Shade variance in ceramic restoration and shade tab: An in vitro study.

BACKGROUND: In clinical practice aesthetics for any restoration needs to consider the parameters of surface form, translucency and colour. To achieve ideal aesthetics the colour replication process for dental porcelain is the most important step which comprises of a shade selection phase followed by shade duplication.
MATERIALS AND METHODS: The ceramic brands Vita VMK95 (classic) and Ivoclar classic V were used for comparison with Vita classic shade tab guide shades A2 and B2. The samples were made of specific shape, size, and were of the recommended dimensions from investing self-cure acrylic strips to casted NiCr specimens Objective. The objective of this study was to quantify the results in CIE AE units system for the colour differences between the Vita shade guide colours and two commercial porcelains for metal ceramic crowns.
RESULTS: The results indicated that the porcelains do not match the shade guides to which they are compared and shade variations exist between different lots of porcelain from the same and different manufacturer.
CONCLUSION: Problems identified that porcelains do not match the shade guides to which they are compared and shade variations exist between different lots of porcelain from the same and different manufacturer.

Shade taking devices are designed to aid clinicians and technicians in the specification and control of tooth color.[1] The shade taking devices are based on the principles of colorimeters, digital cameras as filter colorimeters, spectrophotometers, and spectroradiometers.[1] Visual shade selection is the most common method of color determination in dentistry, but color duplication via this process is plagued by unreliable and inconsistent result.[2]

Although the dental profession has been aware of the shortcomings in shade guide and the corresponding materials for a decade, it is probably the current media-driven emphasis on appearance and an esthetic standard that is primarily responsible for recent advances in dental shade matching.[3] Clinically there is difference in the readymade shade guides available and the porcelain fired for metal ceramic crowns after selecting the shade from the guide. Most brands of porcelain are labeled to match shades of the Vita shade guide, but produce slightly different colors from this guide upon firing.

The clinical focus of color matching in prosthodontics is the beginning and the end, but the overall color replication process is more complicated than this isolated process. The shade duplication phase encompasses many variables that can have isolated or cumulative negative effects on the final outcome. Variables that have been investigated include restoration thickness, type of crown substrate, veneer material choice, firing temperature, frequency, and technical skill.[3]

There have been a number of recent technologic and material advances that offer potential to improve color matching skills in prosthetic dentistry.[3] Hence, a study was designed to compare the color of a custom-fabricated ceramic sample of A2 and B2 shades of Vita classic and Ivoclar classic ceramic material samples fabricated to manufacture's specifications to a Vita classic shade guide.

The development of CIELAB helps in the quantification of the shade duplication process. The color difference of two objects can be determined by comparing the difference between respective coordinate vales for each object. When color is measured and specific color differences are identified, the CIELAB system is frequently used. In 1931, the CIE defined a “standard observer” by a set of three functions x(λ), y(λ), and z(λ). These were carefully prescribed spectral sensitivity curves designed to model blue, green, and red sensitive cone receptors of the eye, respectively. These functions are the key to transformation of spectral energy data into meaningful color data.[4] Visual shade matching is the most used method of selecting shade for crowns from the shade guides available in the market.

The purpose of this study is to evaluate the difference in the shades between the shade tab and fired porcelain. Instrumental color measurement has the advantage of obviating the subjective aspects of color assessment and expediting the determination of color; so, a spectrophotometric evaluation is done.

Materials and Methods

The ceramic brands Vita VMK95 (classic) and Ivoclar classic V were used for comparison with Vita classic shade tab guide shades A2 and B2. The samples were made of specific shape, size, and were of the recommended dimensions from investing self-cure acrylic strips to casted NiCr specimens.[5] The metal samples are milled and grounded for exact dimensions of 1mm thickness, the total dimension of the sample is 1cmx1cm square and the surface is sandblasted. Two layers of opaque were fired, followed by a layer of dentin and enamel and glaze firing cycle according to manufacture's recommendations.[6] For ceramic buildup, a two solid polished mold of different thicknesses of dentin and enamel porcelain was used for the recommended thickness. A total of 40 samples, 10 for each variant, was prepared at a time. The Vita shade tab A2 and B2 and the finished samples were tested in a 1-mm square in the center of the sample for color hue and value in a photospectrometer, and the CIELAB coordinates obtained and the difference among the shade tab and the different brands of ceramic samples were compared and statistically analyzed.

A two-way analysis of variance (ANOVA) was performed to examine the effects of brand and shade on the E. Associated with this ANOVA, a Tukey's Studentized Range (HSD) Test was performed to determine whether significant differences existed at the 95% confidence level. Similar tests were performed for L, C, and H. Color differences were also calculated between the means of the samples prepared using two different brands of ceramic and two different shades and standard Vita shade guide.

Results

The shade tab was valued and read as 2. The average AE values for difference between shades of Vita shade tab and fired ceramic of Vita VMK95 were 2.7 and 2.9 for shades A2 and B2, respectively. The average difference between the two shades of Vita was 2.7. The average AE values for difference between shades of Vita shade tab and fired ceramic of Ivoclar classic were 2.7 and 2.1 for shades A2 and B2, respectively. The average difference between the two shades of Ivoclar classic was 2.5. The AE average between the two brands was 2.8. The average color difference between the Vita shade guide and samples was significantly affected (P<0.0001) by both the brands and the shades of prepared samples. The significance of the interaction indicates that some shades produced better match with some brands. In this study, Ivoclar showed the lowest AE compared to Vita shades.

Discussion

Esthetics is the study of beauty. Knowledge of esthetics helps the dentist achieve a pleasing appearance of effect. Esthetics is the primary motivating factor for patients to seek dental care. In fact, corrections of esthetic problems have a positive effect on the self-esteem. A smile has been said to be one of the most important interactive communication skills of a person. The ultimate objective of esthetics in dentistry is to create a beautiful smile, with teeth of pleasing inherent proportions to one another, and a pleasing tooth arrangement in harmony with the gingiva, lips, and face of the patient.

In addition, color is complex and encompasses both subjective and objective phenomena. The phenomenon of color is a psychophysical response to the physical interaction of light energy with an object, and the subjective experience of an individual observer. Three factors can influence the perception of color, namely, the light source, the object being viewed, and the observer viewing the object. The light source can emit radiant energy of a range of wavelengths and this is characterized by the relative amount of energy emitted at each wavelength in the visible spectrum. The light source that illuminates an object affects color perception, since individual sources contain varying quantities of each of the visible wavelengths of light. The spectral reflectance (or transmittance) of an object characterizes the color makeup of that object.[7]

Knowledge of color is required to communicate accurately the shade of restoration. Color is described by its three attributes: Hue, value, and chroma.

Hue: The quality by which we distinguish one color family from the other (e.g. red, blue, green).

Value: The quality by which we distinguish a light and a dark color.

Chroma: The quality of color by which we distinguish a strong color from a weak one.[8]

To quantify color difference, the CIE LAB color system (1976) is frequently used.[9] This color system has three parameters, L, a*, and b*, to define color. The L coordinate is a measure of the lightness–darkness of the sample. The greater the L, the lighter the sample. The a% coordinate is a measure of the chroma along the red–green axis. A positive a* relates to the amount of redness and a negative a* relates to the amount of greenness of the sample. The b* coordinate is a measure of the chroma along the yellow–blue axis, that is, a positive b* relates to the amount of yellowness and a negative b* relates to the amount of blueness of the sample. The magnitude of the total color difference is frequently represented by a single number, E*. Because the CIE LAB system is based on rectangular coordinates, the equation for calculating total color difference is E. Where

E = (L*)2 + (a*)2 + (b*)2,      (1)

where L*, a*, and b* are the differences in the CIE color–space parameters of the two colors.

Because a single number only gives the size of the total difference rather than the direction or nature of the difference from the standard, valuable information can be obtained by dividing a calculated color difference, E, into its components and examining them separately. It is convenient to separate E into components correlating with hue, value, and chroma. The difference for each of these components can be calculated using equations. The CIE value difference, L, is calculated using the following equation:

L=L2 – L1      (2)

The CIE chroma difference, C, is calculated using the equation:

C=a2 × 2 + b2 × 2 – a1 × 2 + b1 × 2      (3)

The CIE hue difference, H, is calculated using the equation:

H (E)2 – (L)2 – (C)2      (4)

Problems identified were that porcelains do not match the shade guides to which they are compared and shade variations exist between different lots of porcelain from the same manufacturer.[10] Hence, it was suggested that a custom shade guide could minimize these problems. The following difficulties were reported with the fabrication of custom shade guide: (1) the problem of matching a thin piece of porcelain to a shade guide several millimeters thick, (2) the variations of porcelain powder batches that do not match the shade guide consistently, and (3) the difficulty of predicting the final color of the typical layered veneer of opaque, dentin, and enamel. The study results and outcome also come close to those of earlier studies by van Der Burgt T. P., J. J. Ten Bosch, and others. The significance of the interaction indicates that some shades produced better match with some brands. In this study, Ivoclar showed lowest AE compared to Vita shades. The inadequacies of dental shade guide in terms of range and systemic distribution in the tooth color phase have been described. Significant advances in the shade guide organization and coverage of natural tooth color space arrangement of “virtually all existing natural tooth shades”[5] need to be researched and developed for the future of prosthodontics outcome.

Conclusion

When the color of custom shade guide samples prepared from two shades of two brands of porcelain were compared to the color of a Vita classic shade guide, the following conclusions were made as the mean color difference (2.5 for Ivoclar and 2.7 for Vita) exceeded the proposed acceptance limit for dental shade guides. Of the brands tested in this study, Ivoclar showed the lowest AE. The mean AE between the samples prepared from the two brands was 2.8 and illustrates the effect of brand variability when samples are prepared. Problems identified that porcelains do not match the shade guides to which they are compared and shade variations exist between different lots of porcelain from the same and different manufacturer. The following difficulties were reported after fabrication of final prosthesis: The problem of matching a piece of porcelain to a shade guide several millimeters thick, the variations of porcelain powder batches that do not match the shade guide consistently, and the difficulty of predicting the final color of the typical layered veneer of opaque, dentin, and enamel.