorbic acid, and citric acid	3.5	5ºC
Cola	Coca-Cola	Carbonated water, sugar, cola nut extract, yellow dye IV, acidulate INS 338, and natural flavors	2.4	5ºC
Tea	Tetley	Tea leaves	4.9	55ºC

Specimens in subgroup A were immersed in artificial saliva and incubated at 37°C for 125 hours. Specimens in subgroups B, C and D were subjected to 5000 thermal cycles in a thermocycler (MSCT-3; Convel, Aracatuba, Brazil) in orange juice (5ºC), cola (5ºC) and black tea (55ºC) for aging with 30 seconds of transfer time and 30 seconds of dwell time. After each cycle of immersion in the afore-mentioned beverages, the specimens were immersed in artificial saliva (37ºC) for 30 seconds to better simulate the oral clinical condition. After thermocycling, the specimens were cleaned under running water using a brush and dried before surface roughness measurement [32-34].

The surface roughness (Ra) of the specimens was then measured again as described for measurement of baseline surface roughness. The mean and standard deviation of surface roughness were then reported. The data were statistically analyzed using SPSS version 16 (SPSS Inc., IL, Chicago, USA). The effect of beverages and polishing procedure on surface roughness was evaluated using two-way ANOVA and Tukey’s HSD test.

RESULTS

Table 2 shows the mean Ra value and standard deviation of the groups (n=13). The results of two-way ANOVA indicated that the interaction effect of type of beverage and polishing method on surface roughness was not significant (P>0.05). In other words, different beverages had the same effect on polished and glazed specimens (P>0.05).

Table 2.

Mean and standard deviation (SD) of Ra values of glazed and polished ceramic groups

Method	Solution	Mean	SD	N
Glazed	Saliva	.002	.006	13
	Orange juice	.092	.085	13
	Coca-Cola	.077	.051	13
	Tea	.011	.007	13
	Total	.046	.062	52
Polished	Saliva	−.010	.048	13
	Orange juice	.129	.132	13
	Coca-Cola	.087	.046	13
	Tea	.019	.023	13
	Total	.056	.091	52
Total	Saliva	−.004	.034	26
	Orange juice	.110	.110	26
	Coca-Cola	.082	.048	26
	Tea	.015	.017	26
	Total	.051	.078	104

According to the results, there was no significant difference in the mean Ra value of glazed and polished groups. The mean Ra value was 0.046 for the glazed group and 0.056 for the polished group. The type of beverage had a significant effect on the mean surface roughness.

The Ra values of both polished and glazed groups significantly increased after immersion in orange juice and cola (P<0.05). The surface roughness also increased in the saliva and tea subgroups but not significantly (P>0.05, Table 3).

Table 3.

Comparison between solutions using Tukey’s HSD test

Solution	Solution	Mean Difference	Standard Error	P-value	95% Confidence Interval
					Lower Bound	Upper Bound
Orange	Saliva	.114	.017	<.001	.072	.157
Cola	Saliva	.086	.017	<.001	.044	.128
Tea	Saliva	.019	.017	.562	−.022	.061

DISCUSSION

Ceramics are known as inert dental materials, which can retain their integrity in different environments. However, previous studies have reported degradation of some types of porcelain ceramics in different solutions [24-28,30]. Butler et al. [24] showed that 1.23% acidulated phosphate fluoride treatment increased the surface roughness of feldspathic porcelain, low-fusing porcelain and aluminous porcelain. Esquivel-Upshaw et al. [25] evaluated the ion exchange of glass ceramic veneers in acidic environments and concluded that ceramic veneers may be susceptible to considerable degradation in low- and high-pH buffering solutions. Junpoom et al. [26] evaluated the surface changes of fluorapatite-leucite and fluorapatite ceramics using various storage media and concluded that pineapple juice, green mango, cola and 4% acetic acid significantly increased the surface roughness of both ceramics.

Similarly, the results of the current study demonstrated that ZLS ceramics were not as impervious as they are thought to be. The results of measuring and comparing the surface roughness of specimens in this study revealed that the surface roughness of ZLS ceramics increased following immersion in different solutions, which rejected the null hypothesis of the study. Orange juice, cola and tea are some of the commonly consumed soft drinks worldwide that contain some acidic agents (which can cause dental erosion) and some other organic acids. Orange contains ascorbic acid, also known as vitamin C. Other acids that contribute to the nutritional profile oforange include folic, pantothenic, hydroxycinnamic, citric, malic and oxalic acids. The same acidity applies to juice from orange as well, which is also acidic. Black tea is mildly acidic, since its average pH level ranges from 4.9 to 5.5. More water may decrease its acidity [35]. Oxalate and citrate are the major anions in tea. It also contains anions such as fluoride, acetate, chloride, sulfate, nitrate and phosphate, which can cause dental erosion [36]. The three most common acids in cola include citric, carbonic and phosphoric acids. These acids can decalcify the tooth structure [37]. The chelating effect of these acids can cause degradation, ionic dissolution and release of alkaline lithium and aluminum ions and result in dissolution of the ceramic silicate network, which can be toxic [27,28,31,38].

According to the current findings, immersion of ceramics in orange juice, cola and tea created some degrees of surface roughness, albeit the surface roughness caused by immersion in tea was low and not significant. Saliva, as the control group, also caused roughness of ceramic, but not significantly. Precedent studies have demonstrated that increased surface roughness of dental porcelain due to degradation can decrease its strength [23,28, 39].

Other studies have claimed that higher surface roughness can cause higher bacterial plaque accumulation and gingival inflammation [11,14-19]. Ceramic failure may occur as the result of increased surface roughness due to repeated exposure to the studied solutions.

Kamala and Annapurni [40] evaluated the surface roughness changes of glazed and polished ceramic following exposure to fluoride gel, bleaching agent and aerated drink and reported significantly higher mean Ra values for glazed surfaces before exposure to the acidic solutions. They also showed that glazed surfaces had higher mean Ra values after exposure, but not significantly [40]. In the present study, glazed specimens had lower surface roughness before and after immersion, compared with polished specimens, although this difference was not significant. The efficacy of polishing to achieve a smooth ceramic surface is still a controversial topic. Some authors believe that roughness of polished surfaces is equal to that of glazed surfaces, while some others disclaim this statement [14, 41–44].

CONCLUSION

Based on the findings of this study, the following conclusions were drawn:

Orange juice and cola significantly increased the surface roughness of both polished and glazed ZLS ceramics. The surface roughness of polished and glazed ceramics was not significantly different following immersion in different solutions.