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Table of Contents   
ORIGINAL RESEARCH  
Year : 2012  |  Volume : 23  |  Issue : 6  |  Page : 838
The influence of metal substrates and porcelains on the shade of metal-ceramic complex: A spectrophotometric study


1 Department of Prosthodontics, Sree Mookambika Institute of Dental Sciences, Kulasekharam, Tamilnadu, India
2 Department of Prosthodontics, Govt Dental College, Thiruvananthapuram, Kerala, India

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Date of Submission09-Aug-2011
Date of Decision13-Mar-2012
Date of Acceptance20-Apr-2012
Date of Web Publication3-May-2013
 

   Abstract 

Background: The final esthetic outcome of a metal-ceramic restoration is influenced by several factors including the type of the underlying metal as well as the brand of the ceramic.
Settings and Design: An in vitro study.
Aims: The purpose of the in vitro study was to investigate the influence of four types of metal-ceramic alloys and two porcelain systems on the color co-ordinates of the metal-ceramic complex. It also aimed at establishing a color index which correlated the color of the metal-ceramic combination to the yellow-blue axis.
Materials and Methods: Twenty-four disc-shaped metal specimens of 12 mm × 1 mm were cast from base metal alloys, nickel-chromium (Ni-Cr) (Wiron 99), cobalt-chromium (Co-Cr) (Wirobond C), a palladium (Pd)-rich noble metal alloy (Spartan Plus), and a high noble gold (Au) alloy (Pontostar). These discs were covered with two commercially available feldspathic porcelains (Vita Omega and Shofu Vintage) of A3 shade to a total thickness of 1.2 mm. Each group had six specimens, of which three were coated with Vita Omega and the remaining with Shofu Vintage. Opaque and dentin layers were applied to a standardized thickness of 0.2 mm ± 0.05 mm and 1 mm ± 0.05 mm, respectively onto the metal surface after air abrasion with 50 μm alumina particles. The reflectance spectra were measured with a spectrophotometer and data were recorded in L*, a* and b* coordinates in the CIE Lab Color Order System.
Statistical Analysis: Analysis of variance (ANOVA) and path analysis were the statistical tools employed to analyze the data. A critical difference (CD) value was calculated for each color co-ordinate to make comparisons between each metal-ceramic combination. A color index for each metal-ceramic complex was also calculated from the color co-ordinates obtained.
Results: ANOVA revealed that significant differences existed between the metal-porcelain combination at 0.01 ( P < 0.01) level. The L* value was significantly higher for Au and Co-Cr alloys with Vita Omega porcelain. The a* value was highest for Ni-Cr alloy when combined with Shofu Vintage ceramic. The b* value of Au alloy with Vita Omega porcelain was significantly higher than any other metal-ceramic combination. The color co-ordinates of Pd alloy with both porcelain systems did not show any significant differences. Gold alloy with Vita Omega showed the highest color index value.
Conclusion: The variations in metal-ceramic alloy and porcelain can influence the shade of a metal-ceramic restoration. Color index value was the highest for gold alloy.

Keywords: CIE LFNx01aFNx01bFNx01 coordinates, color index, metal ceramics, spectrophotometry

How to cite this article:
Janardanan K, Pillai ST, Karunakaran H. The influence of metal substrates and porcelains on the shade of metal-ceramic complex: A spectrophotometric study. Indian J Dent Res 2012;23:838

How to cite this URL:
Janardanan K, Pillai ST, Karunakaran H. The influence of metal substrates and porcelains on the shade of metal-ceramic complex: A spectrophotometric study. Indian J Dent Res [serial online] 2012 [cited 2020 Dec 4];23:838. Available from: https://www.ijdr.in/text.asp?2012/23/6/838/111279
Fabrication of a crown that matches the natural tooth in color is one of the most challenging aspects of dental restorations and esthetic dentistry. The two crucial steps for a natural-looking restoration are the selection of the shade in the operatory and the duplication of the shade in the laboratory. Successful shade matching requires an understanding of the science of color, proper communication with the laboratory, and a thorough knowledge of the factors affecting selection of the shade. Even then, variations between intended color and final color can result because of the limitations faced while duplicating a shade tab. [1] Although the advent of all-ceramic restorations has reduced the popularity of metal-ceramic restorations, certain factors such as increased cost, increased amount of tooth reduction required, and difficulty in retrieval of a metal-free restoration have shifted the graph in favor of metal-ceramic restorations. The critical factors that can affect color match between natural teeth and restorative materials are the perception of color by the individual and the lighting in the operatory. Factors that are relevant in influencing the final color of a metal-ceramic restoration are the firing temperature of porcelain, [2],[3] glazing cycle, [4] mixing ratio between powder and liquid, [5] thickness of ceramic layers, [6] the type of metal substrate, [7],[8],[9],[10] and the porcelain system [11] employed. Although a search of the literature [12] has shown the influence of metal substrate on the final color of restoration, it is still obscure as to which type of alloy produces the desired chromatic shift on a particular porcelain system. Only a few studies have documented the role of cobalt-chromium (Co-Cr) alloys on the color of porcelain.

Visual assessment of shade using a shade tab, although the most common practice to evaluate and reproduce the natural color of the tooth, often results in subjective errors and does not allow for quantification of color. Hence, instrumental determination of shade using a colorimeter or a spectrophotometer provide a numerical classification of color and a more precise communication with the laboratory, and was made use of in this study. [13],[14]

The present study was an attempt to determine the variations in the color co-ordinates L*, a* and b* (CIE Lab color ordering system) of a metal-porcelain combination using a spectrophotometer, to provide a realistic estimation of the influence of the type of metal on the ceramic to produce the final shade of the restoration. An attempt was also made to numerically categorize the final color obtained for each combination by means of a color index. The null hypothesis was that the variation in the metal and ceramic has no effect on the color co-ordinates.


   Materials and Methods Top


The metal-ceramic alloys employed in the study were two base metal alloys, a nickel-chromium (Ni-Cr) alloy and a Co-Cr alloy, and two noble metal alloys, a palladium (Pd)-rich noble alloy and a high noble gold (Au) alloy. The manufacturers, brand names and composition of the alloys, and the classification according to the American Dental Association [15] are shown in [Table 1]. Six metal discs were fabricated from each of the metals. Each disc had a diameter of 12 mm and a thickness of 1 mm. The two types of ceramics employed were Vita Omega (VITA Zahnfabrik, Bad Sackingen, Germany) and Shofu Vintage (Shofu Inc., Japan). Half of the six discs of the metal specimens were covered with Vita Omega ceramic and the remaining half were covered with Shofu Vintage ceramic. The reflectance spectra of the total twenty four test specimens were measured using a spectrophotometer.
Table 1: Composition and commercial names of metal alloys as provided by the manufacturers

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A circular steel mold [Figure 1] was prepared to make wax patterns at specified dimensions of 12 mm diameter and 1 mm thickness. Twenty-four wax patterns [Figure 2] were prepared and cast in a pressure casting machine. After bench cooling, the cast specimens were retrieved and sandblasted with 250 μm alumina to remove the investment material clinging to the castings.Then, the sprues were cut in a high-speed lathe and the metal discs were adjusted with stones and airborne particles abraded with 50 μm aluminium oxide. The thickness of each disc specimen was rechecked to ensure that the original dimensions had been maintained. After measuring the thickness, the samples were cleaned with steam in a steam cleaner (Bego Inc., Bremen, Germany) followed by ultrasonic cleaning (Bego Inc., Bremen, Germany) in distilled water for 10 minutes and then oxidized in the ceramic furnace (Programat P500), at 950°C in vacuum for five minutes. Six specimens were prepared for each type of alloy.
Figure 1: Circular steel mold

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Figure 2: Wax patterns with sprue attached

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The metal specimens were covered with one of the two porcelains, Vita Omega or Shofu Vintage. Porcelain shade A3 was used for all opaque and dentin layers and was applied in two layers. The thickness of the opaque layer was 0.2 mm ± 0.05 mm [Figure 3] and [Figure 4]. The thickness was determined with a micrometer (Mitutoyo Corporation, Kawasaki, Japan) with an accuracy of 0.05 mm [Figure 5]. After the application of opaque porcelain, the dentin porcelain was added in two layers to a thickness of 1 mm ± 0.05 mm. The total thickness of the ceramic layer was 1.2 mm. All specimens were subjected to four firings. The firing parameters for the two porcelain systems tested are given in [Table 2] and [Table 3]. Enamel layer was not applied and the specimens were not glazed.
Figure 3: Application of Vita opaque layer

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Figure 4: Application of Shofu opaque layer

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Figure 5: Micrometer for standardization of ceramic thickness

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Table 2: Firing schedule for Vita Omega porcelain system


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Table 3: Firing schedule for Shofu Vintage Halo porcelain system

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The optical reflectance of the specimens were measured with an ultraviolet-visible (UV-Vis) spectrophotometer (Shimadzu, UV-2450) using barium sulfate as a reference, set to the standard illumination source of a halogen 50 watt deuterium lamp. Each specimen was analyzed thrice and the average value was taken. The data were displayed in L*, a*, and b* values according to the CIE Lab system.

The treated samples were subjected to a one-way analysis of variance (ANOVA) (at 0.01 level of significance) to compare the changes in color observed between the different metal-ceramic combinations. The critical difference (CD) is the maximum difference expected between two treatment means at 5% level of significance. [16]

CD = t error df ξ √2MSE/r, where MSE stands for mean square value of error and t error df is the difference in treatment error.

If the observed difference between two treatment means is higher than this CD value, then it can be concluded that a significant difference is detected between these treatments at 95% confidence level. The mean square value for error (MSE) component of variation provides an estimate of error variance, or within treatment variation. MSE/r is a measure of the variance of a treatment mean ('r' being the number of replications) and its square root the standard error of the mean. Utilizing this information, a color index was formulated as follows:

I = w1Ĺ*+ w2á* + w3b~*, where I is the color index and w 1, w 2 , and w 3 are the reciprocal of the variance of the mean which are given as weights assigned to each mean.

The interrelations among L*, a*, b*, and color index were estimated and used as the input matrix for the path analysis. Path analysis is a method to study the direct and indirect effects of several independent variables on a dependent variable and is equivalent to standard regression analysis. This analysis helps to measure the direct and indirect contribution of color co-ordinates L*, a*, and b* on the color index.


   Results Top


ANOVA, to compare color differences in metal-ceramic complexes, has been described in [Table 4]. ANOVA revealed significant differences at 0.01 level (P < 0.01) in the main effect of porcelain or alloy and for the interaction between alloy/porcelain combination.
Table 4: Analysis of variance of metal-ceramic complexes

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In [Table 5], mean values of the color co-ordinates of groups under study are listed. CD values for each co-ordinate are also provided at 5% significance. If the pairwise mean differences of the color co-ordinates exceeds the CD value, then the superiority of individual treatments over one another becomes evident and it is considered to be statistically significant.
Table 5: The mean values of color co-ordinates

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The L* values of Au alloy and Co-Cr alloys were found to be significantly higher (CD > 0.351) than Pd alloy and Ni-Cr alloy with both porcelains. The Vita Omega showed increased L* values with all four alloys compared to Shofu Vintage specimens, with the exception of Pd alloy, where the difference was not statistically significant.

Ni-Cr alloys showed a* values that were significantly higher (CD > 0.155) than Au and Co-Cr alloys with both porcelains. Pd alloy also showed significantly higher a* values than Au and Co-Cr when combined with Vita Omega porcelain. Both Au alloy and Pd alloy showed similar values with Shofu ceramic. Shofu Vintage ceramic showed higher a* values with all alloys except Pd alloy, which showed similar values with both porcelains.

Noble metal alloys, Au and Pd, showed significantly higher (CD > 0.231) b* values than Ni-Cr and Co-Cr alloys with either of the two ceramics. The Au alloy with Vita showed the highest b* value which was significantly higher than the b* value of Au with Shofu ceramic. No significant difference in b* value was observed for Ni-Cr, Co-Cr, and Pd alloys, when combined with Vita Omega or Shofu Vintage ceramic.

In [Table 6], the color index values of the two groups under study are arranged. It was observed that the highest color index values were for Au with Vita ceramic and Au with Shofu ceramic, followed by Pd with Shofu ceramic and Pd with Vita ceramic.
Table 6: Color index values of the alloy-porcelain combinations

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[Table 7] shows the inter-correlations between color co-ordinates and color index. Positive correlation was observed for L* and b* with color index at 5% significance level. The correlation of a* with color index was negative.
Table 7: Inter-correlations among color co-ordinates and color index

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   Discussion Top


This in vitro study measured the variations in color co-ordinates between various metal-porcelain combinations. The color properties are described in terms of CIE Lab 1976 color scales in which the elements are arranged in an approximately uniform three-dimensional color space. L* represents the value or lightness co-ordinate of an object; the greater the L*, the lighter the specimen. Values a* and b* represent the chromaticity coordinates, a* the measurement along the red-green axis, and b* the measurement along the yellow-blue axis. When the co-ordinate a* is positive (+a*), the color of the object tends to be red and when a* is negative (-a*), the shade is closer to green. Similarly +b* indicates the direction toward yellow and -b* toward blue. The use of photometric instruments for evaluating differences in color has been well documented, [17],[18] and it provides objective information about the magnitude and direction of color differences that occur between various metal-porcelain combinations. [19]

The two porcelain systems selected were the commonly used Vita Omega and Shofu Vintage. As firing parameters, [20],[21],[22] temperature, and thickness [23],[24],[25] of porcelain layers affect the final shade, they were kept constant to determine the effect of metal substrate and overlying porcelain. A 0.1 mm thickness of opaque porcelain was not adequate to mask the color of base metal alloys. [26] To overcome the color differences caused by various factors, an opaque thickness of porcelain of 0.2-0.3 mm [27],[28] and body thickness of porcelain of 1 mm [29] appeared ideal.

The null hypothesis of this study was rejected, as differences in the CIE Lab color coordinates were noted between different metal-ceramic complexes. ANOVA showed that significant differences existed between alloy-porcelain combinations with respect to the color co-ordinates. Au alloy and Co-Cr alloy showed the highest L*value with both porcelains, that is, these metal substrates produced a lighter color with the overlying porcelains [Figure 6]. Pd alloy showed no significantly different values with both porcelains. Ni-Cr alloy showed the highest a* value with both the porcelain systems [Figure 7]. Hence, the Ni-Cr alloy produced a comparatively darker shade and reddish hue for the restoration, regardless of the two porcelain systems tested. Although a* value of Pd alloy with Vita Omega porcelain was higher than Au and Co-Cr alloys, when combined with Shofu Vintage ceramic, no significant difference was noted between Pd and Au alloy. The b* values of Au and Pd alloys were significantly higher than the values of Ni-Cr and Co-Cr with both porcelain systems [Figure 8]. Au and Pd alloys produced a yellow hue when compared with base metal alloys.
Figure 6: Bar diagram depicting the mean values of L* for different porcelains and alloys

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Figure 7: Bar diagram depicting the mean values of a* for different porcelains and alloys

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Figure 8: Bar diagram depicting the mean values of b* for different porcelains and alloys

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The L* values were greater with Vita Omega porcelain compared to Shofu Vintage for all alloys, except Pd where the difference was not statistically significant (<0.351). Shofu Vintage ceramic showed a higher a* value compared to Vita Omega with all alloys except Pd which showed similar values with both porcelains. Vita Omega ceramic showed higher b* value with Au alloy. But with all other alloys, there was no difference in b* co-ordinate when combined with either of the ceramics.

From the above-mentioned facts, it can be seen that Au alloys are the materials of choice for replication of color of metal ceramic restorations, as they impart a yellowish hue to the alloy-porcelain combination and can be easily masked by an opaque layer. The use of Pd-rich noble alloy in place of high noble Au alloy did produce significant differences in color co-ordinates L*, a*, and b* This is in contrast with the studies conducted by Crispin et al., [9] who found no significant difference between Pd-rich noble alloy and high Au alloys. This study also contradicts the results obtained by Brewer et al., [8] who found no significant differences in color values between Au and Ni-Cr alloys. In a similar study conducted by Kourtis et al., [6] it was found that alloys behaved in the same manner with Vita Omega porcelain. But instead of Ceramco, the porcelain used in the present study was Shofu Vintage and showed similar b* values with all alloys except Au. Currently, the use of titanium alloys for metal-ceramic restorations were found to be more accurate in reproducing the color of the selected shade tab. [30] More research is needed in this direction.

Another interesting observation was that Pd-rich noble alloy did not produce any significant differences in L* a*, and b* values when combined with either of the porcelain systems, that is, the use of Vita Omega or Shofu Vintage ceramic produced similar color co-ordinates for high noble Pd alloy.

Instrumental determination of shade is subjected to the phenomenon of edge loss, occurring as a result of light 'lost' primarily through the translucent ceramic enamel layers. [31] This has been considerably reduced in the present study because a translucent enamel layer was not applied and the specimens were illuminated from the front as well as from the periphery, yielding a more accurate result. But in a clinical situation, the accuracy of the target shade obtained from measurement is only as good as the database and its distribution of reference shade.

From the results obtained, that is, the mean L*, a*, and b* co-ordinate values of each metal-porcelain combination, it was possible to develop a color index which had a strong inter-relation with the color co-ordinates. Path analysis showed that L* and b* co-ordinates had a positive correlation with the color index, that is, the higher the color index value, the lighter and more yellowish the tested specimen, but the correlation between color index and a* co-ordinate was negative. Hence, it could be inferred that the lower the color index, the more the shifts are toward the red-green scale. In this study, it was found that high noble Au alloy showed the highest color index value [Figure 9]. This was greater when Au alloy was combined with Vita ceramic. Interestingly, color index value was lowest for Ni-Cr alloy which was in accordance with the high a* value obtained with this alloy. Noble metal alloys with higher b* values had higher color index values than base metal alloys. Hence color index can be considered as a useful tool in the selection of an alloy-porcelain combination. The relevance of the study is that if the color index values can be calculated for a shade guide, then based on the shade selected clinically, the appropriate alloy-porcelain combination can be determined.
Figure 9: Comparison of color index values between different metal-porcelain combinations

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   Summary and Conclusion Top


The use of different metal-ceramic alloys as well as various porcelain systems has the potential to alter the perceived color of a dental restoration. This study was an attempt to investigate the role played by alloys and porcelains, on the final shade of metal-ceramic complexes. The study has furnished the following inferences:

  • Au and Co-Cr alloy specimens showed higher L* values with both porcelains.
  • Vita Omega ceramic showed increased L* values with all alloys, compared to Shofu Vintage, with the exception of Pd alloy, where the difference was not statistically significant.
  • With both porcelains, Ni-Cr alloy showed higher a* values compared to Au and Co-Cr alloys. There was no statistical difference between Pd alloy and Au alloy combined with Shofu Vintage ceramics.
  • The a* values of all Shofu Vintage specimens were higher than Vita Omega a* values except Pd alloy which showed similar values with both ceramics.
  • The b* values of Au and Pd alloys were higher than the values of Ni-Cr and Co-Cr alloys, with significant differences.
  • The Au alloy with Vita Omega porcelain showed the highest b* value. For all other alloys, the use of either of the porcelains did not produce significant differences in b* values.
  • Pd alloy did not exhibit any significant difference in color co-ordinates with both the porcelain systems.
  • A color index value was computed which had a positive correlation with L* and b* co-ordinates. Noble metal alloys exhibited greater color index values than base metal alloys. Au alloy with Vita Omega porcelain showed the highest color index value. Hence, it could be inferred that this combination was lighter and produced a greater shift toward the yellow-blue axis in the final color of metal-ceramic complexes.


 
   References Top

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Correspondence Address:
Kavitha Janardanan
Department of Prosthodontics, Sree Mookambika Institute of Dental Sciences, Kulasekharam, Tamilnadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.111279

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]

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