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The whitening of teeth has been a human desire dating back centuries. People have gone to great lengths to improve the appearance of their teeth, especially to make them as white as possible. For example, ancient Egyptians considered beauty to be important to both men and women. To improve their appearance, they wore makeup and whitened their teeth. White teeth also had social and economic implications. A person with white teeth had higher social ranking. White teeth were also an indication of wealth and health.1
Methods for tooth whitening have evolved over the centuries. The ancient Egyptians used a frayed stick to apply a mixture of ground pumice and white vinegar. Ancient Romans used a unique substance for teeth whitening: human urine. The ammonia in urine was mainly responsible for the whitening effect.2 In the 17th century, the barber not only took care of hair but also performed dental procedures. To whiten teeth then, the barber would use a file to roughen the facial surfaces of the anterior teeth and then apply nitric acid. The acidic effect on the teeth would lighten the shade, but at the same time, this practice jeopardized the health of the teeth.3
Materials purposed for tooth whitening in the past have been repurposed for the present. For example, in the mid-19th century, oxalic acid was used for teeth whitening.4 Today, oxalic acid is used to bleach wood. Over time, hydrogen peroxide in higher concentrations replaced oxalic acid.5 It was used in a liquid form and applied to tooth surfaces. In 1895, pyrozone was the first commercially available whitening product; it consisted of five parts 25% hydrogen peroxide and one part diethyl ether.6
The home-bleaching technique was discovered by accident in the late 1960s by an orthodontist, William W. Klusmeier, DDS. To improve the gingival health of his patients, he designed a custom-fitting tray where the patient, at home, placed an over-the-counter (OTC) oral antiseptic, Gly-oxide® (Marion Merrell Dow/Medtech Products, prestigebrands.com) in the tray and wore it at night. Dr. Klusmeier found not only did this technique improve gingival health of the patient, it also had a whitening effect on the teeth. Gly-oxide contains 10% carbamide peroxide. This technique was later described as nightguard vital bleaching3 and was introduced commercially as White & Brite™ (Omni/3M ESPE, 3m.com).2Since then, a plethora of whitening products have come on the market.
Indications for Teeth Whitening/Bleaching
The terms tooth "whitening" and "bleaching" have been used synonymously, but they have separate meanings. According to the US Food and Drug Administration, whitening refers to restoring teeth to their natural color, whereas bleaching refers to making teeth lighter than their natural color.7 In this article, for simplification purposes, the term "whitening" will also include the term "bleaching."
A familiar phrase heard in dental offices from patients is, "I wish my teeth were whiter." Obviously, the indication for teeth whitening is discoloration. However, the cause of the discoloration should also be addressed in order to select the most effective mode of treatment. Tooth discolorations can be extrinsic and/or intrinsic in nature and stem from a number of different sources. Materials that produce extrinsic stains, also referred to as external stains, can be divided into two types of compounds: those that are absorbed into the acquired dental pellicle8 that create a stain associated with its basic color and have more of an environmental basis, and those that create a chemical interaction on the tooth surface that leads to staining.9 The basis of the actual color of the stain is thought to be from the polyphenolic compounds that provide the color in food.10 For example, there are a number of stain-producing foods that can discolor teeth, such as beets and dark sauces (soy sauce, curries, and tomato sauce). Stain-producing beverages include black tea, coffee, and red or white wine.11 Tobacco smoking and smokeless tobacco products also can lead to discoloration. Extrinsic staining can occur from poor hygiene, which creates a more acidic oral environment, leaving the enamel more susceptible to staining.2
Sources of intrinsic staining can be hereditary disorders or medications, especially certain antibiotics used during tooth development, such as tetracycline-related antibiotics. In the latter case, the type of discoloration is dependent on the type of tetracycline.12 For example, tetracycline and oxytetracycline produce a yellowish discoloration, whereas chlortetracycline is associated with a greyish shade.9 A semisynthetic tetracycline, minocycline, produces a greenish-blue discoloration.13 If, during tooth development, another tetracycline derivative, tigecycline, is administered, a yellowish/brownish effect can be seen.14Interestingly, the color of tetracycline-affected teeth can darken (brown) from sunlight exposure from the effects of the photo-oxidative process. Under ultraviolet light, affected teeth can appear bright yellow due to fluorescence.13
Other sources of intrinsic staining are levels of systemic fluoride greater than the norm of 0.05 to 0.07 mg/kg of body weight during odontogenesis. This condition is referred to as fluorosis.15 Intrinsic staining can occur later in life from parafunctional habits (bruxism) that wear away the enamel, exposing the underlying dentin tubules.16
Teeth Whitening: How It Works
Teeth will appear whiter by physically removing color-producing stains and/or creating a chemical reaction to lighten the shade of the teeth.8Color-producing stains are organic compounds referred to as chromophores. Chromophores can be divided into two categories: large organic compounds that possess extended conjugated chains of alternating single and double bonds, and compounds containing metals.17
The whitening process is a combination of diffusion, interaction, and changes to the tooth surface.18 The lightening of a tooth shade occurs by diffusion of hydrogen peroxide through the permeable interprismatic spaces in the enamel and dentin and then interaction with the color-producing chromophore molecules.8Degradation of these chromophores results in teeth whitening. The main ingredients in teeth whitening gels are peroxide-based materials, either hydrogen peroxide or carbamide peroxide. Hydrogen peroxide and urea are produced when carbamide peroxide comes in contact with water.19
The efficacy of tooth whitening is based on the interaction of the whitening material and the stain molecule, the chromophore.20 When hydrogen peroxide comes in contact with tooth structure or organic material, it undergoes dissociation,21,22 producing free oxygen radicals.2These highly reactive free oxygen radicals break one or more of the double bonds or oxidize the chemical moieties in the conjugated chains of the chromophores,2resulting in a lighter-shaded compound from the loss of light-absorbing properties. The oxidation process creates smaller, less pigmented constituents.23 However, stains containing metallic compounds are more difficult to lighten, and the use of alternative restorative means (composite or porcelain veneers) may be necessary.8 The pH of the gel can affect dissociation. Whitening gels that are more alkaline form radicals faster than acidic versions.
In the case of tetracycline-stained teeth, the discoloration results from photo-oxidation of the tetracycline molecule present in the tooth structure.24 The chemical degradation of the unsaturated quinone-type structure present in tetracycline accounts for lesser amounts of colored molecules,25 which allows weaker light-reflecting properties.26
There are also micromorphologic changes that occur on the tooth's surface and within the tooth that affect the optical properties.27Even though enamel is inert, there still is a continuous ion exchange with the oral biofilm. To maintain a proper mineral balance, calcium phosphate apatite crystals can move in both directions. Extrinsic color-changing stains can become internalized,28making the removal of the stain more difficult.29
The semipermeability of tooth structure allows water-soluble, low-molecular-weight molecules to infiltrate from the outer layer of the tooth to the pulp cavity through interprismatic spaces in the enamel and dentinal tubules in the dentin.20,27 Bowles and Ugwuneri30 found in their study that a significant amount of hydrogen peroxide was detected in the pulp chamber after exposure in an in vitro model. It has been suggested that diffusion of hydrogen peroxide into the pulp chamber follows Fick's second law of diffusion, which is relative to the surface area available for diffusion. The amount of diffusion is also dependent on the concentration and diffusion coefficient and inversely proportional to the diffusion distance.20,31-33
A common post-whitening complaint is tooth sensitivity. It has been suggested that the cause of sensitivity may arise from the effect of hydrogen peroxide penetration into the pulp chamber causing a release of inflammatory mediators that stimulate sensory nerves, or from higher temperatures with light-activated in-office power bleaching techniques.34Greater amounts of hydrogen peroxide were found in the pulp chamber with the use of higher concentrations of whitening gels,30 prolonged whitening time,35and the use of light activation.36
Manufacturers of whitening products have added compounds to help reduce post-whitening sensitivity. For example, the addition of potassium nitrate 5% has an anesthetic effect by stopping the nerve from repolarizing after it has depolarized in the pain cycle.2,37This additive can decrease postoperative sensitivity without diminishing the whitening effect.38,39Amorphous calcium phosphate-casein phosphopeptide (ACP-CPP) is another additive that has been included in whitening gel. Desensitization occurs from deposition of calcium and phosphate ions in the exposed dentinal tubules. The bleaching effect from ACP-CPP-containing whitening gels is less than potassium nitrate-containing gels but with similar reduction in sensitivity.40
The Effect of Bleaching Lights
There have been various investigations of the effectiveness of adding some type of activation to accelerate the whitening process. Sources of activation include light energy, addition of chemicals, and mechanical energy. Several different light sources have been used, including light-emitting diode (LED), plasma arc light, metal halide lamp, xenon-halogen lights, and lasers.8,41,42An addition of chemical sources includes ferrous and manganese sulfate to reduce whitening time.16Mechanically, the use of ultrasonic energy is thought to increase production of free radicals.16
The effectiveness of adding a light source to a whitening procedure remains controversial. There are sources in scientific literature that support the addition of a light source; other studies support the use of certain types of light sources to be effective, and yet others report that light activation has no effect on whitening results.43-45
Studies that support the addition of an energy source to a whitening technique are based on the concept that light will increase the speed and effectiveness of the treatment by increasing the reactivity of the hydrogen peroxide. The light, also, will increase the formation of free radicals when the light source matches the wavelength of the photo initiators in the whitening gel. The light can also increase the temperature of the whitening gel, which can increase the free radical formation.8,19,46
However, studies point to the lack of benefits, namely the amount of whitening achieved, that can result with light activation. Epple et al47stated that from a chemical viewpoint, the oxidative effect from hydrogen peroxide would not be changed due to irradiation, but, due to a temperature increase, the reaction rate would increase. Viscio et al48 stated in the year 2000 that there lacked clinical validation for activation of hydrogen peroxide by irradiation. In 2008, Marson et al49reported that light irradiation during 35% hydrogen peroxide treatment had no significant effect. Carey8 concluded in a 2014 study that the use of different light sources did not have an effect on the amount of whitening achieved.
In 2019, Maran et al performed a systematic review and a network meta-analysis to answer the following question: "Is there any light-activation protocol capable of improving color change efficacy when associated with an in-office bleaching gel in adults?" Their conclusion found that no light-activated in-office whitening protocol, regardless of the light source or concentration of whitening gel, was superior to a light-free in-office whitening protocol.50
The controversy of the benefits of light activation in tooth whitening procedures remains. The use of non-standardized protocols in these different studies may account for the different conclusions.51
Vital Teeth Whitening Methods
There are three basic paths to whitening teeth: OTC materials that are used at home, teeth-whitening custom trays and whitening gels provided by the dental office, and in-office or power whitening completed by dental professionals.52
Since the introduction of whitening strips in the late 1980s, the popularity of OTC whitening products has increased.8 Today, OTC products include whitening toothpastes, rinses, strips, gels, and do-it-yourself whitening trays. Carbamide peroxide syringes up to 44% can be purchased as well as home teeth-bleaching kits that include LED lights from websites online.
The tray method for whitening is available in both OTC and professionally fabricated trays. The OTC trays are stock trays that may not conform to every mouth and can be uncomfortable in some mouths. People using the OTC tray method do not have professional monitoring and use whatever whitening gel concentration is provided in the OTC kit.2,8 Professionally made whitening trays are custom fitted with better adaptation of the tray wall to the facial aspect of the teeth. These trays are more comfortable, resulting in more compliance, and usually include professional supervision. Professional monitoring is important in selection of the whitening gel concentration and support for any problems that may develop. To diminished whitening gel-induced tooth sensitivity, a lower gel concentration should be used for longer periods of time and increasing the number of sessions.53
The third method is in-office or power whitening with or without light activation. Home whitening methods have been shown to be effective, but there are patients who for a number of different reasons (eg, time, compliance, gastrointestinal disorders)53,54 would rather have the whitening procedure completed in the dental office.
The two different whitening materials for the in-office procedure are hydrogen peroxide (20% to 40%) and carbamide peroxide (up to 38%). Usually, the hydrogen peroxide is provided in a two-syringe delivery system, which is activated by combining the two syringes. Carbamide peroxide usually is provided in a single syringe system and does not require an addition chemical for activation. Carbamide peroxide is activated by contact with saliva, which then breaks down to release hydrogen peroxide and urea.27,55,56
Concentrations of 10% hydrogen peroxide or higher have been shown to be corrosive to soft tissue, resulting in a burning sensation and tissue damage.57 Therefore, in-office whitening requires isolation of the teeth with a rubber dam, gauze, and/or retractors to protect the soft tissue from the whitening solution. A safety barrier should be placed along the gingival margins.
Rezende et al58 demonstrated that the average risk of tooth sensitivity for the at-home whitening technique was 51%, whereas for the in-office technique it was 63%. The lower incidence of sensitivity was associated with the reduced concentration of peroxides.59However, lowering the concentration to decrease the sensitivity also reduced the whitening effectiveness.60
Because carbamide peroxide is associated with lower levels of tooth sensitivity, manufacturers have introduced these products for in-office whitening. However, when carbamide peroxide is used for in-office bleaching, the isolated teeth do not come into contact with saliva, diminishing the whitening effect compared with using hydrogen peroxide.60
Vaez et al61 compared two in vitro alternative techniques to improve the whitening effect when using carbamide peroxide for the in-office procedure. As stated earlier, the breakdown of carbamide peroxide to hydrogen peroxide and urea is moisture dependent. In both alternative techniques, the teeth were isolated. The first alternative procedure was to humidify the tooth surfaces with wet gauze for 1 minute before application of the whitening gel, which was immediately applied over the moist enamel and left undisturbed for 45 minutes. Replenishment of the gel was not initiated. The second alternative procedure was to apply the whitening gel to the dry enamel surface and then cover the whitening gel with moist cotton pellets, leaving them undisturbed for 45 minutes. Again, replenishment was not initiated. They found the first alternative to be most effective because moisture (saliva) is more effective at the whitening gel-enamel interface.
The clinical significance of this study provides an alternative procedure for patients who have sensitivity issues or who have had problems with previous in-office whitening procedures. Humidifying the enamel surface before application of the whitening gel can accelerate and improve the effectiveness of the whitening procedure.61 Offering the use of 37% carbamide peroxide while incorporating the above-mentioned in-office protocol may provide the desired outcome to patients without the sensitivity.
Teeth whitening or bleaching continues to be popular with the general population. Patients have several options to improve the color of their teeth. OTC products can be obtained from the local drug store and online websites. Professionally made custom whitening trays from the dental office can provide a safer environment because the dental team is available for monitoring. In-office bleaching, or power bleaching, is available from the dental team as well. Tooth sensitivity can result from any one of these methods. This article has presented an alternative method of using 37% carbamide peroxide in an in-office setting that results in whitening with little or no sensitivity.
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