The Optical Characteristics of Natural Teeth
Understanding the properties of light and color will help communicate shade prescriptions to the laboratory for optimal results.
Matching one or two artificial restorations to a highly characterized natural dentition can be a challenging procedure for the dentist/technician team. The capability to appraise and fully share the appropriate information from the operatory to a distant laboratory can be greatly enhanced by learning the language of color and the optical characteristics specific to teeth. It is difficult to duplicate nature in ceramics if the clinician cannot describe or fully illustrate what he or she sees in the shade-matching process. This article should give the reader a better understanding of how light interacts with a natural tooth, and to give the dentist/technician team the nomenclature to best describe it.
Synonyms for hue are color, cast, shade, tint, or tone. Hue is specified as the dominant range of wavelengths in the visible spectrum that yields a perceived color (Figure 1). Because hue is a biologic and psychological interpretation of the combination of the wavelengths reflected back from an object to the observer, the exact wavelength of the perceived color may not even be present.1 The lower the intensity of the hue, the harder it is to distinguish.2 In this age of highly bleached teeth, where little color exists, it is becoming less important to document the hue.
Chroma is the measure of how much color is present.2 When brown pigment is placed into white powdered porcelain, the mixture takes on a brown hue. Each time more brown pigment is added, the strength or saturation (chroma) of the brown color increases but the mixture is still the same brown hue. As the powder becomes more saturated with brown pigment, the mixture appears darker, so the increase in chroma has a corresponding change in value. As chroma increases, the value decreases; they are inversely related.3
The chroma of a natural tooth comes mainly from the dentin, and the thickness and opacity of the overlying enamel determines how much chromatic influence the dentin has.4 When the enamel is thin at the gingival third but thick incisally, a chroma gradient is created. Increasing opacity of the enamel, as seen with dehydration and bleaching, can exaggerate the chroma gradient.
The words value and brightness are synonymous. Value is the sum total amount of light that is returned from an illuminated object. Lowering value means diminished light returns from the illuminated object; thus, more light is absorbed, scattered elsewhere, or transmitted through and away from the object. Using porcelain with less chroma and less translucency can increase a dental restoration’s brightness.
The characteristics of brightness and translucency work counter to each other. As tooth bleaching becomes even more popular, dentistry will continue to search for porcelain that is both bright and translucent. Choosing porcelains with higher opalescence,5 optical density, and fluorescence will manifest brighter qualities.
In the dental context, translucency can be described as the gradient between transparent and opaque. Enamel and dentin have varying degrees of translucency. Areas within a tooth or a restoration with higher translucency will have a lower value because light transilluminates through and away from the viewer. When evaluating enamel translucency, the observer will often focus on the opalescent blue areas (Figure 2). Translucent enamel displays the characteristic of opalescence. Opalescence causes tooth enamel to reflect blue light back to the observer. Blue light tends to bend/refract more or to scatter within the enamel body. Longer red-yellow wavelengths do not bend as much in the enamel; therefore, a higher percentage will trans-illuminate the tooth. The blue accumulates within the enamel, giving it a bluish appearance from a frontal view even though it is intrinsically colorless.6-9
This optical phenomenon is named after the appearance of opals. Natural opals are aqueous disilicates that break light down into its component spectrum of wavelengths by refraction. Opals act like prisms and refract or bend different wavelengths to varying degrees. Enamel has a primary mineral makeup of hydroxyapatite, which is crystalline calcium phosphate. Hydroxyapatite crystals align in organized, tightly packed masses to form prismatic enamel rods.10 Refraction will occur as light passes through each enamel rod and also at the internal (eg, cracks) and external surfaces of the tooth. An example of an internal surface within a porcelain-bonded restoration is the interface between a pressed coping and the superficial veneering porcelain. The natural halo effect in teeth is a byproduct of opalescent bending of transilluminating light at the lingual–incisal external enamel surface (Figure 3). In dental ceramics, the appearance of the tooth is successfully imitated as a sum of all its visual dimensions. It is better to build restorations with porcelain with the same opalescent characteristics as enamel rather than to create an artificial incisal halo, for example, with opaque yellow stains.
Optical Density and Fiber-Optic Properties
When light enters a natural tooth it gets bounced around the enamel like a fiber-optic cable. If one side of a tooth is illuminated with a curing light, the entire crown is lit. When light travels from one translucent material to another, light will be either reflected at the surface, or it will pass through the surface but bends (refracts) as it passes. The more optically dense a translucent material is, the more light will enter and stay within its body rather than transilluminating. Enamel is an optically dense material bordered on either side by air or dentin, both with significantly lower optical densities. For light to escape an optically dense material, such as enamel, the light must hit the external surface at an incident angle close to perpendicular, otherwise it will be reflected back into the enamel body. If the incident angle is not close enough to perpendicular to escape, it is said to lack the “critical angle” of incidence to escape. This means that any light rays closer to parallel to the external surface of the enamel will not cross that surface, and will stay within and be completely reflected back internally. This effect is exploited with fiber-optic cable, which confines the light within the fibers. Light travels along the fiber, bouncing off the external walls. Because the light must strike the external boundaries at an angle greater than the critical angle to escape, once the light enters the fiber at a low angle virtually parallel to its walls, it will travel down the fiber without leaking out. By increasing the optical density of the ceramics used to build the enamel layer, the fiber-optic properties of natural enamel can be replicated, resulting in a prosthetic crown that is brighter and more translucent at the same time.
The opalescent effects of enamel brighten the tooth and give it optical depth and vitality.11 The property of opalescence will cause a tooth to appear to be one color when light is reflected from it and another color when light is transmitted through it.5 The more opalescent a tooth or porcelain is, the more anisotropic light qualities it has. Optical anisotropy is the change of visual appearance depending on the angle of view or the angle of illumination. An example of this can be seen in the change of width, chroma, and hue of the incisal halo depending on the angles of illumination (Figure 4).
A percentage of the red-yellows fail to escape the lingual surface of the tooth and can be trapped by the fiber-optic properties of the enamel. This increases the anistropy of the enamel. These longer wavelengths will travel around the circumference of the tooth and can manifest themselves as an ever-changing light show at the incisal edge.
The anisotrophic properties of natural teeth will yield infinite opalescent optical effects depending on ambient lighting conditions. What appears to be a good match in static chairside photographs taken with a macro flash next to the lens and at the same perpendicular buccal view will not necessarily match in the real world.
Metamerism is the unfortunate characteristic of restorations matching well in operatory lighting or in photographs but then displaying differently when the patient smiles in other light conditions.12 Perhaps you have matched clothing under one lighting type and were shocked to find the mismatch under different lighting. One object may have the ability to reflect more blue than another. However, if there is no blue range in the lighting source, they will appear the same; then, when viewed under a light source containing blue, the differences will appear. The perceived color depends on the nature of the light source illuminating the object and what wavelengths are reflected. The closer the sum is of the reflecting wavelengths of the two materials to be matched, the more successful the color match will be.13 Using opaque surface stains to correct mismatches will increase metamerism. When reconstructing the dentin and enamel layers with dental porcelain, selecting porcelains with the same optical properties will minimize metamerism.
Fluorescence might be categorized as a type of reflection by opaque material. Fluorescence by definition is the absorption of light by a substance and the spontaneous re-emission of lower-energy light of a longer wavelength.14 Fluorescence in a natural tooth occurs primarily in the dentin because of the higher amount of organic material present.7,14-16 Ambient, non-visible, near-ultraviolet (UV) light is absorbed and then fluoresced back as visible light primarily in the blue end of the spectrum. Why this is noteworthy in dentistry is that the more the dentin fluoresces, the higher the value of the tooth.17 To increase the fluorescence in stacked dental porcelains, opaque metallic-oxide powders with fluorescent qualities can be added to increase the quantity of light returned back to the viewer, to block out discolorations, and to decrease chroma.17 Unfortunately, these powders will reduce the perceived translucency if not kept to the deeper dentin layers or to the coping. The porcelains commonly used today all vary in their natural fluorescent qualities. Zirconium exhibits little or no fluorescence.
The brightness of natural teeth changes significantly when illuminated with UV light, which can have a dramatic affect on the level of perceived vitality exhibited by porcelain restorations. Imagine how a crown made of porcelain with low fluorescent qualities will stand out in bright sunlight.
Light Sources and Color Rendering
If a wavelength is not part of the ambient light spectrum, it is not there to reflect off the tooth. To properly assess chroma and hue, a full-spectrum light source is needed. Dental unit lights are commonly used for color rendering. Most are incandescent lights that emit light high in the red-yellow spectrum and low at the blue end (Figure 5). The ambient light quality of the operatory must be maintained with artificial lighting (natural light conditions vary). There are ceiling fluorescent bulbs that have full color content and render color more accurately. The quality of ambient light is commonly measured by the color temperature and the color-rendering index (CRI), which measures the percentage of the near-UV and visible light spectrum that a light emits. Any light source over 91 is adequate for dentistry. Quality SLR macro-flash systems have excellent CRIs greater than 93, and are the best way to document color to the laboratory. Ideally, both the dentist and the laboratory technician should have balanced, full-spectrum lighting conditions. Color temperature is an inadequate measure of light quality for shade rendering.
All teeth have surfaces with morphological variations. Surface morphology affects how light will reflect. When light hits a surface perpendicular to your eyes, a significant portion of that light can reflect back to your eyes. Anterior teeth have surfaces perpendicular to a viewer; thus, these reflections have a significant influence on appearance. The “perceived” shape, length, and width of an anterior tooth is significantly influenced by the specular reflections coming off the heights of contour of the buccal surfaces.17 Documenting the surface morphology of posterior teeth is of less importance.
Surface Texture and Luster
The surface textures of maxillary incisors can be described as vertical, horizontal, and varied.18 Vertical surface textures are primarily composed of the heights of contour of the marginal ridges and the developmental lobes. Fine, transverse, wavelike grooves called perichymata or the striae of retzius16,19 create most of the horizontal textures. These horizontal undulations never cross each other, and they go circumferentially (Figure 6). These horizontal textures are formed on top of vertical textures, meaning the horizontal patterns follow into the concavities formed by the vertical textures but the vertical textures are not affected by the horizontal textures. The varied textures are cracks, chips, and other surface aberrations or patterns such as “orange peel” (Figure 7).
At eruption, teeth have their roughest surface texture. A roughened surface texture will not yield as well defined an image and will scatter the light.20 With age, these surface features gradually wear. As the wear process continues into the later years of life, usually all signs of the perikymata are lost and even the definition of the developmental lobes is obliterated (Figure 8).
Luster is often described as surface polish. Reducing the surface luster of window glass by sand blasting will produce a frosty white look. As light hits the surface of the roughened glass surface, it scatters or bends irregularly. This scattering of the light at the surface causes an increase in opacity. The light does not pass the surface but is reflected, causing an increase in brightness (Figure 9 through Figure 11). As the glass becomes less translucent, the value goes up. The net effect is that more light returns to the viewer as the luster/polish goes down. Polishing a porcelain restoration is a subtle way to lower value by making the porcelain clearer and more translucent.19,21
Matching a natural tooth with an artificial restoration will always remain an artistic challenge. The dentist/technician team is responsible for artfully re-creating the natural tooth anatomy, alignment, and wear patterns of the lost tooth structure while also matching the optical parameters that generate the overall visual appearance of the tooth. Beyond talent, the ultimate likelihood of faithfully re-creating nature is limited only by how complete the communication process is. Knowing the language of color as well as understanding how the optical properties of the human dentition are displayed will improve the outcomes of our restorative endeavors.
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About the Author
James Fondriest, DDS
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