To effectively overcome the challenges associated with accurate shade selection in aesthetic restorative dentistry, it is essential for dentists to understand both the science and art of color. This is difficult, because color and its perception have both subjective and objective elements. Furthering the dilemma is the inability of the human eye to perceive color in a clear, concise, and consistent manner; color perception varies from person to person.
|Figure 1. Understanding color requires comprehension of the dimensions of color, with a specific focus on hue, value, chroma, and translucency.
The inconsistent results in shade tab fabrication from a color-control perspective were identified in the early 1900s.1-3 Attention to this issue intensified in the 1980s and has continued to the present time.4 For fabrication of aesthetic dental restorations, the emphasis has been on close cooperation between the clinician and the laboratory technician, and the development of new restorative materials5 and dental adhesives.6 These innovative
restorative materials have enabled the clinician to enhance the vitality and translucency of dental restorations. Research has maximized the strength of all-ceramic restorations while maintaining optimal color and translucency. The importance of this research is apparent today, as an attractive smile is no longer considered a luxury but rather an essential part of a person’s lifestyle, and the natural visual appearance of restorations has become imperative.
To balance the science of dentistry with the artistry of an individual smile, the dental profession must continue to advance the field of aesthetic restorative dentistry. Approximately 50% of remakes for aesthetic restorations are the result of a failure to match shades accurately.7 Clearly, miscommunication and/or misinterpretation of critical information occurs between the clinician and the laboratory. Typically, dentists require 15 minutes to properly take a shade for a central incisor.8 Additionally, some require three to ten appointments to achieve a correct aesthetic match.9
Determination of the proper color and shade for a restoration are subjective assessments dependent on illumination, environment, and the receiver’s eye. The stakes are high for balancing realistic goals against the patient’s aesthetic expectations, and mistakes can prove troublesome for the dentist and the patient.
THE DIMENSIONS OF COLOR
Understanding color requires comprehension of the dimensions of color, ie, hue, value, chroma, and translucency. Translucency is not addressed in Munsell’s color analysis (Figure 1),10 but it may be the single most important factor in the ultimate result of an aesthetic restoration.
•Hue is the color tone (ie, red, blue, yellow, etc). The term “hue” is synonymous with the term “color,” and is used to describe the color of a tooth or dental restoration.
•Chroma is the intensity or saturation of the color tone (hue), ie, light blue or dark blue. For instance, chroma is used to describe the orange or yellow hue of a tooth or a restoration.
•Value is the relative lightness (brightness) or darkness of the hue.
•Translucency is the three-dimensional representation of value. Translucency is abstract and intangible, and is currently difficult to measure and standardize.
With our current low-cost, low-technology visual assessment tools—the shade guide—hue and chroma are easily discerned, but value is not determined correctly. In fact, 75% of improper shade selection involves deviation in value.7 In many cases, the perfect match is a combination of several shade tabs. While widely accepted, these traditional guides have limitations and are considered to provide only 30% of the answer.11-13
|Table. Individual Shade Selection Differences
Combining dentistry with the science of color poses significant challenges. In the quest for optimal aesthetics, new systems and devices must be considered. While visual perception in humans is a biological marvel, it does have its limitations. The Table presents the results of a study on individual color perception.14,15 Three dentists were asked to select a shade for a prosthesis and the adjacent natural teeth using the conventional shade guide tabs. There was no agreement 34% of the time; two of three dentists were in agreement 52% of the time; and only 14% of the time were all three dentists in agreement. Therefore, in 86% of the cases, all three dentists could not agree on the same shade. These data clearly illustrate the problem.
CRITICAL ELEMENTS AFFECTING COLOR PERCEPTION
Controlling variables affecting color perception are:
•the light source.
•the tooth, including textures and layers.
•the receiver (ie, eye).10
Creating the ideal environment for the perception of color is a challenge, and this environment does not routinely occur in the dental operatory. Even in the ideal environment, the following limitations can make accurate color assessment problematic16:
•Retinal fatigue is rapid. The inability to accurately distinguish hue and chroma is most noticeable at times of fatigue, and the color may be perceived as faded.
•Background effects trick the eye/brain interpretive process.
•Binocular difference in color results in a perception variance between the right and left eyes.
•Poor color memory prevents accurate communication.
•Age affects the retina. Over time, the lens of the human eye becomes more yellowish-brown, thereby imparting a yellow-brown bias.
•Color blindness affects more than 10% of US males (1/3% of females, Figure 2).
|Figure 2. If you do not see the number “56” revealed among the dots, you, like more than 10% of US males, may have some degree of color blindness.
|Figure 3. Are the horizontal lines parallel or do they slope? While your eye may initially find them to be sloping lines, they are precisely parallel.
|Figure 4. Look at the chart and say the COLOR of the word, not the word itself. It’s difficult because the image creates a left-right conflict—the right half of your brain is trying to say the color, while the left side of your brain is trying to say the word.
In addition, optical illusions can impact perception. Many books and articles have been written on this topic. Since childhood, we have been exposed to these visual “tricks” (Figures 3 and 4).
Spectral colors are the colors of light in the visible light spectrum. Even though white light appears colorless, it is formed by distinct electromagnetic energy. When light passes through a prism, it is refracted, and the light energy is dispersed into the various wavelengths of white light, described by the acronym ROY G BIV—red, orange, yellow, green, blue, indigo, violet (Figure 5). After a rain shower, we can see a rainbow, as the water droplets in the sky act as natural prisms. The color of the sky also appears different at various times of the day. The cones of the human eye can perceive only these wavelengths of light, hence the term “visible light spectrum.” In physical terms, the wavelengths of visible light range from approximately 400 to 700 µm. Each hue is accurately defined by its wavelength or frequency.
|Figure 5. When light passes through a prism it is refracted, and the light energy is dispersed into the various wavelengths of white light—red, orange, yellow, green, blue, indigo, violet. After it rains, this light energy can be viewed as a rainbow, with the water droplets serving as a natural prism.
In dentistry, we must understand pigment colors, because the restorative materials (ceramics, composites, and acrylic resins) have color. The interaction of colors has a critical role in aesthetic dentistry. Knowledge of primary, secondary, and complementary colors is necessary in order to control and alter shades for a predictable aesthetic restorative outcome.
•Primary colors: Red, yellow, blue—are those that cannot be formed by mixing of other colors; they occur naturally.
•Secondary colors: Secondary colors are formed by mixing primary colors (ie, red + yellow = orange; yellow + blue = green; blue + red = violet).
•Complementary colors: Complementary colors look well together; they enhance the appearance of one another. When complementary colors are mixed together, they form the achromatic color gray.
The additive principle of complementary colors may be used to alter the value of restorations. For instance, if we want to lower the value (increase gray or darkness) of a restoration, the complementary color can be added to that hue (eg, A3 shade contains: orange hue + blue stain = lower value). Adding gray stain to lower the value will only make the restoration look dull. Adding violet (purple) stain to a B shade (yellow hue) restoration will also lower the value. In contrast, the value of a restoration (the brightness) cannot be easily raised.
OBJECTIVE VERSUS SUBJECTIVE SHADE TAKING
Shade taking is a demanding exercise, with few standardized methods to allow transfer and interpretation from the dentist to the laboratory technician. A recent advancement in shade selection has been the development of technology-based shade guide systems; several systems are currently available. Therefore, both a subjective and objective approach to color treatment planning are now available to the clinician. The subjective approach—the use of shade tabs—remains the most common practice, but contains uncertainties because of the human factors surrounding the perception of color. The objective approach—technology-based computer imaging and analysis—overcomes many of the human factors.
Technology-based shade guide systems are based on digital cameras, spectrophotometers, and colorimeters. As with any device, benefits and limitations exist, and the clinician must consider how the technology relates to expectations and needs. Following is a brief review of the three different categories of technology-based shade guide systems.
•Most consumer video or digital still cameras acquire red, green, and blue image information that is utilized to make a color image; these devices are commonly referred to as “RGB” cameras. Various approaches have been used to translate this concept into technology that can be used in clinical dentistry.17 The inherent problem with these systems is that they do not control some of the key variables associated with accurate color determination. Typically, color is synthesized from RGB data using various assumptions about the camera, and also utilizing reference materials within the captured image.
•Spectrophotometer-based devices measure the fundamental reflectance of an object across the visual light spectrum. These instruments typically divide and measure the visual spectrum into multiple parts, resulting in 16 to 32 data points across that range. To date, all instruments offered to dental practitioners have been spectrocolorimeters.18,19 A spectrocolorimeter performs a second data processing step to convert the reflectance data to colorimetric data.
•Colorimeters are engineered to directly measure color as perceived by the human eye. A colorimeter filters light in three or four areas of the visible spectrum to determine the color of an object. Colorimeters are difficult to design, and if the technology is not precise the result will be a reduction in accuracy compared with a spectrophotometer. Properly designed colorimeters can provide greater data efficiency, as they only store the needed three data points of hue, value, and chroma instead of the more basic 16 to 32 data points of reflectance.20
Technology-based shade guide systems have advantages over the conventional shade tab assessment. The reports are less subjective; the capture of an image takes less time, eliminating the potential problems associated with dehydration of a tooth; and the shade of a restoration can be verified prior to intraoral placement. In 2000, Chu and Tarnow21 used a clinical case report to describe the subjective deficiencies in the conventional shade selection process, and compared that conventional method with computer-aided information collected with “artificial vision” technology (RGB digital camera technology, which infers color through mathematical analysis of the image).
Shade determination is rapidly evolving toward a more objective standard. The clinical importance of correct shade selection in aesthetic dentistry cannot be overemphasized. Unless an appropriate shade is selected, the most careful attention to the material, structure, and other aspects of the restoration will not produce an optimal final result.
Determining an accurate shade match is one of the most critically important procedures in aesthetic restorative dentistry, and has always been one of the greatest challenges in clinical dentistry. In addition, dental schools do not provide adequate training of dental students in color education,22,23 and most laboratory technicians have never had the opportunity to work directly with patients.
The development of new shade-matching systems may herald a major advance in clinical practice. Shade determination can be divided into the areas of analysis, communication (of information to the laboratory), interpretation, fabrication, and verification. Technology-based systems address analysis and communication quite well; interpretation and fabrication of restorations are still inherently subjective. In sum, efforts in the direction of objective evaluation of shade via unbiased technology-based information systems hold promise for the future.
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Dr. Chu is a recipient of the Columbia Dentoform Corporation Award in operative dentistry and fixed prosthodontics from the University of Pennsylvania and the Granger-Pruden Award for excellence in prosthodontics research from the Northeastern Gnathological Society in 1988. He has published numerous articles on implant, restorative, and aesthetic dentistry, tissue management, bleaching, and dental ceramics in national dental journals. Dr. Chu maintains a private practice in New York City, and has a part-time teaching appointment at New York University as a clinical associate professor, Department of Implant Dentistry and Postdoctoral Continuing Education series. He is director of the advanced and international aesthetic dentistry program at NYUCD.