Chapter 15 Getting the appearance correct

Introduction

When making restorations that are visible in the smile line, and for some patients even when the restorations are not readily visible, appearance is critical. Creating a good appearance in a restoration depends upon matching the colour, shape and surface texture to adjacent teeth. Whilst selecting the correct shade requires an understanding of light and the science of colour, creating the shape and texture of teeth needs knowledge of tooth morphology, dimensions and proportion. This chapter addresses these issues so that the optimum appearance of aesthetic restorations can be achieved.

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.

Figure 15.1 Electromagnetic spectrum.

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:

• Reflection. Light is reflected by mineralized tissue, mainly enamel rods and extratubular dentine. The amount of reflected light helps to determine the brightness of a tooth: the more light that is reflected, the brighter the tooth will appear.

• Scatter. Light entering the tooth tissue hits various tooth structures and is dispersed in all directions. Some light is returned towards the source and emerges from the tooth surface again in all directions. Scattering therefore reduces the intensity of the radiation being returned from the material. What causes the majority of scatter is different in each dental tissue. For example, in enamel a large amount of scatter is due to the enamel prisms, specifically the hydroxyapatite crystallites. However, most of the scatter in a tooth occurs in dentine and this is due to the dentinal tubules.

• Absorption. Light is absorbed by teeth and occurs to a greater extent in dentine than enamel. Energy that is absorbed does not emerge from the material and is converted to another form of energy. The amount of absorption is dependent on the light source and tooth.

• Refraction. The change in direction of a beam of electromagnetic radiation due to a change in the conveying medium is termed refraction. This occurs to a small extent in enamel (97% mineralized by weight) and to a greater extent in the less mineralized and tubular structure of dentine (approximately 70% mineralized by weight)

• 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.

• Translucency. Translucency is defined as the ability to allow radiation to pass with little scatter. The degree of translucency varies with the wavelength of the light. Enamel is translucent due to the high level of mineralization and crystallite orientation. However, when enamel is hypomineralized as a result of developmental (fluorosis) or acquired (carious white spots – see Chapter 1 on caries) defects, greater scattering of light occurs and the enamel becomes opaque and less translucent.

• Fluorescence. Fluorescence is the property of a material to absorb light of a particular wavelength and then to emit light of a different wavelength and therefore colour. Teeth fluoresce bluish-white when exposed to ultraviolet radiation.

• Metamerism. Metamerism can have an effect on the appearance of a tooth or restoration. Metamers are objects which match each other under one set of light conditions but mismatch under another as the light has different spectral properties (Figure 15.4).

• Opalescence. Opalescence is defined as having the property of opal stone. Natural teeth have the same properties as these stones: they appear yellowish-red in transmitted light but blue in reflected light.

Figure 15.2 Opaqueness of the metal–ceramic crown on the upper left lateral incisor compared to the more translucent teeth and resin composite restorations.

Figure 15.3 Image of teeth that have dehydrated under a rubber dam; the arrow shows where the rubber dam sat.

Figure 15.4 Metamerism: The same shade tab and tooth are matched under different light sources. It is observed that the shade match is better when viewed under the centre picture.

Human anatomy and colour vision

The retina, on the internal posterior wall of the human eye, contains a complex network of nerve endings capable of detecting light. There are over 120 million light-sensitive receptors in the 0.2 mm thick retina. Two types of receptor cell are present: rods and cones. Rods far outnumber cones and are responsible for night vision which is monochromatic. Cones are responsible for medium to high level light vision in full colour and are found in the centre of the retina. Colour judgement is theoretically impaired if the operator views an object from the side of the eye. Three types of cone exist; each one has a different sensitivity to different wavelengths of light (blue, red and green).

Colour vision deficiency (colour blindness)

Colour blindness is usually a sex-linked inherited condition but is rarely due to an acquired defect of the retina; as a result, the majority of people who suffer from this condition are male. Approximately 8–10% of the male population suffers from a congenital colour vision defect (CVD) whilst the prevalence among females is much less (approximately 1%). Differences in prevalence of CVD have also been found between different racial groups. The affects of a CVD on shade selection is discussed below.

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).

Figure 15.5 Framework of Munsell’s colour order system.

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.

Chroma

Chroma describes the intensity of the colour (hue) and distinguishes a strong colour from a weak one. The chroma scale starts at /0 and extends outwards; its maximum varies with each hue. The purest colours are found at the extremes of the colour cylinder (x axis, Figure 15.5). Reduced thickness or mineralization of dentine usually results in decreased chroma. The chroma of teeth is usually found within the range /1 to /5 but can range from /0 to /7.

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/.

Tooth shade selection

‘Selecting the shade’ for a restoration belittles the complexity of the process of determining the shade and form for a restoration. Providing the technician with only the correct shade will not enable them to fabricate an aesthetic restoration, a substantial amount of additional information is required. Additionally, the clinician has to prepare teeth in such a way that the technician is able to recreate the desired shape and shade. (See chapter 10, 11 and 12). Methods of traditional and instrumental shade-matching will be described in this chapter, together with the supplemental information required for an aesthetic restoration.

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15: Getting the appearance correct