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Spectrophotometric Analysis of All-ceramic Materials and Their Interaction with Luting Agents and Different Backgrounds

V.S. Barath^1,*, F.-J. Faber², S. Westland³ and W. Niedermeier¹

¹ Department of Prosthetic Dentistry, Dental School (Zentrum für Zahn, Mund und Kieferheilkunde), University of Cologne, Kerpener Strasse 32, 50931 Cologne, Germany;
² Department of Preclinical Dentistry, Dental School, University of Cologne;
³ Colour & Imaging Institute, University of Derby;

Correspondence: ^* corresponding author, vs.barath{at}uni-koeln.de

	Abstract

TOP Abstract Introduction Materials & Methods Results Discussion Conclusion Future Work References

 
In this study, two All-Ceramic (AC) materials—Empress 2 (EMP) (Ivoclar Vivadent AG, Schaan, Liechtenstein) and In-Ceram ALUMINA (ICA) (Vita Zahnfabrik, Bad Säckingen, Germany)—were analyzed, along with the effects of 3 luting agents—viz. Zinc Phosphate cement (ZNPO, PhospaCEM PL, Ivoclar Vivadent AG, Schaan, Liechtenstein), Glass Ionomer Cement (GIC, Ketac-Cem Radiopaque, ESPE Dental AG, Seefeld, Germany), and Compolute (COMP, ESPE Dental AG, Seefeld, Germany)—on the final color, using the CIELab system. Color differences (DeltaL, Deltaa, Deltab, and DeltaE) were calculated for samples with luting agents and for samples without luting agents with standard white and black backgrounds, with the use of a spectrophotometer, Luci 100 (Dr. Lange, Berlin, Germany). One-way ANOVA for DeltaL, Deltaa, Deltab, and DeltaE within both the AC systems, with and without luting agents, showed significant contributions of the background (p < 0.05). EMP was seen to be more translucent than ICA. Darker ceramics showed less color variation. Luting agents altered the final color of the restoration. ZNPO was least translucent, followed by GIC and COMP. Marginal increases in thicknesses of ICA samples (0.4 mm) do not show a statistically significant color difference. No method exists to predict the outcome of an AC restoration based on consideration of the luting agent and the background color.

Key Words: Ceramics • dental cements • luting agents • color • colorimetry

	Introduction

TOP Abstract Introduction Materials & Methods Results Discussion Conclusion Future Work References

 
The dental profession has long been concerned with the problem of matching the appearance of the ceramic restorations with a patient’s natural dentition (Seghi et al., 1986). As far as the appearance of a tooth-colored restoration is concerned, the color of a metal ceramic or all-ceramic (AC) dental restoration is decisive. AC restorations without a metal substructure allow for greater light transmission within the restoration, thereby improving the color and translucency of the restoration, but still a perfect esthetic tooth-colored restoration cannot be ensured (Wee et al., 2002). It has been suggested that color training for dentists should be a part of the course in prosthetic dentistry (Huang et al., 1989). The CIELab color system (Commission Internationale de l’Éclairage) was first published in 1976 (CIE, 1976). This system facilitates color space presentation (Fig. 1).

View larger version (25K): [in this window] [in a new window]   Fig. 1 — Framework of the CIELAB color model. Kindly provided by http://www.handprint.com/. L - 0 (black) to 100 (white); a - –80 (green) to +80 (red); b - –80 (blue) to +80 (yellow); C - difference of a specific color in relation to gray color of the same lightness. H : H = 0 red, H = 90 yellow, H = 180 green, H = 270 blue.

All-porcelain veneers provide a masking effect for the background shade when luted to the substrate with a luting agent (Davis et al., 1992). The shade is determined not only by the color of the porcelain, but also by the thickness of the porcelain, the thickness and the color of the luting agent, and the color of the underlying tooth structure (Vichi et al., 2000). The ceramics are translucent at clinically relevant thicknesses (Heffernan et al., 2002), and with different core materials, the translucencies vary within the ceramics.

The primary purpose of this study is to investigate the effects of background color and luting agents on the final color in AC samples.

	Materials & Methods

TOP Abstract Introduction Materials & Methods Results Discussion Conclusion Future Work References

 
In this study, 2 AC materials—IPS Empress 2 (EMP) (Ivoclar Vivadent AG, Schaan, Liechtenstein), a heat-pressed glass ceramic which has lithium disilicate as the core material, and VITA In-ceram Alumina (ICA) (Vita Zahnfabrik, Bad Säckingen, Germany), which has aluminum oxide as the core material—were studied. For the study, 15 ceramic sample discs for each group, 16 mm in diameter, were prepared by the manufacturer. Three shades were arbitrarily selected, visually, to represent a cross-section of various degrees of pigmented porcelain.

For Empress 2 (EMP, n = 45), the combinations we used were: 100-110-S1 (EMP1, n = 15), 300-320-S2 (EMP2, n = 15), 500-520-S3 (EMP3, n = 15), with 100, 300, and 500 being the core, 110, 320, and 520 the dentin, and S1, S2, and S3 the enamel (Fig. 2).

View larger version (63K): [in this window] [in a new window]   Fig. 2 — Ceramic samples (left to right). First row: In-Ceram ALUMINA (1.0 mm), ICA1, ICA2, ICA3. Second row: In-Ceram ALUMINA (1.4 mm), ICA1, ICA2, ICA3. Third row: IPS Empress 2 (1.4 mm), EMP1, EMP2, EMP3.

The luting agents (CEM) used as an intermediate layer, when luted to the inner (core—non-glazed) surface, were: Zinc Phosphate cement (ZNPO), shade Neutral (PhospaCEM PL, Ivoclar Vivadent AG, Schaan, Liechtenstein); Glass Ionomer Cement (GIC), shade Universal (Ketac-Cem Radiopaque, ESPE Dental AG, Seefeld, Germany); and Compolute Aplicap (COMP), shade A3, a Resin Luting Agent (ESPE Dental AG, Seefeld, Germany). The luting agents were pressed onto the inner (non-glazed) surface of the ceramic sample by means of a micrometer (Mitutoyo, Neuss, Germany), with a glass slide used as an intermediate layer between the micrometer and the "All Ceramic Luting agent" (ACLA) unit, to press the luting agent onto its non-glazed surface, to produce a thickness of 0.08 to 0.18 mm with various (arbitrary) pressures.

Spectrophotometric evaluation
The colorimeter we used for the measurements was LUCI 100 (Dr. Bruno Lange GmbH, Berlin, Germany), a spectral two-beam spectrophotometer with mobile measuring head. The measurements were recorded on a PC equipped with LUCIQC software (Dr. Bruno Lange GmbH, Berlin, Germany). The spectral range of the colorimeter was between 380 and 720 nm. Distance between two measured points was 10 nm, and the measuring geometry was d/8° according to DIN (Deutsches Institut für Normung) 5033.

The black-and-white standard discs were used for calibration of the spectrophotometer and then served as the standard backgrounds for the sample discs during the CIELab measurements before and after application of the luting agents to the surfaces of the ceramic samples.

We determined the difference between two colors by comparing the differences between respective coordinate values for each coordinate, as shown by the following equation:

(Eq. 1)

E ("Empfindung" = sensation [in German]), the total color difference, and L (also represents translucency of the material), a (redness-greenness), and b (yellowness-blueness) are:

(Eq. 2)

(Eq. 3)

(Eq. 4)

Here, "w" represents the white background and "b" represents the black background due to different luting agents as the intermediate layer, with standard backgrounds, respectively.

E values were calculated for the samples with black and white backgrounds (EWB) for the 3 sample groups of each product (n = 15). The samples were then divided into 3 subgroups; luting agent was applied to the non-glazed surface, and E (EWCBC) values were calculated for the samples with different luting agents as the intermediate layer (n = 5), with white and black backgrounds. We calculated this to see the effect of the background on the final color. To see the effects of the luting agents on the final color, we calculated the E values for samples with the same background, with and without luting agents (EWWC-with white background and EBBC-with black background).

Last, we calculated E values for the individual luting agent samples (n = 10) with standard black and white backgrounds.

Statistical analysis
The significance of the results was tested by one-way ANOVA, followed by Duncan’s (DUN) multiple comparison at the 0.05 level. Pearson’s correlation coefficient was used for evaluation of correlation of translucency (LBW) with the color change (EWCBC) due to background, thickness of the luting agent (CTh), and the effects of luting agents on the final color (EWWC and EBBC).

	Results

TOP Abstract Introduction Materials & Methods Results Discussion Conclusion Future Work References

 
The mean thickness (STh) and standard deviation (SD) of the EMP samples and ICA sample groups, without luting agents, and the mean thickness and standard deviation of luting agents on the samples (CTh) are shown in Table 1.

View this table: [in this window] [in a new window]   TABLE 2 — L, a, b, and E Values between the Samples without Luting Agents and Those with Luting Agents, with Standard Black and White Backgrounds

View larger version (16K): [in this window] [in a new window]   Fig. 3 — Box plots showing L, a, b, and E values between the samples without and with luting agents with standard black and white backgrounds for Empress, 100, 300, and 500 being the core shades.

View this table: [in this window] [in a new window]   TABLE 3 — L, a, b, and E Values between the Samples without Luting Agents and Those with Luting Agents, with the Same Backgrounds

The luting agents were also analyzed without the AC samples, i.e., so that the translucency of the materials could be studied separately. The L and thus the E values are highest for the COMP and least for ZNPO, thus showing that COMP is the most translucent material and ZNPO the least (detailed CIELab values not shown).

	Discussion

TOP Abstract Introduction Materials & Methods Results Discussion Conclusion Future Work References

 
This study simulated a clinical situation where the samples are not individual layers of the components of the ceramics—for example, core, dentin, and enamel—but rather a complete unit, with the dentin and enamel layers fired onto the core layer. The application of luting agents onto the samples was done to simulate a clinical situation, resulting in different thicknesses of luting agents (Morando et al., 1995).

The development of the CIELab color system has been a cornerstone for the measurement and evaluation of color differences by dental materials scientists. Over the years, CIELab has been an accepted method for color measurement, since each color occupies a unique location in the three-dimensional CIELab color space (CIE, 1976) (Fig. 1). CIELab color space gives the actual visualization of the color but not the color properties of the material. The E values are graded as follows (Kuehni, 1976; Johnston and Kao, 1989; O’Brien et al., 1991; Touati and Miara, 1993):

E > 3.7=very poor match
E > 2=clinically unacceptable
E 2=clinically acceptable
E < 1=not appreciable

For our purposes, we consider E > 2 as clinically unacceptable (Tables 2,3).

We clearly demonstrated that the color of the luting agent does play a significant role in the final color in cases of translucent AC materials. The EMP samples are seen to be more translucent than the INC samples, due to the more translucent lithium disilicate core material in EMP as compared with the less translucent aluminum oxide core material in INC samples (Carossa et al., 2001; Heffernan et al., 2002). As the darkness of the samples increases from EMP1 to EMP3 and ICA1 to ICA3, the translucency decreases, and a similar pattern is also observed when luting agents are placed (L values in Table 2). This is due to the increase in pigments in the ceramics to make them darker, thus reducing translucency. For EMP2 and INC2, with the high a value and its shift toward red, i.e., when a standard black background is used, the color coordinates move markedly toward red. This unexpected change altered the end E values significantly and needs to be explained by studying the reflection curve of the materials according to the Kubelka-Munk (KM) theory (Cook and McAree, 1985). This theory gives information about scattering and absorption of incident light, and has been used to predict with accuracy the colors of AC systems with several backgrounds (Miyagawa and Powers, 1983; Cook and McAree, 1985; Davis et al., 1994). The clinically acceptable E values are seen only with dark shades—EMP3 and INC3—with opaque luting agent ZNPO, which is only 11% of all cases. This supports the fact that translucency (LBW) had a statistically significant correlation to color change (EWCBC) in dark shades only in cases of ZNPO.

In Table 3, when the L values for the most translucent agent COMP with a black background are compared with those for COMP with a white background, the final color with black background is further darkened. This means that, when the most translucent agent used in this study (COMP with black background) was placed, the shade of the final restoration was further darkened. This unexpected shift with the black background needs further explanation by study of the reflectance curve and application of KM theory. GIC shows the least change in L values with white and black backgrounds, fraught within the limitations of experimental errors, making the color brighter. Thus, ZNPO has the maximum effect on translucency, making the highest shift in L values with white and black backgrounds. L decreases with the increase in the thickness and the darkness of the samples. The influence of the cement on the final color is dependent on the translucency of the core material (Heffernan et al., 2002).

With a black background, in the case of ZNPO, the E is highest, which is due to the fact that it is less translucent than the GIC and COMP. Over half of the samples, 52%, with luting agents show clinically relevant results which are exclusively due to GIC and ZNPO, as also seen statistically, thus proving that translucent luting agents have less effect on color change with dark background colors. The color of the background—with the black background, clinically, for example, where the background could be a discolored tooth, metal post or core, etc.—is the dominant color with translucent luting agents. With a white background, 33% of the samples show clinically relevant results (EWWC, clinically unacceptable), which are due to all three luting agents, i.e., clinically relevant color differences are observed with all luting agents, whereas, statistically, only ZNPO has showed to have a significant effect (Table 4c). The color of the luting agent is clinically relevant in determining the final color of the restoration, in the case of light background colors—for example, in the case of dentin, dentin-colored esthetic posts and cores (Vichi et al., 2000). The EMP samples with GIC show unexpectedly large E values. This is seen with both white and black backgrounds. This could possibly be due to variations in mixing conditions, since the thickness factor can be ruled out because of low deviation in the thicknesses of GIC in samples (Table 1). Overall, the less translucent AC (e.g., INC) when compared with the more translucent ones (e.g., EMP) show less color change due to luting agents and background color (Carossa et al., 2001; Heffernan et al., 2002). A 0.4-mm increase in the thickness of INC samples does not significantly affect the color properties and does not significantly improve shade matching, as shown in the study and also proven earlier (Carossa et al., 2001).

	Conclusion

TOP Abstract Introduction Materials & Methods Results Discussion Conclusion Future Work References

 

1. EMP is more translucent than ICA. When AC restorations are used, the darker shades are less translucent than the lighter shades, and therefore the color properties of the luting agents and the background color must be considered.
   
2. Luting agents, in combination with the background shade, influence the final color of the restoration.
   
3. A more opaque luting agent cannot be used to mask the color of a darker background color, because of the predominance of the background color. Background shade can be masked only in cases where dark ceramics and opaque luting agents are used. With light background color, luting agents also have a clinically relevant effect on the final restoration of the ceramics.
   
4. With a marginal increase in ceramic thickness (0.4 mm, in the case of INC), the color properties do not statistically significantly differ but are clinically relevant.
   
5. Color cannot be predicted accurately with differing AC thicknesses, differing luting agent thicknesses and colors, and different background shades.
   

	Future Work

TOP Abstract Introduction Materials & Methods Results Discussion Conclusion Future Work References

 
Due to the absence of standard methods of color matching and color prediction in clinical dentistry for "all-ceramic luting agent" units for specified backgrounds—which could be a post, core, a discolored tooth, or dentin—an in silico model for color matching, prediction, and simulation will be developed, taking into account the background shade and the shade of the luting agent, based on the Kubelka-Munk (KM) theory and Parallel Evolutionary Programming (PEP) of Artificial Neural Networks (ANN) (Angeline et al., 1997).

A database with the color properties of the materials will be developed, along with an algorithm for calculation of color properties of dental restorative materials and final color predictions with various combinations and various background colors. The algorithm will be programmed in Java^TM (Java Sun Microsystems), mainly due to its portability. Thickness and mixing conditions will be ignored in the first instance for simplicity reasons.

The main purpose of this would be to eliminate traditional color matching and color prescription, thereby eliminating errors in human eye-color matching and prescription (Yap et al., 1999). A visualization tool will also be developed so that clinicians can see the outcome of the restoration in silico.

	Acknowledgments

 
This work was partly supported by a scholarship to V.S.B. from the Department of Prosthetic Dentistry, Dental School, University of Cologne, Germany, and by the Graduiertenförderung, University of Cologne. This work is based in part on a doctoral thesis to be submitted to the graduate medical faculty of the University of Cologne (Medizinischen Fakultät der Universität zu Köln). The authors acknowledge with appreciation the cooperation of our industrial partners for providing us with the materials and the samples: Ivoclar Vivadent AG (Schaan, Liechtenstein); Vita Zahnfabrik (Bad Säckingen, Germany); PhospaCEM PL (Ivoclar Vivadent AG, Schaan, Liechtenstein; and Ketac-Cem Radiopaque (ESPE Dental AG, Seefeld, Germany).

	Footnotes

 
Publication supported by Software of Excellence (Auckland, NZ)

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TOP Abstract Introduction Materials & Methods Results Discussion Conclusion Future Work References

 

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Advances in Dental Research, Vol. 17, No. 1, 55-60 (2003)
DOI: 10.1177/154407370301700113


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