INTRODUCTION

In-office bleaching is a treatment that offers excellent color stability; moreover, since the procedure is under the control of a dental surgeon, there is less risk of exposure to soft tissue.¹ Hydrogen peroxide (HP) is used in this treatment as a dental oxidizing agent, resulting in effective color change.² However, the concentration of HP and the duration of its application can influence its absorption into dental tissues, causing tooth sensitivity.³

High concentrations of HP promote evident color change at the first application session.^4,5 The mechanism of bleaching can generate a greater number of by-products capable of causing cell stress, clinically reflected as sensitivity.^6-9 Moreover, the clinical protocols for these products require, on average, 30 to 50 minutes of contact of the bleaching gel with the tooth for each clinical session.^10-12

To maintain efficacy in color as well as alleviate sensitivity, bleaching gels with lower concentrations of HP¹³ are used for in-office bleaching. However, low concentrations may require a greater number of sessions to achieve effective bleaching.¹⁴ The efficacy of low-concentration bleaching gels is also affected by the addition of a semiconductor agent. Nanoparticles of titanium dioxide doped with nitrogen have been introduced with the aim of enhancing safety, providing less damage to the dental structure and tooth sensitivity.^14-16

Considering the variety of products available on the market, it is challenging for dentists to identify a bleaching gel that is efficient and does not cause discomfort or sensitivity to patients.^17,18 Therefore, the aim of the present study was to conduct a systematic review of the literature to evaluate whether high concentrations of HP (≥35%) used in an in-office bleaching procedure can influence tooth sensitivity and color change in comparison to low concentrations (6%, 15%, and 20%). The first and second null hypotheses, in relation to tooth sensitivity and change in tooth color, respectively, were that there is no difference between high concentrations (≥35%) and lower concentrations (6%, 15%, and 20%) of HP.

METHODS AND MATERIALS

RESULTS

The search of the databases led to the retrieval of 1478 articles: 722 in Scopus, 597 in PubMed/MEDLINE, and 159 in the Cochrane Library. After the removal of duplicate references, the titles and abstracts of the articles were analyzed regarding the eligibility criteria, and 51 articles were preselected for the reading of the full texts. After the full-text analysis, 14 studies were selected for the qualitative analysis,^{2,12-15,24-32} and seven of these were selected for the quantitative analysis^{12-15,27,29,31} (Figure 1).

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Study Characteristics

Design of the Studies

Table 1 lists the characteristics of the studies. Fourteen randomized controlled trials were selected; four of these studies used a split-mouth design.^14,24,25,31 A total of 649 patients were submitted to bleaching treatment (mean age: 36.32 years; range: 13.9 to 31 years). The most common patient's inclusion criteria in the studies were being caries free,^2,12,15 good oral hygiene,^2,26 absence of periodontal disease,² absence of anterior teeth restorations,^{12-15,26,28,32} and teeth without bleaching experience.^{18,22,26,28,29,31} Three^14,26,31 reported loss of the patient during the follow-ups; this means that 15% of the patients in the included studies dropped out.

Table 1 Characteristics of the Studies Included

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Table 1 Characteristics of the Studies Included (ext.)

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The most common patient exclusion criteria in the studies were systemic diseases,^2,26,32 change in tooth structure,² smoking,² bruxism,^2,12,28 presence of dental sensitivity,² poor oral hygiene,^2,12 presence of fluorosis or tetracycline stains,^{12-15,28,29,31,32} use of orthodontic treatment,^{2,13-15,26,31} taking analgesic or anti-inflammatory drugs,^13,14,31 and pregnancy or breast-feeding.^{2,12-15,25,26,32} The follow-up period ranged from one week to 12 months. The studies compared a higher concentration of HP (35%) to lower concentrations (6% to 20%). Two studies^2,30 performed a comparison of high-concentration HP bleaching gels.

Bleaching Protocol

In-office bleaching was performed in all included studies.^{12,13,15,25,26,28,29,31} The bleaching protocol varied according to the concentration of the bleaching gel, and all bleaching protocols are described in Table 1. Some studies mentioned the use of gingival isolation^2,13-15,28 and lip retractor^2,29 and previous prophylaxis before the treatment.^{13-15,24,26,27,29,33} The whitening gel was applied on the vestibular surface of the teeth (1 to 2 mm thick) or according to the manufacturer's instructions.^13,15,24 Also, other studies used the LED light associated with two low-level lasers at red (660 nm) or infrared (780 nm) wavelengths,²⁴ laser of low intensity (808 nm/infrared light),¹⁸ LED light (470 nm),^13,26-28 LED/laser hybrid cold-light,^13,14,31 or infrared laser diodes (830 nm).²⁶ Only two studies^2,12,30,32 did not report the use of light. The bleaching agent at the end was removed with gauze, suction, and/or water.² Desensitizing agent was used after the bleaching session in the studies.² The patients were instructed to avoid acids and food dyes^2,14,25 and the use of whitening agents.³¹

Tooth-Sensitivity Evaluation

The visual analog scale was used to measure tooth sensitivity.^{14,15,24,26,28,29} The degree of tooth sensitivity was measured from 0 to 10, where 0 represents a total absence of pain, 5 moderate pain, and 10 the maximum level of pain.²⁴ Other measures were also used, such as the Five-Step Scale, which was applied after the bleaching, and the scores were measured with the following criteria: 0: none; 1: mild; 2: moderate; 3: considerable; 4: severe.^2,27 The Five-Point Verbal Scale^12,13,27,29 and a sensitivity questionnaire³⁰ were used, where four levels were considered: none, mild, moderate, or severe. Tooth sensitivity was evaluated before bleaching and after bleaching and at 24 hours, 48 hours, 72 hours, and one week, according to each study's follow-up. Also, some studies^12-15,27,29 described the absolute risk rate and the number needed to treat.

Color Evaluation

The subjective color analysis was based on shade guide units (ΔSGU), and the analysis was made from the color shade scale in some of the included studies.^{12,24,25,27,29} Color was also evaluated in seven studies using an objective color analysis (ΔE) from a spectrophotometer^{2,13-15,25,27,30} either by the confection of transparent soft trays with standardized markings² or without them in the middle third of the labial surface²⁴ or by a guide made with high-viscosity silicone putty^13,31 in the periods before bleaching and after bleaching and at 24 hours, 48 hours, 72 hours, one week, one month, six months, and 12 months, according to each study's follow-up. Objective color differences were calculated from the equation ΔE = [(ΔL*)² + (Δa*)² + (Δb*)²]^1/2.

Risk of Bias

All studies were qualified using the Cochrane Collaboration's tool for the risk of bias (Figure 2) and showed low risk of bias. Some studies were classified as “unclear” for the following items: sequence generation and allocation concealment;^2,24,30 blinding of participants, personnel, and outcome assessor;²⁴ and incomplete outcome data.^25,26,28

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Meta-Analysis

Tooth Sensitivity

Six studies^{12-15,27,29,31} were selected for the quantitative analysis of the concentrations of bleaching gel divided into two groups: lower concentration of HP (6% to 20%) and higher concentration (35%). A significant difference between groups was found, indicating less tooth sensitivity when a lower concentration of HP (6% to 20%) was used (0.67; 95% CI: 0.44 to 1.03; p=0.04; I²: 56%) (Figure 3).

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Color Change (ΔE)

About the color change, some studies^{13-15,25,27,31} found a significant difference between a high concentration of HP (35%) and a low concentration (6% to 20%) (1.53; 95% CI: 2.99 to −0.08; p<0.0001; I²: 82%) (Figure 4).

Color Change (ΔSGU)

Some studies^{12,14,25,27,29,31} performed a subjective evaluation of the change in color, but no significant difference was found between a low concentration (6% to 20%) and a high concentration of HP (35%) (0.24; CI: −0.75 to 1.23, p<0.00001; I²: 89%) (Figure 5).

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DISCUSSION

The first null hypothesis, that there is no difference with regard to tooth sensitivity between a high concentration of HP (35%)^34-36 and lower concentrations (6%, 15%, and 20%),^34-36 was rejected. This was because a meta-analysis revealed a significant difference (0.38; 95% CI: 0.16 to 0.92, p=0.04; I²: 56%), revealing less tooth sensitivity for a lower concentration of HP. This agrees with the findings of Moncada and others²⁸ and other studies^18,26,37 reporting greater tooth sensitivity when high concentrations of HP were used. The major number of dropouts were between the first and second bleaching sections. This is in accordance with the literature^13,25,28 reporting a high level of sensitivity in the early sections.

It was not possible to carry out meta-analyses for all studies that were selected for the present systematic review due to a lack of data in some articles. In the evaluation of tooth sensitivity stemming from bleaching sessions, a reduction in sensitivity was reported one week after the procedure. This agrees with the findings of Mondelli and others,¹¹ who evaluated the effects of high concentrations of HP (35% and 38%) and found that sensitivity increased immediately after treatment and then diminished gradually over the course of a week.

According to Matis and others,³² tooth sensitivity is related to the contact time of the bleaching gel and not the concentration of HP. In the analysis of bleaching gel application among the studies selected for the present systematic review, either the protocol was found to be the same for low or higher concentrations tested^13,14,25-28 or application was performed with low-concentration gels over a longer period (contact time and/or number of sessions).^15,24,29 This indicates that a longer application time does not exert an influence on tooth sensitivity but enables a low-concentration bleaching gel to perform as effectively as a high-concentration bleaching gel.¹³

The sensitivity caused by low-concentration gel (15%) is lower than that found with high-concentration (35%) gel; this may be related to the association between HP and nitrogen-doped titanium dioxide semiconductor nanoparticles (N_TiO₂). In the presence of visible light, this enables the formation of O₂ without hydroxyl (OH⁻) radicals, which are a risk factor in tooth bleaching procedures. Therefore, lower-concentration gels are reported as a less irritating but equally effective tooth bleaching option.²⁶ However, in the subgroup analysis, when comparing a 6% HP concentration with a 35% HP concentration, there is no difference between them, even with the 6% HP concentration having the N_TiO₂, and it may be related to the use of LED/laser hybrid light in high concentrations (35%), which is reported³³ as an important factor to provide less tooth sensitivity.

In the present systematic review, meta-analysis was performed for both objective color change (ΔE) (−1.53; 95% CI: −2.99 to −0.08; p<0.0001, I²: 82%) and subjective color change (ΔSGU) (0.24; 95% CI: −0.75 to 1.23; p<0.00001; I²: 89%). A statistically significant difference in ΔE was found; however, there was no significant difference for ΔSGU. Hence, the second null hypothesis, which affirms that there is no difference in terms of the change in tooth color between a high concentration of HP (35%)^34-36 and lower concentrations (6% to 20%),^34-36 was partially accepted.

The results used in the meta-analysis to analyze ΔE were acquired from those for the longer follow-up period reported in each study. Researchers^14,38 believe that there is color maintenance after a longer period following bleaching, indicating that the peroxide diffuses through the tissues of the tooth and remains there.¹⁴ Even TiO₂, when used as a catalyst at lower HP concentrations associated with LED light, presents strong evidence of color rebound.^16,31 These results indicate that color was maintained when lower concentrations of HP were used, in comparison to previous controls (ΔE), for up to one year of follow-up.^2,14,25 Some authors^14,25 also reported that the bleaching protocol is important for color change and maintenance.^14,39,40 A protocol with 120 minutes of contact time would be ideal for complete effectiveness of a bleaching gel with a 6% concentration of HP.⁴¹ However, the time reported in the studies cited was lower yet still presented better results.

Mena-Serrano and others²⁷ obtained good results by using a 20% concentration of HP with regard to ΔSGU, in comparison to that achieved with 35% HP, but found no difference when color was evaluated using a spectrophotometer (ΔE). This result was obtained in the first 30 days after the bleaching procedure and may be related to the combined use of 35% HP and light. Since 35% HP alone produces radicals in excess, light would not lead to faster bleaching due to the oxidation mechanism of the radicals.²⁷ Moreover, in the studies with a split-mouth design, the optical effect of the incisors exerted an influence on ΔSGU. The subjective analysis of color depends on physical and environmental variables,²⁸ which can lead to inconsistencies in the measurement of color. Some researchers,^40,41 when evaluating the color change by people who were not trained after bleaching or even for people who were trained,⁴¹ reported that objective color evaluation using a spectrophotometer is more effective than using visual scales (ΔSGU). Other studies included in the present systematic review^30,42 performed comparisons of bleaching gels with high concentrations of HP (25% to 38%)¹ and demonstrated effectiveness at the 12-month follow-up evaluation.² In a study involving adolescent patients (mean age: 13.9 years), Bacaksiz and others² found no difference with regard to tooth sensitivity between gels of different concentrations, even when the pulp chambers were wider and more sensitive to irreversible pulpitis.⁴³

The risk of bias among the studies included in the present review was low. However, further randomized clinical trials should be performed with uniform bleaching gel application protocols to ensure greater safety for both the operator and the patient and to minimize tooth sensitivity while maximizing the effectiveness of the treatment.

CONCLUSIONS

The findings of the present systematic review and meta-analysis indicate that a lower concentration of HP in tooth bleaching gels results in less tooth sensitivity and more efficacy of objective color (ΔE) as measured by the spectrophotometer, while the same effect between the high and low concentrations was found regarding subjective color (ΔSGU) as measured by the color shade guide.