Tooth Sensitivity and Whitening

Tooth sensitivity and gingival irritation are the most common side effects of tooth whitening procedures.¹ Of the two, tooth sensitivity occurs more frequently but is poorly understood by clinicians and researchers.

Holland et al² described dentin hypersensitivity as being “characterized by short, sharp pain arising from exposed dentin in response to stimuli typically thermal, evaporative, tactile, osmotic, or chemical and which cannot be ascribed to any other form of dental defect or pathology.” This type of discomfort most often is associated with exposed cervical dentin. Tooth sensitivity from whitening procedures manifests itself in a similar manner but can and does occur in the absence of any exposed dentin, so it is not known whether it represents precisely the same phenomenon.

This article reviews general concepts regarding tooth sensitivity of the classic exposed-dentin type and addresses the incidence and severity of tooth sensitivity associated with patient-applied whitening agents. It also discusses the possible causes of tooth sensitivity with whitening procedures and the various predisposing factors that have been identified.

Basic Concepts of Tooth Sensitivity

The most widely accepted explanation of dentin hypersensitivity is Brännström’s “hydrodynamic theory.”³ According to this theory, various thermal, mechanical, evaporative, and osmotic stimuli can cause a rapid outward flow of fluid in the dentinal tubules.^2-5 Rapid fluid flow causes a pressure change across the dentin, stimulating pulpal A-d nerve fibers and resulting in the perception of pain. Cold is the most common stimulus for dentin hypersensitivity.^2-5

Dentin hypersensitivity is far more likely to occur when the tubules are open and in communication with the oral environment. A study by Absi et al has shown that hypersensitive dentin contains larger and more numerous tubules than normal nonsensitive dentin.⁶ The combination of greater and larger tubules permits a substantial increase in dentinal fluid flow and therefore can amplify the severity of the response to an external stimulus. The increase in fluid flow with increasing tubule size is greater than might be expected because the area of a circle (which dentinal tubules are, in cross section) increases by the square of the radius.⁵

Studies concerning the prevalence of cervical dentin hypersensitivity have reported that 4% to 57% of adults experience cervical dentin hypersensitivity in one or more teeth.^7-12 The prevalence of hypersensitivity is substantially higher (60% to 98%) in periodontal patients.^11-13 The most common locations for dentin hypersensitivity are cervical areas on the facial surfaces of the canines and first premolars, followed by the second premolars and the incisors.^4,14

Because such large ranges are reported in the literature, the real prevalence of dentin hypersensitivity is not precisely known; however, it probably is not only widespread but also underdiagnosed and undertreated. A 2002 international survey of 11,000 adults reported prevalence rates of 37% to 52%, depending on the region.⁴ This survey also revealed that only half of affected individuals reported that they had talked to their dentist about their sensitive teeth, and that only half of those individuals received a treatment recommendation.

Many patients believe that this problem is a minor annoyance not worthy of the dentist’s attention, and some dentists might feel the same way. A surprisingly large proportion of dentists lack knowledge of the problem, its etiology, and treatment alternatives.¹⁵ A better understanding of dentin hypersensitivity is important for dental professionals, especially as more people are experiencing a similar type of sensitivity with tooth whitening. Because of the already high prevalence of dentin hypersensitivity and the increasing use of whitening products, dentists and hygienists routinely should not only ask their patients whether they are experiencing this problem but should also provide appropriate treatment recommendations.

Etiology

Gingival recession frequently is cited as a “cause” of cervical dentin hypersensitivity. However, it should be considered a predisposing factor rather than a direct cause.⁴ Gingival recession does not cause cervical hypersensitivity but rather sets the stage for it by exposing root surfaces to the oral environment.

Similarly, loss of enamel in the cervical area predisposes a person to, but does not cause, dentin hypersensitivity. Enamel can be lost through friction (attrition or abrasion), chemical erosion, or stresses generated by occlusion (abfraction). The processes of attrition, abrasion, erosion, and abfraction do not occur in isolation, and most cervical wear probably results from a combination of factors.⁴

Exposure of dentinal tubules—either through gingival recession and subsequent loss of cementum or through wear of enamel—is required for cervical dentin hypersensitivity to occur. However, dentin exposure does not inevitably result in hypersensitivity. A number of factors contribute to hypersensitivity with exposed dentin, including ingestion of acidic beverages and foods, use of abrasive or tartar-control dentifrices, overzealous or poor brushing technique, and brushing immediately after ingesting an acidic beverage or food.^4,14,15

Treatment

Treatment methods for cervical dentin hypersensitivity can be classified either by method of application (self-applied or professionally applied) or by mechanism of action (nerve depolarization or occlusion of dentinal tubules).With very few exceptions, self-applied products are designed to depolarize nerves and professionally applied products occlude tubules.

Proper application of a desensitizing dentifrice can provide relief for many cases of cervical dentin hypersensitivity. The active ingredient in these dentifrices is 5% potassium nitrate, which is believed to penetrate the dentinal tubules, depolarize the nerves, and prevent them from repolarizing.^16,17 Therefore, the dentinal tubules remain patent and rapid fluid flow still occurs in response to stimuli, but the nerves are not activated. For best results, desensitizing dentifrices must be used twice daily as part of an ongoing brushing routine.^4,5

Dental professionals tend to have differing opinions on the efficacy of desensitizing dentifrices, and the relevant clinical data do not demonstrate absolutely conclusive proof of effectiveness. For example, a Cochrane meta-analysis of four randomized clinical trials of potassium nitrate dentifrices, while noting that the studies did report reductions in sensitivity, concluded that there was no clear evidence of significant efficacy.¹⁸

However, numerous individual clinical trials have reported that potassium nitrate dentifrices are effective for reducing cervical dentin hypersensitivity.^17,19-21 It should be noted that a placebo effect is quite possible in studies of this type, and that most studies directly comparing different dentifrices are sponsored by one manufacturer or another.^4,14 Such factors, and the subjective nature of pain, make the results of some studies difficult to interpret. Nevertheless, a 2003 Canadian consensus paper on the diagnosis and management of cervical dentin hypersensitivity listed potassium nitrate dentifrices as a treatment method likely to provide improvement in a majority of cases.¹⁵ These dentifrices are an attractive first option for treatment because they are noninvasive, treat multiple teeth simultaneously, and are very inexpensive compared to professional treatments. They are indicated especially for cases of mild to moderate hypersensitivity.¹⁴ As mentioned previously, a desensitizing dentifrice must be used regularly to achieve the best results.

In contrast to desensitizing dentifrices, almost all professionally applied desensitizers work by occluding the dentinal tubules. Many products contain compounds such as potassium oxalate that react with ions in the dentin surface to produce a crystalline precipitate that occludes the dentinal tubules.^22-24 This is a reasonable approach, because the hydrodynamic mechanism of cervical dentin hypersensitivity requires the presence of open tubules.^3-6 Laboratory studies have shown that these agents can reduce dentin permeability,^25,26 and clinical trials have demonstrated efficacy for some.^27,28

Resin-based dentin adhesives are another method for sealing dentin surfaces.²⁹ These materials impregnate the dentin, occlude the tubules, and form a polymeric coating on the surface. The coating tends to be relatively thin (about a few microns) and therefore is susceptible to abrasion. In clinical trials, several resin adhesives have demonstrated significant reductions in cervical dentin hypersensitivity.^30-32 While not an adhesive, one resin-based desensitizing material, originally developed as the primer component of an adhesive system, is an aqueous solution of 35% ydroxyethylmethacrylate (HEMA) and 5% glutaraldehyde. It is believed that this desensitizer occludes the dentinal tubules internally, perhaps by coagulation of plasma proteins in the dentinal fluid.³³ Clinical trials have shown that the combination of HEMA and glutaraldehyde is an effective treatment for cervical dentin hypersensitivity.³¹

Interestingly, Clinical Research Associates conducted a field test of various professionally applied desensitizers and found that all of them reduced sensitivity for most patients.³⁴ However, none provided relief to 100% of the patients treated.

Sensitivity with Tooth Whitening

Clinical Presentation and Incidence

The modern era of tooth whitening began with Haywood and Heymann’s 1989 article on nightguard vital bleaching.³⁵ Their article described a method of whitening the teeth using a custom-fitted plastic mouthguard to apply a 10% carbamide peroxide gel (equivalent to approximately 3.5% hydrogen peroxide). Since that time, numerous tray-applied whiteners containing either carbamide or hydrogen peroxide have been introduced, and dentist-prescribed, patient-applied, at-home tooth whitening has proved to be safe and effective.^1,36-41

More recently, strips coated with hydrogen peroxide have been developed as both over-the-counter (OTC) and professionally dispensed, patient-applied whitening products. Like the tray-applied gels, the strips have proved to be safe and effective.^42-44 Numerous other OTC products, including paint-on materials, are available but have been studied less extensively.^45,46

In-office or “power bleaching” methods, typically using a high concentration of hydrogen peroxide in conjunction with an intense light source, are another alternative for tooth whitening. However, this article will address sensitivity issues only with the patient-applied whitening products.

Tooth sensitivity is the primary side effect associated with at-home tooth whitening procedures.¹ Based on the author’s observations, sensitivity typically begins early in the whitening process, sometimes during the first or second day. It is most commonly associated with thermal changes (especially cold) and usually is mild in severity and transient in nature. The transient nature of this particular type of tooth sensitivity is 3-fold. First, when the sensitivity occurs, it is typically very brief. Second, most subjects in clinical trials who report sensitivity also report that it occurs only during portions of the treatment, not continuously or even every day. Third, when the whitening regimen is completed, any lingering sensitivity resolves almost immediately.

Although tooth sensitivity associated with tooth whitening has been described in the literature as a “reversible pulpitis,” the precise cause of the sensitivity is unknown.⁴⁷ Studies have shown that application of a peroxide gel to the external surface of a tooth will result in very rapid penetration of peroxide to the pulp chamber.^48,49 The peroxide concentration can greatly exceed the level at which damage to pulpal enzymes potentially could occur.⁵⁰ However, one clinical study—in which teeth were whitened, extracted for orthodontic reasons, and examined histologically—revealed that some minor pulpal irritation did occur but that it was resolved within 2 weeks after cessation of treatment.⁵¹

This histologic observation is in accordance with the clinical observation that tooth sensitivity may last for as little as 1 day and normally resolves completely within a day or two after the whitening regimen is completed. In fact, it is not uncommon for the sensitivity to occur early in the whitening process and then resolve spontaneously, well before the treatment regimen has finished.

Studies of pulpal responses to stimuli such as electric pulp testing or cold have shown no differences in whitened and unwhitened teeth during treatment or—as reported in the longest published post-bleaching study—up to 12 years later.^52-54 Clinical trials of extended whitening regimens (eg, daily whitening of tetracycline-stained teeth for 6 months or more) have reported a high prevalence of tooth sensitivity, but it does not persist after treatment, and these studies have revealed no evidence of irreversible pulpal damage.^55-57

As reported in the clinical trials literature, the incidence of tooth sensitivity associated with at-home tray-based tooth whitening varies greatly. Of course, it is probably worth noting that the primary outcome measure of most clinical studies has been effectiveness—ie, how well does Product X whiten the teeth? Sensitivity often is mentioned only as a secondary outcome and sometimes is not reported at all.

The reported incidence of tooth sensitivity in clinical trials of whitening varies widely, from as low as 0% to 7% to as high as 75%.^{1,36,38,39,58,59} Although some studies have reported occasional subject dropouts because of tooth sensitivity,⁵³ nearly all sensitivity (~80% of occurrences) is described as “mild.” A recent clinical trial and literature review concluded that mild sensitivity can be expected to occur in 54% of patients, moderate sensitivity in 10%, and severe sensitivity in fewer than 5%.⁶⁰ In other words, about two thirds of patients are likely to experience at least some tooth sensitivity at some point during the whitening process. (As a point of reference, this study evaluated a 15% carbamide peroxide gel containing fluoride that was applied 3 to 4 hours per day over a 4-week period.)

Interestingly, tooth sensitivity rates of up to 20% to 30% have been reported with placebos, which suggests that the sensitivity is not related strictly to the peroxide content of whitening gels.⁴⁷ One example of this was reported in a clinical trial by Matis et al, who compared a 10% carbamide peroxide gel with a placebo gel.³⁹ This study included 5 categories of subject-reported sensitivity—none, slight, moderate, considerable, and severe. With the whitening gel, the percentages of subjects in each category were 45%, 10%, 28%, 7%, and 10%, respectively. For the placebo gel, the percentages were 80%, 10%, 10%, 0%, and 0%. In summary, 55% of subjects in the active group experienced at least some tooth sensitivity, but so did 20% of subjects in the placebo group. A similar study by Leonard et al reported tooth sensitivity in 58% of subjects in the active group and in 34% of subjects in the placebo group.⁵⁸

Predisposing Factors

The exact etiology of tooth sensitivity associated with whitening is unknown—is it a reversible pulpitis of the sort associated with a deep carious lesion, for example, or does it more closely resemble a dentinal tubule phenomenon of the sort associated with exposed cervical dentin? It is this author’s opinion that whitening sensitivity is more like cervical sensitivity, in that neither can “be ascribed to any other form of dental defect or pathology.”² Undoubtedly, we will have a better understanding of the histological events underlying this phenomenon in the future but, for the moment, we can only identify factors that contribute to tooth sensitivity with whitening. Such factors can be classified into two groups—those related to the patient, and those related to the treatment agents. In any given case, the etiology of sensitivity probably is multifactorial.

With regard to the patient, several factors have been ruled out as potential contributing factors in tooth sensitivity. Among these are gender, age, tooth condition (with one possible exception, as discussed below), pulp chamber size, and systemic allergies.⁶¹ The best predictor of tooth sensitivity with tooth whitening appears to be a history of tooth sensitivity. Therefore, Leonard et al recommend asking patients before initiating whitening whether their teeth are sensitive to hot or cold or after receiving a prophylaxis.⁶¹ They also note that a gentle blast of compressed air might help to determine the likelihood of sensitivity.⁶¹

Most studies have found no association between gingival recession (ie, exposed root surfaces) and tooth sensitivity with whitening. This is counterintuitive, because it is expected that teeth with exposed root surfaces would be more susceptible to irritation by the whitening gel. The lack of a clear association between exposed dentin and tooth sensitivity with whitening might be related to the fact that peroxide penetrates the tooth very easily, regardless of whether enamel is present or not. It should be noted, however, that at least one study has reported that gingival recession predisposes a person to sensitivity.⁶⁰

With regard to the whitening treatment agent, several factors have been identified as possible contributors to tooth sensitivity. The first is more frequent application of the gel during the period of use, specifically using more than one fresh application of gel during a single day.⁶¹ Also, longer applications and higher peroxide concentrations can cause more frequent and severe sensitivity. While some studies have reported the greater sensitivity experienced with higher concentrations only as a trend, others have demonstrated a clear correlation between concentration and sensitivity.^{40,41,56,62,63}

Some of the early literature on tooth whitening mentioned the pH of the whitening gel as a factor in tooth sensitivity. Dentist-supervised whitening products do have a slightly acidic pH (mean of 6.48, range of 5.66 to 7.35 in one study).⁶⁴ However, nothing in the literature appears to link pH and tooth sensitivity with dentist-supervised, patient-applied products.

Other aspects of whitening gel composition might have an effect on tooth sensitivity. Products vary not only by active ingredient—ie, type (carbamide or hydrogen) and concentration of peroxide—but also by inactive ingredients such as the base vehicle, thickeners, and flavorings.⁶⁵ The fact that placebo gels cause sensitivity in some patients suggests that the inactive ingredients contribute to sensitivity, at least to a minor extent. For example, most at-home whitening gel products contain glycerin, a desiccant that could potentially cause fluid movement in the dentinal tubules, resulting in tooth sensitivity.

A few whitening gels contain additives such as potassium nitrate and fluoride that are specifically intended to reduce tooth sensitivity and have been shown to do so in clinical trials.^66,67

Some evidence indicates that the simple act of wearing a tray to apply the whitening agent can result in tooth sensitivity.⁶² This problem was more prevalent in the early days of at-home whitening when more rigid trays were commonly used. Today, most custom trays are made of a flexible, soft vinyl material. The use of reservoirs in these trays, which can reduce the tightness of the fit, has been cited as a potential means of reducing the likelihood of sensitivity. However, there is no hard evidence to support this; in fact, one study reported identical sensitivity rates using trays with and without reservoirs.⁶⁸

A variety of methods to prevent and treat sensitivity associated with tooth whitening have been proposed, and these are discussed elsewhere.

Conclusion

Dentin hypersensitivity is more prevalent than many dentists believe. Desensitizing dentifrices provide a convenient, inexpensive, and effective first line of defense for treating cervical dentin hypersensitivity. A variety of professionally applied topical agents also are available, but no single method has proved to be 100% effective.

Tooth sensitivity is the most frequent side effect associated with vital bleaching procedures. Although it tends to be mild and transient, it is also very common and is annoying to patients. Its presentation is similar to that of cervical dentin hypersensitivity and may involve a similar mechanism.

Disclosure

The author has received grant or research support from 3M ESPE, Heraeus Kulzer, DENTSPLY Caulk, and The Procter & Gamble Company. He also is a consultant for Procter & Gamble and has received honoraria from GlaxoSmithKline, Kerr, and DENTSPLY.

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About the Author

Edward J. Swift Jr, DMD, MS, Professor and Chair, Department of Operative Dentistry, University of North Carolina School of Dentistry, Chapel Hill, NC