Decolorization of Stains: Mechanisms of Oxidative Tooth Whitening
By far the most comprehensive and often the most effective form of tooth whitening involves decolorization of stained/colored components on and in teeth. This is primarily achieved though oxidative tooth whitening – also called bleaching. Oxidation achieves decolorization through chemical reactions with stain components.
Common organic compounds in teeth that produce yellowish color include porphyrins. Porphyrins are a class of natural biological chelators that chelate ions like iron in our body to control their levels. As it turns out, iron is also a catalyst for peroxide reactions. Because porphyrin is often associated with iron, the stains often have a catalyst with them that helps enable the peroxide bleaching process
When peroxide encounters a stain such a porphyrin, it takes electrons from the molecules in a process called oxidation. When an electron is removed from a molecule, most often it changes its ability to filter light, rendering it colorless. However, for this process to occur, the peroxide must be right next to the stain. Hence, the peroxide must have sufficient time to travel to the stain during the treatment process. This process of traveling to the stains is called diffusion. Diffusion is driven by concentration. The higher the concentration, the faster a molecule will diffuse to areas of lower concentration. This phenomena was discovered by Adolf Fick in 1855 and commonly referred to as Fick’s Law. Fick’s Law governs the efficiency and efficacy of all bleaching products. Concentration and contact time are essential to achieve whitening!
Vital tooth bleaching works through oxidation reactions. What is unique about peroxide-based tooth bleaching is the ability to decolorize (whiten) the tooth internally – by reacting with chromogens within the existing tooth structure, within enamel, and at the dentin enamel interface, without oxidizing the tooth structures themselves. Depending on how the peroxide is applied (concentration, volume, etc.) diffusion occurs within the tooth and penetrates through enamel and dentin. The diagram below (Figure 9) shows peroxide penetrating the tooth during the application of peroxide.
Figure 9. Peroxide diffusion into tooth from a whitening gel applied with a strip to exterior of tooth.27
In addition to decolorization of chromogens it is possible that oxidation reactions may change other optical properties of the tooth including light scattering and fluorescence.28
Obviously, sources of extrinsic staining – coffee, tea, nicotine, etc., – can be oxidized directly. For intrinsic stains such as tetracycline stains to be eliminated, however, typically requires extended treatment. It should be noted that other forms of oxidation are used in tooth bleaching, such as peroxyacids or chlorite. Ultimately however, the efficacy of these other forms may be affected by the formulation and diffusivity of the agents. This may affect the level of efficacy observed, also the type of tooth whitening which can be achieved (for example focusing on certain types of stains, or stains on the exterior of the teeth). The agent with the longest-term proven record of safety and efficacy is peroxide-based whitening.