TEETH BLEACHING - COLOUR OF NATURAL HEALTHY TEETH

INTRODUCTION 

Tooth discoloration is a common problem. People of various ages may be affected, and it can occur in both primary and secondary teeth. The aetiology of dental discoloration is multifactorial, while different parts of the tooth can take up different stains. Extrinsic discoloration increases with increasing age and is more common in men (Eriksen and Nordbo 1978); it may affect 3JO!c, of men and 21 % of women (Ness et a11977). The result is a complex of physical and chemical interactions with the tooth surface. The aim of this chapter is to assess the aetiology of tooth discoloration and the mechanisms by which teeth stain.

COLOUR OF NATURAL HEALTHY TEETH 
Teeth are polychromatic (Louka 1989). The colour varies among the gjngival, incisal and cervical areas according to the thickness, reflectance of different colours and translucency in enamel and dentine (see Figure 1.2). The colour of healthy teeth is primarily determined by the dentine and is modified by:

• the colour of the enamel covering the crown

• the translucencv of the enamel which varies with different degrees of calcification

• the thickness of the enamel which is greater at the occlusal/incisal edge of the tooth and thinner at the cervical third (Dayan et a11983).

concerning the exact definitions of these terms. Feinman et al (1987) describes extrinsic discoloration as that occurring 'when an agent stains or damages the enamel surface of the teeth, and intrinsic staining as occurring when internal tooth structure is penetrated by a discolouring agent. According to his definitions, the terms staining and discoloration are used synonymously. However, extrinsic staining will be defined here as staining that can be easily removed by a normal prophylactic cleaning (Dayan et al 1983). Intrinsic staining is defined here as endogenous staining that has been incorporated into the tooth matrix and thus cannot be removed by prophylaxis.

Some discoloration is a combination of both types of staining and may be multifactorial. For example, nicotine staining on teeth is extrinsic staining which becomes intrinsic staining. The modified classification of Dzierkak (1991) and Hayes et al (1986) and Nathoo (1997) will be used as a guide.