James Fondriest, DDS
Accurately reproducing the unique characteristics of a single maxillary central incisor so that the artificial replacement is perceived as “natural” can be the biggest challenge in restorative dentistry. This challenge comes at several levels. First, the practitioner needs to have an understanding of what factors go into achieving a good match as well as some basic knowledge of the nomenclature of light science in order to communicate what is seen. Second, the practitioner is responsible for creating a protocol to accurately assess what is happening when light hits the surface of the tooth to create its appearance. There are ways to enhance what we see visually or photographically in the mouth by lessening metamerism, afterimages, and other visual distortions.
Third, the practitioner needs to develop written, graphic, and photographic communication devices that are more comprehensive and less confusing. For example, there is no common dental standard for communicating the degree of translucency, hypocalcification of enamel, or varying degrees of surface luster. If we had to describe in great detail the teeth shown inFig 1 without photography, how many words would it take to deliver a nonconfusing synopsis? And last, laboratory staff needs to develop their skills along with the practitioner partner because all of the levels of communication conveyed to the lab must also be recognized and understood in the photographs.
It is important to realize that matching the hue and chroma is fifth or sixth in importance on the list of things to match when constructing a prosthetic replacement.1 A person would have to be fairly close to detect subtle differences in hue; yet disparities in surface morphology, value, and opacity can be seen from 4 or 5 feet away or more. Disparate tooth silhouettes or perimeter shapes of the teeth can be seen from even 10 feet away. The order of importance while matching a single maxillary central is (1) silhouette or perimeter shape, (2) surface morphology and texture, (3) value, (4) translucency or opacity, (5) chroma, and (6) hue.
Shade Assessment Systems
Shade tabs from any vendor are helpful. Ideally practitioners should have the same shade guide the laboratory uses, but if not, the tab can be shared while the restoration is being completed.] The Vita classic shade guide (Vita Zahnfabrik, Bad Säckingen, Germany), at this time, is used by about 90% of practitioners. This guide unfortunately represents a minority of the natural teeth and unnaturally brightened teeth to be matched. It is hoped that a nonproprietary, universal, full-spectrum guide will be available in the not-too-distant future that the dental material manufacturing industry will adopt. Better shade tab systems that cover more of the hue and value spectrums, such as the Vitapan shade guide, are currently on the market; however, Vita-pan is tied to a proprietary porcelain system.
The current mechanical shade-assessing systems based on colorimeters, spectrophotometers, or camera sensors (charge-coupled device or CMOS) will not rival the results achieved when the practitioner/technician team utilize well-drawn shade maps and quality multi-image photography.13
Measuring Low-light Value
Value is best evaluated in low or subdued light. When the Vita classic shade guide is arranged by order of value (as suggested by the manufacturer) and evaluated in good light, the order might be considered suspect. Some of the darker-appearing tabs seem to be in the middle. If viewed in subdued light (the amount of light present during an ominous thunderstorm with dark clouds), the order seems perfect. The discrepancy occurs because of color confusion. In more light the color-perceiving cones in our eyes are stimulated and the color in the tabs becomes more evident. The colors confuse the observer’s ability to assign value intensities. In lower light, the cones are not stimulated and only the rods are activated. The rods in our eyes are sensitive to lightness and darkness, or grayscale. Rods, which are used in night vision, are very sensitive even with small amounts of light. The cones are activated only with higher light levels. (Consider that, except for colored lights, we do not see colors at night when driving.) In summary, ambient light levels should be low enough so that colors are not that obvious and only the rods in your eyes are used to assess the brightness of the target tooth. Some authors have suggested squinting as a way to assess value.14,15 The practitioner should instruct the laboratory to confirm in low light the overall value of the final restoration.
Low-light value should be the first parameter evaluated in the restorative procedure. By assessing low-light value first, the pupils have not been closed down by the glare of the bright dental unit light.16 Also, the tooth has not become dehydrated. Dehydration increases opacity of the enamel. Light no longer can go from hydroxyapatite crystal to crystal. Intraoperative dehydration causes significant changes in value, translucency, chroma, and hue. Less translucency causes more reflection, so the tooth is brighter.12 Chroma, being inversely related to value, is reduced, and the hue becomes more the color of the light source, which is assumed to be white. Once the low-light value is determined, hue and chroma tabs can be selected.
Selecting Shade Tabs
Create a neutral-colored environment. Complementary-color afterimages of any bright color will occur in milliseconds. The ideal background when assessing color is neutral gray.17,18 Neutral gray has no complementary color and is restful to the cones. A neutral gray environment is more critical with aged teeth that have a glossy surface that reflects the shade of any color placed in close proximity (Aiba N, personal communication, 2001).18-20 The color of the walls in the operatory and laboratory can alter color perception and should be subdued. In a blue room more orange is seen than is actually present because the complement of blue is orange. A gray bib can be used to cover the patient’s clothes,21 and lipstick should be removed or covered. The amount of red tissue seen in the background can be limited by cropping it out with intraoral gray backgrounds (Pensler Shields, #50009211, Kulzer. These disposable cardboard backgrounds can be shaped easily to match the arch form. If the background card is positioned too far behind the teeth and out of focal distance, the gray will darken to black, which increases glare.7,22
The most important parameter of selecting hue is the lighting condition. Due to the variability of daylight, blinds should be used. The practitioner and the laboratory should have color corrected artificial lighting that 1) approaches 5500 degrees K, 2) has a color rendering index of greater or equal to 93, and 3) proper luminosity. Viewing teeth under diffuse illumination will minimize the distortion of reflected light. Reflection from the specular surfaces of a tooth reveals more of the color of the illuminating light than the color of the tooth.23 The average recommended luminosity for dental shade matching is 150 foot-candles.15,19,24-27 To have light of this intensity in the operatory at the level of the dental chair, 8 to 10 4-ft fluorescent bulbs would be needed in a 10 x 10-ft room with 8-foot ceilings.19,26 The diffusion panels covering fluorescent bulbs are important because they can screen out wavelengths. As they age, the panels change what wavelengths they absorb. The best diffusers, preferably the “egg-crate” type, are those that do not filter out any wavelengths of the spectrum. Using 10 color-corrected bulbs on the ceiling will yield more light in the operatory than would be considered comfortable. Portable high-quality light units, such as the Vident light, are ideal.
First impressions are the best, because of the effects of eye fatigue. To prevent hue accommodation, practitioners should not stare at the teeth for more than 5 seconds.15 Miller has suggested using a Vita classic shade guide arranged by hue with the A and B hues at opposite ends and C and D in the middle. C and D have hues in between A and B28 on the linear rainbow (chroma and value are manipulated to yield different looks). When choosing the hue family, a clinician can use the A4 and B4 or A2 and B2 tabs, which facilitate the process of elimination by using tabs with the greatest hue spreads.15The chroma is very low for shades A1 and B1. It can be difficult to distinguish the proper hue family using these tabs. When choosing the hue with a shade tab, the practitioner should look to the midbuccal area of the tooth. Differences between the shade tab and the natural colors of the tooth increase near the root. Compared to the Vita classic shade guide, natural teeth exhibit increased redness and lower translucency at the cervical aspect.20,29 If in doubt about the hue family, the A family can be chosen,30 since perhaps as many as 80% of natural teeth are a closer match to this hue family (Miller L, personal communication, 2001). Most natural teeth have more red than is in the B family. Another guideline for shade matching is to hold the incisal edge of the shade tab to the incisal edges of the teeth. This position effectively isolates the shade tabs from the teeth so they do not reflect onto each other (Aiba N, personal communication, 2001)20; it also reduces afterimages.
Most humans experience eye dominance, and one eye will preferentially perceive shade.16 It is wise to hold the shade guide on both sides of the tooth at each vector (Aiba N, personal communication, 2001). In addition, difficulties can arise when the tooth being examined differs considerably in size from the specimen on the shade guide. A variation in color perception can occur with the relatively larger area appearing brighter and more vivid than the smaller area.31
In dental ceramics, clinicians and technicians try to imitate the appearance of the tooth as a sum of all its visual dimensions. In addition to providing excellent photographic images for the technician, it is extremely valuable to provide a written graphic with an interpretation of these dimensions in the drawing. If no shade tab matches what is seen, then a tab can be customized by applying surface stains. Caution must be used with this technique because the lab is encouraged to duplicate this surface staining, which will increase metamerism in the final restoration.32,33
All that is seen should be shade-mapped in full- page, three-dimensional drawings or on printed photographs of the target tooth and other proximal teeth. Several views (eg, 90-degree straight buccal, 135-degree angle from the buccal-incisal aspect, and straight incisal/occlusal) can be used. The labial face of the crown is then divided into zones. The low-light value from gingival to incisal should be noted, and the base hues should be mapped, along with the chroma stops in the various areas of the buccal surface. A chroma stop is an arbitrary measurement of hue saturation and is designated by the number of the Vita classic shade guide. Do not hesitate to alter these chroma-stop designations. For example, it is acceptable to note an area to be A2.5 or A3.75 even though there are no tabs that have these chroma intensities.
The surface anatomy must be described. The preoperative models will help in duplicating the contours. Although the luster and texture can be better determined photographically, the clinician should describe it on the prescription form and include the age of the patient. Surface texture and luster can be described as heavy, moderate, or light, and different combinations of surface characteristics can be given. Because these surface features determine the character of light reflection and affect the amount of light that enters the tooth, the surface morphology of a crown should be designed to simulate the light transmission and reflectance pattern of adjacent teeth.4,5,29,34-38
When the practitioner is mapping the translucency of the target tooth, he or she looks for the opalescent blue areas. They are more visible with the use of a black background, which limits the reds reflecting from the back of the mouth recombining with the blues to yield white light again.39,40 When drawing proximal translucence, the clinician should ask the patient to turn from right to left, which allows a better analysis. This reevaluation at different angles is called vectoring.16,36,41,42
The practitioner and technician should build a collection of shade guides and tabs that can be shared between the team. Some proprietary guides have tabs that represent different levels of enamel opacity, frost, occlusal staining, etc.
Photographic Documentation Protocols
Developing expert photographic skills is worthwhile because better images yield more information. The practitioner is responsible for creating an environment and protocol to assess what is happening when light hits the surface of the tooth to create the appearance of the tooth. There are ways to increase what is seen photographically in the mouth,40,43 and some fairly simple choreographed images serve to communicate the more important parameters for matching. Communicating with photography will always be better than with written or verbal descriptions. Almost all quality levels of images are better than nothing. That alone should encourage more photographic documentation.
To produce a lifelike restoration, it is important for the clinician and technician to develop their skills at describing the features of a tooth. A sequential protocol for selecting value and shade tabs, shade mapping, and then photographing for shape and surface morphology, translucency, and finally chroma and hue has been described. This protocol allows the clinician to capture detailed and accurate information about the appearance of the tooth, which will aid the dental technician in creating a faithful reproduction. The steps described in this article should be followed before any restorative procedures are performed and even before the dental unit light is turned on. The laboratory needs to develop itself along with the practitioner because all the levels of communication conveyed to the lab must be recognized and understood by the technician.