Tooth whitening or tooth bleaching is the process of lightening the colour of human teeth. Whitening is often desirable when teeth become yellowed over time for a number of reasons, and can be achieved by changing the intrinsic or extrinsic colour of the tooth enamel. The chemical degradation of the chromogens within or on the tooth is termed as bleaching.
Hydrogen peroxide () is the active ingredient most commonly used in whitening products and is delivered as either hydrogen peroxide or carbamide peroxide. Hydrogen peroxide is analogous to carbamide peroxide as it is released when the stable complex is in contact with water. When it diffuses into the tooth, hydrogen peroxide acts as an oxidising agent that breaks down to produce unstable free radicals. In the spaces between the inorganic salts in tooth enamel, these unstable free radicals attach to organic pigment molecules resulting in small, less heavily pigmented components. Reflecting less light, these smaller molecules create a "whitening effect". Peroxyacids are an alternative to hydrogen peroxide and also contribute to the breakdown of pigment molecules. There are different products available on the market to remove stains. For whitening treatment to be successful, dental professionals (dental hygienist or dentist) should correctly diagnose the type, intensity and location of the tooth discolouration. Time exposure and the concentration of the bleaching compound determines the tooth whitening endpoint.
Natural shade The perception of tooth colour is multi-factorial. Reflection and absorption of light by the tooth can be influenced by a number of factors, including specular transmission of light through the tooth, specular reflection at the surface, diffuse light reflection at the surface, absorption and scattering of light within the dental tissues, enamel mineral content, enamel thickness, dentine colour, the human observer, the fatigue of the eye, the type of incident light, and the presence of extrinsic and intrinsic stains. Additionally, the perceived brightness of the tooth can change depending on the brightness and colour of the background.
The combination of intrinsic colour and the presence of extrinsic stains on the tooth surface influence the colour and thus the overall appearance of teeth. The scattering of light and absorption within enamel and dentine determine the intrinsic colour of teeth and because the enamel is relatively translucent, the dentinal properties can play a major role in determining the overall tooth colour. On the other hand, extrinsic stain and colour is the result of coloured regions that have formed within the acquired pellicle on the enamel surface and can be influenced by lifestyle behaviours or habits. For example, dietary intake of tannin-rich foods, poor tooth brushing technique, tobacco products, and exposure to iron salts and chlorhexidine can darken the colour of a tooth.
With increasing age, teeth tend to grow darker in shade. This can be attributed to secondary dentin formation and thinning of enamel due to tooth wear which contributes to a significant decrease in lightness and increase in yellowness. Tooth shade is not influenced by gender or race.
Staining and discolouration Tooth discolouration and staining is primarily due to two sources of stain: intrinsic and extrinsic (see Figure 2). In essence, tooth whitening primarily targets those intrinsic stains in which cannot be removed through mechanics such as a debridement (clean) or prophylaxis, in the dental office. Below explains in-depth the differences between the two sources of which contribute to such discolouration of the tooth's surface.
Extrinsic staining Extrinsic staining, is largely due to environmental factors including smoking, pigments in beverages and foods, antibiotics, and metals such as iron or copper. Coloured compounds from these sources are adsorbed into acquired dental pellicle or directly onto the surface of the tooth causing a stain to appear. Dental plaque: Dental plaque is a clear biofilm of bacteria that naturally forms in the mouth, particularly along the gumline, and it occurs due to the normal development and defences of the immune system. Although usually virtually invisible on the tooth surface, plaque may become stained by chromogenic bacteria such as Actinomyces species. Prolonged dental plaque accumulation on the tooth surface can lead to enamel demineralisation and formation of white spot lesions which appear as an opaque milk-coloured lesion. The acidic by-products of fermentable carbohydrates derived from high-sugar foods contribute to greater proportions of bacteria, such as Streptococcus mutans and Lactobacillus in dental plaque. Higher consumption of fermentable carbohydrates will promote demineralisation and increase the risk of developing white spot lesions. Calculus: neglected plaque will eventually calcify, and lead to the formation of a hard deposit on the teeth, especially around the gumline. The organic matrix of dental plaque and calcified tissues undergo a series of chemical and morphological changes that lead to calcification of the dental plaque and therefore leading to the formation of calculus. The color of calculus varies, and may be grey, yellow, black, or brown. The colour of calculus depends on how long it has been present in the oral cavity for; it typically starts off yellow and over time the calculus will begin to stain a darker colour and become more tenacious and difficult to remove. Tobacco: tar in the smoke from tobacco products (and also smokeless tobacco products) tends to form a yellow-brown-black stain around the necks of the teeth above the gumline. The nicotine and tar in tobacco, combined with oxygen, turns yellow and over time will absorb into the pores of enamel and stain the teeth yellow. The dark brown to black stains along the gum line of the teeth are the result of the porous nature of calculus immediately picking up the stains from nicotine and tar. Betel chewing. Betel chewing produces blood-red saliva that stains the teeth red-brown to nearly black. The extract gel of betel leaf contain tannin, a chromogenic agent that causes discolouration of the tooth enamel. Tannin is also present in coffee, tea, and red wine and produces a chromogenic agent that can discolor teeth. Large consumptions of tannin-containing beverages stain the dental enamel brown due to the chromogenic nature. Certain foods, including curries and tomato-based sauces, can cause teeth staining. Certain topical medications: Chlorhexidine (antiseptic mouthwash) binds to tannins, meaning that prolonged use in persons who consume coffee, tea or red wine is associated with extrinsic staining (i.e. removable staining) of teeth. Chlorhexidine mouthwash has a natural liking for sulphate and acidic groups commonly found in areas where plaque accumulates such as along the gumline, on the dorsum of the tongue and cavities. Chlorhexidine is retained in these areas and stain yellow-brown. The stains are not permanent and can be removed with proper brushing. Metallic compounds. Exposure to such metallic compounds may be in the form of medication or occupational exposure. Examples include iron (black stain), iodine (black), copper (green), nickel (green), and cadmium (yellow-brown). Sources of exposure to metal include placing metal into the oral cavity, metal-containing dust inhalation, or oral administration of drugs. Metals can enter the bony structure of the tooth, causing permanent discolouration, or can bind to the pellicle causing surface stain.
Removal of extrinsic staining Extrinsic staining may be removed through various treatment methods: Prophylaxis: dental prophylaxis includes the removal of extrinsic staining using a slow-speed rotary handpiece and a rubber cup with abrasive paste, mostly containing fluoride. The abrasive nature of the prophy paste, as it is known, acts to remove extrinsic staining using the action of the slow-speed handpiece and the paste against the tooth. Adversely, the action of the rubber cup together with the abrasive nature of the paste, removes around one micron of enamel from the tooth surface every time a prophylaxis is performed. This method of stain removal may only take place in the dental office. Micro-abrasion: allows a dental professional to make use of an instrument which emits a powder, water and compressed air to remove biofilm, and extrinsic staining. This stain removal method can only be undertaken in a dental office, not at home. Toothpaste: there are many available on the market that implement both peroxide as well as abrasive particles, such as silica gel, to help remove extrinsic stains, while the peroxide acts on intrinsic staining. This method of stain removal may take place at home as well as in a dental office.
Intrinsic staining Intrinsic staining primarily occurs during the tooth development either before birth or at early childhood. Intrinsic stains are those that cannot be removed through mechanical measures such as debridement or a prophylactic stain removal. As the age of the person increases, the teeth can also appear yellower over time. Below are examples of intrinsic sources of stains: Tooth wear and aging: Tooth wear is a progressive loss of enamel and dentine due to tooth erosion, abrasion and attrition. As enamel wears down, dentine becomes more apparent and chromogenic agents are penetrated in the tooth more easily. The natural production of secondary dentine also gradually darkens teeth with age. Dental cavities (tooth decay): The evidence regarding carious tooth discolouration is inconclusive, however the most reliable evidence suggests that carious lesion allows for exogenous agents to enter dentine and hence increased absorption of chromogenic agents causing discolouration to the tooth. Restorative materials: The materials used during root canal treatments, such as eugenol and phenolic compounds, contain pigment that stain dentine. Restorations using amalgam also penetrate dentine tubules with tin over time therefore causing dark stains to the tooth. Dental trauma which may cause staining either as a result of pulp necrosis or internal resorption. Alternatively the tooth may become darker without pulp necrosis. Enamel hypoplasia: Enamel hypoplasia causes enamel to be thin and weak. It produces a yellow-brown discolouration and can also cause the enamel's smooth surface to be rough and pitted which causes the tooth to be susceptible to extrinsic staining, tooth sensitivity, malocclusion, and dental cavities. The evidence regarding enamel hypoplasia is inconclusive, however the most likely cause is infection or trauma caused to the primary dentition. Disturbances to the developing tooth germ during neonatal and early childhood stages such as maternal vitamin D deficiency, infection, and medication intake can cause enamel hypoplasia. Pulpal hyperemia: Pulpal hyperemia refers to inflammation of a traumatised tooth which can be caused by a stimuli such as trauma, thermal shock, or dental cavities. Pulpal hyperemia is reversible and produces a red hue seen initially after trauma which has the ability to disappear if the tooth becomes revascularized. Fluorosis: Dental fluorosis causes enamel to become opaque, chalky white, and porous. The enamel can break down and cause the exposed subsurface enamel to become mottled and produce extrinsic dark brown-black stains. Dental fluorosis occurs due to excessive ingestion of fluoride or overexposure to fluoride during the development of enamel which usually occurs between the ages of one and four. Fluoridated drinking water, fluoride supplements, topical fluoride (fluoride toothpastes), and formula prescribed for children can increase the risk of dental fluorosis. Fluoride is considered an important factor in the management and prevention of dental cavities, the safe level for daily fluoride intake is 0.05 to 0.07 mg/kg/day.