Vickers Micro-Hardness of New Restorative CAD/CAM Dental Materials: Evaluation and Comparison after Exposure to Acidic Drink

CAD/CAM (computer-aided design/computer-aided manufacturing) for indirect restorative materials has been recently introduced in dentistry. The purpose of this study was to evaluate the change of the surface micro-hardness of different restorative CAD/CAM materials after exposure to a carbonated acidic drink (Coca-Cola, Coca-Cola Company, Milan, Italy). One hundred and eighty specimens of identical size (2 mm thickness) were obtained by sectioning each tested CAD/CAM block of four materials: a hybrid ceramic (CERASMART™, GC Corporation, Tokyo, Japan), a resin nano ceramic (Lava™ Ultimate, 3M, Monrovia, CA, USA), a nanohybrid composite (Grandio blocs, VOCO GmbH, Cuxhaven, Germany), and a zirconia-reinforced lithium silicate glass ceramic (VITA SUPRINITY® PC; VITA Zahnfabrik, Bad Sackingen, Germany). Forty-five specimens of each material were tested. Micro-hardness was measured at baseline, after 7 days and after 28 days. The data were analyzed. The micro-hardness of each material varied significantly after immersion in Coca-Cola. The nanohybrid composite had a high initial micro-hardness and the greatest percentage loss after acid exposure. The hybrid ceramic and the resin nano ceramic had similar percentage losses of micro-hardness values even if the second material had higher initial values. The zirconia-reinforced lithium silicate glass ceramic had the highest baseline values and the lowest percentage loss of micro-hardness. The different CAD/CAM materials presented different micro-hardness values before and after acid exposure.


Introduction
The consumption of energy drinks and carbonated drinks is very common, particularly in the age range of 18-35 years. Many studies demonstrated that their consumption can cause dental erosion and consequently restoration erosion [1,2]. The presence of some acids, including organic acids such as acetic, propionic, and lactic acid in the formulation of beverages can lead to a decrease in the micro-hardness values of resin composites [3,4]. A more controlled consumption of soft drinks has a positive influence on diet and on dental and dental materials erosion. Coca-Cola is a widely consumed carbonated beverage, with a pH level of 2.52. In the present in vitro study, 45 specimens of each of the four materials were prepared. For each restorative material, the CAD/CAM block was sectioned to obtain specimens of identical size (2 mm thickness) with a G5 milling machine (Dental Machine Srl, Bobbio PC, Emilia-Romagna, Italy) and the related software (WX Prop View; Matrix Vision GmbH, Oppenweiler Germany).
Before testing their properties, VITA SUPRINITY ® PC was crystallized with VITA VACUMAT 600 M (VITA Zahnfabrik, Bad Sackingen, Germany) and each specimen was polished with super fine Materials 2019, 12, 1246 3 of 11 (600 P) and ultra-fine (1200 P and 1500 P) abrasive sandpaper (SandBlaster, 3M, Monrovia, CA, USA). The polishing procedures were conducted under dry conditions for 30 s of each abrasive sandpaper and before the next polishing step each specimen was rinsed with water and allowed to air-dry [1].
For each restorative CAD/CAM material, the 45 specimens were randomly divided into three groups (n = 15 per group). The micro-hardness of each CAD/CAM material was tested at baseline (T0), after immersion in a sealed vial containing 50 mL of acidic drink (Coca-Cola Company, Milan, Italy; pH 2.52) for 7 days (T1, n = 15) and after immersion in 50 mL of acidic drink (T2, n = 15). The specimens tested at T2 were rinsed with water weekly in order to prevent bacterial adhesion and the solution was changed to maintain the low pH level [22].
As in previous reports that evaluated the micro-hardness of dental materials [1,22], each specimen's micro-hardness was determined with a micro-hardness tester (Isoscan HV2, LTF Spa, Antegnate, Bergamo, Italy) using a Vickers diamond indenter. Three indentations were made equally placed over a circle, each being no closer than 0.5 mm to the adjacent indentation (UNI EN ISO 6507 [23]), using a 1000 g load with a 15 s dwell time. The two diagonal lengths of each indentation were measured by a 40× magnification built-in scale microscope, and were converted into a micro-hardness value (VHN) using the following equation: HV = 1.854 P/d 2 , where HV is micro-hardness in kg/mm 2 , P is the load in kgf and d is the average length of the diagonals in mm. For a given specimen, the three hardness values for each surface were averaged and reported as a single value. Scanning electron microphotographs were taken before and after the indentation test for each group tested (mvBlueFOX3, MATRIX VISION Italia, Brescia, Italia). In the photomicrographs, the different surfaces of the four materials tested were evaluated before and after indentation at the three different immersion protocols (T0: no immersion; T1: 7 days; T2: 28 days). The micro-hardness values were analyzed using Stata 12 software (Stata, College Station, TX, USA). Descriptive statistics including the mean, standard error of mean, median, minimum, and maximum values were calculated for all groups. Statistical analysis of the results of micro-hardness testing [1,14,15] included the Shapiro-Wilk test to assess the normality of the distributions and the nonparametric Kruskal-Wallis analysis of variance (ANOVA) to assess the differences caused by the immersion protocol in the acidic drink for each material. A significant level of α = 0.05 was set for comparison between the groups. The mean percentage of micro-hardness loss after the immersion protocols was calculated for each CAD/CAM material and the groups were compared with a Tukey test (α = 0.05).

Results
The micro-hardness values of the four materials involved in the study are reported in Table 2. Descriptive statistics refer to the micro-hardness values of the three different conditions tested (T0, T1, and T2). In the control group (T0), the values (731.4 ± 2.63) reported for the zirconia-reinforced lithium silicate glass ceramic (VITA SUPRINITY ® PC) were significantly higher than those of the other materials (p < 0.001), while the significantly lowest micro-hardness (68.22 ± 0.52) was measured for the hybrid ceramic block (CERASMART™) (p < 0.001).
The ANOVA test within experimental replications showed that each tested material varied significantly after immersion in the acidic drink (p < 0.05). The main mean percentage micro-hardness loss was recorded after 7 days of immersion, while during the following 21 days the percentage loss was significantly lower (p < 0.05), as displayed in Figure 1 and reported in Table 3.
In the control group (T0), the values (731.4 ± 2.63) reported for the zirconia-reinforced lithium silicate glass ceramic (VITA SUPRINITY ® PC) were significantly higher than those of the other materials (P < 0.001), while the significantly lowest micro-hardness (68.22 ± 0.52) was measured for the hybrid ceramic block (CERASMART™) (P < 0.001).
The ANOVA test within experimental replications showed that each tested material varied significantly after immersion in the acidic drink (P < 0.05). The main mean percentage micro-hardness loss was recorded after 7 days of immersion, while during the following 21 days the percentage loss was significantly lower (P < 0.05), as displayed in Figure 1 and reported in Table 3.  After 28 days of immersion (T2), the mean percentage of micro-hardness loss was not significantly different between the hybrid ceramic block (CERASMART™) (8.41%) and the resin nano ceramic (Lava™ Ultimate) (8.73%) (P > 0.05). The nanohybrid composite (Grandio blocs) showed the highest percentage in micro-hardness loss (13.4%) (P < 0.05). The zirconia-reinforced lithium silicate glass ceramic (VITA SUPRINITY ® PC) showed the lowest mean percentage (2.65%) (P < 0.05).  After 28 days of immersion (T2), the mean percentage of micro-hardness loss was not significantly different between the hybrid ceramic block (CERASMART™) (8.41%) and the resin nano ceramic (Lava™ Ultimate) (8.73%) (p > 0.05). The nanohybrid composite (Grandio blocs) showed the highest percentage in micro-hardness loss (13.4%) (p < 0.05). The zirconia-reinforced lithium silicate glass ceramic (VITA SUPRINITY ® PC) showed the lowest mean percentage (2.65%) (p < 0.05).

Discussion
The purpose of the present study was to test the micro-hardness (Figures 2-5) of four different recently introduced CAD/CAM indirect restorative materials, and the results showed significant differences among various groups.
The lowest micro-hardness values were reported with hybrid ceramic blocks (CERASMART™) (Group 1). The hybrid ceramic was previously tested for mechanical properties [24,25], showing similar indentation values (67.30 ± 2.04). Moreover, micro-hardness and color stability after exposure to an acidic substance (coffee and red wine) have been tested previously [25]. The results indicated that coffee can affect the color and micro-hardness of tested materials. This is in agreement with the results of our study.
In the present study, higher indentation values were reported with resin nano ceramic (Lava™ Ultimate) (Group 2). This material has been previously tested [17,24], including the evaluation of its mechanical properties elastic modulus, flexural strength, and micro-hardness. The reported micro-hardness (112.20 ± 10.2) values were similar (slightly higher) to the results of our report at T0. However, the materials were tested without exposure to an acidic drink, in contrast to the present study. Another report [26] evaluated the effects of simulated gastric juice on the mechanical properties of a CAD/CAM resin composite (i.e., micro-hardness). The acid scenario (immersion in an acidic solution for 6 and 24 h) did not change their micro-hardness. These results are in contrast with the present study because the exposure to an acid scenario was shorter, even if the pH solution was lower (gastric solution pH = 1.2; Coca-Cola pH = 2.52).
Another material tested in our study was the nanohybrid composite (Grandio blocs), which showed higher micro-hardness values than the other two materials previously discussed. This material has been previously tested for edge force [27] and micro-tensile bond strength [27,28], but there are no indentation studies yet. There are many studies on the effect of sports and energy drinks on the surface hardness of different restorative materials [1,29]. However, nowadays there are few studies on the effect of acidic drinks on the micro-hardness of restorative CAD/CAM materials.
In the present report, the highest micro-hardness values were reached with zirconia-reinforced lithium silicate glass-ceramic (VITA SUPRINITY ® PC). This glass ceramic is an innovative material, introduced in 2013, with a dual microstructure composed of lithium metasilicate (Li 2 SiO 3 ) and lithium disilicate (Li 2 Si 2 O 5 ) in a glass matrix containing zirconium oxide in solution [30]. In the literature, there are studies regarding the optical properties [31] and mechanical properties [32] of this material, but there are no reports on its micro-hardness.
After immersion in Coca-Cola (pH = 2.52) for 7 and 28 days, the micro-hardness of all tested materials was negatively affected. Zirconia-reinforced lithium silicate glass ceramic (VITA SUPRINITY ® PC) was less affected, presumably because of its high chemical stability. This is probably due to the composite component or polymeric matrix incorporated in the other three materials (hybrid ceramic, resin nano ceramic, nanohybrid composite), which appeared more affected by the acidic solution than the reinforced glass ceramic. In fact, dental composite resins have a polymeric network that can be negatively influenced by exposure to acidic beverages. It is reported that mechanical properties, in particular micro-hardness, are affected by long-term immersion in an acidic environment [33].
Dental ceramics are considered chemically inert restoration materials; however, exposure to acidic erosive agents can undermine the stability and durability of ceramics. A study [34] demonstrated that exposure to an acid scenario leads to a decrease in the micro-hardness values of ceramics because of the dissolution of the materials; elementary components such silica, aluminum, and potassium are released by the glass phase. This dissolution has also been reported with other dental materials used in conservative dentistry, such as resin composites [35] and fiber-reinforced composites [36]. In fact, the oral cavity is a complex aqueous environment, modified by everything that is introduced daily. For example, food and beverages can lower the salivary pH values. In this way, the physical and mechanical properties of restorative CAD/CAM materials can be changed [37]. The loss of mechanical characteristics, such as flexural strength, flexural modulus, and, particularly for this study, micro-hardness, is caused by factors such as saliva, acids, and bases [38].
The results obtained at T0 confirmed that CAD/CAM resin composites have greater micro-hardness characteristics than other materials tested. In general, the mechanical characteristics of ceramic-based materials are stronger than composite-containing materials [39][40][41]. However, the present report tested only micro-hardness, and this is a limitation of the study. Future research is needed in order to test other mechanical characteristics to complete the overview of the behavior of the materials tested.
The exposure to acidic drink in this study was continuous, without rinsing the specimens daily, in order to simulate long-term exposure to carbonated beverages in the oral cavity. Immersion in Coca-Cola for 24 h is comparable to the consumption of the beverage for one month, considering that it remains in the mouth while drinking [42]. In a recent study [25], immersion in the acidic drink was tested for 7 days (T1) and 28 days (T2); the second immersion protocol is comparable to more than two years of in vivo conditions.
As confirmed by other studies [43], the loss of the mean micro-hardness percentage is higher during the first 7 days of exposure (T0-T1), while during the other 21 days (T1-T2) it is lower. The cited study was performed on resin composites for direct restorations; we obtained analogous results of the mean micro-hardness percentage, although in the present study restorative CAD/CAM materials were used. Our experimental design evaluated the behavior of the materials after 7 and 28 days of acidic drink immersion, as many reports in dental literature evaluated similar time intervals [1,13,43]. It would be interesting in the future to test the same materials with more measurements at shorter time periods. These new tests could be useful to check the hardness evolution along time, thus allowing extrapolation to longer service periods [44].
Immersion in Coca-Cola was performed at room temperature (18 ± 1 • C), even if is reported that acidic drinks have a lower erosive power if they are consumed at a low temperature [45]. The present investigation did not evaluate other drinks (e.g., water, tea, and milk) because of their lower acidic concentrations compared with Coca-Cola [2,4].
The oral cavity includes many variables that cannot be simulated by in vitro conditions. Therefore, further clinical research is needed to confirm the present results. The results obtained at T0 confirmed that CAD/CAM resin composites have greater microhardness characteristics than other materials tested. In general, the mechanical characteristics of ceramic-based materials are stronger than composite-containing materials [39][40][41]. However, the present report tested only micro-hardness, and this is a limitation of the study. Future research is needed in order to test other mechanical characteristics to complete the overview of the behavior of the materials tested.
The exposure to acidic drink in this study was continuous, without rinsing the specimens daily, in order to simulate long-term exposure to carbonated beverages in the oral cavity. Immersion in Coca-Cola for 24 h is comparable to the consumption of the beverage for one month, considering that it remains in the mouth while drinking [42]. In a recent study [25], immersion in the acidic drink was tested for 7 days (T1) and 28 days (T2); the second immersion protocol is comparable to more than two years of in vivo conditions.
As confirmed by other studies [43], the loss of the mean micro-hardness percentage is higher during the first 7 days of exposure (T0-T1), while during the other 21 days (T1-T2) it is lower. The cited study was performed on resin composites for direct restorations; we obtained analogous results of the mean micro-hardness percentage, although in the present study restorative CAD/CAM materials were used. Our experimental design evaluated the behavior of the materials after 7 and 28 days of acidic drink immersion, as many reports in dental literature evaluated similar time intervals [1,13,43]. It would be interesting in the future to test the same materials with more measurements at shorter time periods. These new tests could be useful to check the hardness evolution along time, thus allowing extrapolation to longer service periods [44].
Immersion in Coca-Cola was performed at room temperature (18 ± 1 °C), even if is reported that acidic drinks have a lower erosive power if they are consumed at a low temperature [45]. The present investigation did not evaluate other drinks (e.g., water, tea, and milk) because of their lower acidic concentrations compared with Coca-Cola [2,4].
The oral cavity includes many variables that cannot be simulated by in vitro conditions. Therefore, further clinical research is needed to confirm the present results.         Data availability statement: All the data are available upon request to andrea.scribante@unipv.it.

Conclusions
• Coca-Cola has an erosive power on the surface micro-hardness of restorative CAD/CAM materials, particularly during the first 7 days of immersion.

•
Ceramics have greater mechanical characteristics, particularly in the case of micro-hardness, than resin composites and are less affected by the acidic solution.
Author Contributions:

Conclusions
• Coca-Cola has an erosive power on the surface micro-hardness of restorative CAD/CAM materials, particularly during the first 7 days of immersion.

•
Ceramics have greater mechanical characteristics, particularly in the case of micro-hardness, than resin composites and are less affected by the acidic solution.