Tooth Whitening: What We Now Know

Abstract

Current research about tooth whitening shows that it is safe and effective when manufacturer's protocol is followed, yet there are risks of which the profession and users should be aware. This update provides a summary of current research and assessment of the safety and efficacy of tooth whitening regimens.

Background

Tooth whitening has become one of the most frequently requested dental procedures by the public. The public has come to demand whiter, more perfect smiles and in response many choices for tooth whitening have been made available. These include home-based products such as toothpastes, gels, and films, as well as in-office based systems where products containing highly concentrated bleaching agents are applied under professional supervision. The profession and public have been aware of certain risks related to tooth whitening such as increased tooth sensitivity and gingival irritation. New research has shown that there are other risks such as tooth surface roughening and softening, increased potential for demineralization, degradation of dental restorations, and unacceptable color change of dental restorations. The new research is also focused on optimizing whitening procedures to reduce tooth sensitivity and to increase the persistence of the whitening.

Methods

Current reports in the literature are reviewed that are related to the use of peroxide based whitening methods. These reports include in vitro studies for method optimization and mechanism as well as clinical studies on effects of various whitening regimens.

Conclusions

When manufacturer's instructions are followed, hydrogen peroxide and carbamide peroxide based tooth whitening is safe and effective. Patients should be informed of the risks associated with tooth whitening and instructed on identification of adverse occurrences so that they may seek professional help as needed.

Introduction

The public has been influenced by the portrayal of perfect white smiles in the media. The color quality of television, movies, electronic and print media has raised self-awareness of discolored teeth. In the late 1980's several companies introduced both home-based products and professionally applied tooth whitening products into the U.S. marketplace. These have gained popularity with the public who has come to demand whiter, more perfect smiles.¹ There are several different methods for whitening teeth, each with their own mechanism of action. The efficacy of these different methods is dependent upon the particular tooth discoloration that is being treated. Causes of tooth discoloration can be categorized into two main groups: intrinsic and extrinsic staining (see Figure 1). Intrinsic staining, sometimes called internal staining, can be attributed to factors such as genetics, age (from enamel wear over time exposing yellower dentin), antibiotics, high levels of fluoride, and developmental disorders and can start before the tooth has erupted. After eruption of the tooth some dental restorations can cause tooth staining. Extrinsic staining, sometimes called external staining, is largely due to environmental factors including smoking, pigments in beverages and foods, antibiotics, and metals such as iron or copper. Colored compounds from these sources are adsorbed into acquired dental pellicle or directly onto the surface of the tooth causing a stain to appear. See Figure 1 for examples of the staining from these sources.

Tooth whitening is any process that lightens the color of a tooth. Whitening may be accomplished by physical removal of the stain or a chemical reaction to lighten the tooth color. Bleaching is defined here as the chemical degradation of the chromogens. The active ingredient in most whitening products is hydrogen peroxide (H₂O₂) which is delivered as hydrogen peroxide or carbamide peroxide. Carbamide peroxide is a stable complex that breaks down in contact with water to release hydrogen peroxide. Because carbamide peroxide releases hydrogen peroxide the chemistry of most tooth whitening is that of hydrogen peroxide.

Tooth stains consist of compounds that have color or darker shades called chromogens that are accumulated in the tooth (intrinsic) or on the tooth (extrinsic). Chromogens fall into two categories: large organic compounds that have conjugated double bonds in their chemical structure as shown in Figure 2A; and metal containing compounds. Bleaching of the organic compounds by hydrogen peroxide involves reacting with the double bonds to oxidize the double bond as shown in Figure 2B. This causes the chromogen to become a lighter colored compound. Bleaching of the metallic compounds is much more difficult; better esthetic options may be veneers, bonding, or crowns. There are some professional products that contain sodium hypochlorite (NaOCl) which reacts with the double bonds of the chromogen in much the same way as peroxide as shown in Figure 2C.

Different product types are marketed to address the particular stain to be removed. The broad categories include: cleansers such as smokers' toothpastes which contain larger amounts of detergents and abrasives to aid in the removal of chromogens from the tooth surfaces; bleaches to react with the chromogens to lighten color; and products that have both increased cleansing and bleaching. More severe or complex stains are best lightened professionally, while over-the-counter products can be used for milder cases. The treatment time required for reaching the tooth whitening endpoint is dependent on the time of exposure and the concentration of bleaching compound. This endpoint is dependent on the type of whitening employed, usually 1–2 shades for cleansers and over-the-counter (OTC) gels, and more for professionally applied products.