Understanding Tooth Colours
In order to understand how to quantify shade, we must use a colour model that can accurately describe colours. Most models work in three dimensions, and describe colour using three essential traits of hue, value and chroma. Hue is the attribute of a colour by virtue of whether it is discernible as red, yellow, green, or blue, and which is dependent on its dominant wavelength. In plain English, it means that hue is the main colour, or combination of colours, present. If an object looks red, the hue is red. If an object is green, the hue is a combination of yellow and blue.
With teeth, the differences in hue are pretty subtle. Chroma is the saturation or intensity of hue. Red and pink have the same hue. However, red has a high chroma while pink has weak chroma. Likewise, maroon would have a higher chroma than both red and pink. Finally, value is the relative lightness or darkness of a colour. It is the most important factor in shade matching because the human eye is very sensitive to changes in value. On a scale of black to white, white has a high value, while black has a low value. When light falls on an object, it is either reflected, transmitted or absorbed. If all the light is reflected, the object will appear white. If the light is absorbed, the object will appear black. In the case of teeth, reflection, transmission and absorption all occur simultaneously to varying degrees.
The Munsell Colour Solid below demonstrates the properties of colour in one simple diagram. Hue changes as you move around the centre. Chroma changes as you move from the centre outward. Value changes from top-to-bottom.
Transmission of light through teeth Because teeth are made up of different layers, they have both translucent and opaque qualities. Enamel has translucent properties, in that light passes through it but is dispersed, with some being reflected. Opacity means that light is unable to pass through at all. The dentine of a tooth is an example of an opaque medium. When light hits an anterior tooth, the light is transmitted through the translucent enamel layer. In the incisal third, with no underlying dentine present, and where the enamel layer is thinnest, the light has less to pass through. Here the tooth appears lightest. The cemento-enamel junction of the anterior tooth is the thickest part of the tooth, with the greatest thickness of dentine. Here, the light diffuses through the enamel layer and is either absorbed or reflected by the dentin. The translucency pattern contributes to the shade by affecting value. Over time, due to tooth-brushing and wear, the enamel layer becomes thinner. Loss of enamel thickness, increases translucency allowing the darker dentinal shade to dominate. This is an important factor when trying to anticipate whitening success in patients.
How teeth discolour
Intrinsic Staining The success of tooth whitening depends largely on the type of staining present. Intrinsic staining is discolouration that is incorporated into a tooth, either while the tooth is developing, or after it has erupted – this stain cannot be removed by prophylaxis.
Fluorosis Some of the main causes of intrinsic staining are excessive fluoride intake over the period that the tooth develops. The resulting fluorosis manifests itself as either white and chalky enamel, or brown staining. Either way, bleaching can be used in most cases to reduce the contrast of the mottled enamel and improve the appearance.
Antibiotics Another major cause of intrinsic staining is use of the antibiotics, tetracycline and minocycline. These impart a blue-grey banding on the teeth. Severity of staining will differ depending on the type and duration of use. In both cases the staining is similar, however, minocycline can stain teeth both during development and after eruption. Prolonged whitening is usually required in these cases, but more often than not, it must be used in combination with bonding or veneers to get a satisfactory result.
Trauma The most common cause of intrinsic staining is trauma which manifests itself in a two-fold process. Firstly, inflammation of the pulp causes hemorrhage into the dentinal tubules to give the tooth a pinkish tinge. The haemoglobin then breaks down to iron sulfide to give a dark black colour. This type of staining responds well to prolonged whitening. Likewise, if a tooth becomes necrotic secondary to trauma, a similar process occurs. However, treatment in this case must include root canal treatment to remove the necrotic material.
Hereditary Other common causes of intrinsic staining are hereditary. Imperfections in the formation of either enamel or dentin can cause discoloured teeth. Both amelogenesis and dentinogenesis imperfecta, along with enamel hypoplasia are examples of hereditary causes of intrinsic staining. Diseases like porphyria can also cause discoloured teeth due to excess porphyrins in the blood during mineralization of the teeth. Affected teeth are usually pinkish brown.
Age Age also causes intrinsic discolouration of teeth. This is due to changes in the physical composition of the tooth. Over time, layers of enamel are lost, exposing the darker underlying dentine. Sclerosis and secondary dentine can often take on a darker hue, which also contributes to age related discolouration.
Extrinsic staining is far more common than intrinsic staining. Extrinsic stains are those that only affect the surface of the teeth and can be removed by prophylaxis. Mostly these arise from the interaction of pigments from food and beverages with the plaque covering the enamel. This can easily be removed by dental prophylaxis and polishing. When this type of staining penetrates the microscopic cracks and fissures of the teeth, it cannot be removed by tooth-brushing or dental prophylaxis. In these instances, whitening is required. The most common causes of extrinsic staining are coffee, tea, red wine and smoking.
VITA SHADE GUIDE AND TOOTH WHITENING
The classic Vita shade guide is the most widely used guide for assessing tooth colour. The various letters denote the underlying hues that make up the colour for that particular group, as follows: In terms of predicting whitening success, A shades and B shades whiten the best. As the teeth get darker, and more grey in colour (darker C shades and D shades), more prolonged whitening is usually required.