Shape of Color: Aesthetics from a Physiologic Perspective

One of the most challenging aspects of practicing dentistry is the ability to match natural-looking ceramic res-torations to the existing dentition. To obtain predictable results, sound principles and methodology should be followed during each phase of treatment. There are 2 principles that can give us insight into a biomimetic interpretation of ceramic restorations; these are the principles of color theory and anatomy. Possessing knowledge about how color integrates into anatomy are tools that should be a part of your armamentarium in order to design ceramic dentitions that are indistinguishable from nature.

COLOR LOGIC THEORY
How we perceive color constitutes one of the deepest mysteries when we try to investigate it. It is difficult to apply the principles of the science of color to clinical dentistry. Instead, it is easier to apply the principles of color theory utilized by artists. As described by photographer John Paul Caponigro,¹ “Color theory is a language that conceptually and perceptually describes the elements of color and their interactions. Part physical, part biological, part cognitive, and part psychological, color theory is perceptually based. Color theory often describes optical responses, some of which are described as ‘illusions,’ but are nonetheless perceived.”¹ Color theory can easily be adapted to interpret dental color.
Applying color theory to dentistry illuminates the dynamic interactions between the elements of color, which can be used to guide decisions in selecting and adjusting color relationships. Color theory is best used to inform color choices rather than to make them. Theory is the sum of what we know, but it does not contain what we do not yet know. It can prime conditions for a breakthrough, but it cannot make one. This can be accomplished by creating an ideal model from which to compare.
Color is the combination of 3 elemental dimensions: hue, saturation, and value. Value is also called brightness or luminosity, and saturation refers to intensity or chroma. The challenge in matching restorations to the existing dentition lies in the architecture of the restoration itself, which is expressed in layers of color; ie, primarily dental hues. Physicists define color as being derived from varying wavelengths of particles of light in an electromagnetic field. Red is the lowest frequency, violet the highest frequency, and yellow lies in between. Neurophysiologists tell us that the photosensitive cells of the retina can determine varying degrees of 3 colors; ie, red, yellow, and blue. Even though these insights are useful, they fall short of explaining the miracle of vision. The eye as an extension of the brain constantly monitors our perceived world in a highly selective fashion.² For example, we see high-resolution images only at the center of our field of vision and low-resolution images at the periphery. Yet, we feel that we see everything clearly. Also, our eyes emphasize straight-lines over fuzzy boundaries. The eye is an active agent, not a passive instrument like a camera. Every color we observe in nature is enhanced by the presence of a color from the other end of the visual spectrum.³ Nature mixes different colors together so that each is distinct while at the same time none seem out of place. It is a natural phenomenon that makes the ordinary seem beautiful. Warm colors (dentin) and cool colors (enamel) interplay with each other, drawing the eye from one range to another, resulting in the harmony and completeness seen in nature. When we look at teeth in every circumstance of observation, the interplay among light, surface, and our perception creates new hues. Leonardo stated that an object’s color imparts and partakes of its surroundings. In effect, color is an active process that is always changing. When thinking of this argument, you can see why the challenge of fabricating ceramic restorations to match the natural dentition is difficult. The architecture of ceramics is color, which is an active 3-dimensional process. Mimicking this active process requires a knowledge base of biomimetics. One fundamental concept of biomimetics is that we must observe the interaction of light with the intact tooth and compare it to the interaction of light with ceramic.

From this fundamental biomimetic concept a breakthrough is achieved in creating an aesthetic keyway to the foundation of color in aesthetic dentistry (Table 1).

THE SHAPE OF COLOR
The identification of color in natural teeth can be justified based on anatomic form. If an ideal tooth form is analyzed, certain predictable patterns of color can be ascertained based upon the physiologic form of the tooth. These patterns are independent of your color selection of the tooth. When matching a crown to an existing dentition, the color pattern and its visualized location are more important to the success of the crown’s biomimetic appearance as a natural tooth than is the color chosen. Color variations exist within in all dentitions, but it is important to consider an ideal tooth structure unto which variation can be interpreted. The observation of multiple dentitions reveal certain constants with regards to color (Figures 1 and 2). Tooth color interpretation is based on physiologic form of the tooth, and not a shade guide. Chroma, value, and hue variations also exist within any given dentition. Physiologic analysis provides methodology for implementing color into anatomic shape, position, and composition in ceramic.
A rationalization for tooth structure is necessary to consider if color is to be perceived as part of the physiologic form. The perceived visual interaction between dentin and enamel is described according to the rule of 3 and is constant with regards to any light interaction.

Figure 1. Anatomic shape of color. Ideal anatomic composition of incisal edge halo, translucent zone, translucent wash, mamelons, basic shade high-value area, interproximal and cervical neck hues.	Figure 2. Shape of color landmarks average-value dentition. Physiologic landmarks are easy to identify in average value dentitions.

The rule of 3 describes the anatomical development of teeth.4 The rule states that all cusps are composed of 3 developmental lobes. Every cusp is composed of 2 proximal developmental lobes and one central developmental lobe. The lobes are separated by 2 developmental grooves. Using this model, certain anatomic accuracies exist that are constant with regards to anatomical position of color in enamel and the anatomical position of color in dentin.

The Shape of Color From the Facial Perspective
Using a maxillary central incisor as a model, the following anatomic landmarks can be described based on color justification for the facial surfaces of teeth (Figures 1 to 3). Landmarks are described from the incisal to cervical. The incisal edge of the maxillary central incisor is marked by the appearance of a halo. A halo is a prismatic illusion that is the result of a convergent angle incisal edge enamel that is unsupported by dentin. The convergent edge allows enamel to interact with light to reflect and refract a high-value pattern. The halo is described as white and/or a vanilla hue. The halo shape varies in incisocervical thickness, value, and chroma. It will extend from contact point to contact point. For every high-value area identified, there is a corresponding low-value area to complement and create visual harmony. Apically to the halo, the unsupported enamel is visualized as a translucent zone. The buccal and lingual surfaces of the enamel are near parallel and allow light transmission through the form. The translucent zone is of low value and is composed of only enamel. The translucency of enamel that is unsupported by dentin makes the halo visible incisal to its (translucent zone) position. The translucent zone extends from contact to contact. The hue of the translucent zone is described as blue, azure, or gray. The translucent zone can vary in shape, value, and chroma. The translucency of the zone extends apically into the body of the tooth at the developmental grooves. Apical to the enamel translucent zone is the first high value color of dentin located in the mamelon formations. Mamelons are the most incisal aspect of the 3 dentin developmental lobes and offer the first incisal enamel support. The fingerlike projections of the mamelons will be one of 3 hues. Mamelons can be an orange, a vanilla, a salmon, or the basic shade of the tooth, which would be determined by default. In addition, mamelon projections may be “tipped” with white. Mamelons can vary in color within the cusp. Each developmental lobe may produce a mamelon of a different color and an individual mamelon may be composed of more than one of the 3 colors, as depicted in Figure 3. This variability amongst the mamelons allow individual character within tooth to tooth dental compositions.

Figure 3. High-value dentition. Anatomic landmarks are more muted because of the milking effect of the outer enamel shell.

Proximal to the developmental lobes, a translucent wash of blue or gray exists in the enamel. The character of the wash is more subtle, or less intense in color than the enamel translucent zone existing apical to the halo. Physiologically, this appearance of blue is best defined as a thickness of interproximal enamel that gives the illusion of being unsupported by dentin by creating a prismatic effect in this area. The prismatic effect of the translucent wash is best seen from the oblique view.
The body of the tooth is most often determined as the basic shade of the tooth. The basic shade of the tooth is the dominant hue of the dentin in the body and cervical areas of the tooth. Often the basic shade is best understood and described by ceramists and dentists as a Vita shade. The basic shade of the tooth is best simplified as being white, yellow, or orange. The highest value area of the facial surface is in the middle of the cervical third. The color is most often a creme or vanilla hue whose value is higher than its surrounding basic shade. Variety does exist with regards to the shape of the high value. The location of the high value is described as that area where enamel is supported by the most dense dentin thickness. The neck of the tooth is an ochre, copper, orange, khaki, or sunset yellow hue. In general, the neck hue is defined by the density of the dentin structure and its close proximity to root form and has little enamel effect. Utilizing the physiologic form of color in an ideal tooth analysis allows the effects of wear to be interpreted.

INTERPRETING EXTERNAL WEAR
External wear can affect the interpretation of color. Horizontal incisal wear will decrease the size of the translucent zone of unsupported enamel (Figure 4). If the incisal wear pattern is extensive enough to expose dentin at the incisal edge, the resultant effect on the incisal third color pattern will eliminate the translucent zone and the halo. This is because the enamel will be totally supported by dentin (Figures 4 and 5). Incisal edge examination should determine if dentin exposure exists. Enamel facial surface wear can affect the color. The outermost shell of enamel, or outer enamel shell (OES), is 96% mineral content.5 This character of mineral imparts a high value to the outermost enamel layer (Figure 6). It is often called a “milking effect,” because the hue, value, and chroma assimilate the effect of adding milk to water⁶ (Figure 7). The innermost layer of enamel as it approaches the dentino-enamel junction is 64% mineral and is called the inner enamel complex (IEC). The low mineral content imparts a medium to low value to the IEC. As the OES wears, the character of enamel will change from a characteristically white to a neutral or gray color as the surface wears to deeper layers of the IEC (Figure 8). Analyzing the degree of facial and incisal wear can give cognitive insight into how to restore dentitions.

Figure 4. Incisor color map. Physiologic landmark color selection. Note the basic shade very seldom determines the final color characteristics of the finished restoration.

Figure 5. Interpreting incisal wear. Wearing of the incisal edge removes the unsupported enamel of the translucent zone, and therefore, the halo.	Figure 6. Worn central incisor comparison. Incisal edge wear comparison of the central incisors identifies a lack of translucent zone and halo of the right central incisor; the physiologic anatomy of worn and unworn dentition.
Figure 7. The milking effect. Although the color anatomy is present, the milking effect of the outer enamel shell (OES) controls the chroma of a high-valued dentition.	Figure 8. Facial wear removing OES. Facial wear removes the OES and exposes the inner enamel complex, resulting in a low-valued dentition.

BASIC SHADE SELECTION
Translating physiologic color into ceramic language requires an understanding of ceramic fabrication. Choosing a shade tab from a shade guide often leads to a poor result, because not enough information is gathered. To achieve lifelike results, the dentist first must formulate in his or her own mind how the tooth to be matched was developed 3-dimensionally by interpreting the shape of color and diagramming it.
The dentist must then visualize how the dentition has matured and interpret the external wear of the dentition. Thorough medical and dental histories play key roles in determining what environmental forces have been placed on the dentition.
Last, the dentist must know the ceramist’s techniques in order to mimic this creation.
To discover what colors to prescribe to match an anterior dentition, the dentist should work within a framework of ceramic technique. This offers a common ground of communication with the ceramist. Therefore, it is necessary for the dentist to have a working knowledge of the ceramist’s technique of fabrication. For purposes of explanation, consider that you and the ceramist are going to create a work of art. There are 3 material selections to make in order to construct an anterior work of art. The 3 material selections are the canvas, painting, and protective over-layer.

The Canvas: Selecting a Basic Shade
A canvas is needed on which to place a painting. The canvas, also known as the basic shade of the tooth, is selected by a method of deduction, and not by a matching method. The basic shade can be deduced by asking 2 pertinent questions. In general, all teeth are a basic underlying color of white, yellow, or orange.

Figure 9. Matching single central prep. Matching a high-value dentition requires planning for a higher than normal outer enamel shell.

The first question you should ask yourself to deduce the basic shade is, “Are the patient’s teeth white, yellow, or orange?” This question should be asked without holding the shade guide up to the mouth. Instead, use the information within the shade guide itself. All shade guides are grouped, or can be grouped according to color. For example, the Chromoscop (Ivoclar Vivadent) shade guide is grouped according to series 100 (white), 200 (yellow), 300 (orange). The VITA shade guide (Vident) can be rearranged according to white, yellow, and orange groups (Table 2). Examine the dentition and determine if you see an underlying white, yellow, or orange color. Once a selection is made with regards to white, yellow, or orange, you will have eliminated four fifths of the shade guide. The second question will determine one of the final colors in the group. The first question asked is to designed to isolate a group (white, yellow, or orange). The second question is designed to pinpoint which basic shade best applies. The second question to ask is, “Are the patient’s teeth light, dark, or somewhere in between?” As a working example, let’s say by asking the first question, you determined the patient’s teeth to be yellow. The next question to ask yourself is, “Are the patient’s teeth a light yellow or a dark yellow, or a shade somewhere in between?”
In summary, there are 2 questions to deduce the basis shade:

1. Is the patient’s dentition white, yellow, or orange?
2. Are the patient’s teeth light, dark, or somewhere in between?

You would think that exceptions to these questions like tetracycline stained and other intrinsically stained teeth would create a different set of criteria. However, intrinsic stains do not create the basic shade. Tetracycline-stained teeth, in general, will offer a glimpse of the true basic shade of the mouth in an incisal, body, or cervical third. Somewhere other than where the banding pattern is located. Also, the basic shade may reveal itself in other areas of the mouth, because rarely do tetracycline stains encompass an entire mouth. Tetracyline stains are mostly mahogany and brown intrinsic stains. Remember, it is only the basic shade you are selecting. It is only the canvas or backdrop, and there will be an entire painting placed on the canvas you selected. Rarely does the basic shade selection affect the final outcome and blending of a restoration with the existing dentition.

CLINICAL APPLICATION
Clinical application of this knowledge can be used to match existing dentitions. Matching a single central crown to a high-value dentition begins with the determination of the basic shade to be white (Figure 9). A diagrammatic map (Table 3) of the shape of colors underlying the OES can then be determined (Figure 10). The crown can be created based upon the shapes determined and the control of chroma based upon the IEC and thickness of the OES layer (Figure 11). The created crown relates well to the surrounding dentition and is lifelike in appearance (Figure 12).

Figure 10. Color map for single central. Even for high-valued dentitions, the shape of anatomic color remains consistent compared with normal-valued dentitions.

Figure 11. Completed restoration. The close proximity of complimentary colors creates a lifelike appearance.	Figure 12. Postoperative result. Single central exhibits vitality as it interacts with light.

Figure 13. Color mapping for multiple units. Pre-op condition: Patient wants to change dark orange basic shade of dentition.

Figure 14. Desired color map of multiple units. Basic shade change to A1 and color map. The patient desires more ideal appearance of dentition. Treatment planning for crowns allows change of basic shade from orange to white.

When patients desire a change in their dental appearance, this knowledge can be used by extrapolating how a lighter dentition would be mapped (Figure 13). By altering the basic shade of an existing dentition from orange to white, the underlying canvas will increase the chroma of the design. Diagramming the shape of color in each sequence of proposed layers helps the ceramist realize the transition of shapes between tooth-to-tooth relationships (Figure 14). The formulated crown compositions can then be created (Figures 15 and 16). The completed crowns allow internal colors to create a lifelike realism (Figure 17). An aesthetic composition is created and looks much improved compared to the preexisting condition (Figures 18 and 19).

Figure 15. Dentin build. Creation of dentin colors of copper, orange, sunset, creme, white, and azure.	Figure 16. Enamel build. Creation of the inner enamel complex allows for value control to see through to internal colors.
Figure 17. Completed ceramic restorations. OES application raises dentition value and makes color map more subtle.	Figure 18. Post-op condition. Dentition is energized by improved appearance.

Figure 19. Post-op closeup. Subtle effects of color mimicking nature.

TO SUMMARIZE
In order to create a lifelike result, it is important to determine the shape and location of the hues present, the degree of external wear on the enamel, and the basic shade. The interplay of colors you see can be laid down in the form of a simple drawing called a color map.
Possessing a knowledge of the physiologic shape of color will improve the lifelike realism of your aesthetic restorations and will increase your communication skills with the dental ceramist.

References

1. Caponigro JP. Color theory. johnpaulcaponigro.com/downloads/technique/documents/tech_color_theory.pdf. Accessed August 16, 2011.
2. Birren F. Color Perception in Art. New York, NY: Van Nostrand Reinhold; 1976:105-110.
3. Livingstone M. Vision and Art: The Biology of Seeing. New York, NY: Harry N. Abrams; 2002.
4. Kataoka S, Nishimura Y, Sadan A. Nature’s Morphology: An Atlas of Tooth Shape and Form. Chicago, IL: Quintessence Publishing; 2002.
5. Cuy JL, Mann AB, Livi KJ, et al. Nanoindentation mapping of the mechanical properties of human molar tooth enamel. Arch Oral Biol. 2002;47:281-291.
6. Schwartz JC. The biomimetic dentoenamel junction: a paradigm shift of ceramic thought. Quintessence Dent Technol. 2006;29:183-189.

Dr. Schwartz is both a dentist and ceramist. In addition to his private practice devoted to aesthetics, he serves as clinical assistant professor in the department of Prosthodontics at Louisiana State University School of Dentistry and as director of the Integra Institute Center for Advanced Dental Learning. He can be reached at johnschwartz@drjohnschwartz.com.

Disclosure: Dr. Schwartz reports no disclosures.