Effect of Finishing and Polishing Agents on the Surface Texture and Colour of Pressable Ceramic Restorative Material - An In Vitro Study

 Introduction

Ceramics have been widely used in dentistry since the 1980s.¹ Ceramic restorations are known for their superior esthetics, making them a material of choice both among patients and clinicians. Ceramics exhibit excellent biocompatibility, high wear resistance and are highly esthetic. All ceramic crowns can be fabricated through conventional sintering, casting/ with a pressure technique or by using various direct milling techniques.² Pressed ceramic material mimics natural teeth by exhibiting excellent light transmission, color reproduction and texture.³

The superior esthetics and surface finish of all ceramic restorations is achieved by the process of glazing. Not only does glazing provide a smooth surface, it also helps retain high luster for many years. This glazed superficial layer shows low thermal expansion, reduce depth and surface width flaws, thereby strengthening the material.⁴

Many a times, the all ceramic restorations require occlusal correction prior to their cementation. This can result in loss of glaze layer, creating a rough surface. In such scenario, the dental ceramics are either polished or re-glazed. Re-glazing involves preparing the ceramics for another cycle of firing, which can result in structural changes that make them more prone to breakage and can be time consuming.⁵ Also, re-glazing is not possible if adjustments are to be made after cementation. Therefore, polishing ceramic restorations chairside could be a suitable substitute.

Literature indicates that polishing the surface is effective similar to glazing. Rosenstiel et al., reported that fracture resistance of polished porcelain was high compared to glazed porcelain.6 Polishing not only reduces the chair side time, but also makes it less abrasive to the opposing tooth.³

Although literature supports finishing and polishing of the restoration as an alternative to glazing, there are no studies available studying the impact of finishing and polishing agents on feldspathic porcelain material layered on aluminum based pressable ceramics.

Therefore, this in vitro study was carried out to evaluate and compare the effects of different finishing and polishing agents on the surface texture and colour of feldspathic porcelain material layered on aluminum based pressable ceramics.

Materials and Methods

Fabrication of Specimens

Forty-five disc shaped specimens of pressable ceramic (Cergo Kiss Degudent, Dentsply, Germany), in dimensions of 10 mm diameter and 0.7 mm thickness were fabricated. The wax patterns were sprued (Figure 1A) and pressing was done at 960°C under a vacuum pressure of 2.7 pressure bar for 40 minutes in the ceramic furnace following the manufacturer’s instructions (Figure 1B). Restorative sample specimens were retrieved (Figure 2), finished using a medium-grit diamond bur and measured using a Vernier caliper.

 Layering of ceramic specimens

Dentin layer was built on ceramic specimens using polyvinyl siloxane mold to achieve a thickness of 0.5 mm. Specimens were then subjected to a firing temperature of 800°C, under vacuum (50hPa) as per the manufacturer’s instructions. After firing, specimens were withdrawn from the furnace and were cooled. Similarly, enamel porcelain of 0.5 mm was layered on dentin porcelain using a polyvinyl siloxane mold and fired at a temperature of 800°C, under vacuum (50 hPa). Irregularities were removed on the disc surface using ceramic finishing burs and then were overglazed using low fusing ceramic glaze material at 800°C.

Specimens were grouped under three different categories of 15 each, named as Group A, Group B and Group C (Table 1). Each specimen was demarcated into two halves by marking a line passing along the center using a diamond cutting wheel, one side of which was used as control (glaze) and the other side was used for finishing and polishing. To differentiate the glazed and polished surfaces, each specimen was marked with one notch using diamond cutting wheel on the periphery of glazed surface and two notches on periphery of polished surface.

Finishing and polishing procedures

All the procedures were performed by a single individual using the same pressure and time for each specimen (Figure 3). The finishing and polishing agents used were grouped as,

Group A: Porcelain adjustment kit (Shofu Kyoto, Japan)

Group B: 15 μ yellow band bur (TR 11EF Mani burs)Silicon rubber cup impregnated with diamond (Sr106M, Dianfong) Silicon carbide impregnated brush (1102F Swiss made)

Group C: Diamond polish mint 1.0 μm, Robinson bristle brush (Ultradent, USA)

Quantitative and Qualitative Analysis of Surface Texture

Surface Roughness (Quantitative analysis) was quantified using a Profilometer (Form Talysurf series 2, Taylor-Hobson) with a transverse section of 0.002 mm radius. The testing was done on both control and polished surfaces and this was repeated three times. Mean values were considered for quantifying the specimens [Ra (μm)].

Surface Roughness (Qualitative analysis) - To appreciate the surface morphology of the specimens, Scanning electron microscope photomicrographs were made under 500X magnification.

Colour Measurements

The colour measurements were made using Spectrocolorimeter on both the surfaces of each specimen in all the three groups. Values were recorded in accordance with the Commission Internationale de l’ Eclairage L*a*b*(CIELAB) system.

The obtained data were subjected to statistical analysis.

Results

Surface texture

Table 1 presents the descriptive statistics of the mean Ra values (μm), showing the differences in average surface roughness between polished and glazed surfaces of Groups A, B and C. The lowest mean Ra value was recorded for Group A (Ra=00.32±10) and the highest was recorded for Group C (01.13±23).

One-way ANOVA test was carried out to indicate significant interactions among the three polishing methods. Post-hoc test indicated statistical significant difference between two groups of all three polishing methods analyzed (Table 2).

Colour measurements

The descriptive statistics of mean ΔE values of difference in L*a*b* values of polished and glazed surfaces of the Group A, B and C was recorded. The lowest mean ΔE value was recorded for Group A (ΔE=1.33±67) and the highest mean ΔE value was recorded for Group C (ΔE=2.06±07) (Table 1).

One-way ANOVA test was carried out to indicate significant interactions among the three polishing methods. Post-hoc test indicated statistical significant difference between the two groups of all the three polishing methods analyzed (Table 2).

Discussion

The growing demand for aesthetics from both patients and clinicians has led to the development of tooth-colored restorative materials with desirable characteristics, like superior aesthetics, wear resistance, smooth surface, biocompatibility, etc. These properties lead to enhanced interest in ceramics, making it one of the most popular restorative materials in the field of Prosthodontics.

In certain instances, such as presence of high points or occlusal interferences, it becomes necessary to perform chair-side occlusal corrections by grinding the glazed occlusal surface. Occlusal adjustments on adhesive ceramic restorations can be performed only after cementation. This removes the glazed layer, producing a rough surface. Re-glazing to smoothen the ceramic surface is not a practical solution and studies have suggested that finishing and polishing procedure provides a suitable surface texture to minimize the bacterial plaque accumulation and also to re-establish the colour of the ceramic restoration.^5,7-10 A rough surface can lead to plaque accumulation, which may cause gingival inflammation, inducing soft tissue reaction and increased wear of opposing teeth.^11-14

Patterson CJW et al., in 1992 stated that the smoothness of ceramic surface helps in maintaining the chemical inertness of the material, and glazing ceramic increases the surface hardness.^14,15 If the ceramic is not adequately polished, micro-cracks present may cause catastrophic fractures in restorations in the future.¹⁶ On the contrary, Ruschel et al., reported that finishing and polishing does not influence the flexural strength of the restoration.¹⁷

Albakry M et al., reported that, among the available ceramic systems, pressed ceramics possess properties such as light transmission, colour reproduction and texture that are comparable to the properties of teeth.³ Various studies that compared the polishing of ceramic surfaces as against glazing suggested that, polishing can serve as an alternative to glazing.¹⁰ However, there is an ongoing debate about the optimal choice of material, as well as the finishing and polishing methods for materials used in fixed restorations. Therefore, this study was carried out to evaluate and compare the surface roughness and color of pressable ceramic restorative material after finishing and polishing with different finishing and polishing agents.

Based on the mean difference in average surface roughness values of polished and glazed surfaces, the minimum mean surface roughness value was observed in Group A (Ra=0.32±0.10 μm) and maximum in Group C (Ra=1.13±0.23 μm) (Table 1). There was a statistical significant difference in average surface values between groups as determined by One-way ANOVA. (F=99.425, P=000) (Table 2). Post-hoc test revealed statistical significant difference between the groups for all the three polishing methods.

Further, qualitative analysis using Scanning Electron Microscope was carried out to correlate the micro photographic features with profilometric values. In Group A, glazed surface revealed smooth appearance with small pits (Figure 4A) that were not quite prominent. The polished surface showed slight irregularities compared to glazed surface with numerous pits and shallow grooves (Figure 4B).

In Group B, glazed surface appeared smooth with small pits (Figure 4C) that were not prominent. The polished surface revealed numerous small pits and very few medium to deep pits with a few widened grooves (Figure 4D). The surface appeared more irregular when compared to polished surface of Group A.

In Group C, the glazed surface showed small pits with striations (Figure 4E). The polished surface showed highly irregular surface with a greater number of widened and deeper grooves, including a few fissures (Figure 4F) in contrast to Group A and Group B of polished surface.

On comparison, it was clear that all these features were found to be consistent with the profilometer readings in this study. Also, the surface alterations of Group C polished surface were more evident when compared to Group A polished surface.

The mean ΔE value of Group A was 1.33 and Group C was 2.06 (Table 1). One-way ANOVA test indicated that the polishing techniques affected colour change; however, no significant differences were found within the groups. Further, Post hoc analysis was done to compare the results between the two groups (Table 2). Porcelain adjustment kit (Group A) manifested the lowest ΔE values besides the other two groups (Group B and C).

The results of the surface roughness and colour difference evaluations were found to be similar across the groups. These results support the common belief that surface roughness impacts the colour of the ceramic.

In the present study, Group A results revealed that Porcelain adjustment kit produced smoother surface, with less colour change as compared to the other two groups. This finding is similar to that reported by Shetty M et al., and various other studies, which concluded that porcelain adjustment kit yielded a smoother surface with minimal colour difference.^10,18,19,20 Furthermore, Klausner et al., study results agreed with the current study findings as the porcelain adjustment kit produced a smoother surface similar to a glazed porcelain.²¹ This smooth surface produced by porcelain adjustment kit can be attributed to the white stone abrasive particles present in the adjustment kit, which effectively remove the irregularities from the ceramic surfaces.¹⁹

Current study results revealed that Group B polishing agents were inefficient in removing the surface irregularities and the colour difference was greater compared to Group A. Patterson et al., conducted a similar study and found that ceramic surface finished with yellow band diamond point followed by refinishing kit produced a relatively smooth surface, similar to glaze.¹⁴ In another study, Haywood et al., used a sequence of finishing burs and polishing pastes like, finishing grit diamonds with reduced particle sizes (Micron Finishing System), 30 fluted carbide burs and diamond polishing paste intraorally to polish the porcelain restorations, which produced a smooth surface similar to a glazed porcelain.²² However, the results of the present study are in contrary to those reported by Patterson et al., and Haywood et al., This difference could be attributed to variability in instruments, speed and water spray used in the studies.

In the present study, Group C results presented higher surface roughness values compared to the other two groups. The present study results contradict the findings of Camacho et al., as they reported that the diamond polishing pastes used with Robinson bristle brush were effective in reducing surface roughness.²³ Similarly, Boaventura et al., also reported the effectiveness of diamond polishing paste (1 μm) in producing a smooth surface.⁵ The variations in these findings could be explained by the differences in the size and cross-section of abrasive particles that can impact the abrasive potential of diamond pastes.²³

Hence, it may be concluded that in the current study, among the three polishing techniques selected, porcelain adjustment kit showed better results with less surface roughness values and less colour change (ΔE) values. Thus, it can be recommended to be used chair side for finishing and polishing of all ceramic pressable restorations.

However, considering that the present in vitro study did not exactly simulate oral conditions, further studies are advisable to evaluate the properties of polished ceramic materials by subjecting them to ageing for exact simulation of oral conditions. Research can also be conducted to determine the amount of plaque accumulation on these ceramic materials when subjected to different polishing methods.

Conclusion

Within the limitations of the study, it can be concluded that ceramics exhibited highly significant differences in the surface roughness values when subjected to different polishing techniques (P=.000**). When the surface roughness was assessed with different finishing and polishing methods, the lowest surface roughness values were found in Group A (Porcelain adjustment kit), followed by Group B (15 μ yellow band bur, followed by silicon rubber cup impregnated with diamond and silicon carbide impregnated brush) and Group C (Diamond polish mint 1.0 μm and Robinson bristle brush). The colour values obtained for the ceramics were not statistically significant across the three groups. However, within the groups, Group A exhibited the lowest values compared to Group B and C; yet all values obtained remained clinically acceptable.

Conflicts of Interest

Nil