Stain Control/Whitening Agents
Stain control and whitening are key benefits of modern dentifrices. These are accomplished via ingredients that target specific types of tooth stain. Stains can be classified as extrinsic (surface stains) or intrinsic (below the enamel surface), and their management is based primarily on that classification. Dentifrices primarily work against extrinsic stains. Bleaching products that contain hydrogen peroxide (i.e., whitening strips) or carbamide peroxide (i.e., dental office bleaching trays) and allow longer contact time with the dentition address intrinsic stains as well as extrinsic stains.61,117
Despite being extremely hard, the tooth’s surface can be stained. Extrinsic (surface) stains can generally be relatively easily removed daily by proper tooth brushing with a dentifrice. If extrinsic stains are not frequently removed, they can firmly attach to the tooth surface, and may require a professional removal. Surface stains can be removed daily through either physical or chemical action, as described below.61,118
Physical action. Most dentifrices contain mild abrasives that help clean precipitated stain particles from the tooth surface, controlling surface stains with home care. While dentifrice abrasives are desirable to keep stains off teeth, they are also designed to not wear down the tooth enamel over time with repeated brushing.117
The abrasivity of dentifrice is measured in terms of Relative Dentin Abrasivity, or RDA. This rating was introduced in the early 1970s and is used by professional dental societies and boards of health to rate the abrasivity of commercial dentifrices.117-119 RDA values are obtained in the laboratory by comparing the amount of tooth structure worn away by a standardized tooth brushing protocol using any given dentifrice with that of a standard dentifrice. These laboratory values are not intended to replicate real life conditions or predict real world outcomes. Dentifrice with a low RDA value may or may not be less abrasive, also tends to remove less surface stain. The International Standards Organization (ISO) specification states that a dentifrice should not exceed an RDA of 250, which is considered safe for hard tissues for a lifetime of use. Although there is a wide range of RDA values for various dentifrices, there are no relative degrees of safety between 0 and 250. In other words, a dentifrice with an RDA of 200 is as safe as one with an RDA of 50 for daily usage for a lifetime. Having an effective abrasive system in a dentifrice is important for cleaning the teeth and removing extrinsic stain.120Fluoride ions are very reactive and can interact with common dentifrice abrasives, rendering the fluoride inactive for caries control. Also, many dentifrice abrasives have a very porous, negatively charged surface that can bind many dentifrice ingredients, lowering their bioavailability. For this reason, formulating dentifrices with the right abrasives is critical to achieving the desired benefits of other ingredients.
Chemical action. Polyphosphates, which are dentifrice ingredients used to control calculus, also target extrinsic stain. One of the most effective ingredients with this dual action is SHMP, which is used in several marketed dentifrices. SHMP controls stain with a chemical action. Stain molecules, or chromogens, are usually negatively charged molecules; they have an affinity for positively charged ions like calcium (Ca2+) that reside in the tooth enamel and cross-link pellicle proteins. SHMP is also negatively charged, with a strong affinity for calcium. SHMP can displace stain molecules from calcium binding sites. It binds to the tooth surface and integrates into the pellicle to prevent additional stain molecules from binding (Figure 19).61,62,121
Figure 19. Stain Removal and Prevention.