INTRODUCTION

Tooth whitening nowadays is one of the most frequently requested cosmetic dental procedures by patients who want a “perfect white smile.”¹ Usually, vital tooth bleaching can be categorized generally as in-office (professionally administered), at-home (professionally dispensed) or over-the-counter (self-administered).

Advantages of in-office dental bleaching over at-home or over-the-counter bleaching techniques include professional control, avoidance of soft tissue exposure and material ingestion, reduced total treatment time, and the possibility of immediate results.^2,3 Most bleaching solutions contain hydrogen peroxide or carbamide peroxide as the active ingredient delivered through various carrier designs⁴ that are applied with or without light activation.

Proponents of light-activated bleaching claim that this procedure reduces total in-office bleaching time by energizing hydrogen peroxide through the use of various light sources.^1,2,5 The theoretical advantage is the ability of the light source to heat the hydrogen peroxide, thereby increasing the rate of decomposition of oxygen and accelerating the release of free radicals with higher kinetic energy, thus enhancing the rupture of stain-containing molecules.^6,7

Despite the fact that many curing lights have been introduced onto the dental market for the purpose of accelerating the bleaching process, the effectiveness of such an approach has been controversial.^1,8-16

One of the most common side effects associated with vital tooth bleaching is tooth sensitivity. Reports and estimates of bleaching-induced tooth sensitivity incidence range from 55% to 100%,^1,3,13,17,18 and the degree of tooth sensitivity in these reports ranges from very mild to intolerable. Existing literature reveals that activation of bleaching agents by heat or light (halogen, light-emitting diode [LED], or laser) may have an adverse effect on pulpal tissue.^19,20 It was already reported that the use of intense lights does elevate bleach temperature, but it results in increased intrapulpal temperatures,²¹ which may further impact patient sensitivity and pulpal health.

As dental professionals, we have an obligation to pursue scientific knowledge about what is available to treat our patients' teeth, so that we can differentiate between effective and safe bleaching methods and those that are marketed on the basis of promotional speculation.²² Therefore, the aim of this clinical study was to evaluate the effects of LED/laser activation on bleaching effectiveness and tooth sensitivity during in-office bleaching.

MATERIALS AND METHODS

This clinical investigation was approved (protocol number 05531/09) by the Scientific Review Committee and by the Committee for the Protection of Human Subjects at the local university. On the basis of pre-established criteria, 30 students from the same university were selected for inclusion in this study. All patients had anterior teeth with initial shade C2 or darker, as judged by comparison with a value-oriented shade guide (Vita Lumin, Vita Zahnfabrik, Bad Säckingen, Germany). Two weeks before the bleaching procedures were performed, all subjects received a dental screening and dental prophylaxis and signed an informed consent form.

RESULTS

All participants completed the study. The kappa statistic agreement between the two evaluators was 89%. Means and standard deviations of Δ SGU are depicted in Table 1. Both bleaching therapies were effective in terms of bleaching, as statistically similar changes in the total number of SGUs were observed after two sessions of in-office bleaching. However, comparison of bleaching speed after just the first clinical appointment showed that the LED/laser-activated group bleached more SGUs than the non-activated group (p<0.05), and was therefore faster.

Table 1:  Tooth Shade at Assessment Points for the Two Treatment Groups^a

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With regard to prevalence of tooth sensitivity, 86.6% of participants from the non-activated group and 100% of participants from the LED/laser-activated group experienced tooth sensitivity immediately after bleaching (p=0.48; Table 2). The percentage of patients with tooth sensitivity decreased significantly after 48 hours for both groups (p<0.05). Approximately 53.3% and 26.6% of participants from the LED/laser-activated and non-activated groups, respectively, showed tooth sensitivity after 24 hours. Around 33% of participants from the LED/laser-activated group reported tooth sensitivity after this period; no participants from the non-activated group presented sensitivity (Table 2). The percentage of patients with sensitivity was similar for the two groups immediately and after 24 hours (p>0.05). However, a significant difference between groups was detected after 48 hours. The percentage of patients with sensitivity was significantly higher in the LED/laser-activated group (33.3%) than in the non-activated group (0%) after 48 hours (p=0.04) (Table 2).

Table 2:  Comparison of Numbers and Percentages (%) of Patients Who Experienced Tooth Sensitivity During the Bleaching Regimen^a

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Figure 1 depicts percentages of participants according to the level of reported tooth sensitivity for both groups immediately after bleaching. One can observe that severe sensitivity was reported by only two participants from the LED/laser-activated group, and two participants from the non-activated group did not present sensitivity. The sensitivity levels of both groups were similar immediately and after 48 hours (median, 2 and 0, respectively) (p>0.05). However, a significant difference between groups was detected after 24 hours. The intensity of sensitivity was significantly higher in the LED/laser-activated group (median, 1) than in the non-activated group (median, 0) after 24 hours (p=0.01) (Table 3).

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Table 3:  Median (Minimum and Maximum) of Intensity of Tooth Sensitivity Immediately After and 24 and 48 Hours After the Bleaching Regimen^a

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DISCUSSION

Comparison of color change after in-office bleaching as described in the existing literature is difficult because of the different methods of measurement (shade guides and spectrophotometers) and the different units of measurement (CIELAB system, shade guide units, etc.) used. However, studies that used 35% hydrogen peroxide (with and without light activation) and reported their results in shade guide units (SGUs) usually observed an overall color change of 5 to 8 SGUs after two bleaching sessions,^1,9,25 which is consistent with results of the present investigation. This wide range of color change is probably the result of different materials employed and the length of time after which color was assessed.

In the current study, faster bleaching was observed for the LA group than for the NA group after the first session of bleaching. The rationale behind the benefits of light activation is that a small fraction of light is absorbed by the bleaching product, and its energy is converted to heat. Most likely, this is the main mechanism of action of all light-activated bleaching procedures, and it leads to increased release of hydroxyl radicals through a rise in temperature (thermocatalysis),^5,26 which may favor the initial whitening effect observed for the LA group after the first bleaching session. It is also reported that release of hydroxyl radicals from H₂O₂ is possible through direct excitation by light (photolysis).

Different light sources have been used to activate bleaching agents, including halogen curing lights, ultraviolet and infrared lamps, plasma arcs, light-emitting diodes (LEDs), and lasers (CO₂, argon, and diode lasers). The light source used in this study is made of a matrix of LEDs and three diode lasers. Although the temperature rise was not measured, one can suppose based on previous studies that the LED source produces the least thermal insult during the light activation process^27-29; this light source was employed in the present investigation for this reason. The use of high-intensity light to raise the temperature of the hydrogen peroxide and to accelerate the rate of bleaching was reported in 1918.⁵ Other approaches for heating peroxide, such as heated dental instruments, have historically been described to accelerate tooth whitening. However, these techniques could excessively heat the pulpal tissues, causing irreversible damage.⁵

One cannot rule out the fact that the initial difference between tooth whitening that favored the LA group could be attributed to teeth dehydration caused by the generation of heat from the light source.¹¹ Thus, the recorded change in color appearance during bleaching treatment would be more accurately described as desaturation rather than bleaching/whitening.¹⁴ This hypothesis is supported by clinical findings showing that light alone can produce an initial whitening effect, which, however, is not sustained for a long time.^13,16 This seems to be the most appropriate explanation for faster bleaching after one clinical appointment, because no significant benefit of light activation was found between groups after two bleaching sessions. This is consistent with findings of recent studies reporting that powerful light sources have no effect on the actual speed or extent of bleaching.^1,8-12,15

However, some clinical studies have reported that the use of a light can improve bleaching efficacy.^13,14,16 Closer analysis of these studies reveals that they employed bleaching gels with lower hydrogen peroxide concentrations (15% to 25%). One may hypothesize that light activation may improve the whitening effect in this circumstance, because the quantity of available radicals produced only by the chemical degradation of hydrogen peroxide may be the limiting factor in the bleaching reaction. Because light can definitely improve the degradation rate of hydrogen peroxide,³⁰ a light source may be useful to activate the bleaching procedure for low–hydrogen peroxide bleaching gels; the increase in hydroxyl radicals compensates for the low concentration of the gel.

The beneficial effect of light activation likely is not perceived when 35% hydrogen peroxide is used,^1,9,15 because the quantity of radicals already produced by the chemical degradation of hydrogen peroxide is enough to react with pigments in the dentin structure and any further increase in this quantity will not accelerate bleaching. It is likely that in this situation, the limiting factor is no longer the quantity of the reagents, but the reaction time itself. Moreover, it has been suggested that use of a light source should be considered optional for in-office bleaching when higher concentrations of hydrogen peroxide are used.⁹

Tooth sensitivity seems to result from the easy passage of peroxide through enamel and dentin to the pulp; this takes approximately 5 to 15 minutes to occur.³¹ Further proof of this rapid passage of peroxide is seen in research findings showing that the dentin changes color next to the pulp as fast as it does next to the dentin-enamel junction.³² Hence, sensitivity results from the insult of the peroxide on the nerve and may be considered a reversible pulpitis.

The present investigation noted higher levels of tooth sensitivity for both groups immediately after the bleaching procedure. Based on previous studies, we may report that the overall percentage of patients who experience tooth sensitivity after in-office bleaching ranges from 50% to 80%, depending on the type and concentration of material used.^13,16,25 However, tooth sensitivity seemed to be less persistent when the light source was not used. Although tooth sensitivity in most patients resolved within 24 hours for the NA group, LA patients still presented higher levels of tooth sensitivity up to 48 hours. This finding was also reported by previous investigators.^8,13,14,16 Kugel and others¹³ noted that although the use of light with peroxide gel resulted in immediate incremental whitening, this was largely offset by the increased occurrence and severity of tooth sensitivity.

It was previously demonstrated that in-office bleaching treatment seems to result in increased tooth sensitivity when compared with at-home bleaching,⁹ mainly when light activation was used.^9,13,14,17 Although some in vitro studies have not found differences in the viability of pulp cells when 35% hydrogen peroxide was associated with light sources,^20,33 other in vitro studies have indicated that application of the bleaching gel associated with halogen light produced more severe cytotoxic effects to the cultured odontoblast than application of hydrogen peroxide alone.¹⁹ Besides this, laser-activated tooth-bleaching systems increase expression of substance P (SP) in human dental pulp to significantly higher than normal values.⁶ Substance P is, among others, an important neuropeptide that has the primary function to induce vasodilation, increasing pulpal blood flow and allowing rapid and large arrival of inflammatory cells. These increased levels of SP may be clinically relevant because they could cause a neurogenic inflammatory reaction in pulp tissue.⁶ Perhaps this hypothesis can explain the higher levels of tooth sensitivity observed when the LED/laser was applied in the current study.

As yet, it is still debatable whether activation results in superior tooth brightening as compared with non-activated bleaching therapies. Therefore, application of heat and light-activated bleaching procedures should be critically considered, while the physical, physiologic, and pathophysiologic implications are kept in mind. If heat or light activation is applied, it is strongly advised to follow the manufacturer's recommendations with limited duration of heat activation, to avoid undesired pulpal responses.²⁶

CONCLUSIONS

After two bleaching sessions, the use of light activation did not improve bleaching speed. Persistent tooth sensitivity and higher tooth sensitivity levels after 24 hours of bleaching were observed when LED/laser light activation was used.