Chapter 7 Color and Shade
The study of color can be extremely complex and involved. Although it is not the purpose of this chapter to be a complete report on the art and science of color, certain aspects of color have a direct impact on the clinical practice of porcelain laminate veneers. In fact, the understanding of color and its manipulation is fundamental to the ability to create esthetically pleasing restorations in the mouth. At first this may seem a formidable task. In fact, just describing a given color can be a heroic pursuit, because there are literally millions of discernible colors. By turning to the Methuen Handbook of Colour one can, if one has the time and inclination, learn the names of over 8000 of them. These include some of the less-than-classic colors, such as “fancy free,” “wafted feather,” and “heart’s desire.” These names may evoke poetic images, but they certainly do little to clearly communicate any of the dimensions of the shade they represent, particularly when they are communicated through linguistic translation. Clearly there is a need for some distillation of color science, and it is obvious that some sort of order is vitally necessary if one is to develop skill in the manipulation of color without having to spend a lifetime developing a “feel” for what seems to work.
The search for such an understanding has been a preoccupation for centuries. In 1666 Isaac Newton discovered that white light can be broken down into a rainbow of color, but it was not until the nineteenth century that German physiologist Ewald Hering first described the now familiar color circle.
Although this created some organization of the color perception experience, it was only a beginning. At the time of its discovery, the color circle seemed to describe some basic law of physics, but it turns out that the organization of color into a color circle has more to do with the physiology of the eye and psychology of the observer. It does, however, form the basis for several present-day workable systems of color. In 1905, Albert Henry Munsell, an American artist and art teacher, further modified the color circle, devising a system of color organization that centered around three unique aspects of color: hue, chroma, and value (Figure 7-1). Using these three aspects, Munsell was able to construct a three-dimensional color wheel (Figure 7-2).
The Munsell system is not unique. Numerous other color wheels are available. Each is of different national origin, with versions from Britain, France, Germany, Argentina, and Sweden. Naturally enough, each system finds its greatest usage in its country of origin. Unfortunately, although such systems provide a good way to describe color, they actually do little to teach how to manipulate and control color in a clinical situation. In other words, rather than the Munsell system being a method used to control color, it merely serves as a relatively precise “language” to verbalize what is being done. In fact, it is even of limited value in describing tooth color, because it is primarily involved with surface reflection. It does not make any distinction between one color that is relatively translucent and one that is opaque.
Differences in surface texture also are not addressed by such a system. All dentists have seen the differing appearance of porcelain crowns and plastic temporaries of the same color. Subtle differences in appearance can even be discerned among various brands of porcelain, variations that can transcend the qualities of hue, chroma, and value. Obviously, then, there are dimensions to the appearance of tooth shade beyond mere color. This situation is not unique to dentistry.
In the fifteenth century, Flemish painters such as Jan van Eyck worked with a method of painting known as the “wet-on-wet” technique, in which layers of wet paint are applied to previous layers of wet paint (Figure 7-3). Different details of the paintings are placed in different strata over the canvas, and the layers of paint toward the outside are usually increasingly translucent, creating an illusion of depth and vitality that would be impossible if only opaque paints were used (Figure 7-4). In some paintings as many as 30 layers are used to create maximum effect. This is not that different from the technique a master dental technician uses when different shades of porcelain along with flecks of inlaid color are employed to imitate the dentinal and enamel layers.
(Copyright © The National Gallery, London.)
(Copyright © The National Gallery, London.)
Thus in some respects dentists are not much farther along than the artists of the Renaissance in the ability to control the chromatic appearance of their work. Nonetheless, there are several principles that guide practice. Today’s dentist must understand that there are several completely unique systems for understanding and manipulating color, and although each of them provides a workable framework for understanding, they also may often seem to contradict the teachings of the other systems.
Over the last decade, computerized shade-matching systems have appeared on the market (Figure 7-5). This innovative technology offers better accuracy, improved efficiency, and esthetic benefits to patient, dentist, and technician. Based on technology imported from the painting industry, these systems analyze the color of the natural teeth; calculate the exact ratio of hue, chroma, and value for a multitude of points on the tooth surface; and display this information on the dentist’s computer screen. The process illuminates the guesswork often associated with reading the shade tabs and greatly facilitates the communication of information to the lab technician. This improved flow of information encourages the fabrication of predictably accurate, highly esthetic restorations. From the lab’s perspective, the frequency of remakes is reduced.
(Courtesy Vident, Brea, California).
Computerized shade-matching systems are available in a variety of formats. Most include hardware and software that identifies the variously colored, translucent, reflective, and characterized areas of a tooth. This information provides a computerized shade map, one that offers significantly more information and detail than traditional shade matching and communication.
Computerized shade matching eliminates the subjectivity and frequent perception errors associated with traditional shade taking. It improves the accuracy and predictability of restorative procedures, particularly those difficult situations in which a single anterior tooth is being replaced (Figure 7-6). There is no requirement for standardizing the light environment of the dental operatory, and thus accurate shades can be taken anywhere, anytime.
FIGURE 7-6 A, MHT SpectroShade Micro. B and C, Screenshots showing how the SpectroShade reads the color of the tooth and indicates the closest available chromatic standard. It calculates the numeric difference between the natural tooth and the selected color in terms of brightness, chroma, and hue.
(A courtesy MHT Optic Research AG, Niederhasli, Switzerland.)
Although they offer many advantages, these systems can be expensive. The value of this technology to the individual practice must be determined not in terms of a single cash outlay, but as a purchase that saves time and improves the quality and predictability of treatment. The cost of the system should be amortized over the useful life of the product, and the daily cost determined. It is then a simple matter to compare the benefits of the system with the costs.
The major driving force in dentistry today is the demand for esthetic services. This has been the case for the last two decades, and this trend is likely to continue for at least a decade more (if not longer). For the first time in dental history, the patient is involved in diagnosis, treatment planning, and outcome evaluation. Much of the growth in dentistry is patient driven. Although this phenomenon has altered the traditional patient-dentist relationship, it has also brought greater patient interest, cooperation, motivation, and compliance.
The public expects that the cosmetic or esthetic dentist can recreate nature accurately, repeatedly, and rapidly. The dentist is faced with the tasks of determining the shade, communicating it to the lab technician, and maintaining it through the cementation procedure to the final (and, one hopes, esthetic) result. These steps presume predictable shade determination and very clear and accurate communication between the dental office and lab. These requirements place tremendous pressure on eliminating the guesswork from color matching and communication in any treatment.
Overall the indications for color and shade matching can encompass both the direct and the indirect procedures that are undertaken by the dentist on an everyday basis. Much of today’s dentistry is esthetically related or expected to be esthetic by patients. Whether the restoration is a direct composite filling or an indirect ceramic or ceramic porcelain-fused-to-metal procedure, it is expected to match the coloration of all the existing teeth.
There are really no contraindications to shade matching because shade matching is not an invasive procedure. It does not affect the tooth structures that are being worked on. It does not change anything. It does not cause any discomfort unless the patient is subjected to a very long and tedious procedure. If the procedure is done incorrectly, then the resulting restoration will not match the teeth; and, of course, in that case the procedure will have failed even before the restoration is placed in the patient’s mouth.
There are two primary techniques for acquiring shade-matching data in dentistry. Both involve assigning ceramic and/or composite color analogs to the existing dentition shades to describe the natural colors as accurately and completely as possible. These analog (shade) maps enable the lab technician to create an esthetically compatible crown at a distance. The differences in the two techniques lie in the technology and the cost.
Traditional shade taking involves matching one or more selected colors from a range of shade tabs to the teeth adjacent or contralateral to the teeth to be restored. This serves as a guide to the lab technician fabricating the crown or the bridge. The more information (and accuracy) that the dentist can provide in the prescription, the more lifelike the technician’s output can become. Thus the dentist who provides a drawing of a tooth color map, indicating the various shades within the tooth and their borders, is more likely to have a positive result than the dentist who describes the shade as a single generic color (Figure 7-7).
Earlier shade guides (Figure 7-8, A) were developed haphazardly, with no infrastructural relationship among the various shades. Today’s advanced guides (Figure 7-8, B) have been developed logically, with incremental relationships among the various shade tabs, an organizationally structured series of family and color groupings, and a range that covers the entire tooth-visible color envelope. These modern guides are designed for ease of learning and ease of use. The entire shade guide system is available at less than $100.
These clinical options for shade matching include a variety of shade tabs that are used intraorally, with the dentist and/or one of the dental team members using his or her own visual perception to determine the color of the teeth relative to the system tabs predetermined color. The other major category is the electronic shade-taking devices.
The disadvantages are many. The problems associated with perceiving color with the naked eye include first and foremost the environment. The environment of any color-taking procedure will affect the results. The coloration of the patient’s clothing, the dental and operator chairs, the wall decorations, and even the color of the equipment may affect the perceived perception of the tooth color, as can sunlight or even snow reflections streaming in through an open window. Furthermore, sunlight at different times of the day will have different qualities and different underlying tones. Therefore it would seem that shade taking must be done in a room that has no windows and is totally color neutral. Another problem is metamerism—the same color appears different when viewed at various angles or reflecting off unlike surfaces.
Color perception can be unpredictable. The observer is instrumental in correct color evaluation or shade taking. Typically females see colors or hues better than males, but males see value or the grayness of an object better than females do. Younger individuals of both genders see color better than older individuals, and many people are affected by color blindness. In fact, up to 8% of all males (knowingly or unknowingly) may be affected by various levels of red-green color blindness. Fatigue can also affect the ability to take shades accurately, and there is little doubt that shades taken earlier in the day are more accurate than those attempted at the end of a long and difficult working day. Various medications also affect the ability to see color accurately, and although they may only affect color perception slightly, this still makes a major perceptive difference. Furthermore, eyes can be mistaken by illusions. Optical illusions and contrast effects often tend to hide the true nature of a color.
On the other hand, the advantages of a digital shade-matching system include objective readings and accuracy. There are two types of digital shade-matching devices commonly used in dentistry: the spectrophotometer and the colorimeter.
The spectrophotometer consistently and accurately measures natural tooth coloration in reference to any known specific color or can be based on any shading system. It measures the color characteristics of the natural tooth precisely and scientifically, indicating the deviations and gradations of value, chroma, and hue from a standard and provides all the information that is necessary to create an accurate restoration, or to modify an existing one such that it will accurately match the tooth. The spectrophotometer develops an accurate interpretation of the tooth shade on a given color system, which can then be related to an existing shade tab within dentistry or to a color that is interpolated between the shade tabs. In either case a lab technician is given all the color clues to recreate a shade that is very natural in appearance and very close to the target coloration.
The colorimeter analyzes the tooth coloration based on preloaded data that is related to a shade system. It determines the shade tab that is closest to the actual color of the tooth. The colorimeter is typically less accurate than the spectrophotometer but may suffice in most dental situations.
Because both spectrophotometers and colorimeters tend to eliminate ambient light by standardizing the immediate environs of the target tooth, the shade can be taken in any operatory with any kind of lighting streaming in through the window. Digital shade taking therefore is far easier, far more practical, and far more accurate than shade taking using color tabs and the naked eye in a variable environment.
The current best approach to shade taking is the spectrophotometer. It provides the most accurate method for matching the coloration of the tooth. Some systems provide readings of translucence and reflectivity as well. Spectrophotometers provide consistent shade measurement regardless of the environment, lighting conditions, or other operatory variables including the dental team member who is conducting the shade-taking process. With some systems, a further comparative analysis can be undertaken on shade scans taken before and after treatment to provide the color difference between the two measurements. This is particularly useful for tooth-whitening procedures (Figure 7-9).
Knowledge of this system (the so-called “additive” system of color) (Figure 7-10) has enabled the creation of such devices as the color television. Using only three phosphors, one each of the three primary colors, the color television is able to produce a seemingly unlimited range of shades. One such television monitor boasts a palette of 16,777,216 colors that are available on the screen. In the additive system, white is the balanced mixture of all the colors, and black is the absence of color. Yellow is a balanced mixture of red and green. Because the additive system of color does such a laudable job of organizing color, it may seem that there is no need for any other approach.
In the subtractive system, black is the result of a mixture of the three primaries, and white is the absence of color. This system is popular because it is perhaps the easiest to use when dealing with pigments (Figure 7-12).
There are other systems as well. Each color system has its own strengths and deficiencies. Yet despite the apparent contradictions in the various color schemes, each popular system of analyzing color is correct within its own framework. Because dentists work with pigments when dealing with porcelain, the easiest system for clinicians to use is the subtractive system.
The subtractive system is not only the easiest to use, but also the one with which the dentist may be most familiar. The subtractive system is the one used in children’s crayons; children learn at an early age that when they mix red and blue, for instance, violet is the result.
This system works the way it does because the pigments within the crayons absorb certain parts of the spectrum and reflect others. Thus the pigments are “light traps.” Red pigment, for instance, absorbs all parts of the light spectrum except red (Figure 7-13 and Table 7-1).
If pigments displayed perfect efficiency, the mixture of any two primary colors would result in the production of black. If in the crayon example one crayon absorbed all the spectrum except red, and the other one absorbed everything except blue, there would be nothing left over. Normal pigment concentrations, however, are notoriously inefficient in this regard and are almost always thinned out to create a relatively low saturation. Thus a red crayon selectively absorbs certain wavelengths and reflects those centering around the 700-nm (red) range (Figure 7-14).
Because of the profession’s familiarity with the rudiments of this system, and because of its easy applicability to the dental applications, all subsequent color discussions in this chapter take place within the framework of the subtractive system of color.
In the subtractive system, when the three primary colors are arranged in the traditional color wheel, diametrically opposed colors are called complementary colors. Yellow and violet, for instance, are complementary colors. The mixture of two highly saturated complementary colors results in the elimination of color and the production of black. Because the pigments used in dentistry are poorly saturated and imperfect, the mixture of the stains usually produces some shade of grey instead of black (Figure 7-15).
There is a list of difficulties the dentist must overcome when trying to make a perfect match of a tooth’s color. Not the least of these problems is establishing the actual color of the tooth being matched. As every dentist knows, this is easier in theory than it is in practice.
If the source of light changes, however, the apparent color can change dramatically. The normal transmission curve of a typical incandescent light bulb (Figure 7-17) contrasts with the normal transmission curve of a fluorescent light tube (Figure 7-18).
Figure 7-19 shows the two different reflectance curves a tooth would display under these two different sources of light. Even with a constant source of light, the light that actually reaches the tooth can be affected by the colors in the environment at the moment. A dark shade of lipstick or an intensely colored outfit can easily affect the available spectrum for reflection by the tooth.
In addition, there are variations in operator sensitivity. Color blindness is no small problem. It is a fact that nearly one in 10 dentists in the United States has some degree of color deficiency in the red and green areas. If other color deficiencies are also included, the percentage of visually deficient operators goes up even further. The likelihood of a male dentist being color deficient is more than 10 times that of his female counterpart (Table 7-2).
|INCIDENCE IN MALES||INCIDENCE IN FEMALES|
|Caucasians||8.08 ± 0.26%||0.74 ± 0.11%|
|Asiatics||4.90 ± 0.18%||0.64 ± 0.08%|
Others (e.g., Korean, Filipino)
|Other Racial Groups||3.12 ± 0.40%||0.69 ± 0.07%|
Fortunately, most of these operators are not color “blind” but only color deficient. Unfortunately, this means that most of them are not even aware of their problem. In most situations, such a deficiency is of little importance, but in the case of esthetic dentistry, even minor weaknesses in color perception can compromise the intended results.
Obviously, then, it is to each dentist’s advantage to be tested for color sensitivity. Even if a minor deficiency is found, a simple solution may be to have a colleague or staff member who is not color deficient confirm all color choices.
Even when the dentist’s innate visual color sensitivity is found to be optimal, however, there is still no guarantee of consistent color judgments. The eye can sustain a decrease in sensitivity from nerve fatigue, the same as any other sensory organ. For this reason, it is important to avoid staring at the tooth and shade guides when taking a shade. Instead, short glances should be employed, with the first reading considered the most accurate. One other important point: during the cementation step for veneers, the area is often isolated. If many laminates are involved, the isolated teeth have time to desiccate during the extended procedure. After only a few minutes of drying, the appearance of the teeth begins to change. The dried teeth are markedly whiter, and their surfaces more opaque. To demonstrate this, a patient with perfectly matched anterior teeth had a rubber dam placed, exposing the maxillary teeth to air for 20 minutes. When the rubber dam was removed, the difference in appearance was clearly evident (Figure 7-20).
Obviously, any shade decisions must be made while the natural teeth are not desiccated. When laminates are placed in the mouth and the shade is perfect, the neighboring teeth may be slightly whiter at the completion of the procedure. If this is anticipated, the patient should be warned in advance that the laminated teeth will appear a bit dark for one day but that they will color match as soon as the unlaminated teeth rehydrate.
Most evidence points to the fact that the eye is a tristimulus colorimeter. Like the color television, which is capable of producing thousands of colors from only three basic color phosphors, the eye can discern a nearly infinite range of color using receptors for only three wavelengths. Also like the color television, these three receptors seem to have their greatest sensitivity around the colors of red, green, and blue. Although this design may be conservative for the number of required receptors for color vision, it does lead directly to the problem of metamerism, the effect that is achieved when two samples of color appear to match in one type of light but do not match in another.
Simply put, the eye is incapable of distinguishing between certain combinations of light stimuli. Both of the spectral curves shown in Figure 7-21 are perceived as yellow-green. Under full-spectrum lighting, the surface that reflects light centered around 540 nm is indistinguishable in hue from one that reflects two loci of reflectance, one centered around 490 nm and the other around 650 nm.
When the lighting source changes, however, the perceived color of the objects also changes. Sunlight on an average day produces a spectral distribution similar to that seen in Figure 7-22. Contrast this with the curve shown in Figure 7-17 for typical tungsten light. As can be seen, when a surface is illuminated by a tungsten source, there is very little light in the 490-nm range available for reflection. In this situation, the samples no longer match.
This example is by no means unique. A nearly infinite number of combinations can be computed that create metameric pairs. Figure 7-23 demonstrates one of the many pairs that would appear to be blue under full-spectrum light but that would not match under other light sources. The metameric pair in Figure 7-23 results from the fact that the eye cannot distinguish between a pure blue hue at 475 nm and the combination of 440 nm (reddish blue) and 490 nm (greenish blue), one of the many pairs that would appear to be blue under full-spectrum light.
The pair shown in Figure 7-24 would be seen as yellow. Unfortunately, metamerism is a common illusion in the dental field. A factor that complicates this even further is the fact that human vision is most acute in the yellow range—a color range of particular importance in dentistry. In other words, not only is it impossible to accurately determine a nonmetameric color match, but human eyes are uniquely most sensitive to this error in the yellow range.
Still another type of metameric pair can be created as a result of the fluorescent nature of teeth. It is well recognized that when natural teeth are exposed to ultraviolet light, they seem to glow. The apparent glow of the teeth is a result of their own natural fluorescence. Early attempts to achieve natural-looking fluorescence in porcelain involved the inclusion of small amounts of radium into the porcelain mixtures, a practice that is no longer used. Instead, certain fluorescing rare earths are incorporated into the porcelain.
Tooth fluorescence is not uniform across all shades. Early this century, when several dentists made a careful study of the fluorescent properties of teeth, it was noted that certain teeth were more fluorescent than others. Usually teeth with the lighter shades were the most fluorescent. This led directly to the development of dental porcelains with variable fluorescing properties.
When a fluorescent porcelain is used in place of the nonfluorescent one, the shade match can even carry over to situations with intense ultraviolet light (Figure 7-25). Clearly there is a distinct advantage to creating the porcelain laminate veneer crown or bridge out of a variable fluorescing porcelain. There may be some advantage to using a luting agent that displays variable fluorescence as well.
True-to-life shade matching is the key to creating both indirect and direct esthetic restorations. Obtaining the precise shade is always dependant on a number of factors that are less than objective, including the age, the gender, and the experience of the observer. To put it simply, not everyone is equally adept at taking accurate shades. This in itself need not prevent any dentist from doing very successful esthetic dentistry. After all, individuals on staff can assist in this part of the procedure as they assist in other aspects in overall treatment. Electronic shade matching, on the other hand, introduces an objective, readily reproducible and easily communicable data system that provides a predictable result every time.