James Fondriest, DDS
Closely matching natural teeth with an artificial restoration can be one of the most challenging procedures in restorative dentistry. Natural teeth vary greatly in color and shape. They reveal ample information about patients’ background and personality. Dentistry provides the opportunity to restore unique patient characteristics or replace them with alternatives. Whether one tooth or many is restored, the ability to assess and properly communicate information to the laboratory can be greatly improved by learning the language of color and light characteristics. It is only possible to duplicate in ceramic what has been distinguished, understood, and communicated in the shade-matching process of the natural dentition. This article will give the reader a better understanding of what happens when incident light hits the surface of a tooth and give strategies for best assessing and communicating this to the dental laboratory. (Int J Periodontics Restorative Dent 2003;23:5 466-479.)
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Every opaque object receives light, the three primary light colors in some ratio. Some objects reflect all of the light they receive, and others absorb it almost totally.1 Most “opaque” objects absorb partially and reflect the rest. The dominant wavelengths reflected back to the eye is the perceived color of the object. Natural teeth have many optical characteristics that increase the complexity of what we see. Understanding these characteristics will improve our ability to describe them. Albert Munsell described color as a three-dimensional phenomenon. He described the three dimensions as hue, value (brightness), and chroma (saturation).
Hue is the quality that distinguishes one family of colors from another. It is specified as the dominant range of wavelengths in the visible spectrum that yields the perceived color, even though the exact wavelength of the perceived color may not be present.2 Hue is a physiologic and psychologic interpretation of a sum of wavelengths. In dental terms, hue is represented by A, B, C, or D on the commonly used Vita Classic shade guide.
Value, or brightness, is the amount of light returned from an object. Munsell described value as a whiteto- black gray scale. Bright objects have lower amounts of gray, and low-value objects have larger amounts of gray and will appear darker. The brightness of a crown is usually increased in two ways: by using lighter porcelain (lowering chroma), or by increasing the reflectivity of the surface. Lowering value means diminished light returns from the object illuminated; more light is being absorbed, scattered elsewhere, or transmitted through.
Chroma is the saturation, intensity, or strength of the hue. Envision placing red food dye into a glass of water. Each time more of the same color dye is added, the intensity increases, but it is the same red color (hue). As more dye is added, the mixture also appears darker, so the increase in chroma has a corresponding change in value. As chroma is increased, the value is decreased; chroma and value are inversely related. Higher numbers on the Vita Classic shade guide represent increased chroma.
Other Optical Properties
In dental ceramics, we try to imitate the appearance of the tooth as a sum of all its visual dimensions. Human teeth are characterized by varying degrees of translucency, which can be defined as the gradient between transparent and opaque. Generally, increasing the translucency of a crown lowers its value because less light returns to the eye. With increased translucency, light is able to pass the surface and is scattered within the body of porcelain. The translucency of enamel varies with the angle of incidence, surface luster, wavelength, and level of dehydration. With a translucent enamel layer, the ceramist achieves color depth and the illusion of a vital natural tooth.
Ultraviolet (UV) light can have a dramatic effect on the level of vitality exhibited by restorations. With the characteristic of fluorescence, they look brighter and more alive. Fluorescence is the absorption of light by a material and the spontaneous emission of light in a longer wavelength.3 In a natural tooth, it primarily occurs in the dentin because of the higher amount of organic material present.1,4-6 Ambient near-UV light is absorbed and fluoresced back as light primarily in the blue end of the spectrum, but it will occur at all wavelengths. The more the dentin fluoresces, the lower the chroma.1,7Fluorescent powders are added to crowns to increase the quantity of light returned back to the viewer, block out discolorations, and decrease chroma.7This is especially beneficial in high-value shades, as it can raise value without negatively affecting translucency when placed within the dentin porcelain layers.
Opalescence is the phenomenon in which a material appears to be one color when light is reflected from it and another color when light is transmitted through it.8 A natural opal is an aqueous disilicate that breaks transilluminated light down into its component spectrum by refraction. Opals act like prisms and refract (bend) different wavelengths to varying degrees. The shorter wavelengths bend more and require a higher critical angle to escape an optically dense material than the reds and yellows. The hydroxyapatite (HA) crystals of enamel also act as prisms. Wavelengths of light have different degrees of translucency through teeth and dental materials. When illuminated, opals and enamel will transilluminate the reds and scatter the blues within its body; thus, enamel appears bluish even though it is colorless.1,4,9 The opalescent effects of enamel brighten the tooth and give it optical depth and vitality10 (Fig 1).