Chapter 18 Dental bleaching systems
Over the past 20 years, the general public and therefore dental patients have become much more conscious of the appearance of their teeth. Their awareness of the treatment options dentists can offer has also increased owing to the increased media attention to dental health and cosmetic dentistry: there are many television programmes showing patient transformations using cosmetic surgery including cosmetic dentistry; and the public is bombarded with photographs of models with very white teeth in advertisements and glossy magazines. Although these photographs are very often ‘adjusted’ with the use of digital software tools, the perception that white, straight, perfect teeth are the norm and therefore desirable, has in many countries, contributed to a cultural shift, particularly in the USA. The market for cosmetic products and treatments has greatly increased, and it is now very common for patients to enquire about whitening their teeth during their dental appointment. Tooth bleaching is now the most commonly requested cosmetic service. Frequently, this appears in the patients’ minds to be more important than the treatment of dental disease. Beauty salons and hairdressers are also offering tooth whitening treatments although there are legal issues (in the UK) concerning non-dental care professionals carrying out any form of dental treatment.
Successful tooth bleaching can greatly improve the patient’s self-image, self-confidence and physical attractiveness. This can then lead to improved employment prospects and increased social confidence. Many products which are designed to lighten teeth can now be bought over the counter (OTC) from pharmacies and chemists and sometimes even in supermarkets. Other products are available only to dentists. Treatments with these products need to be carried out under professional supervision either directly by the clinician or by the patient carrying out the treatment at home, and returning to the dental clinic from time to time to allow the dentist to monitor their progress. Some treatment regimes combine both processes.
Chemical Reaction: An Oxidizing Process
The currently available products used to bleach teeth are based on hydrogen peroxide. This is a chemical which, on decomposition, produces species which can take part in an oxidizing reaction on tooth tissue, that is oxygen-free radicals and water. Many products also contain carbamide peroxide or sodium perborate, which both break down to release hydrogen peroxide.
Carbamide peroxide is a compound of hydrogen peroxide and urea. In the presence of water, carbamide peroxide breaks down into its two main constituents. Carbamide peroxide products have been shown to be active even after 10 hours of use so enhancing their efficacy. Carbamide peroxide is also referred to as urea peroxide, perhydrol urea and carbamyl peroxide.
Carbamide peroxide is inherently unstable and starts to decompose as soon as it has been manufactured. This decreases its oxidizing power and whitening effect. It is highly recommended therefore that tooth whitening products are stored in a refrigerator at the recommended temperature prior to use to maximize the product’s shelf-life. Some products are now supplied in refrigerated packs to achieve the same end (e.g. Evolution ICE, Enlighten).
Sodium perborate reacts with water to form sodium borate and hydrogen peroxide. The amount of hydrogen peroxide produced using sodium perborate is less than that from a similar amount of carbamide peroxide.
Mode of action
The free radical oxygen species produced by these compounds pass through the pores in the enamel and later the dentine by diffusion that is, moving from areas of high concentration to areas of low concentration until equilibrium is achieved or the source of the species is exhausted. This starts within a matter of 15 minutes of applying the product. It is possible that the active ingredient may eventually reach the pulp. To reduce this risk only a small quantity of the material should be used as the active species penetrates all coronal tissues. The diffusion process in the cervical region of the tooth is more rapid as dentine is more porous in this region. Penetration beneath restorations may occur and there is now some evidence of interactions between amalgam and the active species, leading to mercury release.
The free radical oxygen species break down the high molecular weight coloured complex organic molecules which cause staining. The smaller molecules so produced reflect less light from or they are lost from the tooth tissue with the result that the tooth tissue appears lighter in colour. Generally speaking, after an hour’s clinical use these breakdown products are rendered inactive (Figure 18.1).
The concentration of hydrogen peroxide in products varies, depending on whether the product is available to the public as an OTC product or is licensed for use under the direct supervision of a dentist. Concentrations range from 3% to 38%, respectively. The concentration of carbamide peroxide in any commercial product is three times the concentration of the hydrogen peroxide liberated, i.e. 10% carbamide peroxide will break down to release 3.3% hydrogen peroxide.
Common Ingredients in Tooth Whitening Products
|Chemical||Reason for inclusion|
|Hydrogen peroxide||Active ingredient|
|Carbamide peroxide||Source of hydrogen peroxide|
|Sodium perborate||Source of hydrogen peroxide|
|Urea||Stabilizer, and increases the pH, which is less irritant to soft tissue|
|Increased antibacterial effect|
|Glycerine||Increases viscosity, so that the product is retained in the bleaching tray|
|Carbopol (polyacrylic acid polymer)||Increases viscosity, decreases breakdown in saliva and slows release of oxygen|
|Alcohol ethoxylates or sodium xylene sulphonate||Surfactant – promotes wetting by lowering surface tension or to solubilize one of the ingredients, such as an insoluble fragrance|
|Amorphous calcium phosphate (ACP)||Decreases sensitivity by occluding the dentinal tubules with calcium phosphate|
|Improves enamel smoothness and restores lustre|
|Potassium nitrate||Decreases sensitivity by altering nerve conduction|
|Fluoride (e.g. sodium fluoride)||Decreases sensitivity by occluding the dentinal tubules|
|Provides caries resistance|
|Neutralizers||Alkaline substances to create neutral pH|
|Flavourings||Increases patient acceptability|
|Carotene||Converts light energy to heat so increasing the activation of hydrogen peroxide by speeding up its dissolution into free radicals in products intended to be exposed to light energy|
Indications and Contraindications
Clearly the main indication of tooth whitening products is to lighten teeth which have darkened physiologically or due to smoking or staining by chromogenic materials. The treatment is much more effective in those stains that are just below the enamel surface. Staining caused by some dental conditions is also amenable to bleaching:
Side Effects, Risks and Hazards
The most common side effect of bleaching treatments is sensitivity to thermal stimuli and occurs in between 25% and 50% of patients. As mentioned earlier, the oxygen species diffuse slowly through the hard dental tissues, eventually reaching the pulp. The volume reaching this site is dependent on the initial concentration and amount used. Many patients experience peak sensitivity on day 3 of treatment. The sensitivity tends to be transient and usually abates 2 days after discontinuation of treatment. Sensitivity has been reported more commonly in the lower than the upper arch, probably because of with relatively smaller size and volume of the tooth crowns. For this reason, many clinicians treat the upper arch before embarking on the lower. This also allows the clinician and patient to see what final result may be achieved (Figure 18.2). Long-term studies have shown no detrimental effects of this sensitivity after a period of 7 years.
Fig. 18.2 Postoperative view of bleaching. Teeth 13, 12, 11, 22, and 23 were bleached prior to the placement of ceramic veneers on 13 and 12 (after the original failed ceramic veneers had been removed from 13 and 12). Note the colour difference between the bleached upper teeth and the lower arch, which was not treated. Also note that the Directa (Svenska Dental Instrument AB) temporary crown rebased with Trim II (Bosworth) has reacted with the bleaching agent 16% PF Carbamide Peroxide gel (White Dental Beauty, Optident) and turned orange (see p. 314).
Some products have desensitizing chemicals included in their formula. Some clinicians advocate the application of desensitizing agents (such as sensitive dentifrices or mouthwashes containing potassium nitrate or neutral sodium fluoride) alternately with the bleaching chemicals in an attempt to reduce sensitivity.
Some patients have to discontinue treatment due to sensitivity although they are small in number. Simple analgesics such as paracetamol or ibuprofen will control any symptoms. Sometimes a reduction in the frequency or length of time of application may allow patients to continue with treatment. Likewise, changing the product to one with a lower concentration of active chemical may also help.
Gingival and soft tissue irritation
The next most common side effect is that of irritation of the oral soft tissues, occurring in 33% of cases. It is more commonly associated with self or home treatment. It is usually caused by the patient placing too much bleaching agent in the tray, which is displaced from the tray on insertion, or the bleaching gel may leach out of the tray during use. Both these result in the gel contacting the gingival tissues. Any soft tissue damage usually resolves uneventfully a few days after discontinuation of the treatment. The fabrication of a well-fitting bleaching tray should help to reduce gingival irritation together with careful placement of gel within it.
If a large amount of the bleaching gel is inadvertently swallowed, gastric irritation may occur due to release of large volumes of gas and gastric bleeding. Ingestion of large amounts of both hydrogen peroxide and carbamide peroxide can be fatal and these products should be stored securely and out of the reach of children.
Altered taste sensation
Some patients have reported a metallic taste during treatment. The strong oxidizing effect of the active species will cause mucosal changes and may have an effect on the tongue mucosa, altering the taste for short period of time. It may also be associated with a reaction between restoratives and the oxidizing agent. This is observed most commonly when the tray is removed in the morning after overnight bleaching. The taste usually disappears after a couple of hours. Many of the products used for bleaching are supplied in various flavours and the exposure to different flavours may also compromise the ability to taste other foods.
External cervical resorption
The aetiology of external cervical resorption is still not fully understood. There is a higher incidence of this condition in teeth that have undergone tooth whitening treatment. The prevalence is between 6% and 8% and it usually only affects non-vital teeth or those which have a history of trauma. The risk increases as the concentration of hydrogen peroxide rise above 30%. Clearly, if cervical resorption does occur, it will potentially compromise the prognosis of the tooth and should be treated without delay.
Risk of mutagenic effects
There has been some suggestion of an association between dental bleaching and an increased risk of developing neoplastic changes either in the oral cavity or elsewhere. The aetiology of this is that hydrogen peroxide may initiate or promote mutagenic change; it is known to be genotoxic in vitro but not in vivo. The experimental studies which showed these changes used artificially high concentrations of the chemical. The extreme effects produced by these high concentrations are of low significance considering the low concentration of hydrogen peroxide used to whiten teeth. Together with the short application time and short duration of treatment, this risk is very low, even in patients who smoke or drink alcohol, both known risk factors for the development of oral cancer.
Adverse structural changes in the dental hard tissues and changes in translucency of enamel
Carbamide peroxide at a low concentration has no effect on the surface or surface microhardness of enamel or dentine when used in a formulation with a neutral pH. However, some studies have shown that exposure to 10% carbamide peroxide for a significant part of the day over a month resulted in the enamel losing its aprismatic layer. Carbamide peroxide at a/>