Color Science and Shade Matching in Direct Composite Restorations

Color Science

Color science is an area of study that many dental schools have neglected and often only provide a few hours of instructional time devoted to the subject. There is a movement currently by The Society for Color and Appearance in Dentistry along with the help of numerous industry partners to correct this deficit in clinical education. Early leading clinicians and color researchers have spoken to the lack of shade matching and color science knowledge of clinical dentists.^1-6

Color is perceived when the human eye, using two sensory structures (rods and cones), receives visible light wavelengths which were first described by Isaac Newton. The rods perceive brightness or intensity and the cones perceive color, with separate cones for sensing red, blue, and green. This color information is then translated by the brain to create a perception. No two humans will perceive color the same way due to the numerous factors that affect its perception. Factors such as age, gender, fatigue, diet, color-blindness, binocular differences, lighting conditions, and personal experience will all affect an individual’s perception of color.⁷ The visible light spectrum spans 390 nm to 700 nm, from violet, indigo, blue, green, yellow, orange, and red. Pure white light embodies all wavelengths together. In the context of clinical dentistry, for color to exist there must be a source (light), an object (teeth), and receptor (eye). Light emitted from a source, such as a light bulb, will then either pass through an object (transmission), be reflected (reflection), or emitted (emission from source) to the receptor, the rods or cones of the eye.⁸

Current shade guide tab systems, such as Vita’s Classical shade guide, still order their tabs based on the original descriptions of the dimensions of color by Munsell, who established the dimensions of hue (color tone), value (relative lightness to darkness of color also known as gray scale) and chroma (saturation of color).⁹ One significant component that has been left out with current shade matching tabs or electronic shade matching technology is the importance of translucency. Translucency relates to the degree of light which is transmitted versus the degree it is absorbed. Translucency is the inverse of opacity as it defines the degree of light to pass through an object. Opacity defines light being reflected or absorbed. These components of color are the ones most associated with a restoration having a natural character.^10-12 Other elements for consideration when shade matching are the type and quantity of illumination, metamerism, and contrast.^13-16

One component necessary for optimal color perception is the type and quantity of illumination when shade matching. The type of illuminant used can significantly affect the perception of color. The quantity of light is critical; too much light and it overwhelms the observer and colors will appear higher in value and less chromatic. Light intensity that is too low will cause a decreased perception of value and increased chroma. The optimal light intensity can be checked with a light meter and should be between 150 and 200 foot-candles (1614 to 2152 lux) with the reading measured at the reflected object. It is important that the lighting source used when shade matching best reflect natural daylight. The color temperature that best represents this is 5,500 Kelvin (K). The CIE (Commision International de l’Eclarirage)¹⁴ has set standards for lighting at the 5,500 K temperature as a D50 illuminant. The D50 illuminant best matches natural light and also emits ultraviolet components; this allows for fluorescents to occur when viewing teeth, which is important for natural esthetics. There are also handheld lights such as the Rite-Lite (AdDent, Danbury, CT) that produce a 5,500 K that can be used in clinical applications for environments where optimal lighting cannot be achieved. When natural light is to be used it is best to take the shade in an open area at 10:00 AM or 2:00 PM on a clear bright day with the patients teeth in line to the source (sun).¹⁵ There are several industry groups and researchers that consider a D65 illuminant with a color temperature of 6,500 K to offer a better color rendering index. The handheld application available for use in dentistry with a D65 illuminant is the Demetron Shade Light (Kerr, Orange, CA).¹⁷

Metamerism is the optical phenomenon where two objects appear to match under one lighting source and condition and then appear different under another light source or condition. An example is the classic case of a patient liking the composite match under operatory lighting but noticing a significant color difference in photographs or in their normal work environment. To reduce the effects of metamerism, biomimetic materials act to emulate the spectral curve of natural teeth. The replication of the spectral curve has allowed the newest hybrid and nanocomposites to decrease the color variance under different lighting conditions. These hybrid and nanocomposites exhibit a chameleon effect blending into the adjacent natural dentition.¹⁵

Contrast is an important consideration in shade selection. Contrast is the opposition or juxtaposition of different forms, lines, or colors. There are contrasting colors found in the visual environment of the dentist. Items such as wall color, bib color, lipstick, and inflamed gingival tissues can all affect the clinician’s ability to accurately assess the color of a restorative material.^4,5 Contrasts of value, hue, and chroma can cause the clinician to select the incorrect restorative material color. Areal (objects of differing sizes), spatial (objects in foreground and/or background) and successive contrast (viewing one object immediately after another) can also confuse the human eye when attempting to select the appropriate color of restorative material.¹⁰

Composite Shade Matching

Although standard shade guides are useful to derive a general hue, chroma, and value, they should not be implicitly used when selecting a direct composite. The manufacturers have made great efforts in matching the Vita Classical Shade Guide but the variability of each restoration does not allow for a simple singular shade selection approach.^18,19

Electronic shade meters are even less beneficial in composite selection as they simply give a displayed measurement based on Vita Classical standardized shade tabs. There is a possible use for electric measuring devices using the custom shade tabs made from the restorative composite in use, but it only adds complexity to shade matching and it still leaves out the other critical color components of translucency, opacity, and florescence.⁵ The current generation of electronic shade-matching machines cannot quantify these color components that are attributed to the natural appearance. Factors such as the size of preparation, underlying natural tooth, adjacent tooth color, and variations of color all have to be considered when selecting the most appropriate composite for an optimal esthetic outcome. Implementing alternative surface treatments during the final finishing steps can alter opacity, translucency, value, and chroma. These elements of color are known as the polychromatic nature and optical character of a composite.²⁰

An alternative process for reducing the variability that exists between standardized shade guides and composites is to remove the standardized guide from the process and create a guide using the restorative composite of the dentists’ choice (Figure 1). In this process a putty matrix is used from a mold of an existing shade guide (Figure 2 through Figure 6). These custom shade tabs are then placed on a non-contrasting matte-finished backing stick to ease handling and to reduce any negative contrast effects (Figure 7). These custom tabs can then be finished in the same method used by the clinician (Figure 8). If you have alternative methods for finishing they can be employed on each tab to show the various changes that can be used to match the tab (Figure 9). The first tab was polished with laboratory pumice and a universal polishing paste. The second tab was finished with a fine diamond and an Occlubrush (Kerr, Orange, CA) and the third tab was finished with a green point (Kerr) and PoGo (Dentsply Caulk, Milford, DE). With each method, subtle characteristics are played up and other characteristics are decreased. This method of shade tab creation will allow the practitioner to achieve a desired esthetic outcome regardless of composite used and help direct them to the surface treatment that will yield the best result given their specific armamentarium.^21-25

Direct test strips are also used before preparation to help narrow down the composites of choice and allow the practitioner a glimpse of the chameleon effect and other color properties prior to final placement of the direct composite. Prior to the etching and bonding sequence, strips of composites determined to be the best match in color properties are applied next to each other directly on the tooth in a thickness equivalent to the final restorative layer. The composite test strips are then light-cured. A determination is made to the best color match after curing. The strips are cured due to the optical alteration of resin polymerization. This process, although different based on each composite’s chemistry, typically exhibits an increase in opacity and decrease in value. Since there has been no application of etch or bond, these test strips can be easily removed and the surface preparation and bonding steps can then follow.

Conclusion

The understanding of color science and its parameters, components, and environmental effects are important tools for optimizing the esthetic outcome of all restorations. A general understanding of color science can provide one with some important tools to realize the dentist’s and patient’s desired esthetic outcome. It is particularly important to understand how to achieve an optical outcome in a direct composite restoration. By utilizing a custom shade guide, proper lighting, test strips, and finishing protocol, a clinician can fine-tune the opacity, value, and chroma of a composite. This process allows the practitioner to create a beautiful biomimetic restoration that will have long-term color stability.

References

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About the Authors

Chad J. Anderson DMD, MS, Research Instructor, Tufts University School of Dental Medicine, Boston, Massachusetts; Private Practice, Fresno, California

Gerard Kugel, DMD, MS, PhD, Associate Dean for Research, Professor, Tufts University School of Dental Medicine, Boston, Massachusetts