Surface Roughness And Colour Stability Of Dental Porcelain Treated With Different Polishing Methods

Introduction

stability in the oral environment. They have been used as a restoration material in restoring carious and missing teeth, such as in crowns, veneers, inlays, onlays, denture teeth, and bridges [1, 2]. Dental porcelains possess a superior strength compared to other restorations such as amalgam, composite restoration and resinmodified glass ionomer cement [3-5], and the porcelain glazed gives a natural appearance of a restored tooth, reducing the wear of opposing teeth, as well as facilitate good oral hygiene [6-8]. However, in a situation like a high bite, crack, chipping, over contour or overhanging margin, a chairside adjustment has to be carried out on porcelain glaze before cementation of the restoration. In the past, chairside porcelain surface adjustment has been associated with increase abrasive wear and plaque accumulation [9, 10]. If a chairside adjustment has to be done, it should be followed by a re-glazing procedure to get the optimum advantage of the porcelain restoration. Nevertheless, not all dental clinic setting has porcelain glazing facilities, and to get it done in the laboratory will inevitably delay the treatment procedure. In the advent of technology in dental porcelain, more efficient porcelain polishing kits have been developed.

Studies have shown that the chairside porcelain polishing kits either used alone or in combination can restore the glaze surface smoothness of adjusted dental porcelain surface [11-14]. On the contrary, studies also showed contradicting findings, that some of the porcelain polishing procedures produced inferior smoothness of dental porcelain compared to the glaze surface [15-17]. Apart from that, studies also showed that a porcelain surface smoothness superior to porcelain glaze can be achieved using the porcelain polishing procedures [18, 19]. These contradicting findings were assumed to arise from the difference in the polishing method, such as pressure and time applied, types of lubricant used and speed of the handpiece. There are also other possible contributing factors such as properties of porcelain, characteristics of abrasive materials, and type of abrasive carrier [20]. Unfortunately, the inadequate porcelain surface smoothness from the chairside polishing procedure has also been associated with the reduced ability of the dental porcelain to withstand staining from common beverages like tea and coffee [14, 21]. Since the literature on the effect of chairside porcelain polishing methods were still limited, and the findings were inconclusive, this study is hoped to give some contribution in understanding the role of porcelain polishing methods in chairside procedure. The aim of the study was to evaluate the effect of dental porcelain polishing methods on the porcelain surface roughness. At the same time, the study also assessed the impact of porcelain surface roughness on the tea extract staining.

Materials and Methods

The lithium disilicate IPS e.max Press glass-ceramic ingots (Ivoclar Vivadent, UK) were used to prepare the porcelain specimens. A total of 40 disc-shaped porcelain specimens measuring 10 mm in diameter and 2 mm in thickness were made by a single investigator. The lost wax casting technique was used in the porcelain specimen preparations. For each specimen, a disc-shaped wax pattern was prepared using a silicone mould. A digital calliper (Mitaka, Japan) was used to ensure consistency in the wax’s dimensions. The wax specimens were then invested in a phosphate-bonded investment material (Interfine K+B Speed, Interdent, Slovenia). The porcelain ingots were pressed into disc-shaped porcelains using the lost-wax casting technique. The press furnace, Programat EP 3000 (Ivoclar Vivodent, UK) was used in the pressing process following the manufacturer’s instruction. All specimens were allowed to cool and then finished with a medium-grit diamond bur to remove any irregularities. The specimens were then sprayed with Triton SLA steam blaster (BEGO, USA) before placed in a furnace to obtain an auto glazed surface. Specimens were inspected under a light microscope (Leica, Germany) to ensure a smooth surface, and the defective specimen was removed and replaced.

The specimens were then randomly divided into 4 groups of 10 specimens each according to polishing methods: Group A (Glaze), the specimens were untreated and served as control; Group B (Polishing paste), the specimens were polished using slow speed green stone bur (Edenta, Germany) and followed by diamond polishing paste (Shofu, Japan); Group C (NTI), the specimens were polished using NTI CeraGlaze polishing kits (Kerr, Switzerland); and Group D (Identoflex), the specimens were polished using Identoflex Diamond Ceramic Polisher (Kerr, Switzerland). The specimens were then subjected to a baseline measurement for surface roughness and colour shade using surface profilometer (Tokyo Seimitsu Co. LTD, Japan) and digital spectrophotometer, VITA Easyshade Advance 4.0 (VITA, Germany) respectively. Group A (Glaze) served as a control, and no surface treatment was done. As for the test group, the glaze layer was removed using a medium-grit diamond rotary bur with a slow speed handpiece under water spray to simulate clinical repair procedure. The test group specimens were then subjected to the mechanical polishing procedure as assigned to each group for 60 seconds. After completing the surface treatment, the specimens were subjected to a second surface roughness measurement.

The specimens were then subjected to cyclic thermal stress for 2000 cycles using distilled water between 5°C and 55°C with a dwell time of 30 seconds and a transfer time of 15 seconds. The procedure was carried out to simulate the service life of dental porcelain in the oral cavity. The specimens were subjected to staining for 30 days in a tea extract solution at room temperature. The tea extract solution was prepared according to the company’s instruction by adding 5 tea bags (BOH, Malaysia) into 1000 ml of freshly boiled water and brewed for 2 minutes. Teabags were then removed, and the tea extract solution was left to cool at room temperature. This is followed by a second measurement of colour shade.

Surface Roughness Measurement

The measurement of surface roughness was made using profilometer (Tokyo Seimitsu Co. LTD, Japan), where Ra was chosen as the roughness parameter. Ra was calculated as the roughness average of a surface measured microscopic peaks and valleys that have been traced by the profilometer [22]. A higher Ra value indicates a rougher surface. The stylus used in this profilometer has a tip radius of 2 μm. The profilometer was calibrated before the measurement of each group. For the measurement process, the profilometer was set to a cut off value of 0.25 mm for detecting surface irregularities. A sample measuring distance was 2 mm, and a constant measuring speed and force were set at 0.06 mm/s and 0.7 N, respectively. During the surface roughness measurement, the specimen was stabilised with silicone impression material into a Perspex mould. Three readings were taken for each sample, where all measurements were carried out as close as possible to the sample centre. The Ra value for each sample was calculated from the average of the three measurements.

Colour Shade Measurement

The measurement of colour shade was made using a spectrophotometer, VITA Easyshade Advance 4.0 (VITA, Germany). Before colour measurement, the spectrophotometer was calibrated against the calibration block as directed in the instruction manual. Colourimetric measurements of the specimens were performed according to the CIELAB colour scale, recording the L*, a*, and b* values. For each specimen, three measurements were performed on a white background to get a mean value for the L*, a*, and b*. The colour changes (ΔE) was calculated using the following formula [23], ΔE = [(ΔL)2 + (Δa)2 + (Δb)2]1/2.

Statistical Analysis

Since the data were not normally distributed when analysed using Shapiro-Wilk test (p < 0.05), comparing more than two variables was carried out using the Kruskal-Wallis test, which was followed by pair-wise comparisons using a series of Mann-Whitney U tests. Significance level was set at p = 0.05.

Result

Table 1 shows the surface roughness of dental porcelain after treated with different polishing methods. The surface roughness of dental porcelain was significantly higher in the Polishing paste group when compared to the Glaze group (p < 0.05). However, in the NTI and Identoflex group, the surface roughnesses were found not significantly different from the Glaze group (p > 0.05).

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Additionally, Table 2 shows the colour changes (ΔE) of different porcelain polished surface condition after staining in tea extract solution. In all test groups, the Glaze group shows superiority in colour stability. The ΔE value was statistically significantly higher compared to all test groups (p < 0.05). The Polishing paste group was statistically significantly higher in ΔE values compared to NTI and Identoflex group (p < 0.05).

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Discussion

Our study showed that polishing the roughened porcelain surface using a slow speed green stone bur (Edenta AG, Germany) and diamond polishing paste (Shofu Inc., Japan) produced a significantly higher surface roughness compared to the glazed porcelain. Polishing a roughened porcelain surface using different types of diamond polishing paste alone has been shown to give higher surface roughness compared to the glazed porcelain [13, 24]. The finding appears similar when the diamond particle-impregnated wax (diamond wax) was used [25]. Seemingly, the diamond paste or wax possibly only beneficial in reducing the height of the maximum surface peak and it is not meant for clearing the mass of roughened surface. It is not clear if the diamond paste or wax on its own efficient enough in reducing surface roughness (Martinez- Gomis et al., 2003). It was shown that for the diamond paste or wax to be effective, the roughened porcelain surface has to be prereduced sufficiently using either porcelain adjustment kit, polishing wheel, polishing stick, or Sof-lex discs [13, 25, 26]. Diamond polishing paste has been shown to produce a smooth porcelain surface comparable to the glazed porcelain when used in combination with other porcelain adjustment kits such as polishing disks [24]. Although a slow speed green stone bur was used in this study to pre-reduce the porcelain surface roughness, the coarse finishing of the stone probably too rough to be eliminated by diamond polishing paste.

This study also revealed that the NTI CeraGlaze polishing kits and Identoflex Diamond Ceramic Polisher kits were able to smoothen the roughened porcelain surface comparable to the glazed surface. This is in line with the previous study that showed the capability of the NTI CeraGlaze polishing kits in smoothing the roughened porcelain surface to the level similar to glazed porcelain [11, 27]. Also, a profilometer analysis revealed that Identoflex system was capable of producing porcelain surface roughness equal to the glazed surface. However, when the analysis was extended to the microscopic level using the atomic force microscope (AFM), the Identoflex system was shown incapable of producing the surface roughness comparable to the glazed porcelain [17].

In this study, the polished porcelain surface produced by different mechanical polishing procedure was evaluated for their ability to resist tea extract staining in comparison to the glazed surface. Our finding showed that the ΔE value of all polishing groups; Polishing paste, NTI and Identoflex revealed the median ΔE values of 3.70 or higher, which is categorised as clear noticeable colour difference [28]. On the other hand, the glazed surface presented with the median ΔE values of less than 2, which is characterised as noticeable only by an experienced observer. Statistical analysis also showed that all the polishing groups (Polishing paste, NTI and Identoflex) presented with significantly higher median ΔE values when compared to the glaze surface. The Polishing paste group showed statistically significantly higher median ΔE values when compared to the NTI and Identoflex groups (p < 0.05).

Different porcelain polishing procedures have been shown to produce polished porcelain surfaces that have a distinct ability to withstand staining from common beverages like coffee and tea [29]. This could be due to the variation in surface roughness created by different polishing techniques. Moreover, porcelain glazed surface has been shown to have a better ability in resisting tea or coffee extract staining when compared to mechanically polished porcelain surface [30-32]. Evaluation using methylene blue staining also showed better colour stability of the porcelain glazed surface when compared to various methods of mechanical porcelain polishing methods (Yilmaz et al., 2008).

Within the limitation of this study, we observed that with a good selection of porcelain polishing technique and material, a fractured or roughened porcelain surface can be polished to achieve the surface roughness of at least equal to the glaze surface when evaluated using a profilometer. However, it is known that at a microstructure level, the smoothness of the glazed surface is superior compared to the mechanically polished surface. This is probably the best explanation to describe the inferiority of the mechanically polished porcelain colour stability compared to the glaze surface. Hence, perhaps, it is better when performing porcelain adjustment on the aesthetic concern area, the porcelain is re-glazed before cementation. The main limitation of our study was we only evaluate one type of dental porcelain. Studies showed that different types of dental porcelain produced a varying degree of surface roughness after mechanical chairside polishing, which also had various ability in resisting beverage staining [11, 14, 21]. This study also can be improved by evaluating the surface roughness degree at the microscopic level using atomic force microscope (AFM).

Acknowledgement

This study was supported by the Universiti Sains Malaysia (USM) through the USM research grant, Grant no: 304/PPSG/61313149 and 304/PPSG/61313137.