TOOTH WHITENING MATERIALS

Linda Greenwall

INTRODUCTION

In the quarter century since the introduction of tooth whitening materials, there have been numerous modifications and improvements. We are currently using third-generation home whitening materials. The first-generation materials were in liquid form. These materials leaked out of the trays and needed continual replenishment over time, hence the use of reservoirs in the whitening trays. The second-generation materials were more viscous and in gel form. This viscosity allows the retention and adherence of the gel. The second-generation materials varied in concentrations of active ingredients and were known as either night products or day products. The third-generation materials differ in their concentration and incorporate soothers such as potassium nitrate, fluoride, and amorphous calcium phosphate (ACP). In general, quality control by the manufacturers and dental companies has improved, together with changes in the packaging, containers, and patient instructions, to make these products more patient-friendly. New legislation requires that tooth whitening syringes be labeled for specific use as a tooth whitening product, and in Europe they are not to be used in patients younger than 18.

Over the last 200 years, numerous attempts have been made to whiten teeth using all types of chemicals. The most effective products have been hydrogen peroxide and carbamide peroxide. These materials have different properties. This chapter will discuss the different materials that are used for tooth whitening treatments (Tables 3.1 and 3.2). Several other products that have been used for tooth whitening are discussed briefly.

THE CHEMISTRY OF WHITENING MATERIALS

CARBAMIDE PEROXIDE

Carbamide peroxide (CH₆N₂O₃) in a 10% gel formulation is the most commonly used home whitening material. The material is normally supplied in a syringe for ease of application, although some products are supplied in blister packs (see Figures 3.23–3.25). This breaks down to a 3.35% solution of hydrogen peroxide (H₂O₂) and a 6.65% solution of urea (CH₄N₂O). Fifteen percent and 20% solutions of carbamide peroxide are also available for dentist-supervised home whitening procedures. The 15% carbamide peroxide solution (CPS) yields 5.4% hydrogen peroxide, and the 20% solution yields 7% hydrogen peroxide (Fasanaro 1992) (Table 3.3).

A 35% carbamide peroxide gel is available as Quickstart (DenMat, Lompoc, CA) and 45% gel as Opalescence Quick (Ultradent Products, South Jordan, UT). These are used in an in-office procedure to enhance and accelerate the whitening process. The patient can see immediate results and is then motivated to continue the whitening treatment with 10% carbamide peroxide at home. This 35% gel yields 10% hydrogen peroxide. It can cause soft tissue damage and so should be used with a rubber dam or soft tissue protectant. The higher the concentration of materials, the faster the speed of action and the greater the likelihood of sensitivity (Table 3.4).

HYDROGEN PEROXIDE

Most of the whitening agents contain hydrogen peroxide in some form. The hydrogen peroxide breaks down into water and oxygen. It is the oxygen molecules that penetrate the tooth and liberate the pigment molecule, causing the tooth to whiten. (see Figures 3.1, 3.2, and 3.5).

The chemical hydrogen peroxide has been used for over 200 years to whiten teeth. Originally it was used as a liquid, applied directly onto a toothbrush, or as a cleansing agent in mouthwash form; now it is used in a gel formula to lengthen and preserve its short shelf life. Hydrogen peroxide is used in all types of whitening procedures—in office, chairside, and home whitening.

THICKENING AGENTS

Carbopol (carboxypolymethylene)

Carbopol is a polyacrylic acid polymer. Concentrations range from 0.5–1.5%. Trolamine, which is a neutralizing agent, is often added to Carbopol to reduce the pH of the gels to 5–7.

1.  The solutions containing Carbopol (e.g., Opalescence [Ultradent Products]) release oxygen slowly, whereas those without it are fast oxygen-releasing solutions. The rate of oxygenation affects the frequency of solution replacement during whitening treatment. The fast oxygen-releasing solutions release a maximal amount of oxygen in less than 1 hour; the slow solutions require 2–3 hours for maximal oxygen release, but remain active for up to 10 hours (Matis et al. 1999).

2.  Carbopol enhances the viscosity of the whitening material. The thixotropic nature of Carbopol allows better retention of the slow-releasing gel in the tray. Less whitening material is required for treatment (approximately 29 mL per arch). The viscosity also improves adherence to the tooth. The currently available formula of Opalescence has more Carbopol than previously.

3.  Carbopol retards the effervescence because it retards the rate of oxygen release. The thicker products stay on the teeth to provide the necessary time for the carbamide peroxide to diffuse into the tooth.

4.  The increased viscosity seems to prevent the saliva from breaking down the hydrogen peroxide, which might achieve more effective results according to Haywood (1991b). The partial diffusion into the enamel may also allow the tooth to be whitened more effectively deeper within its enamel and dentin layers (Garber et al. 1991).

Polyox

Polyox (Union Carbide, Houston, TX) is the thickener used in the Colgate Platinum system. The composition of the Polyox is a trade secret (Oliver and Haywood 1999). The additive influences the activity of the material and the tray design.

UREA

Urea occurs naturally in the body. It is produced in the salivary glands and is present in saliva and the gingival crevicular fluid (Moss 1999). Urea breaks down to ammonia and carbon dioxide either spontaneously or through bacterial metabolism. The effect on the pH depends on the concentration of the urea and the duration of its application.

Urea is used in whitening kits to do the following:

•  Stabilize the hydrogen peroxide (Christensen 1997). It provides a loose association with the hydrogen peroxide, which is easily broken down.

•  Elevate the pH of the solution.

•  Enhance other desirable qualities, such as anticariogenic effects, saliva stimulation, and wound healing properties (Archambault 1990).

VEHICLE

Glycerin

Carbamide peroxide is formulated with a glycerin base, which enhances the viscosity of the preparation and ease of manipulation. However, this may dehydrate the tooth. Many dentists have reported that the tooth seems to lose its translucent appearance; this may be caused by dehydration. The dehydrating effect and the swallowing of the glycerin in the solution may be responsible for the sore throat that is sometimes reported as a side effect when these agents are used.

Glycol

This is anhydrous glycerin.

Surfactant and pigment dispersants

The surfactant functions as a surface wetting agent, which allows the hydrogen peroxide to diffuse across the gel–tooth boundary. A pigment dispersant keeps pigments in suspension (as in commercial water softeners). Gels with surfactant or pigment dispersants may be more effective than those without them (Feinman et al. 1991, Garber et al. 1991). This may result in a more active gel, and dentists who prescribe these particular kits (Nu-Smile and Brite Smile) should caution their patients to adhere to the manufacturers’ suggested wearing time (Feinman et al. 1991).

PRESERVATIVES

Methyl, propylparaben, and sodium benzoate are commonly used as preservative substances. They have the ability to prevent bacterial growth in whitening materials (Alqahtani 2014).

Many of the gels contain a preservative such as citroxain, phosphoric acids, citric acid, or sodium stannate. These preservatives sequestrate transitional metals such as iron, copper, and magnesium, which accelerate the breakdown of hydrogen peroxide. These acid solutions give the gels greater durability and stability. They therefore have a mildly acidic pH.

FLAVORINGS

Flavorings are used in whitening materials to improve the taste of the material, add to the selection of whitening agents, and improve acceptability of the product to the patient (e.g., melon and mint). Many agents also contain sodium saccharine, a sweetener.

DESENSITIZERS

For an in-depth discussion, see Chapter 20).

1.  Fluoride.

2.  Potassium nitrate.

3.  ACP (as in ACP–casein phosphopeptide [CPP] formulations).

4.  Others, used in desensitizing toothpastes—many are recommended for use during whitening.

OVER-THE-COUNTER WHITENING PRODUCTS

Many over-the-counter (OTC) whitening-associated products have been introduced over the last 20 years.

Whitening-associated products are normally used to provide improved oral hygiene for removal of extrinsic stains, with some whitening maintenance to retain the white color. They are used as low-cost alternatives for whitening maintenance (Demarco et al. 2009).

These products include the following:

1.  Whitening kits

2.  Whitening maintenance kits (lower strength products)

3.  Whitening mouthwash

4.  Whitening toothpaste

5.  Desensitizing whitening toothpaste

6.  Whitening chewing gum

7.  Whitening floss

8.  Paint-on applicators

Such products, sold as cosmetics, have escaped rigorous legislation in the United States, the United Kingdom, and Europe. They are freely available through pharmacies, stores, mail order, and the Internet. Despite discussions with Internet mail order companies, these products are unregulated and are available to consumers. This has caused problems for patients and also for dentists, who should be monitoring whitening procedures carefully. There are many variations of these OTC materials (see Chapter 14).

The OTC kits contain the following:

1.  Acid rinse. This is usually citric or phosphoric acid, which may be harmful to the dentition because continued rinsing may cause tooth erosion. The potential for misuse may be considerable (Jay, 1990). The pH of this rinse is between 1 and 2.

2.  Whitening gel. This gel, applied for 2 minutes, has an acidic pH.

3.  Postwhitening “polishing cream.” This is an abrasive toothpaste containing titanium dioxide, which may give a temporary painted-white appearance. The efficacy and structural effects of these systems have not been evaluated in the literature.

H₂O₂ STRIP SYSTEM

The hydrogen peroxide strip system is a trayless whitening system that does not require any prefabrication or gel loading. The delivery system is a thin strip precoated with an adhesive 5.3% hydrogen peroxide gel (Haywood 2000, personal communication; Sagel et al. 2000). The backing is peeled off and the strip is placed directly on the facial and buccal surfaces of the six anterior teeth. Each strip is worn for 30 minutes, removed, and discarded, and the procedure is performed twice a day for 14 days. The manufacturers (Procter & Gamble, Cincinnati, OH) claim that the strip holds the gel in place to whiten the teeth both extrinsically and intrinsically and provides a uniform controlled application of gel. The material was initially supplied to dentists and is now available OTC.

PROBLEMS WITH OVER-THE-COUNTER WHITENING KITS

Cubbon and Ore (1991) reported that the overuse of OTC whitening agents caused erosion of the labial surfaces of the teeth, dissolution of the enamel, and loss of anatomy. The exposed dentin appeared darker than the remaining enamel, and patients overused this agent to reachieve the “white” tooth color. Dentists should be aware of these hazards when questioning patients who show evidence of tooth erosion of unknown cause (see Figure 3.3).

Patients may misdiagnose and self-prescribe whitening treatment, which may be inappropriate for their dental condition. A patient may have pulpal pathology that could be exacerbated by this treatment. In addition, a patient determined to speed up the whitening action may be overzealous in use of the product. Such abuse may lead to further problems and sensitivity (Fischer 2000a, 2000b).

SODIUM PERBORATE

Sodium perborate powder has long been used as a whitening agent. It was often used on its own as a cleansing agent and as a mouthwash (Bocasan, Oral-B, Procter & Gamble). It was used in combination with hydrogen peroxide owing to its synergistic effect to speed up the reaction of the oxidation. In 1965, Nutting and Poe recommended its use in internal whitening in combination with 35% hydrogen peroxide to be sealed inside the root canal. The effect of combining these two products was to create 50% hydrogen peroxide, which is very strong, to seal inside a non-vital tooth. In recent years, Internet suppliers of tooth whitening products were supplying it as a “safe” option because it was considered to contain no hydrogen peroxide, and so it was in effect below the “official legal” whitening limit. However, in the breakdown process of the sodium perborate, hydrogen peroxide is released. It is thus important to look at the chemistry.

Materials containing sodium perborate are Vitint System (Dental Partners, Rotterdam, the Netherlands) and Opalescence SP (Ultradent Products). The gel interacts with the moist tooth structure and is activated. The oxygen complex interacts with the tooth structure and saturates and changes the amino acids and double bonds of oxygen, which are responsible for tooth discoloration. However, sodium perborate breaks down to release hydrogen peroxide.

CHEMISTRY OF SODIUM PERBORATE

There are two main forms of sodium perborate (A. Pala 2014, personal communication): monohydrate (empirical formula NaBO3·H2O, molecular weight 99.81) and tetrahydrate (empirical formula NaBO3·4H2O, molecular weight 153.81).

1.  Sodium perborate is not simply a mixture of sodium metaborate and hydrogen peroxide, but rather an entirely different molecule with its own chemical properties.

2.  Sodium perborate’s whitening-oxidizing capacity increases with temperature. It works best at temperatures over 55°C.

3.  As a whitening agent, sodium perborate is used with organic activators such as tetraacetylethylenediamine (TAED) to allow the use of sodium perborate at lower temperatures. In these cases peracetic acid is formed as an intermediate compound. Moreover, sodium perborate is stabilized with complex ethylenediaminetetraacetic acid (EDTA) in such a way that it is not broken down by ions. Caution should be taken to avoid addition of these chemicals to commercial preparations intended for tooth whitening.

4.  The whitening activity of perborates is probably a result of the presence of not only perhydroxyl anion (HO2)−, but also the peroxoborate anion.

5.  Sodium perborate has been banned in Europe for use in tooth whitening because it is considered to be fetotoxic and cytotoxic. The European Commission’s Scientific Committee on Consumer Safety is of the opinion that sodium perborate and perboric acid can be considered as “hydrogen peroxide”–releasing substances and thus are covered by entry 12 of Annex III of the Cosmetics Directive 76/768/EEC (Council of European Dentists 2014).

MECHANISM OF WHITENING ACTION

Enamel should be considered a semipermeable membrane (Figure 3.15). Peroxide solutions flow freely through the enamel and dentin and into the pulp owing to the porosity and permeability of these structures (McEvoy 1989). The free movement is caused by the relatively low molecular weight of the peroxide molecule (see Figure 3.4) and the penetrating nature of the oxygen and superoxide radicals. It is difficult to set up barriers to prevent the rapid penetration.

The hydrogen peroxide acts as an oxygenator and an oxidant. Its whitening effect has been attributed to both these qualities, although the exact mechanism of action is not fully understood. In general, however, the hydrogen peroxide oxidizes the pigments in the tooth. The yellow pigments (xanthopterin) are oxidized to white pigments (leucopterin) (see Figures 3.1, 3.2, and 3.5). The oxidants react with the chromophores, which are the color radicals, to cleave the double bonds. The hydrogen peroxide must be in situ long enough and frequently enough to release the pigment molecules from the tooth by oxidation.

Carbamide peroxide is a bifunctional derivative of carbonic acid. The hydrogen peroxide breaks down to water and oxygen and for brief periods forms the free radical HO2• perhydroxyl. The free radical is very reactive and has great oxidative power.

•  It can break up a large macromolecular stain into smaller stain molecules, which are expelled to the surface by diffusion.

•  It can attach to inorganic structure and protein matrix (Fasanaro 1992).

•  It can oxidize tooth discoloration.

Carbamide peroxide eventually breaks down to water, oxygen, urea, carbon dioxide, and ammonia (Figure 3.8).

RELATIVE MERITS OF H₂O₂ VERSUS CARBAMIDE PEROXIDE SYSTEMS

Both systems contain hydrogen peroxide and work well. It appears that the H₂O₂ system may be faster than the CPS (Frysh et al. 1991), with a faster treatment and exposure time. Concentration of the hydrogen peroxide is determined by the teeth, not by the soft tissues, because the hydrogen peroxide solution contains mucous membrane protectant. However, there is no scientific literature about this, and these are purely manufacturers’ claims. The H₂O₂ systems are aqueous gel based, whereas the CPS systems are anhydrous gel based. Dehydration of the hard tissue is less likely to occur with hydrogen peroxide treatment. The CPS systems whiten more slowly and need longer exposure times (see Figure 3.9). This is because of the presence and contact of Carbopol, which is a slow-release oxygen agent. The urea that is present in the gels assists with wound healing and contains a soft tissue protectant.

MOVEMENT OF CARBAMIDE PEROXIDE SOLUTION

The CPS moves freely through the tooth in a multidirectional manner. It can laterally diffuse through the tooth and into the pulp within 5–15 minutes. This means that the CPS can be applied palatally to the tooth surface to whiten the color beneath labially placed veneers. The transient pulpal sensitivity that some patients experience may be related to the rapid movement of urea and oxygen through the teeth (Haywood and Heymann 1991).

The whitening effect can be detected around the edges of the incisal area and in the incisal corners. It appears that lateral incisors take up the whitening product most quickly. Upper canines appear to be the slowest to lighten. This is because of the anatomy of the enamel and dentin microstructure. Upper teeth whiten more quickly and have fewer side effects than lower teeth.

Whitening on lower teeth can be slower. A demarcation line may be detected on lower canines, also referred to as a “snow-capped appearance,” during the first few days of the treatment. This effect is most commonly noticed on lower canines, and patients are often concerned that their teeth are whitening unevenly. They can be reassured that this demarcation line usually fades after a few days of further whitening treatment. An in vitro study was undertaken by Oliver and Haywood (1999) to see whether the use of shortened dental trays could cause a demarcation line, but this proved negative and the teeth whitened evenly.

TREATMENT TIMING

There is no significant difference in the whitening efficacy of different concentrations of the material used during whitening treatments. At the end of treatment, it is not possible to know which concentration of whitening gel was used. However, studies have shown that the higher concentration materials may whiten teeth faster. An in vitro study by Leonard, Sharma, and Haywood (1998b) showed that a 16% solution of highly viscous materials (NiteWhite) successfully obtained a two-unit shade change more quickly than 5% and 10%. However, at the end of the 2-week trial period, there were no statistically significant differences among the 5%, 10%, or 16% solutions. A lower concentration of carbamide peroxide will also work, but it will take longer. The researchers observed that canines responded better to a 16% solution than did the central and lateral teeth. This would be beneficial for dentists who are treating an isolated dark tooth or darker canines (Leonard 1998).

PREFERRED REGIMENS

Most of the suggested whitening times are for either overnight use or daytime use, depending on the product. Carbamide peroxide home whitening products are recommended to be used overnight. If the patient cannot manage overnight use, then 2–3 hours is recommended in the evening. This is related to the whitening potential of the Carbopol slow-release oxygen agents, which can remain active for 10 hours during the whitening treatment (Matis et al. 1999).

In a randomized double-blind clinical trial, Cibirka and colleagues (1999) tested two different 10% carbamide peroxide materials used overnight for 2 weeks to whiten the upper teeth; the results showed a significant degree of lightening. There was no significant difference between the two materials tested, which were Opalescence and NiteWhite. In another clinical evaluation of two carbamide peroxide agents, both caused the teeth to lighten (Heymann et al. 1998). The latter author made the comment that the actual treatment time may not represent the active concentration time because the carbamide peroxide is decomposing relatively rapidly in the initial phase of any treatment.

Thirty-eight patients participated in a Nightguard whitening study using two first-generation whitening materials. Participants were instructed to wear the guard at night or during the day for 2–4 hours for 6 weeks of total treatment time. Haywood et al. (1994) reported that 92% experienced successful lightening of their teeth. There were several categories of patients: (1) aging patients with inherent discoloration, (2) patients with trauma, (3) patients with fluorosis, and (4) patients with tetracycline staining. Of the first category, 100% of patients experienced tooth lightening compared with 80% of those with brown fluorosis and 75% of the tetracycline patients. Successful results could be seen within 20 hours of treatment.

The application technique relies on the mouthguard to keep the whitening agent in contact with the tooth so that the whitening product can penetrate through the enamel. The whitening process is thus dependent on the time for which the whitening agent is in contact with the tooth. It was noted that the more times the whitening tray was replenished, the greater the likelihood of sensitivity. Increase in the incidence of sensitivity was noted when the whitening technique was used twice per day rather than once per day (Leonard 1998).

Using Opalescence whitening material, Matis and coworkers (1999) have shown that carbamide peroxide degrades in an exponential manner after the first hour and that the degradation rate is higher in areas closer to the tooth structure. After 2 hours, 50% of the active ingredient of the whitening material was available and 10% was still available after 10 hours (Matis et al. 1999; see Figures 3.8 and 3.9). Longer treatment times may thus be advisable—that is, the agent needs to be active for extended periods of time; to get maximum use out of the whitening agent, it is preferable to sleep with the tray in position. The whitening potential of a material is thus an important factor to consider. Once this level has been determined, tray wearing times can be scientifically calculated.

Patients who have tetracycline staining may need to whiten their teeth for 3, 6, or 9 months or longer to achieve successful lightening. With extension of the treatment time, the efficacy rate for patients with tetracycline staining improved to 90% (Haywood et al. 1997).

Table 3.5 lists the properties of the ideal whitening agent, and Table 3.6 summarizes the factors affecting the rate of color change.

EFFECTS ON ENAMEL

Surface texture

Most scanning electron microscopy studies of enamel surfaces treated with CPS whitening agents have shown little or no change in morphology (see Figures 3.16–3.18).

It is thought that the enamel surface remains intact and unaffected by the CPS and the whitening process (Haywood et al. 1991). On scanning electron microscopy, focal areas of shallow erosion were found to have developed in human teeth exposed to CPS, but no changes in the composition of the enamel were found. One study testing 16% and 35% CPS, however, reported significant changes in the enamel, including loss of the aprismatic layer, exposure and demineralization of enamel prisms, and pitting (Bitter 1995).

Surface hardness and wear resistance

Enamel surface hardness is apparently unaffected by the whitening agent (Zalkind et al. 1996, Kelleher and Roe 1999). However, a study using a whitening/remineralization cycle showed that 10% carbamide peroxide treatment significantly decreased enamel hardness. The application of fluoride improved remineralization of enamel. The reduction of hardness may reflect the loss of mineral from enamel, which could also result in reduced wear resistance (Seghi and Denry 1992). The researchers also showed that there was a change in the fracture toughness of the enamel (McCracken and Haywood 1996). Whitening may reduce the hardness of the enamel surface, and that may be more readily detected with instrumented low-load testing systems. This hardness reduction may arise as a result of degradation or denaturation of enamel matrix proteins by the peroxide oxidation (Elfallah and Swain 2013).

Enamel microhardness

A systematic review conducted by Attin showed interesting results. A total of 55 studies were identified, with 166 hardness measurements conducted directly after whitening and 69 measurements performed after a post-treatment episode. Directly after whitening, 84 treatments (51%) showed microhardness reduction compared with baseline, whereas 82 (49%) did not. After the post-treatment episode, 20 treatments (29%) showed hardness reduction and 49 (71%) did not. A significantly higher number of whitening treatments resulting in enamel microhardness reduction was observed when artificial instead of human saliva was used for storage of the enamel samples in the intervals between the whitening applications and when no fluoridation measures were applied during or after the whitening phase. Significantly, in those studies that simulated the intraoral conditions as closely as possible, the risk of enamel microhardness decrease as a result of whitening treatments seems to have been reduced. Nevertheless, more in situ and in vivo studies are needed to verify this observation (Attin et al. 2009).

Chemical composition

There may be loss of organic components from treated enamel surfaces—carbon, hydrocarbon, and tertiary amine groups replaced by oxygen, calcium, and phosphorus. The calcium/phosphate ratio of dentin was significantly decreased by whitening with 30% hydrogen peroxide and 10% carbamide peroxide in a study by Rotstein and colleagues (1996). In a study by McCracken and Haywood (1996), teeth exposed to CPS for 6 hours lost an average of 1.06 µg/mm² of calcium. This amount was significantly greater than in controls. However, this amount is small and the results may not be clinically significant. Drinking one can of a cola drink produced a comparable calcium loss of about 1 µg/mm². These results are consistent with calcium loss from enamel after 2-min exposures to carbonated cola, orange juice, apple juice, or diet carbonated cola (Grobler et al. 1990). The potential for remineralization occurs in vivo and may counteract these effects, but these have not been studied for CPS yet.

Some of the OTC whitening agents have a very low pH (5.6), and this may cause erosion of the enamel. The toothpaste provided with the kit may be abrasive to the tooth surface (Jay 1990). There is the potential side effect that the teeth can be etch-whitened (Bartlett and Walmsley 1991).

EFFECTS ON DENTIN

Tooth color is primarily determined by the dentin and can be changed by whitening treatments. In an in vitro study (McCaslin et al. 1999) using 10% carbamide peroxide placed directly onto the enamel to validate the color change in dentin and to assess whether dentin changed uniformly, it was noted that a color change occurred throughout the dentin and the color change was uniform.

Dentin bonding may be altered after whitening (Della Bona et al. 1992) and the smear layer may be removed (Hunsaker et al. 1990). The bonding between glass ionomer and dentin may also be affected (Titley et al. 1991). This may be a result of the precipitate of hydrogen peroxide and collagen that forms on the cut dentin surface after tooth whitening. It is suggested that adhesive dentistry be delayed for 2 weeks after whitening (Powell and Bales, 1991). (See further discussion later.)

EFFECTS ON PULP

Pulp penetration during whitening varies significantly among commercial 10% carbamide peroxide whitening products (Thitinanthapan et al. 1999), which may result in different levels of tooth sensitivity or whitening efficacy. Pulp penetration can occur within 5–15 minutes according to studies by Cooper et al. (1992). The potential for pulpal damage thus exists as a result of enamel and dentin penetration (Powell and Bales 1991). There appears to be less penetration into the pulp from carbamide peroxide than from hydrogen peroxide. A 3% solution of H₂O₂ is capable of causing a transient reduction in the pulpal blood circulation and occlusion of pulpal blood vessels (Robertson and Melfi 1980). The sensitivity is transient and lasts only for the duration of the whitening treatment (Basting et al. 2012). There is no long-lasting sensitivity.

Although the pulp is highly resilient to indirect insult from restorative materials, there is a danger that patients who are overzealous to achieve faster whitening may cause undesirable consequences (Minoux and Serfaty 2008). The most common side effect experienced by patients using the home whitening technique is transient, mild temperature sensitivity (Heymann et al. 1998) during the first hour after treatment. The sensitivity appears to be dose related rather than pH related. In a study by Scherer et al. (1991) the patients who experienced transient tooth hypersensitivity after week 2 had overloaded their trays. The studies appear to support the clinical observation that controlled home whitening is safe to the pulp (Li 1998, Kelleher and Roe 1999).

An evidence-based evaluation (Cochrane review by Hasson et al. 2006) that assessed 416 articles on tooth whitening noted that strips (5.5% to 6.5% hydrogen peroxide) are more effective than gel in tray with 10% carbamide peroxide (mean difference 1.82; 95% confidence interval [CI] 0.26–3.38). Mild to moderate tooth sensitivity and gingival irritation were the most common side effects. The whitening strips and products with high concentrations of hydrogen peroxide caused more users to report tooth sensitivity. There is evidence that whitening products work when compared with placebo or/no treatment. Hasson et al. (2006) concluded that there are differences in efficacy among the products, mainly resulting from the levels of the active ingredients, hydrogen peroxide and carbamide peroxide. All trials were, however, short term, and most were judged to be at high risk of bias and were either sponsored or conducted by the manufacturers.

EFFECTS ON CEMENTUM

It appears from recent studies that the cementum is not affected by the materials used for home whitening (Rotstein and Friedman 1991, Murphy et al, 1992). A study by Scherer et al. (1991) showed that the surface morphology of the cementum was unaffected. Cervical resorption (Latcham 1986, Madison and Walton 1990) and external root resorption (Cvek and Lindvall 1985) have been reported in teeth whitened by the internal whitening technique using 35% hydrogen peroxide. In the latter study most of the teeth were associated with previous trauma, and it is not known whether the trauma predis-posed the tooth to the resorption or whether it was caused by the effects of the whitening agent. pH measurements of the root surface have demonstrated cervical resorption occurring in those teeth that were not previously traumatized.

WHITER, BRIGHTER, OR LIGHTER?

The penetration of oxygen inside the tooth gives the tooth a whiter appearance. When the oxygen becomes saturated inside the tooth, it appears bright. Two weeks after termination of whitening, the tooth appears less bright because the oxygen is dissipating from the tooth, and the shade settles to the lighter shade. There may be a decrease in the translucency of the tooth. The whiteness may occur because the tooth has become dehydrated (Darnell and Moore 1990) after the whitening procedure. This may be a transient effect. It is not apparent what effect or combination of effects is occurring. A brightness index derived from computer analysis of digitized images may be useful for monitoring effectiveness of whitening (Bentley et al. 1999).

PATIENT RESPONSE TO HOME WHITENING

Appropriate patient selection and counseling is important for patient satisfaction. In a longitudinal study by Haywood et al. (1994), at 1.5 years after treatment 74% of patients who had responded to home whitening were satisfied with the shade of their teeth. At 3 years after treatment 62% were satisfied with their color. At 7 years after treatment of the same patient pool, 35% were satisfied with the color of their teeth. No one reported reversion to the original shade (Leonard et al. 1998a).

Patient perceptions of the whitening technique are positive. Ninety-five per cent of patients are genuinely glad they went through the procedure and 97% recommended the procedure to their friends (Leonard 1998). Of the patients surveyed in the latter study, 87% said they would undergo whitening again.

COLOR REGRESSION AND SHADE RELAPSE

Once whitening has been terminated, a slight relapse in color occurs over the following 2 weeks. It has been hypothesized that the tooth is filled with oxygen from the oxidative process and this changes the optical qualities of the tooth to appear more opaque. After 2 weeks, the oxygen has dissipated and the tooth demonstrates the actual lightened shade. Patients should be informed of this phenomenon because they tend to think that the whitening is regressing; in reality the teeth are equilibrating to the new actual shade (Haywood 1999b). Color regression occurred within the first month after whitening in a study by Matis and coworkers (1998).

The process of color regression toward darker shades is poorly described and understood in the literature (Heymann et al. 1998), but is thought to be the opposite of the whitening procedure. Regression occurs over a longer period because some of the previously oxidized substances may become chemically reduced and cause the tooth to reflect the old discoloration or the enamel may become remineralized with the staining molecule of the original stain (Lyons and Ng 1998).

A clinical trial (Leonard et al. 1999) evaluating color stability after 54 months of tetracycline-stained teeth that were treated with 10% carbamide peroxide and extended whitening times showed that it is possible for the color to remain stable for 54 months after whitening treatment. The color stability may be related to the extended treatment time of 6 months. This is the longest post-treatment clinical study published. No patient felt the need to have the teeth re-treated as a result of color regression.

A 4% color regression after 6 months has been reported in nonvital whitening (Ho and Goerig 1989). The considerable application time with home whitening techniques may explain the minimal color regression reported. Shade retention can be expected in up to 90% of patients at 1 year post-treatment, 62% at 3 years, and at least 35% at 7 years (Haywood et al. 1994). Patients in the latter study rewhitened on average at 25 months. The shade never reverts to the original shade.

The success rate for home whitening using a viscous whitening material for 7–10 days is about 95% (Haywood et al. 1994). With changes to the treatment time and/or concentration of the whitening material, the success rate for tetracycline-stained teeth is 90%.

RESPONSE OF THE STAINS TO WHITENING

The initial color of the affected teeth seems to determine the success or failure of the technique (Arens 1989). It appears that the lighter the tooth, the easier it is to whiten. Yellow stains are the easiest to whiten. Less responsive stains in order of decreasing responsiveness are light gray (see Figures 3.10 and 3.11), light brown (see Figure 3.1), dark yellow, dark brown, gray, and black (Swift 1988). Length of whitening time is an important predictor of success in teeth that have fluorosis staining (Seale and Thrash 1985). Seale and Thrash suggested that because of increased porosity, younger teeth would be easier to whiten than older ones (see Figure 3.2). Older teeth, because of the more complex dentin structure, take longer to lighten (see Figures 3.1, 3.10, and 3.11). The whitening period may need to be extended to 6–10 weeks to achieve significant lightening. Some parts of the tooth may be difficult to whiten. A dark black crack line on the central incisors caused by grinding and smoking may not be lightened with whitening (see Figure 3.12). Teeth with extreme wear and older yellow teeth may take longer to whiten. Although patients with severe wear on their teeth may request whitening, further restorative dentistry may be more appropriate. Whitening may be the beginning of the treatment plan, and then further restorative dentistry may need to be carried out (see Figure 3.14). Erosive dentin lesions may not whiten sufficiently, and it may be inappropriate to whiten areas of deep erosion. Other restorative treatment may be more appropriate for these patients (see Figure 3.15).

EFFECTS ON RESTORATIVE MATERIALS

Initially, no effects on restorative materials were reported (Haywood et al. 1991c). Recent studies have shown that surface effects may occur on existing restorative materials (such as composites, glass ionomers and, luting cements). A recent in vitro study of the effects of carbamide peroxide on provisional crowns showed that an orange discoloration occurred with provisional materials containing methacrylate (Robinson et al. 1997). Amalgams may appear to become greenish during the whitening treatment because of the copper inside the amalgam.

BOND STRENGTH TO ENAMEL

Whitening is frequently used in combination with other forms of restorative dentistry that require bonding to enamel. These procedures may include replacement of existing restorations to improve shade matching, diastema closure, and placement of veneers (see Chapter 15). All these techniques rely on adequate adhesion of the composite to the enamel. Any factor compromising adhesion can affect the aesthetics and longevity of the bonded restoration (Cvitko et al. 1991).

As we have seen, whitening with peroxide reduces the bond strength of enamel by about 20% in the immediate effect. Studies by Torneck et al. (1991) have shown that there is a reduced bond strength of composite to enamel immediately after whitening with a 35% solution of hydrogen peroxide. However, the bond strength will improve if etching and bonding are delayed for 1–2 weeks postwhitening (Titley et al. 1991, Godwin et al. 1992). Waiting 2 weeks will also allow the color to stabilize. Hydrogen peroxide appears to change the surface chemistry. Resin tags in whitened enamel are less numerous, less defined, and shorter than those in unwhitened enamel (Titley et al. 1991). The residual oxygen in the whitened tooth surface also inhibits the polymerization of the composite resin (McGuckin et al. 1991) and disrupts the surface (Haywood 1999a).

Studies using 10% CPS also showed that the bond strength of composite to etched enamel was reduced (Cvitko et al. 1991). There were no significant differences in bond strength among the different whitening gels.

The reduced bond strength is transient; it diminishes after 24 hours and disappears after 1 week (Della Bona et al. 1992). The use of topical fluoride after whitening may help to regain the bond strength (Haywood 1991c). The use of acetone- or alcohol-based adhesive systems or roughening the surface may counteract these effects of peroxides on bond strength. More conservative enamel removal may be sufficient to counteract these effects, but further research is needed. Internal whitening of endodontically treated teeth has been shown to result in greater leakage of composite restorations immediately after completion of internal whitening (Barkhorder et al. 1997). It may be more appropriate to wait for 2 weeks before placing the composite restoration and to place a glass ionomer restoration directly into the access cavity immediately after completion of the whitening treatment to prevent the reduction in bond strength.

However, recent studies have shown that application of ascorbic acid in a gel form for 60 minutes may improve the bond strength with immediate effect (Kaya et al. 2008). Miranda et al. (2013) showed that compromised bond strength to whitened enamel was immediately restored with the application of antioxidant gel sodium ascorbate and exposure to human saliva in situ for at least 7 days. Best results were obtained with exposure to human saliva in situ for 14 days. Treatment with sodium ascorbate gel for 60 minutes may be recommended if patients cannot wait for at least 7 days for adhesive techniques to be performed. However, clinically this may be difficult to achieve because patients may not tolerate the application of ascorbic acid to the teeth for such a long period of time. This effect of the acid needs further research to ensure that there is no permanent erosive effect on the teeth.

COMPOSITE MATERIAL

Reports on the effects of whitening on composite resins are conflicting. Some studies have shown that composites are unaffected by CPS (Haywood et al. 1991, Baughan et al. 1992, Machida et al. 1992). Others have shown that surface hardness is altered (Bailey and Swift 1991, Friend et al. 1991), and another study showed that surface hardness was unaffected by whitening (Nathoo et al. 1994). Surface roughening and etching may occur (Singleton and Wagner, 1992) and tensile strength is affected (Cullen et al. 1993). However, these effects are unlikely to be clinically significant (Swift 1998).

Haywood and Heymann (1991) have noted no significant color changes other than the removal of extrinsic stains around existing restorations. The effect is primarily the superficial cleansing of the restoration and a lightening of the underlying tooth structure and not an intrinsic color change of the restorative material itself. Whitening has been shown to increase the microleakage of existing restorations. Restorations may need to be replaced after whitening owing to the color change in the tooth.

AMALGAM RESTORATIONS

Although there have been few reports of effects on amalgam restorations, studies (Hummert et al. 1992) suggest that there may be significantly more mercury released from amalgam restorations during the whitening procedure. Rotstein and coworkers (1997) found than 4–30 times as much mercury was released from amalgams in vitro compared to saline controls. Further clarification of this is required. It appears that prolonged treatment with whitening agents may cause microstructural changes in amalgam surfaces and this may possibly increase exposure of patients to toxic byproducts (Rotstein et al. 1997). Existing amalgams may change color from black to silver (see Figure 3.19). This effect is dependent on the type of dental amalgam used (Rotstein 1999, personal communication). However, not all combinations of amalgam and whitening agents result in higher mercury levels. Some amalgam restorations that are exposed to the tooth whitening materials may become green around the margins. This may be a result of the copper content of the amalgam (Haywood 2007).

OTHER MATERIALS

Fired porcelain showed a slight change after being immersed in the whitening gels for three 2-hour periods a day for 5 weeks (Hunsaker et al. 1990). No effect on gold has been reported. Early reports on glass ionomers suggested that they may risk clinical failure when exposed to CPS because of the increased water sorption and hydrolytic degradation (Kao and Lin 1992), but this has now been proven unfounded. There appears to be an alteration in the matrix of the glass ionomer (Jefferson et al. 1991). Other luting cements may also be affected. Analysis by scanning electron microscopy has revealed erosion of the matrix of the luting cement, and there was some degree of crystalline formation (Jefferson et al. 1991) of the zinc phosphate cement.

Provisional restorations such as those using intermediate restorative material (IRM) may be affected by hydrogen peroxide (see Figures 3.20 and 3.21). Macroscopically, IRM exposed to hydrogen peroxide appears cracked and swollen, whereas carbamide peroxide does not appear to have an effect. Provisional crowns made from methyl methacrylate may discolor and turn orange. Polycarbonate crowns and bis-acryl composite temporary materials do not discolor (Robinson et al. 1997).

OTHER PROPERTIES OF CARBAMIDE PEROXIDE

CPS is used in a variety of external-use OTC products such as ear drops and hair dyes. Patients with known sensitivity or allergy to any of the ingredients should not use the whitening agent. Allergic symptoms have been reported, such as swelling of the lips (Goldstein and Garber 1995).

ORAL USES AND EFFECTS

Carbamide peroxide also has an antibacterial effect (Gugan et al. 1996; see section on therapeutic aesthetics). It reduces plaque adherence and accumulation and therefore has been used for treatment of periodontitis, oral hygiene (Stindt and Quenette 1989), reduction of gingivitis, reduction of caries rate, and aphthous ulceration (Tse et al. 1991). It has been used since 1960 for oral wound debridement (Fasanaro 1992). CPS has been incorporated into a chewing gum with inhibition of plaque formation (Moss 1999). It has also been tried for treatment of recurrent herpes labialis. It has been tested for use as an irrigant or lubricant in root canals and as an adjunctive to sodium hypochlorite (Stindt and Quenette 1989). It was also used for post-operative rinsing after tooth extraction. The studies using CPS have shown promising results.

SYSTEMIC SIDE EFFECTS CARBAMIDE PEROXIDE

The only systemic effects that have been reported have been one case that produced a mild laxative effect, and mild gastric irritation reported by some patients who are using the treatment for whitening of tetracycline discoloration.

Owing to the possibility of overuse of CPS by the patient at home, it is prudent to ensure that the patient follows the prescribed regimen for tray wearing. Encourage the patient to refrain from smoking during treatment (Larson 1990). Dentists should encourage their patients to stop smoking for general health improvement and to enroll in a smoking cessation program. This would reduce the associated staining.

There is also the possibility of an allergy to the whitening materials or to the tray plastic material. One patient experienced allergies to the preservative in the whitening tray. This patient experienced swelling of the face 1 week after whitening the upper teeth when she began to whiten the lower teeth. Whitening was terminated at that point with immediate effect. Occasionally an allergy to hydrogen peroxide may be noted. This is very rare. There is no known cross-reaction between the hydrogen peroxide materials used for highlighting hair and tooth whitening materials. Table 3.7 lists the possible side effects.

The majority of the literature and research indicates that the use of 10% carbamide peroxide for dentist-monitored home whitening is an effective and safe way to lighten discolored teeth (see Figure 3.1).

WHITENING TOOTHPASTES

Toothpastes have become more specialized and sophisticated in the last decade (Koertge et al. 1998) with many designed to perform either therapeutic or cosmetic functions. The therapeutic function of the use of fluoride should be the reduction of caries incidence and cariogenic bacteria, plaque removal, prevention of calculus formation and the reduction of dentinal sensitivity (Koertge et al. 1998). Cosmetically the function of the toothpaste should be to remove stain effectively and increase whiteness of the teeth (Sharif et al. 2000).

The introduction of whitening toothpastes has been very rapid. There has not been that much clinical research conducted on these products, which concerned the American Dental Association Council on Therapeutics (1994). There are many questions about these products, particularly whether they are effective at maintaining a white smile after whitening and delay the color regression. The whitening toothpastes seem effective at removing surface stains on the teeth and may be useful to reduce extrinsic stains that occur from tea and coffee. They may be used to maintain white teeth after whitening.

Table 3.8 shows a classification of whitening toothpastes according to their mechanism of action (Haywood 1996).

MORE ABRASIVE THAN USUAL

The abrasive toothpastes try to remove the surface staining by ‘sanding’ the teeth. The paste toothpaste is more abrasive than the gel toothpaste. The overzealous use of abrasive toothpaste will cause the removal of enamel as well as the stain. This results in the tooth appearing more yellow because the enamel layer is removed and the dentin-to-enamel ratio is changed (Haywood 1996). The combination of a hard toothbrush and an abrasive toothpaste has been recognized as creating further tooth wear problems.

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