15: Getting the appearance correct

Chapter 15 Getting the appearance correct

Light

Visible light forms a small portion of the whole electromagnetic (EM) spectrum (Figure 15.1) and often takes the form of polychromatic light which is composed of electromagnetic radiation of more than one wavelength. The colour of an object that one observes is actually the reflection of the light that strikes it. For example, a red flower appears red because red light is reflected by the flower whilst the other colours of light are absorbed.

The unique appearance of teeth is due to the complex interactions between light and tooth tissue, and this makes shade selection difficult. The interactions that take place are as follows:

Transmission. Light transmission occurs in both enamel and to a lesser extent in dentine. This is evident as neither material is opaque, unlike some materials used in dentistry (Figure 15.2). Transmitted light radiation passes through the incisal edge and approximal areas of a tooth following a number of the reactions described above. The number of these reactions is reduced at the incisal edge compared to the body of a tooth. This results in more light being transmitted at the incisal edge, occasionally giving the appearance of an almost transparent region. Enamel is almost translucent and if no dentine was present to block the transmission of light, teeth would appear glass-like. Dentine, however, provides the colour of a tooth that is evident in the body and cervical areas. When a tooth is dehydrated, it appears whiter and brighter due to dehydration of the collagen matrices. This can occur after impression making, rubber dam placement (Figure 15.3) and bleaching. It is therefore important to shade match at the beginning of an appointment rather than at the end when some temporary colour change might have occurred.

Description of colour

There are three main systems which can be used to describe and quantify colour, namely Munsell’s colour order system, the 1976 C.I.E.L*a*b* uniform colour space system (C.I.E Commission Internationale d’Eclairage (International Commission on Illumination)) or the RGB (Red, Green, Blue) colour space system. The first system is the one most commonly used and quoted in clinical dentistry and is therefore the only system described in this text.

Munsell’s colour order system

The Munsell colour system was devised by a painter, Albert Munsell, in 1905. The system’s attributes are hue, chroma and value (Figure 15.5).

Hue

Hue is the quality by which we distinguish one colour from another – for example, red from yellow, or green from blue. There are 100 Munsell hues: 10 major hues with each placed 10 steps apart in a horizontal plane (z axis, Figure 15.5) around a central axis. Teeth are found in the yellow and yellow–red region but the exact range of hues varies with the method of assessment and is different for extracted teeth. Dentine provides the main source of hue in a tooth but this is modified by the enamel.

Value

Value is the quality by which one distinguishes a light colour from a dark one and is therefore the brightness on a grey scale. The value symbol 0/ is used for absolute black and 10/ for absolute white (y axis, Figure 15.5). In a healthy young tooth there is less dentine thickness due to a reduced amount of secondary dentine and so the ratio of reflected to absorbed light radiation is increased compared to older teeth: as a tooth ages its value therefore decreases. Tooth value is usually found within the range 6/ to 8.5/ but can range upwards from 4/.

Jan 9, 2015 | Posted by in Operative Dentistry | Comments Off on 15: Getting the appearance correct
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