Abstract
The requirement to achieve natural looking restorations is one of the most challenging aspects of dentistry, and the shade matching of dental restorations with the natural dentition is a difficult task due to the complex optical characteristics of natural teeth. Dental porcelain is considered the reference material for prosthetic rehabilitation, but it is not easy to handle and aesthetic excellence is quite difficult to obtain. For these reasons, shade matching with dental porcelain is often considered to be more artistic than scientific. Shade matching is considered unpredictable due to several variables that may influence the final appearance of a restoration. In order to improve this situation, over the last decade new shade guides and instruments have been developed and the aesthetic aspects of dental porcelain have been further investigated. In this review some aspects of color selection and color reproduction have been examined. Color selection has advanced through the development of new shade guides and electronic shade taking devices, although visual assessment has still not been entirely replaced by electronic instruments. Color reproduction with dental porcelain has improved thanks to advances in the performance and knowledge of dental porcelain, but is still not easy to achieve. The difficulties of achieving good aesthetics with PFM restorations and the desire for metal free solutions have resulted in the increased use of zirconia. The unique optical properties of zirconia have introduced new opportunities for achieving superior aesthetics, however further research is required with this material.
1
Introduction
Due to the complex optical characteristics of tooth color, achieving a close shade match with an artificial restoration of the natural dentition is a difficult process. Successful aesthetic restorations require the integration of several critical factors, including an individual’s perception of color, the light source used for color evaluation, the surface and structural characteristics of both the tooth and of the restorative materials used and a knowledge of some basic principles of color perception. To be successful, clinicians require the understanding of color, light and related characteristics of porcelain and resin, as well as the ability to clearly communicate instructions with technicians when an indirect procedure is performed. Dental porcelain, combining wear resistance, strength, toughness and excellent aesthetics is considered to be the reference material for prosthetic rehabilitation. However porcelain is not easy to handle and aesthetic excellence is quite difficult to obtain. In order to achieve a natural looking restoration it is necessary to perform two different steps: (i) to select the best possible shade, using a shade guide and/or an electronic shade taking instrument and (ii) to reproduce this shade with an appropriate dental material.
2
Color measurement
Since Newton’s experiments during the 18th century, it has been difficult to explain the scientific basis of color. The concept of color is difficult to understand, not easy to define, and is often related more to art than science. Within dentistry, color matching between natural teeth and restorative materials, is often a misunderstood and unpredictable procedure. Many authors have tried in the past to address this issue. The paper from Clark of 1933 , based mainly on the Munsell color scale of 1905, was the first attempt to organize dental colors. In the same period, the Commission International de l’Eclairage published the first standards for color matching, establishing some scientific parameters for color evaluation . However the absence of valid scientific instruments for color measurement did not allow significant improvements at that time. In the 1950s, the first dental shade guides based on a rational arrangement of shade tabs were introduced to the dental profession and, even if still based on individual perception rather than on strict scientific criteria, they were rapidly adopted by clinicians. Sproull in the early 1970s published a series of articles in which the three dimensional nature of color and its relationship with dental shades was studied, and a series of theoretical and practical indications were given in order to improve color matching in dentistry. These articles, that represented for a long time the “state of the art” for dental color matching, pointed out how the procedure for shade taking was negatively influenced by several factors, among which shade guides that were regarded as poor and inadequate in relation to the complexity of the appearance of the teeth. Meanwhile, the developing science of color, which was led by the needs of industry, defined colors, measured them and calculated differences in order to allow proper color control in industrial processes. Since the Munsell system as well as the CIE specification of 1931 had an irregular space distribution, this resulted in differences between calculated and perceived color differences, and for this reason it was clear that a new system for color measurement was required. In 1976 and 1978 the CIE developed a new system, called CIE Lab * , in which for the first time it was possible to express color by numbers and calculate the differences between two colors in a way that corresponded to visual perception. In this system, which is regarded as the benchmark for scientific purposes, color is expressed by three coordinates: L * value is the degree of lightness of an object, a * value is the degree of redness/greenness, and b * value is the degree of yellowness/blueness .
The CIE Lab * system was scientifically based and useful for calculating color differences, but not amenable to easy color communication. For color communication, the HSB/HSV (Hue; Saturation or Chroma; Brightness or Lightness or Value) system is most commonly used, even in dentistry. In this system, H (Hue) is defined as the radial component of the cylindrical coordinates CIE Lab * and calculated according to the formula h ab = arctan ( b */ a *); S (Saturation or Chroma) is defined as the radial component of the cylindrical coordinates CIE Lab * and calculated according to the formula C ab = [( a *) 2 + ( b *) 2 ] 1/2 ; L (Lightness or Brightness or Value), corresponds to the L * of the CIE Lab * system and represents the lightness-darkness of a color . Color can be measured in these two systems, as well as in other systems such as CIELuv, XYZ, Lch and RGB. All these systems differ mathematically, but may be correlated to each other by the use of conversion formulae.
In the CIE Lab * system a formula is used to calculate color differences:
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