Bleaching

Bleaching is a procedure which involves lightening of the color of a tooth through the application of a chemical agent to oxidize the organic pigmentation in the tooth.

Contraindications for Bleaching

Dentin Hypersensitivity

Hypersensitive teeth need to provide extraprotection before going for bleaching.

Extensively Restored Teeth

These teeth are not good candidate for bleaching because:

• They do not have enough enamel to respond properly to bleaching.
• Teeth heavily restored with visible, tooth colored restorations are poor candidate as composite restorations do not lighten, in fact they become more evident after bleaching.

Teeth with Hypoplastic Marks and Cracks

Application of bleaching agents increase the contrast between white opaque spots and normal tooth structure: In these cases, bleaching can be done in conjunction with:

• Microabrasion
• Selected enameloplasty
• Composite resin bonding.

Defective and Leaky Restoration

Defective and leaky restorations are not good candidate for bleaching.

• Discoloration from metallic salts particularly silver amalgam: The dentinal tubules of the tooth become virtually saturated with alloys and no amount of bleaching with available products will significantly improve the shade.
• Defective obturation: If root canal is not well obturated, then refilling must be done before attempting bleaching.

Ideal Bleaching agent

• Be easy to apply on the teeth.
• Have a neutral pH.
• Lighten the teeth efficiently.
• Remain in contact with oral soft tissues for short periods.
• Be required in minimum quantity to achieve desired results.
• Not irritate or dehydrate the oral tissues.
• Not cause damage to the teeth.
• Be well controlled by the dentist to customize the treatment to the patient’s need.

Materials used in Bleaching agents

Carbamide Peroxide (CH6N2O3)

It is a bifunctional derivative of carbonic acid. It is available as:

• Home bleaching

1. 5 percent carbamide peroxide.
2. 10 percent carbamide peroxide.
3. 15 percent carbamide peroxide.
4. 20 percent carbamide peroxide.

• In-office bleaching

1. 35 percent solution or gel of carbamide peroxide.

Hydrogen Peroxide (H2O2)

H2O2 breaks down to water and nascent oxygen. It also forms free radical perhydroxyl (HO2) which is responsible for bleaching action.

Sodium Perborate

It comes as monohydrate, trihydrate or tetrahydrate. It contains 95 percent perborate, providing 10 percent available oxygen.

Thickening Agents

• Carbopol (Carboxy polymethylene): Addition of carbopol in bleaching gels results in:

1. Slow release of oxygen.
2. Increased viscosity of bleaching material, which further helps in longer retention of material in tray and need of less material.
3. Delayed effervescence–thicker products stay on the teeth for longer time to provide necessary time for the carbamide peroxide to diffuse into the tooth.
4. The slow diffusion into enamel may also allow tooth to be bleached more effectively.

Urea

It is added in bleaching solutions to:

• Stabilize the H2O2
• Elevate the pH of solution.
• Anticariogenic effects.

Surfactants

Surfactant acts as surface wetting agent which allows the hydrogen peroxide to pass across gel tooth boundary.

Preservatives

Commonly used preservatives are phosphoric acids, citric acid or sodium stannate. They sequestrate metals such as Fe, Cu and Mg which accelerate breakdown of H2O2 and give gels better durability and stability.

Vehicle

• Glycerin: It is used to increase viscosity of preparation and ease of manipulation.
• Dentifrice.

Flavors

They are added to improve patient acceptability.

Fluoride and 3 percent Potassium Nitrate

They are added to prevent sensitivity of teeth after bleaching.

Mechanism of bleaching

Mechanism of bleaching is mainly linked to degradation of high molecular weight complex organic molecules that reflect a specific wavelength of light that is responsible for color of stain. The resulting degradation products are of lower molecular weight and composed of less complex molecules that reflect less light, resulting in a reduction or elimination of discoloration.

Rate of color change is affected by

• Frequency with which solutions are to be changed.
• Amount of time, the bleach is in contact with tooth.
• Viscosity of material.
• Rate of oxygen release.
• Original shade and condition of the tooth.
• Location and depth of discoloration.
• Degradation rate of material.

Bleaching technique

For vital teeth

• Home bleaching technique/Nightguard vital bleaching
• In-office bleaching

1. Thermocatalytic Vital Tooth Bleaching
2. Non-thermocatalytic Bleaching
3. Microabrasion.

For nonvital teeth

• Thermocatalytic Non-Vital Tooth Bleaching
• Walking bleach/Intracoronal bleaching
• Inside/outside bleaching
• Closed chamber bleaching/Extracoronal bleaching

Effects of Bleaching agents on tooth and its supporting structures

Tooth Hypersensitivity

Tooth sensitivity is common side effect of external tooth bleaching. Higher incidence of tooth sensitivity (67-78%) are seen after in-office bleaching with hydrogen peroxide in combination with heat. The mechanism responsible for external tooth bleaching though is not fully established, but it has been shown that peroxide penetrates enamel, dentin and pulp. This penetration was more in restored teeth than that of intact teeth.

Effects on Enamel

Studies have shown that 10% carbamide peroxide significantly decreased enamel hardness. But application of fluoride showed improved remineralization after bleaching.

Effects on Dentin

Bleaching has shown to cause uniform change in color through dentin.

Effects on Pulp

Penetration of bleaching agent into pulp through enamel and dentin occur resulting in tooth sensitivity. Studies have shown that 3% solution of H2O2 can cause:

• Transient reduction in pulpal blood flow.
• Occlusion of pulpal blood vessels.

Effects on Cementum

Cervical resorption and external root resorption in teeth has been seen in teeth treated by intracoronal bleaching using 30-35% H2O2.

Cervical Resorption

More serious side effects such as external root resorption may occur when a higher than 30% concentration of hydrogen peroxide is used in combination with heat. Hydroxyl groups may be generated during thermocatalytic bleaching, especially where ethylenediaminetetraacetic acid has been used previously to clean the tooth. Hydroxyl ions may stimulate cells in the cervical periodontal ligament to differentiate into odontoclasts, which begin root resorption in the area of the tooth below the epithelial attachment. Cervical resorption is usually painless until the resorption exposes the pulp, necessitating endodontic therapy. Intracanal dressings of calcium hydroxide are often successful in halting further tooth resorption, but severe external root resorption often necessitates extraction of the tooth. Moderate root resorption can be treated by orthodontically extruding the tooth and restoring it with a post-retained crown, but the prognosis of this treatment can be doubtful. Mild cervical resorption can be treated by surgical access, curettage, and placement of a restoration.

Effects on Restorative Materials

Application of Bleaching on Composites

• Increased surface hardness.
• Surface roughening and etching.
• Decrease in tensile strength.
• Increased microleakage.
• No significant color change of composite material itself other than the removal of extrinsic stains around existing restoration.

Effect of Bleaching Agents on Other Materials

• No effect on gold restorations.
• Microstructural changes in amalgam.
• Alteration in the matrix of glass ionomers.
• IRM on exposure to H2O2 becomes cracked and swollen.
• Provisional crowns made from methyl methacrylate discolor and turn orange.

Mucosal Irritation

A high concentration of hydrogen peroxide (30-35%) is caustic to mucous membrane and may causes burns and bleaching of the gingiva. So the bleaching tray must be designed to prevent gingival exposure by use of firmly fitted tray that may has contact only with teeth.

Genotoxicity and Carcinogenicity

Hydrogen peroxide shows genotoxic effect as free radicals released from hydrogen peroxide (hydroxy radicals, perhydroxyl ions and superoxide anions) are capable of attacking DNA.

Toxicity

The acute effects of hydrogen peroxide ingestion are dependent on the amount and the concentration of hydrogen peroxide solution ingested. The effects are more severe, when higher concentrations are used. Signs and symptoms usually seen are ulceration of the buccal mucosa, esophagus and stomach, nausea, vomiting, abdominal distention and sore throat. It is therefore important to keep syringes with bleaching agents out of reach of children to prevent any possible accident.

Bleaching is safe, economical, conservative and effective method of decoloring the stained teeth due to various reasons. It should always be given a thought before going for more invasive procedure like veneering or full ceramic coverage, depending upon specific case.

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