Capturing Tooth Color

An invisible restoration, a restoration that does not differ from the natural teeth in size, shape, surface structure, color, and function, is the ultimate achievement in esthetic dentistry. The lost tooth tissue can be replaced with different tooth-colored restorative materials. The materials that have similar optical properties to the natural teeth are dental porcelain and composites. For a successful esthetic restoration a proper color determination is essential. When color of teeth is determined visually, the determination is influenced by the illuminant, the interaction of the light with the tooth tissue, the observer’s response to the color stimuli, and the quality of the dental shade guide used. Due to these reasons, visual color determination is neither accurate nor precise. A possible solution can be the use of electronic instruments, which are, in principle, accurate and precise.

The tooth color is pre-determined by the tooth anatomy, which consists of a pulp chamber surrounded by a dentine and covered with a layer of enamel. The thickness of the dentine and enamel within the tooth and among different teeth is differently, but predictably distributed due to genetic description. A relation exists between their thicknesses and the perceived color distribution of a tooth. Due to this relation it may be possible to predict color of one tooth or tooth part from another tooth or tooth part.
The bluish transparent enamel over the orange-yellow dentine and place depend thicknesses gives the specific appearance of tooth. Fabrication processes of restorations that make use of multiple layers of porcelain can mimic this appearance. The layering technique in combination with CAD-CAM offers a possibility to create the porcelain layers with predetermined thickness, shade, and translucency. Eventually this can be used as alternative for the less predictable manual porcelain build-up technique.

Chapter 1 describes basic knowledge about different factors that play an important role in tooth color determination and color reproduction with dental restorative materials. The variables that influence visual color determination are discussed in detail. 

Chapter 2 describes the way in which Dutch dentists and dental technicians determine tooth color and their opinions about different methods of color determination. It also evaluates the opinion of the dental professionals about the physical conditions in their offices for color assessment and whether they feel a need to change these conditions. It shows that dental technicians make more effort to improve color determination than dentists. The most common method of color determination is visually with the Vita Lumin shade guide, but recently new color-measuring instruments have emerged on the market and many dental professionals feel the need to determine color with an electronic instrument. 

Chapter 3 presents the accuracy and precision of five devices for tooth-color measurement in standardized and clinical conditions. The instruments tested are two CCD/digital cameras (Ikam and ShadeScan), one spectrophotometer (Easyshade), and two colorimeters (ShadeEye and IdentaColor II). The obtained Vita Lumin output was converted to L*a*b* values and the accuracy and precision of these instruments was evaluated by means of ΔE values. In the in vitro conditions the spectrophotometer and the digital cameras performed better than colorimeters. In the in vivo conditions ShadeScan was significantly less precise but its accuracy remained high, whereas Easyshade and Ikam performed same as in the in vitro study. 

Chapter 4 and Chapter 5 deal with the relation in color between different tooth segments, cervical, middle, and incisal and between different teeth, maxillary central incisor, lateral incisor, and canine. It was expected that anatomical configuration of the teeth influences the color distribution in such way that the color of one tooth part could be predicted from the other tooth part. A digital camera was used to measure the color of natural teeth in vivo. The camera was placed in a construction in order to standardize the distance and the angle between the lens and tooth and to diminish the influence of the external light. The obtained RGB data were translated into CIE L*a*b* data so that not only the quantitative analysis was possible but also the ΔE values could be calculated. The three segments of central incisors, lateral incisors, and canines were statistically significantly different from each other in both studies, but their color is related to each other, which indicate that the color prediction of tooth segment is possible. The relation in color between maxillary anterior teeth was weaker than the relation in color between three tooth segments. The best predictable is the color of cervical segment of lateral incisor from central incisor. 

In Chapter 6 the influence of thickness of two porcelain layers on the overall color of a restoration is presented. The overall color, measured with spectrophotometer, can be used to build a color database for the production of layered CAD/CAM crowns with predictable color variation. The specimens consisted of a ceramic core layer with a constant thickness (0.70 mm), and two porcelain layers (more opaque and less opaque version) whose thickness was sequentially varied within 1.00 mm space. The L*a*b* values of different specimens were measured with conventional spectrophotometer. The results indicate that the color of layered specimens can be perceivably influenced by small changes of porcelain thickness. With the thickness of opaque porcelain a* and b* values increase whereas change in L* value depends on the shade of the porcelain. 

Conclusions

• The majority of dentists and dental technicians determine tooth color visually using Vita Lumin shade guide, although they express the need to switch to an electronic system. 
• Commercially available electronic systems for tooth color measurements show different accuracy and precision among each other. Generally colorimeters (ShadeEye, IdentaColor II) are less reliable than spectrophotometer (Easyshade) and digital camera (Ikam). Spectrophotometer (Easyshade) is the most reliable instrument in both in vitro and in vivo circumstances. Digital camera (ShadeScan) is less precise in the clinical set up. 
• It is possible to obtain reproducible L*a*b* values of the whole tooth surface with digital camera. The L*a*b* values of three tooth segments, cervical, middle, and incisal, differ statistically significantly. Their color is related to each other and therefore a prediction of these values of one segment from another segment is possible. The strength of this relation is higher within one tooth than between two different teeth of the same arch.
• There is a statistically significant correlation between the thickness ratio of opaque/translucent porcelain system within 1.00 mm and color coordinates a* and b*. Correlation of the L* value with thickness of opaque/translucent porcelain system is shade dependent. Most of the thickness-ratio-influenced shade changes would be perceptible under standardized viewing conditions.

Future prospects

In 1931 Bruce Clark wrote on “The color problem in dentistry”, “…since we have never had a system of color measurement and color specification, it is difficult for us to appreciate its practical value. We are inclined to believe that it is impossible to accomplish certain standards of perfection in color work, when in fact it becomes a very simple procedure if we are properly equipped to do it.” 

Now, seventy-five years after this was written, we are ‘equipped’ with a variety of dental colorimeters. However, the best way to measure the natural teeth color is still questionable. There is no adequate methodology, which can measure all optical characteristics of natural teeth. Furthermore, the conversion of colorimetric data to a universally recognized color-order system is still not available as standard. Further investigation of these topics is needed before colorimeters can be used as reliable instruments in daily practice. Even if accurate instrumentation was available, the use of this information by dental technicians to formulate ceramic materials and fabricate individualized porcelain restorations does not guarantee a predictable color of this restoration. Therefore, it would be worthwhile to further investigate the possibility to apply the CAD-CAM technique in fabrication of standardized dental restorations with predictable esthetics.