Chapter 15 Getting the appearance correct
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.
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 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 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.
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.
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 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 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/.
‘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.
The basic shade for a restoration is usually selected using a shade guide. Most manufacturers provide a shade guide for use with their materials, whether they are for use with indirect or direct restorations. These allow transfer of inf/>