cosmetic dentistry_ beauty & science

special topic _ shade analysis I _the character of the light; _the observer; and _the object being viewed. Achangeintheconditionofanyofthe three will cause a change in perception of colour.Thus,differingviewingconditions, that is changes in light or changes in po- sition, can alter perception.8 It is impossi- ble to try and match tooth colour under every lighting and positional possibility. One then should try to match under the conditions in which the restoration is most likely to be viewed. Relative to tooth position most people are viewed standing up at conversational distance, so this is the best position in which to place the pa- tient to evaluate shade. Too often, shade is taken with the patient lying back, which increases the chanceofamisperception.Thereasonthishappens is the shade guides do not have the same optical propertiesasthenaturaltooth.Atdifferentviewing angles,theylookdifferent,thatisaperceivedmatch from one viewing angle may not be a perceived match at another viewing angle. Shade-analysis rule 1: take the shade with the patient sitting up, eye to eye at conversational distance. There are many different types of light we are all exposed to, as will be your patients and the restora- tions you make. When the shade guides are man- ufactured, they are compared to a standard in a controlled lighting situation. It is very controversial as to what colour temperature light to use to view shade,thatis5,000,5,500,or6,500K.9–12 Mostshade guidesarefabricatedtomatchastandardina5,500K light source. As already stated, shade guides do not have the same optical properties as natural teeth. This means they do not reflect light in the samemannerinalllightingconditionsasthecorre- sponding shade tooth would. Thus, visual shade matchingshouldonlybedoneinalightingenviron- mentthatiscloseto5,500K.Frommyexperience,if the shade guide is matched to the teeth in a 5,500 K light, then it will match well in most lights, but if it is matched in a strongly biased light (for example blue) the restoration will only match in that light. There are many different companies that sell florescent lights. Full-spectrum, colour corrected with a colour tempera- ture of 5,500 K are the lights best suited for visual shade taking. Ideally, it is best to outfit the operatory with this type of lighting, but an inexpensive way to con- trollightistousetwoOttLites(Fig.1)held at 61 cm from the patient at tooth level. Also, there are several innovative self-contained lighting devices available in dentistry. Optilume Trueshade (Optident Dental Products) works well for this and has a magnified viewer (Fig. 2). There are many other things that could be cov- ered about controlling the viewing conditions. The quantity of light and the hydration of the tooth are very important. Make sure when you are shade matching that there are no overt shadows on the teeth or shade guide and that the light is not so strong as to create specular highlights (reflective white spots). Also, the teeth need to stay hydrated. Saliva dries quickly, especially with cheek retractors inthemouth.Weuseamediumviscosityclearglaze liquid(SmileLineGlazeliquid,SmileLineUSA)towet theteethandtheshadeguide.Itisimportanttowet both, as differences in surface texture between the shade guide and the tooth can create a mispercep- tion.Thesameliquidonbothsurfacescanneutralise this (Fig. 3). Shade-analysis rule 2: use full-spectrum, colour corrected lighting, keeping the teeth ade- quately hydrated. _Understanding colour parameters critical to dental shade analysis A basic understanding of colour terminology is necessary for one to be able to evaluate differences from the shade guide and to communicate colour to the ceramist. Colour has been defined in many Fig. 4_Classical Shade Guide in colour with the correct value relationship. Note how tabs with dissimilar chromas look very different in value. Fig. 5_Classical guide in black and white with the correct value relationship. Fig. 6_Using the Classical guide arranged by value and working by a process of elimination to obtain to four tabs that cover the value range of the tooth being evaluated. Fig. 7_Using the Classical guide to select the chroma level. I 07cosmeticdentistry 1_2012 Fig. 5 Fig. 7 Fig. 4 Fig. 6