Three-Dimensional Optical Study on the Effects of Microwave Glazing on Surface Roughness of Zirconia-Reinforced Glass Ceramic

Introduction

Smooth and esthetically acceptable surfaces are necessary for tooth-colored restorative materials. Glazing ceramic restorations help to produce a smooth surface that lasts a long time and has a high luster. Nonetheless, any ceramic restoration undergoing clinical try-in must have any occlusal interferences checked and adjusted. The restoration is taken back to the laboratory for the final glazing after adjustments. The processes of glazing, glaze firing, and finishing/polishing help to smooth the surface of porcelain restorations [1]. The glazing and glaze firing procedures for porcelain materials must be followed as per the manufacturer’s instructions. While many studies have investigated the effect of conventional oven glazing on the properties of reinforced glass ceramic, to date, effects of microwave glazing of zirconia-reinforced glass ceramic have not been investigated. Surface roughness is important, as the patient’s tongue can discern surface roughness scratches as small as 20 μm [2].

To avoid the patient noticing the restoration, finished restorations should have a maximum roughness of 0.50 μm [3]. An optimal threshold of 0.2 μm should be the surface roughness of hard oral materials to retain bacterial plaque [4]. Consequently, the goal of this work is to examine the surface roughness attained by glazing ceramic specimens made of glass reinforced with zirconia, using both a regular oven and a microwave. Restoration of decayed and fractured teeth requires not only rebuilding but also finishing for esthetic needs. Therefore, this 3-dimensional (3D) optical study aimed to evaluate the effects of microwave glazing on the surface roughness of zirconia-reinforced glass in a conventional oven, hand polishing, and microwave technique. We hypothesized the study results to show that the microwave technique provides a higher quality of glazing than a conventional oven and hand polishing.

Material and Methods

Preparation of Specimen

PREPARATION OF SPECIMEN:

This randomized control study was conducted after approval from the Ethics Committee (Annexue 1) at King Khalid University, Abha. A sample size of 90 ceramic materials tiles was derived according to the standard sample size formula. The 3 commercial dental ceramics used were IPS e.max CAD (lithium disilicate ceramic; IvoclarVivadent), Suprinity (zirconia-reinforced lithium silicate; VITA Zahnfabrik), and Celtra Duo (zirconia-reinforced lithium silicate; Dentsply Sirona) (Figure 1). Ninety specimens of these 3 different materials were fabricated in accordance with ADA Specification Number 69 for dental ceramics. Each ceramic material of uniform size was milled to the size of 14×10×3 mm, following the manufacturer’s instructions using an Isomet1000 precision cutter (Beuhler; Figure 2) by one investigator (M.A.B). The study groups were divided as group A (n=30): IPS e.max CAD (lithium disilicate ceramic; IvoclarVivadent); group B (n=30): Suprinity (zirconia-reinforced lithium silicate; VITA Zahnfabrik); and group C (n=30): Celtra Duo (zirconia-reinforced lithium silicate; Dentsply Sirona).

SURFACE TREATMENTS:

The 30 specimens from each of the above groups were further divided into 3 groups, each with 10 specimens: group 1 (n=10): conventional oven glazing (control group); group 2 (n=10): hand polishing (test group); and group 3 (n=10): microwave glazing (test group).

Following the manufacturer’s instructions, specimens from groups A1, B1, and C1 were glazed in a conventional oven (Ivoclar Vivadent Programat P310 Porcelain Furnace; Figure 3A) while under vacuum. The glazing temperature for specimens was set in accordance with the manufacturer’s instructions. Following the manufacturer’s instructions, specimens from groups A2, B2, and C2 were polished using diamond rotary ceramic polishers, a slow-speed hand piece rotating at roughly 12 000 rpm. The adjustment package had 3 steps: polishers (Ceragloss) with green, blue, and yellow high-luster finishes were used one at a time for 20 s each.

To glaze the specimens of groups A3, B3, and C3, an experimental microwave oven (Samsung model G273V; Figure 3B) equipped with a SiC susceptor for hybrid heating was used. It allowed the temperature inside the microwave oven to be raised to a maximum of 1600°C. The microwave oven could produce 1.25 kW of electricity and run at a frequency of 2.45 GHz.

A pyrometer was used to measure the oven’s internal temperature. Following the manufacturer’s instructions, the glazing procedure was completed in the trial microwave oven, and 10 samples of each variety of porcelain were glazed in the microwave oven at the suggested glazing temperature. The pyrometer and controller box were used to set and track the temperature and heat rate. Each specimen required an average of 6 min to glaze in the microwave.

TESTING OF SPECIMENS: Using a Contour GT 3D Optical Microscope (Bruker) and 3D non-contact surface metrology with interferometry, all 90 samples were assessed for surface roughness (Figure 4). This 3D optical microscope for the topographical study of material surface works on the principle of light, instead of the physical probe, in determining data. The main feature of this technique is directing the light source to detect the 3D data of the surface. The 3D images were obtained from the specimens. To calculate the roughness value in nanometers, each sample was scanned at a certain position, with the focus at the center, the recordings per specimen surface were repeated thrice, and the average values were taken. This was regarded as Ra (arithmetic average for absolute distance of roughness profile), a value that increases with surface roughness. The statistical analysis of the data was done using Microsoft Excel and SPSS Version 20.0 statistical software (IBM Corp, Armonk, NY, USA). Significance was evaluated at P<0.05 for all tests.

Results

After we gathered and entered the recorded data into a Microsoft Excel spreadsheet, we exported it to the data editor of SPSS. The statistical analysis of the data was done using Microsoft Excel and SPSS. The expression for continuous variables was mean±SD. Analysis of variance (ANOVA) was used for multiple comparisons and intergroup data analysis based on surface smoothing technique and ceramic type (Table 1). The test of least significant difference was used. Diagrams were used to visually portray the data. Results with a P value less than 0.05 were deemed statistically significant.

Based on the mean surface roughness between various ceramics used, group A was the smoothest (Ra=66.8), followed by group B, while group C (Ra=72.3) was the roughest (Figure 5). The highest mean value reported was 89.82 nm and the lowest was 49.23 nm. However, the overall differences in the materials and smoothening methods were not statistically significant. The microwave-glazed surfaces (Ra for A3, B3, C3=49, 54, 55, respectively) produced smoother surfaces for all 3 brands of ceramics than did hand polishing (Ra for A2, B2, C2=82, 83, 90, respectively) and conventional glazing (Ra for A1, B1, C1=69, 66, 71, respectively). The hand-polished (group 2) presented with higher surface roughness, followed by conventional glazing (group 1) and microwave glazing (group 3). All of these results were statistically significant. Microwave glazing produced a smoother surface than the other 2 surface treatments. For the materials tested, group C (Ra for C1, C2, C3=71, 90, 55, respectively; Table 2) presented with greater surface roughness than did group B (Ra for B1, B2, B3=66, 83, 54, respectively; Table 3) and A (Ra for A1, A2, A3=69, 82, 49, respectively; Table 4). There was a significant difference between conventional and microwave glazing and between conventionally glazed and hand-polished specimens. Furthermore, a highly significant difference between microwave-glazed and hand-polished specimens was observed.

Discussion

A major prerequisite for a good restoration is that the material should be created with a smooth exposed surface. Various authors advise polishing and glazing to enhance the esthetic appearance of these restorations. The key objective to finish and polish a ceramic restoration is to achieve a highly smooth surface that provides satisfactory patient comfort and clinically acceptable esthetics and slows the rate of plaque formation, to help to maintain oral hygiene.

Ra is the arithmetical average of all absolute roughness profile distances from the measurement length’s center line. It characterizes a surface’s overall roughness [5,6]. Quantitative techniques, such as contact profilometry, and qualitative techniques, such as optical and scanning electron microscopy, are used in the study of surface roughness in dentistry. There are a few issues with the sensitivity of these techniques, which have been well-described by Tholt de Vasconcellos et al [7].

The Ra value given by a contact profilometer is 2-dimensional data relating to roughness height but does not provide any information regarding the surface profile. To obtain this information regarding the surface topography, an image of the surface is necessary, which can be acquired by means of optical and scanned electron microscopy. However, this does not provide qualitative data in 3 dimensions. Bessing and Wiktorsson [8] in 1983 theorized that some profilometer readings of the ceramic surface can be deceptive due to the presence of voids on the ceramic surface that creates inaccuracy in the readings.

Therefore, the present study was conducted using a Contour GT 3D Optical Microscope (Bruker) and 3D non-contact surface metrology with interferometry. The main feature of this technique is directing the light source to detect the 3D data of the surface [9,10].

When the surface roughness mean values were examined, in a progression from smoothest to roughest, the smoothest surfaces were obtained with microwave glazing, followed by conventional glazing, and the roughest were with hand-polishing. The results obtained by statistical analysis indicated that there was a significant difference in surface roughness based on the type of glazing. Thus, the null hypothesis, which stated that there would be no variation in surface roughness for the 3 varieties of porcelain among the 3 treatment groups, was rejected.

The results of the study suggested that regardless of the ceramic used, the microwave-glazed method is a better method than all other methods, with high statistical significance. Similar reports were received from Chu et al [11] and Vieira et al [12]. However, several studies advocated that polishing produces surfaces as smooth as glazing [13–15]. In addition, Klausner et al [16] attributed the similar results between polishing and glazing to the polishing kits.

Findings of the present study supported the results obtained by Prasad et al regarding microwave glazing vs conventional oven glazing [17]. The authors reported nearly similar results as ours, confirming that microwave glazing caused lower Ra values than conventional oven glazing for IPS e.max, Ceram, and Vita Omega 900 porcelain specimens. In contrast to the earlier investigation, a highly significant result was found comparing glazed and unglazed specimens. The reason for this disparity in outcomes could be that glazing lessens the number of surface imperfections and porosities, and that microwave energy in particular promotes better grain dispersion and increased densification. However, Flury et al [18] concluded that the glazing procedure had different effects on different porcelain materials. This can be attributed to the fact that their study used different material than did the present study.

Also, atomic force microscopy showed that microwave-glazed specimens presented with a surface profile characterized by rounded, flat surfaces with shallow valleys, compared with hand-polished specimens that presented highly elongated, sharp peaks, and deep valleys.

In the present study, we concluded that based on the material used, the lowest surface roughness values were obtained with the lithium disilicate ceramic (IPS e.max) group followed by zirconia-reinforced lithium silicate (Suprinity) and zirconia-reinforced lithium silicate (Celtra Duo). This result was not statistically significant; however, when the materials were compared based on the method of smoothening, the results were statistically significant. Almost similar results were obtained by Ramy et al [19]. However, their results differed from our results in regards to the polishing protocol. The results obtained by different authors indicate that there was a significant difference in surface roughness based on the type of glazing and metal ceramic used, as there was a significant interaction between the method of surface treatment and porcelain used.

The image analysis for mean values of surface roughness showed the smoothest surfaces were achieved by microwave glazing, followed by traditional glazing, while the roughest surfaces were created through hand polishing. Based on the kind of glazing, there was a substantial variation in surface roughness, according to statistical analysis of the test results. Based on the material surface roughness, Celtra Duo, followed by the Suprinity porcelain type, were higher than that of IPS e.max CAD ceramic.

Dental porcelains require a vacuum phase during the glazing process in a conventional oven. One potential limitation of this work is that microwave glazing does not include a vacuum phase. A microwave chamber presents a challenge for vacuum achievement because of the possibility of sparking. Glazing under pressure in a microwave could be one way around this restriction. Prospective studies on microwave glazing under pressure are possible in the future. The effect of microwave glazing on 3 dental porcelains – IPS e.max CAD (lithium disilicate ceramic), Suprinity (zirconia-reinforced lithium silicate), and Celtra Duo (zirconia-reinforced lithium silicate) – was the exclusive focus of this study. Before the surface treatments, none of the specimens’ surfaces were assessed for roughness. Consequently, it is unknown if the specimens were initially smoother or rougher and whether the difference shown following the treatments was actually brought about by the variations in traditional oven- and microwave-glazing methods.

Conclusions

In the present study, the surface treatment of the ceramic specimens affected the surface roughness. The specimens that were glazed in a microwave, as opposed to a conventional oven, showed a notable distinction. The porcelain glazed in a microwave had a smoother surface than porcelain glazed in an oven. This could be explained by the fact that there were fewer porosities after microwave sintering. Visual inspection revealed that all 3 surfaces – oven glazed, microwave glazed, and hand polished – appeared clinically acceptable.

Considering the advantages of time and volumetric heating, the energy-saving microwave glazing procedure can be considered as an alternative method. For ceramics, microwave-glazed ceramics were better than conventional oven-glazed ceramics, and hand polishing resulted in a rougher surface than glazing. Irrespective of the surface treatment methods, IPS e.max CAD ceramic showed a relatively smoother surface than did Suprinity and Celtra Duo.