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cosmetic dentistry_ beauty & science

08 I I special topic _ shade analysis different ways. The most widely used colour order- ing or descriptive system used in dentistry was de- veloped by Mussell.13 He defined colour according to three dimensions: _hue, the specific wavelength of light energy that wouldbelabelledred,greenorblueandeverything in between; _chroma,theintensity,concentrationoramountof a given hue (for example lighter yellow or deeper yellow); and _value, or the lightness or darkness of a colour. In real terms, if more light reflects off an object and hits our eyes, it will be perceived as brighter or higher in value. Conversely, if less light reflects off an object and hits our eyes, it will be perceived as darker or lower in value. There is a fourth dimension of colour, translu- cency, that is important when evaluating tooth colour because teeth are translucent and translu- cency is directly related to the perception of value. When evaluating tooth colour, the most important colour dimension to match is the value and a close second the translucent zones. Next in importance are the chroma zones present in the teeth being evaluated. The least important dimension of colour relative to matching natural teeth is the hue. In natural teeth, the hue range is very narrow and in my experience matching the specific hue is unim- portant as long as value/translucency and chroma are closely matched. In the discussion on shade guides and their use, I will give detailed descriptions on how to evaluate value, translucency and chroma in the shade-analysis process. _Ideal set-up and use of current shade guides TheVITAClassicalShadeGuide(Vident) has been the standard shade guide used in dentistry for several decades. More re- cently,theVITA3D-MasterShadeGuideandarecent significantupgrade,theVITALinearguide,havebeen available for shade analysis.14 The 3D-Master guide and Linearguide are based on actual spectropho- tometer analysis of natural teeth15 and are my preferredguide,butmorethan50%ofdentistsstill use the Classical guide, so I will go through its op- timised set-up and use and then detail the use of the newer guides. VITAClassicalShadeGuide Every dentist and ceramist is familiar with the VITA Classical guide. This shade guide was initially developed several decades ago with the last modi- fication or update in the 1960s. It was adequate for that time but analysis of the shade guide shows several problems that lead to the many shade mis- matchesthatstillexist.First,theshadeguidepoorly coversthemeasuredrangeofnaturalteeth.16 Noth- ing can be done about this except either changing the guide or using a different one. Second is the value arrangement. The value arrangement as re- ported by the company is different from what has been measured.16 Figures 4 and 5 show the value arrangement as we measured it in both grey scale and colour images. A1 as we measured is higher in value than B1 and D2 is lower in value than A3. You will probably notice that the colour image of the valuearrangementwillbehardtobelieve,thatisthe tabs right next to each other that have significantly different chromas will appear to have significantly differentvalues,wheninfacttheyarevery similar (view the black and white image). This is a problem with human perception that has not been discussed in dentistry before: if two objects have similar values but different chromas the observer will perceivethehigherinchromatabaslower in value when this is not the case. This is exactly what is happening when A1 is compared with B1 (Fig. 5). As previously stated,A1ishigherinchromathanB1and thus perceived as lower in value when in fact it is higher in value. The same is true for other areas on the Classical guide. This Fig. 8_The 3D-Master arranged with just the M shades from 0 to 5 value. Fig. 9_Choosing the value for a patient case. Fig. 10_The Linearguide used to choose value. Fig. 11_Using the Linearguide to determine value. cosmeticdentistry 1_2012 Fig. 8 Fig. 9 Fig. 10 Fig. 11