THE SCIENCE OF TOOTH WHITENING

Linda Greenwall

INTRODUCTION

Over the past 25 years there has been an explosion in research on tooth whitening and the science behind the process of whitening, including its effectiveness (Table 2.1).

There are still many aspects that have not been researched, and some of these are discussed in this chapter. The purpose of this chapter is to understand the scientific basis behind the tooth whitening treatments available and how the whitening gels whiten the teeth.

HOW DOES WHITENING WORK?

THE CHEMICAL REACTION

Whitening agents mainly consist of hydrogen peroxide. The empirical formula for hydrogen peroxide is H₂O₂. The structural formula is HO–OH (Kelleher 2008). The whitening agents act by a redox reaction with the discolored substrate. When the whitening agent is placed onto the teeth, reactive oxygen is released; the discolored substrate is chemically reduced and transformed into a colorless material (McEvoy 1989). The oxidation reaction occurs when the peroxide diffuses through the dentin structure and into the peritubular dentin (Chng et al. 2005). See Figure 2.1.

The enamel can be considered a semipermeable membrane because it allows the permeation of the whitening gel through the tooth. The surface area of the enamel is important. The active mechanism of whitening agents mainly depends on a complex oxidation reaction, which releases oxygen and other free radicals. The oxygen and free radicals establish their primary mechanism of action in tooth whitening by penetrating through the porosities of the enamel prism to the dentin (Han et al. 2014).

The whitening gel moves from the enamel to the dentin and into the pulp within 5 to 15 minutes of gel application (see Figures 2.1 and 2.2). The whitening gel also penetrates into the weakest part of the tooth, which is a crack (Kwon et al. 2012) or an area of demineralization or hypomineralization, such as a white spot. Patients should be warned that white spots on the tooth get whiter during the initial whitening process (see Figure 2.7).

Minoux and Serfaty recognized that in-office tooth whitening is a very complex process that depends on several factors, including (1) the pH of the whitening agent, (2) the method of application and thickness of the whitening agent applied to the enamel, (3) the fluctuation of light irradiation, (4) the length of photoactivation, (5) tooth size, and (6) selective absorption of the wavelength of light.

HOW DOES WHITENING AFFECT THE ORAL ENVIRONMENT?

ORAL HEALTH BENEFITS OF WHITENING TEETH

It has been shown that whitening teeth improves the oral health of the mouth. This has long been known; the first whitening material, a hydrogen peroxide mouthwash (Peroxyl), was first used to heal gingivae that were swollen after braces (see Figure 2.3) (Haywood 1990).

In this context, it should also be mentioned that salivary proteins adsorbed onto the surfaces of composite materials decrease after whitening with peroxide- containing agents, which is suggested to have an influence on bacterial adhesion of cariogenic bacteria, such as Streptococcus sobrinus and Streptococcus mutans, but not Actinomyces viscosus (Steinberg et al. 1999).

HOW DO THE TOOTH WHITENING GELS WORK ON THE TOOTH? HOW DOES IT GET WHITER?

The tooth whitening gels in the form of hydrogen peroxide were originally used as treatments to reduce the swelling of the gingivae after orthodontic treatment. An orthodontist, Bill Klausmier, noted that when the gingivae were swollen after removal of the orthodontic brackets, the use of glyoxide mouthwash 3% in the retainers resolved the gingival inflammation. The hydrogen peroxide worked by oxygenating the gingival crevice and helping to reduce the inflammation. Hydrogen peroxide rapidly breaks down into water and oxygen; the oxygen is the active ingredient. It travels into the gingival crevicular fluid and diffuses into the gingival margin and into the tooth. The appearance of oxygen bubbles at the gingival margin gives an indication that the hydrogen peroxide is being activated to break down into oxygen and water, and on contact with the tooth the gel becomes activated and starts to penetrate through the tooth. The surface contact area is important. There is discussion about the shortest contact time to produce the quickest effect. The oxygen penetrates into the enamel, the dentin, and the pulp within 5 to 15 minutes of gel application. The oxygen then moves into the pigment molecules that are embedded in the dentin. These yellow pigment molecules become white pigment molecules. The process removes the color from the tooth, which is lightened (see Figure 2.10). The oxygen remains in the tooth for a period of 2 weeks while the gel is continually applied. Oxygen is liberated and whitens the tooth. The appearance of the oxygen makes the tooth appear lighter and whiter.

WHITENING PENETRATION: THE DIRECTION OF THE WHITENING PROCESS

The whitening gel travels into the weakest part of the tooth first. In some instances this can be straight into a crack in the tooth. This was demonstrated in a study by Kwon and colleagues (2012), in which the tooth was stained with rhodamine B dye to assess the pathway of the discoloration and the pathway of the whitening gel. It was noted that the process of discoloration is the same as the process of the whitening gel; it follows the direction of the dentin tubules, the odontoblasts, and the peritubular dentin. It was also noted that the whitening process is multidirectional. Although it appears that the whitening starts from the incisal tip, first it penetrates the tooth in a three-dimensional way, referred to as multidirectional penetration. Often it may appear that the incisal tip is lightening first (see Figure 2.10) and then the whiteness moves up the tooth toward the neck of the tooth. The upper teeth appear to whiten more quickly than the lower teeth.

INTO THE PULP

It is known that the whitening gel penetrates into the nerve within 5 to 15 minutes of gel application. The higher the concentration of hydrogen peroxide, the faster the gel moves into the tooth. This factor also depends on the permeability of the teeth and the tooth anatomy. A recent study assessed vascular permeability in rats (Ferreira et al. 2013) and found that hydrogen peroxide tooth whitening can induce an increase in vascular permeability in rat incisors. Importantly, this increase is more dependent on the length of the whitening procedure than on the concentration of the whitening agent.

In power whitening using higher concentrations of hydrogen peroxide, the whitening penetration into the pulp may occur more quickly depending on the concentration used and the contact time during which the whitening gel stays on the labial surface of the tooth. An in vitro study (Marson et al. 2015) on bovine teeth assessed the effects of different concentrations of hydrogen peroxide on the tooth (Opalescence Xtra Boost, 38%; White Gold In-Office, 35%; Whiteness HP Blue, 35%; Whiteness HP Maxx, 35%; and Lase Peroxide Sensy, 35%). The results showed that all products significantly reduced the concentration of H₂O₂ activates by 45 minutes; however, it was also shown that increasing the time the product remained on the tooth surface enhanced the penetration of H₂O₂. The researchers concluded that the whitening gels retained substantial concentrations of H₂O₂ after 45 minutes of application, and the penetration of hydrogen peroxide was time dependent.

WHITENING OF LOWER TEETH

The whitening of the lower teeth may occur more slowly because the sublingual salivary glands may wash out or reduce the effect of the whitening gel. Often on the lower canines there is a snow-capped appearance—a clear demarcation that occurs between the tip of the tooth, which is white, and the rest of the tooth, which has yet to whiten or “catch up.” The whitening in this situation needs to continue, and it may be 3 weeks before the whole tooth is completely white.

GINGIVAL MARGINS AND ROOT DENTIN

Gingival areas on cervical dentin do not whiten to the same effect. This is because at the gingival level there is reduced enamel thickness, and where there is gingival recession present the root appears more yellow. It is important to warn the patient that this area may not whiten to the full extent. There is debate as to whether extending the whitening tray over the gingival area to the extent of the gingival recession and onto the full extent of the exposed root will help to whiten this area fully. The root dentin will never whiten fully. The root dentin will lighten, but it will not be completely white. It is unrealistic to expect this, and this should be carefully explained to the patient. There are many different tray designs to address this issue, and although many manufacturers have trademarks and patents on the whitening tray design and where to finish the scalloped margin—whether to the edge of the tooth, beyond the tooth onto the gingivae, or 1 mm above the gingivae—the root has not shown a perfect white shade. Some studies have shown that the last 1 mm can be cut off the whitening tray to reduce sensitivity, and this has made no difference to the final whitening effect. Some gingival margins can become irritated from too much gel in this area. Whitening treatments were observed to reduce background luminescence of enamel, dentinoenamel junction (DEJ), and dentin in a study using confocal laser scanning microscopy (Götz et al. 2007).

WHICH TOOTH WHITENS THE QUICKEST?

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