Tooth whitening

Tooth whitening or tooth bleaching is the process of lightening the color of

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.^[4] Additionally, the perceived brightness of the tooth can change depending on the brightness and colour of the background.^[4]

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.^[2] 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.^[4] 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.^[2] 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.^[2]

With increasing age, teeth tend to be darker in shade.^[5] 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.^[5] Tooth shade is not influenced by gender or race.^[5]

Staining and discolouration

Tooth discolouration and staining is primarily due to two sources of stain: intrinsic and extrinsic (see Figure 2).

prophylaxis, in the dental office.^[6] Below explains in-depth the differences between the two sources of which contribute to such discolouration of the tooth's surface.

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.[7]

• gumline, and it occurs due to the normal development and defences of the immune system.^[8] Although usually virtually invisible on the tooth surface, plaque may become stained by chromogenic bacteria such as Actinomyces species.^[9] 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.^[10] 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.^[8]
• 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.^[11] The color of calculus varies, and may be grey, yellow, black, or brown.^[9] 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.^[9] 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.^[12] Betel chewing produces blood-red saliva that stains the teeth red-brown to nearly black.^[13] The extract gel of betel leaf contain tannin, a chromogenic agent that causes discolouration of the tooth enamel.^[14]
• Tannin is also present in coffee, tea, and red wine and produces a chromogenic agent that can discolor teeth.^[15] Large consumptions of tannin-containing beverages stain the dental enamel brown due to the chromogenic nature.^[16]
• Certain foods, including curries and tomato-based sauces, can cause teeth staining.^[17]
• 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.^[18] 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.^[19]
• 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).^[20] 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.^[19]

Removal of extrinsic staining

Extrinsic staining may be removed through various treatment methods:

• 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.^[15] Below are examples of intrinsic sources of stains:

• 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.^[21]
• Dental cavities (tooth decay):^[12] 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.^[21]
• 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.^[21] 
• internal resorption
  . Alternatively the tooth may become darker without pulp necrosis.
• tooth germ during neonatal and early childhood stages such as maternal vitamin D deficiency, infection, and medication intake can cause enamel hypoplasia.^[21]
• 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^[23] and produces a red hue seen initially after trauma which has the ability to disappear if the tooth becomes revascularized.^[21]
• 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.^[25]
• autosomal dominant
  which means that the condition runs in the family.
• Amelogenesis imperfecta:^[20] The appearance of amelogenesis imperfecta depends on the type of amelogenesis, there are 14 different subtypes and can vary from the appearance of hypoplasia to hypomineralisation which can produce different appearances of enamel from white mottling to yellow brown appearances.^[21]
• ultraviolet light. Later, the tetracycline is oxidized and the staining becomes more brown and no longer fluoresces under UV light.^[9][28]
• Porphyria:^[9] A rare metabolic disorder in which the body fails to adequately metabolise porphyrins, which leads to accumulation or excretion of porphyrins into teeth. The excretion of porphyrins produces purple-red pigments in teeth.^[29]
• Hemolytic disease of the newborn:^[9] This disease occurs when a newborn's red blood cells are being attacked by antibodies from the mother caused by an incompatibility between the mother and baby's blood. This condition can produce green staining of teeth due to jaundice, which is an inability to excrete bilirubin properly.^[30]
• Root resorption:^[31] Root resorption is clinically asymptomatic; however, it can produce a pink appearance at the amelocemental junction.^[21]
• buccal mucosa.^[32]

Figure 3. Shade guides

VITA classical A1-D4 shade guide arranged according to value

VITA classical A1-D4 shade guide arranged according to chroma; A: red-brown, B: red-yellow, C: grey, D: red-grey

Before the treatment, the clinician should examine the patient: taking a health and dental history (including

The whitening shade guides are used to measure tooth colour. These shades determine the effectiveness of the whitening procedure, which may vary from two to seven shades.^[33] These shades may be reached after a single in office appointment, or may take longer, depending on the individual. The effects of bleaching can last for several months, but may vary depending on the lifestyle of the patient. Consuming tooth staining foods or drinks that have a strong colour may compromise effectiveness of the treatment. These include food and drinks containing tannins such as; coffee, tea, red wines, and curry.

In-office bleaching procedures generally use a

Bleaching is least effective when the original tooth color is grayish and may require custom bleaching trays. Bleaching is most effective with yellow discolored teeth. If heavy staining or tetracycline damage is present on a patient's teeth, and whitening is ineffective (tetracycline staining may require prolonged bleaching, as it takes longer for the bleach to reach the dentine layer), there are other methods of masking the stain. Bonding, which also masks tooth stains, is when a thin coating of composite material is applied to the front of a person's teeth and then cured with a blue light. A veneer can also mask tooth discoloration.

In-chair whitening is faster and more effective in comparison to the take-home bleaching options.^{[medical citation needed]} Some clinicians also make custom bleaching trays, which can take up to a week to create. After the whitening treatment is completed, the patient is able to use these trays for maintenance of their bleaching with at-home kits or for use with desensitising products.

Light-accelerated bleaching

Power or light-accelerated bleaching uses light energy which is intended to accelerate the process of bleaching in a dental office. Different types of energy can be used in this procedure, with the most common being

The ideal source of energy should be high energy to excite the peroxide molecules without overheating the pulp of the tooth.[37] Lights are typically within the blue light spectrum as this has been found to contain the most effective wavelengths for initiating the hydrogen peroxide reaction. A power bleaching treatment typically involves isolation of soft tissue with a resin-based, light-curable barrier, application of a professional dental-grade hydrogen peroxide whitening gel (25–38% hydrogen peroxide), and exposure to the light source for 6–15 minutes. Recent technical advances have minimized heat and UV emissions, allowing for a shorter patient preparation procedure.

For any whitening treatments, it is recommended that a comprehensive examination of the patient is done including the use of radiographs to aid in the diagnosis of the current condition of the mouth, including any allergies that may be present. The patient will need to have a healthy mouth and free of periodontal disease or cavities and to have had a debridement/clean done to remove any tartar or plaque build up.^[38]

It is recommended to avoid smoking, drinking red wine, eating or drinking any deeply coloured foods after this as the teeth may stain considerably straight after treatment.

Nanoparticle catalysts for reduced hydrogen peroxide concentration

A recent addition to the field is new light-accelerated bleaching agents containing lower concentrations of hydrogen peroxide with a titanium oxide nanoparticle-based catalyst. Reduced concentrations of hydrogen peroxide cause lower incidences^[spelling?] of tooth hypersensitivity.^[39] The nanoparticles act as photocatalysts, and their size prevents them from diffusing deeply into the tooth. When exposed to light, the catalysts produce a rapid, localized breakdown of hydrogen peroxide into highly reactive radicals. Due to the extremely short lifetimes of the free radicals, they are able to produce bleaching effects similar to much higher concentration bleaching agents within the outer layers of the teeth where the nanoparticle catalysts are located. This provides effective tooth whitening while reducing the required concentration of hydrogen peroxide and other reactive byproducts at the tooth pulp.

Internal bleaching

Internal bleaching is a process which occurs after a tooth has been endodontically treated. This means that the tooth will have had the nerve of the tooth extirpated or removed through a root canal treatment at the dentist or by a specialist endodontist. Internal bleaching is often sought after in teeth which have been endodontically treated as tooth discolouration becomes a problem due to the lack of nerve supply to that tooth. It is common to have this internal bleaching done on an anterior tooth (a front tooth that you can see when smiling and talking). A way around this is by sealing off the bleaching agent inside the tooth itself and replacing it every few weeks until the desired shade has been achieved. The amount of time between appointments varies from patient to patient and with operator preference until the desired shade has been achieved.^[40] Even though this is a great option, the disadvantage of this treatment is a risk of internal root resorption of the tooth that is being internally bleached. This may not occur in every patient or every tooth, and its occurrence is difficult to determine prior to completing the treatment.^[40]

At home

At home tooth whitening products are available from dentists or '

The plastic whitening strips contain a thin layer of peroxide gel and are shaped to fit the buccal/labial surfaces of teeth.[1] Many different types of whitening strips are available on the market, after being introduced in the late 1980s.^[1] Specific whitening strip products have their own set of instructions however the strips are typically applied twice daily for 30 minutes for 14 days.^[1] In several days, tooth colour can lighten by 1 or 2 shades.^[1] The tooth whitening endpoint does depend on the frequency of use and ingredients of the product.^[1]

Whitening gels are applied onto the tooth surface with a small brush.^[1] The gels contain peroxide and are recommended to be applied twice a day for 14 days.^[1] The tooth whitening endpoint like that of the whitening strips.^[1]

Rinses

Whitening rinses work by reaction of the oxygen sources such as hydrogen peroxide within the rinse and the chromogens on or within the tooth.^[1] It is recommended to use twice a day, rinsing for one minute.^[1] To see an improvement in shade colour, it can take up to three months.^[1]

Toothpastes

Whitening toothpastes differ from regular toothpastes in that they contain higher amounts of abrasives and detergents to be more effective at removing tougher stains.^[1] Some whitening toothpastes contain low concentrations of carbamide peroxide or hydrogen peroxide which help lighten tooth colour however they do not contain bleach (sodium hypochlorite).^[1] With continuity of use over time, tooth colour can lighten by one or two shades.^[1]

Tray-based

Tray-based tooth whitening is achieved by wearing a fitted tray containing carbamide peroxide bleaching gel overnight or for two to four hours a day.^[1] If manufacturer's instructions are followed, tooth whitening can occur within three days and lighten teeth by one or two shades.^[1] This type of tooth whitening is available over-the-counter and professionally from an oral health professional.^[1]

Baking soda

Baking soda is a safe, low abrasive, and effective stain removal and tooth whitening toothpaste.^[43] Tooth whitening toothpaste that have excessive abrasivity are harmful to dental tissue, therefore baking soda is a desirable alternative.^[43] To date, clinical studies on baking soda report that there have been no reported adverse effects.^[43] It also contains acid-buffering components that makes baking soda biologically antibacterial at high concentrations and capable of preventing growth of Streptococcus mutans.^[43] Baking soda might be useful for cavities-prone patients, as well as those who wish to have whiter teeth.^[43]

Pens

Whitening pens are a convenient way to touch up any dental surface. The plastic, convenient, tube contains a bleaching gel that can be easily applied in hard-to-reach spots. In order to work, the gel needs to stay on the tooth surface for around 20–30 minutes.^[44]

Indications

Tooth whitening may be undertaken for a variety of reasons, but whitening may also be recommended to some individuals by dental professionals.^[45]

• Intrinsic tooth staining
• Aesthetics
• Dental fluorosis
• Endodontic treatment (internal bleaching)
• Tetracycline staining

Contraindications

Some groups are advised to carry out tooth whitening with caution as they may be at higher risk of adverse effects.

• Patients with unrealistic expectations
• Allergy to peroxide
• Pre-existing sensitive teeth
• Cracks or exposed dentine
• Enamel development defects
• Acid erosion
• Receding gums (gingival recession) and yellow roots
• Sensitive gums
• Defective dental restorations
• Tooth decay. White-spot decalcification may be highlighted and become more noticeable directly following a whitening process, but with further applications the other parts of the teeth usually become more white and the spots less noticeable.
• Active periapical pathology
• Untreated periodontal disease
• Pregnant or lactating women
• Children under the age of 16. This is because the pulp chamber, or nerve of the tooth, is enlarged until this age. Tooth whitening under this condition could irritate the pulp or cause it to become sensitive. Younger people are also more susceptible to abusing bleaching.^[46]
• Persons with visible white fillings or crowns. Tooth whitening does not change the color of
  fillings, porcelain, and other ceramics when they become stained by foods, drinks, and smoking, as these products are only effective on natural tooth structure. As such, a shade mismatch may be created as the natural tooth surfaces increase in whiteness and the restorations stay the same shade. Whitening agents do not work where bonding has been used and neither is it effective on tooth-colored filling materials. Other options to deal with such cases are the porcelain veneers or dental bonding.^[47]
• Individuals with poor oral hygiene

Some of the common side effects involved in teeth whitening are increased sensitivity of the teeth, gum irritation, and extrinsic teeth discolouration.[41]

Hypersensitivity