Effect of low-concentration at-home bleaching gels on whitening and tooth sensitivity: a split mouth randomized clinical trial

Abstract

Objective

To assess the bleaching efficacy, tooth sensitivity, enamel surface morphology, and oral health-related quality of life (OHRQoL) of two at-home bleaching products with matched effective hydrogen peroxide (HP) concentrations: Beyke iWhite (8% carbamide peroxide) and Invisalign (3% hydrogen peroxide).

Materials and methods

A randomized, triple-blind, split-mouth clinical trial was conducted with 22 participants undergoing a two-week bleaching treatment. The gel was used daily for 6–8 h. Tooth color and OHRQoL were assessed at baseline, post-bleaching, and 1-month follow-up. Tooth color was measured using a spectrophotometer (ΔE₀₀), while OHRQoL was evaluated using psychological questionnaires (OHIP and PIDAQ questionnaires). Daily tooth sensitivity reports were collected, and enamel morphology was analyzed using scanning electron microscopy. An in vitro experiment tracked the initial pH values of gels and their 8 h changes on bovine enamel. Statistical analysis included Wilcoxon Signed Rank tests for tooth sensitivity and questionnaires, paired t-tests for color change (ΔE₀₀), and ANOVA for pH changes.

Results

Both products presented effective color change (ΔE₀₀ = 5.39 ± 2.50 for 3% HP and ΔE₀₀ = 4.75 ± 2.25 for 8% CP) after two weeks, exceeding clinical acceptability thresholds (ΔE₀₀ >1.8). The 3% HP gel yielded a statistically greater color change than the 8% CP gel (p < 0.05). The 8% CP gel induced statistically lower tooth sensitivity (NRS/VAS, p < 0.05), while both reported mild sensitivity (0 < NRS < 1.5, 0 < VAS < 3). OHRQoL improved significantly (P < 0.05). Invisalign treatment led to more noticeable surface undulations than Beyke iWhite. The in vitro experiment indicated Beyke iWhite gel was initially alkaline, turning neutral, while Invisalign gel remained acidic.

Conclusions

Both products effectively whitened teeth while inducing mild tooth sensitivity, and both had a positive socio-psychological impact. The 3% HP gel provided a bleaching efficacy advantage, while the 8% CP gel demonstrated lower sensitivity and less enamel alteration.

Clinical relevance

Clinicians may consider at-home bleaching techniques with 3% HP and 8% CP to obtain effective bleaching results with slight side effects.

Trial registration

Chinese Clinical Trial Registry ChiCTR2400092807, 24/11/2024, Retrospectively registered.

Introduction

Tooth discoloration is a common aesthetic concern caused by various intrinsic and extrinsic factors, including aging, dietary habits, smoking, and certain medications [1, 2]. In response to the growing demand for aesthetic dentistry, tooth whitening has become one of the most requested cosmetic dental procedures [3]. Broadly, medically supervised tooth bleaching can be classified into in-office and at-home techniques [4]. High-concentration bleaching agents (25%–40% HP or 30–37% CP) are used for in-office bleaching treatments under oral soft tissue protection and isolation, while low-concentration bleaching agents (3.0%–8.0% HP or 10%–22% CP) are commonly used for at-home bleaching treatments via custom-fitted trays under the guidance of dental professionals [5–7]. At-home dental bleaching has gained significant popularity due to its convenience, reduced clinical time, affordability, and minimal side effects [8, 9].

Hydrogen peroxide (HP) and carbamide peroxide (CP) are the two primary ingredients in at-home tooth-bleaching gels [10]. HP is an unstable compound that decomposes into water and reactive oxygen radicals [11]. Conversely, CP is a stable structural complex that decomposes into urea and HP [11]. HP operates as a direct and potent oxidant, offering a more immediate and rapid release of active peroxide compared to the slower decomposition of CP [11–14]. This leads to a rapid onset and shorter bleaching process with HP [15–17]. Additionally, urea can decompose into ammonia and carbon dioxide, resulting in an increase in the pH of the solution. The HP solution is acidic [18]. The pH of bleaching gels is closely related to patient sensitivity and enamel surface morphology [11–13, 18–22].

Historically, the gold standard for home bleaching has been the 10 ± 1% carbamide peroxide (CP) [23, 24]. Products having 10% CP (3.5% HP) were awarded the American Dental Association (ADA) Seal of Acceptance [25]. However, research indicates that 10% CP is not free of undesirable effects, involving increased sensitivity, increased roughness, as well as alterations in the chemical composition of dental enamel [21, 23, 24, 26, 27].

Mild to moderate tooth sensitivity is the most common side effect, affecting approximately 51% of patients undergoing at-home tooth bleaching treatments. The issue is that sensitivity often causes patients to discontinue treatment [27–31]. Although the industry has developed some desensitizing agents such as remineralizing substances and anti-inflammatory medications, research findings indicate that these interventions offer only marginal alleviation [30, 32, 33, 49]. Consequently, patients’ satisfaction and compliance can be compromised, discouraging them from completing the prescribed regimen [30, 34]. According to the latest research findings, the cause of tooth sensitivity lies in the relatively high concentration of active hydrogen peroxide, a breakdown product of CP or HP [15, 22, 35, 36]. Studies have shown that even a slight decrease in peroxide concentration can lead to a great reduction in tooth sensitivity [34, 35].

The increased enamel surface roughness is also a frequently reported side effect [18, 21, 28, 37]. Multiple clinical investigations have indicated that higher concentrations of bleaching agents result in greater enamel surface roughness [18, 21, 28, 37, 38]. The Scanning Electron Microscopy (SEM) results observed that the enamel surface in the 10% HP group had more rough unevenness than the 3.6% HP group [38]. A smooth enamel surface is crucial for maintaining the bleaching effect, as a polished enamel surface is less prone to discoloration and color rebound compared to a rough enamel surface [39–41].

Thus, low-concentration gels were proposed for at-home use and have demonstrated promising results [22, 31, 35, 36, 42, 43]. This newer generation of products, including formulations such as = 8% CP or = 3% HP, is designed to mitigate side effects without sacrificing clinically perceptible whitening. Mohammadipour et al., in their clinical trial on 3% HP, observed an effective whitening effect over a two-week evaluation period [43]. Cordeiro et al. conducted an in vitro experiment that compared 10% CP with lower concentrations of CP gel (4%, 5% and 7% CP), revealing that low concentration gels are as effective as 10% CP [42].

Given that subtle changes in concentration have little impact on whitening efficacy, it is pertinent to consider whether the choice of active ingredient, HP versus CP, could yield distinct effects at low concentrations. HP and CP exhibit differences in physicochemical properties, decomposition rates, and byproducts-induced whitening microenvironments. To date, direct clinical comparisons between low-concentration HP and CP under matched conditions remain scarce, and their relative effects on whitening efficacy, tooth sensitivity, enamel integrity, and patient-reported outcomes are not well established. Therefore, the aim of this triple-blind, split-mouth, randomized controlled clinical trial was to directly compare two low-concentration at-home bleaching gels, 3% HP and 8% CP. 8% CP is almost equivalent to 3% HP in effective hydrogen peroxide concentration (2.9%) based on the conversion formula [44]. Besides, the in vitro experiment measured the initial pH values and their changes over 8 h on bovine enamel surfaces for both gels. Our hypotheses were as follows: (1) Both CP and HP bleaching products (= 3% HP) would demonstrate effective bleaching results with low levels of tooth sensitivity and there would be no significant inter-group differences in key clinical outcomes, including bleaching efficacy (ΔE₀₀), tooth sensitivity, enamel surface morphology; (2) There would be a positive impact on the quality of life for patients who received bleaching using low-concentration gels.

Materials and methods

Ethics approval and protocol registration

This clinical study was approved by the Ethics Committee of the Ninth People’s Hospital, with the protocol number SH9H-2023-T342-2. It has been registered on the Chinese Clinical Trial Registry (ChiCTR) website under the code ChiCTR2400092807 and on the Chinese Clinical Research website with the registration number MR-31-24-016296. We followed the guidelines provided by the Consolidated Standards of Reporting Trials (CONSORT) for reporting this randomized clinical trial [45].

Study design, Setting, and data collection site

This research employed a randomized, triple-blind clinical trial design, where patients, operators, and evaluators remained blind. A separate researcher, who was not involved in any evaluation activities, handled the randomization process. All participants were informed about the study’s nature and purpose. This randomized clinical trial was conducted from March 2024 to August 2024 at the Ninth People’s Hospital affiliated with Shanghai Jiao Tong University, fully complying with the latest revision of the Declaration of Helsinki by the World Medical Association.

Recruitment

Recruitment was carried out by displaying written advertisements on hospital walls. Prior to participation, all volunteers provided their consent by signing an informed consent form.

Eligibility criteria

To minimize potential confounding factors, strict inclusion and exclusion criteria were applied.

Inclusion Criteria

Participants were eligible to enroll in the study only if they met all of the following conditions:

• Aged between 18 and 50 years;

• Good general and oral health, without active systemic disease or untreated oral pathologies;

• Possessed at least 12 natural anterior teeth (maxillary and mandibular incisors and canines), all of which were free from caries, restorations, or prosthetic crowns;

• Displayed baseline tooth shade of A2 or darker on canines, as determined using the VITA Classical Shade Guide (Vita Zahnfabrik, Bad Säckingen, Germany).

These criteria were established to ensure consistent substrate conditions for bleaching and reliable baseline color assessment.

Exclusion Criteria

Participants were excluded if they met any of the following conditions:

• Use of removable dentures, fixed orthodontic appliances;

• Presence of severe intrinsic discoloration, including but not limited to tetracycline staining, dental fluorosis;

• Current pregnancy or breastfeeding;

• Diagnosed bruxism, tooth attrition, enamel fractures, or dentin exposure, with or without symptoms of dentin hypersensitivity;

• Consumption of analgesic, anti-inflammatory that could interfere with tooth sensitivity reporting;

• Tobacco use, regardless of frequency (active smokers);

• Underwent any form of tooth bleaching or professional whitening within the last 12 months.

These exclusions were designed to remove factors known to affect bleaching outcomes, tooth sensitivity, and enamel response, as well as to prevent pharmacological or lifestyle-based interference with colorimetric evaluations or patient-reported outcomes.

Sample size calculation

The sample size was determined using Statistics Analysis System 9.4 software, applying a significance level of 5%, a statistical power of 80%, and a minimum effect size of 2 [46, 47]. In this split-mouth study, the procedures were conducted on the left and right dental arches within each of the 22 participants’ mouths. To account for potential participant dropout or refusal, the sample size was increased to 26 participants per group.

Randomization and allocation concealment

To compare the two bleaching gels, a split-mouth design method was carried out. Different treatments were randomly applied to the left and right sides of the maxillary and mandibular arches within the same patient. This study employed cluster randomization. A biostatistician used the SAS statistical software package (version 9.4 or later) to generate random letters A/B on a computer, with the blinding operation performed by professionals who were not part of the research team. The letter A or B represents that Beyke iWhite was assigned to either the left or right arch, while the other side of the arch received treatment of Invisalign. The random allocation table was created by the statisticians, and packaging was done according to the table and blinding principles. After blinding, the assignment codes were sealed separately. Once a subject met the conditions and completed all preliminary assessments, the envelope was opened immediately after the procedure to reveal the assignment.

Blinding

This study is a triple-blind clinical trial, where patients, operators, and evaluators were unaware of the group assignments. The randomization, gel delivery, and administration instructions were carried out by a third researcher, who was not involved in either the implementation or assessment phases of the study. Before the experiment began, all labeling, packaging, and any other identifiable factors of the bleaching gel products were removed and labeled with the numbers 1 (Beyke iWhite) or 2 (Invisalign). Both gels were transparent and viscous, exhibiting similar colors and textures.

Study interventions

The bleaching procedure was conducted by dentists who have more than three years of clinical experience. Alginate impressions of both the upper and lower arches were taken for every participant and filled with dental stones. Bleaching trays were trimmed 1 mm beyond the marginal gingiva. Participants were provided with two bleaching products and given instructions. The hemi-arches of dental stone models labeled with numbers 1 and 2 were given to patients for reference to ensure accurate application of the designated product in the correct quadrant (Fig. 1).

Fig. 1.

(a) Dental stone models labeled with numbers 1 and 2; (b) Bleaching tray on the dental stone

To ensure standardized application across both groups and to address potential discrepancies in peroxide exposure, all participants were instructed to apply an equal amount of bleaching gel per side. Specifically, a 2–3 mm diameter bead of gel was dispensed onto the buccal surface of each anterior tooth (from canine to canine) within the tray and excess material was gently removed to minimize soft tissue exposure, reducing irritation risk. This protocol promoted uniform gel distribution and comparable active agent exposure across hemiarches, in both the 3% HP and 8% CP groups, thereby minimizing dosage as a confounding variable. 1

The bleaching gels used in this study were 8% CP (Beyke iWhite, Hangzhou City, China) and 3% HP (Invisalign, South Jordan, Utah, USA). Table 1 outlines the composition of the bleaching gels according to the manufacturers’ instructions. It also includes guidance on wearing protocols. The gels were used daily for 6–8 h over a 14-day period [43, 48, 49]. Following each daily application, patients were advised to take out the bleaching trays, rinse them under tap water, and incorporate tooth brushing into their regular oral care routine. Toothpastes that contain bleaching agents or were designed for desensitizing were not allowed to be used.

Table 1.

Bleaching gel (manufacturer, composition, and implementation approach)

Manufacturer	Composition	Implementation approach
Beyke iWhite, Hangzhou City, China	Carbamide peroxide 8%, polyacrylic Acid, peppermint oil and distilled water	A drop of gel was applied to each tooth in the tray, targeting the areas corresponding to the buccal surfaces of the teeth to be whitened. The gels were used daily for 6-8 h over a 14-day period.
Invisalign, South Jordan, Utah, USA	Hydrogen Peroxide 3%, glycerin, carbomer, PEG-6 and distilled water

Assessment of tooth sensitivity (TS)

To record their tooth sensitivity (TS), patients were instructed to complete daily forms during the 14-day treatment. Before beginning the bleaching procedure, each patient was provided with a paper form depicting the upper and lower dental arches. They were instructed to rate their pain intensity using two scales. The first scale was the five-point Numeric Rating Scale (NRS), where 0 = none, 1 = mild, 2 = moderate, 3 = considerable, and 4 = severe. Using this scale, patients reported sensitivity levels separately for each quadrant of the dental arches. The second scale employed was the Visual Analog Scale (VAS), consisting of a 10 cm horizontal line labeled 0 (no sensitivity) at one end and 10 (severe sensitivity) at the other. Patients marked the intensity of TS by drawing a vertical line on the scale. The distance from the zero to the patient’s mark was then measured in millimeters.

In this study, participants were not instructed to indicate the specific tooth causing discomfort but could do it as if they wished. They were instructed to complete the form whenever they experienced discomfort. For instance, they did not feel any TS, participants were informed to record the intensity as zero. These completed forms were collected by researchers after two weeks of treatment. Participants who consistently reported a score of 0 (no sensitivity) across all assessments were categorized as insensitive to the bleaching procedure. In other cases, participants were deemed to have bleaching-induced TS. To determine TS intensity, the worst NRS score and the highest VAS value reported by each patient during the two weeks of treatment were recorded with only one value per patient being used.

Assessment of color

A trained dentist conducted clinical evaluations at baseline, after bleaching, and 1-month follow-up. These dentists were not involved in the randomization process. A spectrophotometer, VITA Easyshade (VITA Zahnfabrik, Bad Säckingen, Germany), was used to conduct the color assessments. Color evaluation was performed on a total of 12 teeth, including the upper and lower central incisors, lateral incisors, and canines. The examiners utilized the spectrophotometer to measure color by taking an impression of the maxillary and mandible arches with dense silicone putty (DMG, Hamburg, Germany). The impression, featuring a 6 mm diameter window on the buccal surface of each tooth being evaluated, served as a standardized guide for color measurement. This diameter exactly matched the tip of the spectrophotometer. By inserting the tip into the guide, the spectrophotometer measured the L*, a*, and b* color parameters. The L* value indicates lightness, ranging from 0 (black) to 100 (white), while the a* value measures the red-green axis, and the b* value measures the yellow-blue axis. Using a transformation program, convert the L*, a*, and b* color parameters to the L*, C*, and H* color parameters. The ΔE₀₀ color difference was calculated using the CIEDE2000 formula from the International Commission on Illumination (CIE). The formula for calculating color difference is as follows [50, 51]:

Oral health impact profile (OHIP-14)

To evaluate patients’ aesthetic perceptions, the OHIP-14 questionnaire was translated into Chinese and validated [52, 53]. The OHIP-14 questionnaire comprises seven dimensions with 14 questions designed to evaluate the patient’s quality of life. This questionnaire was used at baseline, post-bleaching, and at a 1-month follow-up. It consists of 14 questions, each scored on a Likert scale. Each question is scored from 0 to 4 (0 = never, 1 = hardly ever, 2 = occasionally, 3 = fairly often, 4 = very often). The seven dimensions included functional limitations, physical pain, psychological discomfort, physical disability, psychological disability, social disability, and handicap. A lower average score across the seven categories signifies a greater improvement in the person’s OHRQoL. This questionnaire was completed at baseline and one-month follow-up.

Psychosocial impact of dental aesthetics questionnaire (PIDAQ)

The PIDAQ questionnaire [54, 55] consists of 23 questions organized into four categories (three negative and one positive). These categories cover the following aspects: (1) social impact, (2) psychosocial impact, (3) aesthetic concern, and (4) dental self-confidence. The self-confidence dimension comprises six questions, while the social impact dimension includes eight questions related to the social dimensions of the patient’s quality of life. The psychosocial impact dimension comprises six questions focusing on how dental aesthetics affect psychological well-being, and the aesthetic concern category consists of three questions [56]. Participants filled out the questionnaire at the beginning of treatment. Each question was rated on a five-point Likert scale, ranging from 0 (no effect of dental appearance on the quality of life) to 4 (maximum effect of dental appearance on the quality of life). The response options included: 0 = not at all, 1 = a little, 2 = somewhat, 3 = strongly, and 4 = very strongly. The questionnaire was also translated and validated in Chinese, achieving a Cronbach’s alpha reliability coefficient of 0.94 [57]. This questionnaire was completed at baseline and one-month follow-up.

Scanning electron microscopy (SEM)

Two patients were randomly selected for SEM observation based on the criterion that the enamel surfaces of the maxillary central incisors appeared visually similar before bleaching, with no noticeable scratches, indentations, or protrusions. Prior to the bleaching process, their upper incisors were cleaned using prophylactic paste for 10 s, followed by thorough rinsing and drying for another 10 s. A negative control impression was made with polyvinyl siloxane impression material applied to the enamel surface. After completing two weeks of bleaching treatment, another impression was taken from the right upper central incisor. These impressions were then coated with a thin layer of gold and examined using a cold field emission scanning electron microscope (SU9000, Hitachi High-Technologies Corporation, Japan) at magnifications ranging from 500x to 2000x. As positive controls, impressions were obtained from two premolars without caries that had been extracted for orthodontic purposes [49]. Immediately following extraction, the vestibular surfaces of these teeth were treated with 32% phosphoric acid for 30 s. The impressions were then created using the same procedure as described earlier.

Measurement of pH levels over time

This study was conducted in vitro to evaluate the pH levels of two bleaching products at five-time points: before bleaching, 2 h, 4 h, 6 h, and 8 h during application. To ensure accuracy, three samples were analyzed at each time point for both products. Thirty extracted bovine central incisors were allocated randomly into two groups (n = 15) to undergo different bleaching procedures [58]. To simulate the moist environment of the oral cavity, a humidifier was utilized. A measured 0.1 ml of bleaching gel was applied to the enamel surface using a graduated syringe, and the gel was stirred hourly to maintain continuous contact with the material. A pH meter (Mettler Toledo, Greifensee, Switzerland) was employed to measure the pH of the bleaching gels. The pH meter was calibrated according to the manufacturer’s guidelines before reading each group. To obtain the average pH value, each product was evaluated three times following the methodology detailed in a previous study [59].

Statistical methods

All participants were randomly allocated (Fig. 1). Group assignments were concealed from the statisticians to maintain blinding. Each dental arch was examined independently. Data on TS intensity, collected using the VAS and NRS scales, were presented as histograms and tested for normality. As the data did not follow normal distribution, the Wilcoxon Signed Rank test (a = 0.05) was employed for intergroup comparisons. The mean and standard deviation of ΔE₀₀ color changes, measured from baseline to the 1-month follow-up, were computed. We applied paired t-tests to evaluate the differences in ΔE₀₀ values between the groups, determining the efficacy of the bleaching treatments. The significance threshold for all statistical tests was set to 5%. Comparisons of the OHIP-14 and PIDAQ scores were also performed using the Wilcoxon test. Additionally, ANOVA was utilized to examine variations in pH levels.

Results

Participant characteristics

The study initially assessed 31 participants based on predefined inclusion and exclusion criteria (Fig. 2), but only 22 met the requirements to participate in the clinical trial. Information regarding participants’ ages and gender distribution is summarized in Table 2.

Fig. 2.

Flow diagram outlining the phases of the study design, detailing the enrollment process and allocation criteria

Table 2.

Age and the distribution of the genders of the participants

Age (years; mean ± SD)	21.6±3.3
Gender (female; %)	81.8

* Abbreviations: SD Standard deviation

Throughout the bleaching protocol, all participants attended the scheduled recall visits, and no one withdrew from the treatment. The flow of participants through each phase of the study design is illustrated in Fig. 2.

Objective measurement of color changes

Both treatments demonstrated improvements in tooth shades by more than 4.5 units in ΔE₀₀ value. When considering the acceptability limits for simulated clinical environments based on ΔE₀₀, as proposed by Paravina et al., both bleached groups exceeded the perceptibility threshold (ΔE₀₀ >0.8) and acceptability threshold (ΔE₀₀ >1.8) [60]. The color change showed a normal distribution (P >0.05). There is no statistically significant difference intragroup (t-test, p >0.05) between after bleaching and 1-month follow-up in terms of ΔE₀₀ (Table 3). There was a significant difference between these two groups in terms of ΔE₀₀ (P<0.05) (Table 3).

Table 3.

Color change by ΔE₀₀ (mean ± SD) at two-time points

Tooth color parameters	Bleaching products	Color change by ΔE00		P value
		After bleaching	1-month follow-up
ΔE00	3% Hydrogen peroxide (Invisalign)	5.39 ± 2.50	5.32 ± 2.03	0.125
	8% Carbamide peroxide (Beyke iWhite)	4.75 ± 2.25	4.42 ± 1.86	0.761

Statistically significant difference between Invisalign and Beyke iWhite (t-test, p < 0.05).

Intensity of sensitivity

Using the five-point Numeric Rating Scale (NRS), where 0 = none, 1 = mild, 2 = moderate, 3 = considerable, and 4 = severe, both bleaching gels yielded scores close to 1, indicating mild sensitivity (Table 4).

Table 4.

The intensity of tooth sensitivity (mean ± SD) for both groups

	Pain scale	3% Hydrogen peroxide (Invisalign)	8% Carbamide peroxide (Beyke iWhite)	P value
Upper arch	NRS 0-4	1.2 ± 0.6	0.8 ± 0.6	0.007
	VAS 0-10	2.4 ± 1.7	1.7 ± 1.7	0.032
Lower arch	NRS 0-4	1.3 ± 0.6	1.0 ± 0.7	0.008
	VAS 0-10	2.5 ± 1.8	1.9 ± 1.7	0.008

A notable difference in TS intensity was identified between the groups on both pain scales for the upper and lower dental arches (NRS and VAS; p < 0.05; Table 4). These data are shown in Table 4.

Oral health impact profile (OHIP-Aesthetic)

The OHIP-14 scores (Table 5) showed a significant difference when comparing the baseline assessment prior to treatment with the results after two weeks of bleaching (p < 0.05). To ensure greater reliability, the assessment was repeated one month later, and the findings continued to demonstrate a statistically significant difference (p < 0.05). Notably, significant changes were observed between the baseline and post-bleaching in the dimensions of functional limitation, psychological discomfort, and handicap (Table 5). These dimensions above remained a statistically significant difference between the baseline and 1-month follow-up. This shows an evident bleaching effect remaining for one month.

Table 5.

OHIP scores (mean ± SD) at different time points

Dimension/scores per dimension (1 better – 10 worse)	Baseline	After bleaching	1-month follow-up
Functional limitation score	5.41 ± 2.06	4.18 ± 1.50a	4.00±1.48a
Physical pain score	4.00 ± 1.35	4.41 ± 1.71	3.36±1.47b
Psychological discomfort score	4.82 ± 2.09	3.55 ± 1.47a	3.45±1.44a
Physical disability score	3.45 ± 1.44	2.95 ± 1.25	3.05±1.33
Psychological disability score	3.86 ± 1.88	3.18 ± 1.44	3.18±1.65a
Social disability score	2.91 ± 1.23	2.36 ± 1.05	2.36±0.85a
Handicap score	3.09 ± 1.41	2.41 ± 0.73a	2.59±1.10a
Sum/scores (0 better – 70 worse)	27.54 ± 8.64	23.05 ± 6.64a	22.00 ± 7.33a

^aStatistically significant difference (Wilcoxon test, p < 0.05) compared to baseline

^bStatistically significant difference (Wilcoxon test, p < 0.05) compared to after bleaching

Psychosocial impact of dental aesthetics questionnaire (PIDAQ)

The PIDAQ questionnaire results, shown in Table 6, revealed significant differences between the baseline and after two weeks of bleaching, and 1-month follow-up controls (p<0.05). The dimensions of social impact, psychological impact, and dental self-confidence remained a statistically significant difference between the baseline and 1-month follow-up. This shows a sustained effect during the survey and after one month.

Table 6.

PIDAQ scores (mean ± SD) at different time points

Dimension	Baseline	After bleaching	1-month follow-up
Social impact (better 0–40 worse)	17.27 ± 5.39	12.68 ± 3.72a	13.55 ± 3.98a
Psychological impact (better 0–30 worse)	16.05 ± 5.08	11.82 ± 3.72a	12.45 ± 3.74a
Esthetic concern (better 0–15 worse)	5.73 ± 3.01	4.14 ± 2.19a	4.50 ± 1.71
Dental self-confidence (worse 0–30 better)	15.95 ± 5.14	19.23 ± 5.49a	19.18 ± 3.84a

^aStatistically significant difference (Wilcoxon test, p < 0.05) compared to baseline

Effect on enamel surface

The surface morphology of teeth treated with Invisalign exhibited greater roughness and more pronounced surface undulations compared to those treated with Beyke iWhite (Fig. 3a, b, c, and d). In comparison, the positive control (enamel etched with phosphoric acid) exhibited more pronounced morphological changes than the surfaces treated with either bleaching gel (Fig. 3e and f).

Fig. 3.

(a-b) Scanning electron microscope (SEM) analysis at 2000× magnification showing the middle third of an upper incisor treated with Beyke iWhite bleaching gel. (a) Enamel surface before application; (b) Enamel surface after application. (c-d) SEM analysis at 2000× magnification showing the middle third of an upper incisor treated with Invisalign bleaching gel. (c) Enamel surface before application; (d) Enamel surface after application. (e-f) SEM analysis at 2000× magnification of enamel etched with 32% orthophosphoric acid. e Surface of unetched enamel; (f) Surface of etched enamel displaying noticeable alterations

Measurement of pH levels over time

To monitor the trend of pH changes during the process, the average readings for each group were recorded at 0 h, 2 h, 4 h, 6 h, and 8 h using a digital pH meter (Table 7; Fig. 4). Initially, the Beyke iWhite bleaching gel was slightly alkaline, while the Invisalign bleaching gel was slightly acidic. In more detail, the Beyke iWhite bleaching gel had an initial average pH of 8.08. As the reaction proceeded, the pH gradually shifted towards neutrality and stabilized at a neutral level during the latter stages. At 8 h, the average pH had decreased to 7.26, which is near neutral. In contrast, the Invisalign bleaching gel began with an initial average pH of 6.91, remaining relatively stable throughout the reaction, with a final average pH of 6.82 at 8 h. This indicates that the Invisalign bleaching gel maintained a slightly acidic pH with minimal variation during the reaction. From these results, a declining trend in pH was observed with the Beyke iWhite bleaching gel, moving from alkaline to neutral by the end of the process. On the other hand, the Invisalign bleaching gel exhibited no significant changes in pH, maintaining a consistent acidic environment.

Table 7.

The changes in pH value (mean ± SD) at different assessment time

	Assessment time	3% Hydrogen peroxide (Invisalign)	8% Carbamide peroxide (Beyke iWhite)
pH value	0 h	6.91 ± 0.02A	8.08 ± 0.06A
	2 h	6.86 ± 0.04AB	7.86 ± 0.02B
	4 h	6.78 ± 0.04B	7.41 ± 0.03C
	6 h	6.82 ± 0.02AB	7.43 ± 0.03C
	8 h	6.82 ± 0.06AB	7.26 ± 0.02D

Comparisons are valid only within columns. Distinct letters indicate statistically significant differences (repeated measures ANOVA and Tukey’s test, p < 0.05)

Fig. 4.

Graph of pH levels over 8 h for Beyke iWhite and Invisalign bleaching gels

Discussion

This study is a randomized, triple-blind clinical trial designed to evaluate the effects of two at-home bleaching products, each containing a concentration of hydrogen peroxide (HP) slightly less than or equal to 3%. The study specifically focused on commercially available products with 3% HP and 8% carbamide peroxide (CP), which represent the lowest concentrations commonly found in at-home bleaching gels. To compare the efficacy of these two bleaching gels, a split-mouth design was employed, with treatments randomly applied to the left and right sides of the maxillary and mandibular arches in each participant. This design is based on the assumption that homologous teeth should have similar baseline shades. During the participant selection process, we also ensured that no visible color discrepancies existed between the homologous anterior teeth on either side. Given the similarity in the active ingredient concentrations between the two products, comparing homologous teeth allowed us to detect subtle differences in the bleaching effect.

The findings of this study led to the rejection of the null hypotheses regarding tooth color change, tooth sensitivity, and enamel surface morphology. Within the limitations of this study, it can be concluded that: (1) Both CP and HP bleaching products (= 3% HP) demonstrate effective bleaching results. Specifically, a statistically significant difference in whitening efficacy (ΔE₀₀) was observed between the two groups, with the 3% HP gel demonstrating a slightly better whitening effect. The intensity of the sensitivity of the two bleaching products was mild. 3% HP group reported a slightly higher intensity of tooth sensitivity, and qualitative SEM analysis revealed more pronounced alterations on enamel surfaces compared to 8% CP. (2) For patients who underwent bleaching using low-concentration gels, there was a positive impact on their quality of life.

Our study assessed the tooth bleaching effects (ΔE₀₀ values) after two weeks of bleaching and one-month follow-up. To further minimize measurement errors, a custom-made silicone guide was employed during color assessments, helping to standardize positioning and shield the teeth from external environmental factors. This contributed significantly to the stability and reliability of the experimental results [48, 51].

A significant bleaching effect was observed in both groups using a spectrophotometer, shown by reduced b* values (reduction in yellowness) and increased L* values (increased lightness). We assessed the mean ΔE₀₀ values (overall color change) after two weeks of bleaching. In our study, after two weeks of bleaching, the ΔE₀₀ values were 5.39 ± 2.50 for 3% HP and 4.75 ± 2.25 for 8% CP. When considering the acceptability limits for simulated clinical environments based on ΔE₀₀, as proposed by Paravina et al., both bleached groups exceeded the perceptibility threshold (ΔE₀₀ >0.8) and acceptability threshold (ΔE₀₀ >1.8) [60]. Both products have similar effective concentrations, yet their bleaching effects show statistically significant differences. The Invisalign gel demonstrates slightly better results. Meanwhile, the average difference of the ΔE₀₀ values between the two bleaching gels is 0.65, meaning it was not perceptible to the naked eye [60]. Furthermore, no statistically significant difference was found intragroup (p >0.05) between the results after bleaching and the one-month follow-up regarding ΔE₀₀, indicating that the color remained stable one month after treatment completion.

It is particularly noteworthy that the whitening efficacy of these low-concentration agents is comparable to that of traditional high-concentration formulations over similar treatment periods. 3% HP and 8% CP low-concentration agents achieved a level of whitening efficacy comparable to that reported for traditional high-concentration formulations over a similar treatment period. The mean ΔE₀₀ values of 3% HP and 8% CP after two weeks of bleaching are slightly lower than or comparable to the reported results when using 10% CP gel (ΔE₀₀ = 5.3 ± 2.3) [23]. This observation aligns with a systematic review and network meta-analysis: Most at-home bleaching agents exhibit similar efficacy, with differences being observed only when the highest concentrations were contrasted with the lowest ones [61]. When total exposure time and cumulative peroxide dose are matched, both low concentration and high concentration agents can achieve comparable long-term whitening results [10, 61].

Sensitivity is a major side effect that leads patients to discontinue bleaching treatment [22, 28, 30]. The shift toward lower concentrations of bleaching gels also reflects an increasing patient demand for comfort and reduced tooth sensitivity. Research indicates that while various desensitizing agents have been developed, their effectiveness in reducing tooth sensitivity is marginal and not statistically significant compared to desensitizer-free bleaching gels [30, 32, 33]. Regarding the intensity of bleaching-induced tooth sensitivity, a decreased concentration of peroxides has been linked to a reduced incidence of tooth sensitivity [27–30]. The results of our study show that the intensity of the sensitivity measured by the NRS and VAS scales was mild. On a 0–10 rating scale, the average pain intensity was below 2.5 units, while on the NRS scale, the average pain intensity was below 1.3 units. The underlying mechanism of bleaching-induced tooth sensitivity involves the penetration of peroxide into the tooth structure, with the diffusion rate of peroxide from the enamel and dentine to the pulp chamber being concentration-dependent [42]. A recent systematic review and the study by Chemin et al. both support these findings, revealing that lowering the concentration results in a reduction in tooth sensitivity intensity during at-home bleaching [24, 27]. Although a statistically significant difference in TS intensity was observed between the groups on both pain scales (p < 0.05), the clinical relevance of this finding is likely limited, given that the TS intensity was both very mild (0 < NRS < 1.5, 0 < VAS < 3). We suspect that this difference may be due to the slight variations in the effective concentration or differences in pH between the two products.

Regarding the pH value of the gels, the Invisalign bleaching gel is slightly acidic while the Beyke iWhite bleaching gel is slightly alkaline. The results of this study revealed that teeth treated with Invisalign exhibited more pronounced surface undulations and relatively higher sensitivity compared to those treated with Beyke iWhite. This difference may be closely related to the pH values. Previous research has demonstrated that bleaching gels with acidic pH promotes irregularities in enamel morphology [18, 21, 26]. An in vitro study conducted by Acuña et al. revealed that bleaching agents with pH value 5.1 exhibited higher permeability of hydrogen peroxide than the ones with pH values 6.3 and 7.0 [62]. Xu et al. demonstrated that bleaching gels with pH values 7.0 and 8.0 caused no significant alterations in enamel morphology or chemical composition in a 1 h in vitro experiment [63]. These findings highlight the significance of pH value in selecting bleaching agents, suggesting that neutral or alkaline formulations are preferable for maintaining enamel integrity and reducing sensitivity while effectively achieving whitening results. Our findings further validated their results through in vivo experiments.

During use, we expect the surface morphology to remain flat and smooth, which is beneficial in preventing tooth staining. A polished surface is less susceptible to staining and color rebound than a rough enamel surface in the long term [39, 41]. Our study found that both low-concentration bleaching gels exhibited less pronounced morphological changes than the surfaces treated with positive control (enamel etched with 32% phosphoric acid). Grazioli et al. conducted an in vitro study on at-office gels and demonstrated that 25% and 15% hydrogen peroxide cause less significant changes in enamel hardness and surface morphology than 35% ones [64]. Navarra et al. studied at-home gels with 10% carbamide peroxide and the SEM analysis confirmed the absence of relevant alterations of the enamel surface after 2-week in vivo treatment [49]. However, our study observed slight but noticeable enamel surface morphological change when applying low-concentration gels. We noticed that the study by Navarra et al. first took an impression of the tooth surface using polyvinyl siloxane impression material and then replicated it by pouring impressions with epoxy resin. In contrast, our study directly examined polyvinyl siloxane impressions using scanning electron microscopy, and this may explain the difference.

HP and CP exhibit differences in physicochemical properties, decomposition rates, and byproducts-induced whitening microenvironments. According to our measurement, the HP gel maintained a mildly acidic pH of approximately 6.7 to 6.9, which favors faster reactive oxygen species (ROS) release but may also increase enamel demineralization [11, 65]. Acidic conditions enhance proton-driven dissolution of hydroxyapatite, promoting outward diffusion of calcium and phosphate ions which leads to irregularities in enamel morphology [18, 21, 22, 26]. HP molecules, being small (34 Da) and highly reactive, diffuse into dentin when enamel permeability is increased by demineralization or by acidic contact [13, 14]. In contrast, the 8% CP gel undergoes slower hydrolysis and ultimately delivers the same active oxidant, with urea acting as a stabilizer [6, 11, 16, 49]. Urea decomposes into carbon dioxide (CO₂) and ammonia (NH₃), maintaining the gel’s local pH toward neutrality [7, 11]. This scenario reduces enamel demineralization risk and lowers the potential for tooth sensitivity [33, 34, 38, 49].

The psychological questionnaires serve as a comprehensive evaluation of bleaching effects and sensitivity. Regarding the social-psychological effect on tooth bleaching, this study demonstrated a positive social-psychological effect of tooth bleaching. This is measured by two complementary questionnaires: the Oral Health Impact Profile (OHIP) and the Psychosocial Impact of Dental Aesthetics Questionnaire (PIDAQ) [53–55]. Patient satisfaction was measured using the OHIP Questionnaire. OHIP identified a positive shift in their perceptions of oral health and its impact on their lives. Compared to baseline, patients reported less functional limitations, psychological discomfort, and social disability after bleaching treatment. The PIDAQ focuses more specifically on dental aesthetics [56]. After two weeks of bleaching, our patients scored better on the four scoring dimensions, which showed increased confidence and reduced anxiety regarding patients’ appearance post-treatment. During the one-month follow-up, both questionnaire results remained at a favorable level, demonstrating the sustained positive effects of the bleaching treatment on patients’ oral aesthetic confidence and quality of life.

Given the distinct properties of HP and CP gels, different protocols are worth considering to optimize both efficacy and safety. The protocol in this study included wearing times of 6–8 h, which is consistent with the product instructions. While this ensured equivalent contact time, recent published reviews suggest that a “one-size-fits-all” protocol may not be optimal for leveraging the unique characteristics of each agent [8, 16, 22]. The slightly better efficacy and higher sensitivity observed with the 3% HP gel over 6–8 h period suggest that its therapeutic window could be shorter. Given HP’s rapid release kinetics, it is plausible that most of its whitening action occurs within the first 1–2 h [15, 16]. This implies that for low-concentration HP agents, future clinical protocols might be optimized by recommending shorter application durations to balance efficacy and patient comfort.

The primary limitation of this study was that we evaluated only two bleaching products. The presence of other additives in the bleaching formulations may also have an impact on these outcomes. Future studies should utilize experimental gels with customized formulations to explore the effects. Furthermore, although the sample size of 22 participants is sufficient for statistical validity, it may limit the generalizability of our findings. To enhance clinical applicability, these results should be validated with larger sample sizes. Additionally, since the low-concentration gels exhibit low sensitivity in this study, future research could explore the feasibility of time-extended whitening treatments and its impact on whitening outcomes, enamel surface morphology, and long-term color stability.

This study provides valuable insights into the clinical performance and patient-reported outcomes of two low-concentration commercial bleaching products recently introduced to the market, 3% HP and 8% CP. Our findings contribute to the growing body of evidence supporting the use of low-concentration bleaching agents, which offer a gentle and effective approach to tooth bleaching. The change of color remained stable one month after treatment completion. Moreover, products at a concentration of hydrogen peroxide (HP) slightly less than or equal to 3% showed low tooth sensitivity and had a favorable impact on patients’ oral health-related quality of life.

Conclusion

Both Beyke iWhite (8% carbamide peroxide) and Invisalign (3% hydrogen peroxide) effectively whitened teeth while inducing mild tooth sensitivity. The use of these products also had a positive socio-psychological impact on participants. The 3% HP gel provided a bleaching efficacy advantage, while the 8% CP gel demonstrated lower sensitivity and less enamel alteration. This study evaluated two bleaching products and a sample size of 22 participants, which may limit the generalizability of the findings. Given the low sensitivity observed in the current study, future research could investigate the feasibility of prolonged treatment period with low-concentration gels and its impact on whitening outcomes, enamel surface morphology, and long-term color stability.

Authors' contributions

S.J. Chu and Y.H. Wang contributed to the study conceptualization. S.J. Chu, Y.Z. Wang, Z.H. Duan and P. Wu contributed to the methodology and investigation. S.J. Chu, Y.Z. Wang, Z.H. Duan, P. Wu, and Y.H. Wang contributed to the literature survey. S.J. Chu, Y.Z. Wang, and Y.H. Wang contributed to the data curation, formal analysis, and original draft. S.J. Chu, Y.Z. Wang, and Y.H. Wang contributed to the visualization, manuscript editing, and review. Y.H. Wang contributed to project administration. All authors contributed to the article and approved the submitted version. All authors read and approved the final manuscript.

Funding

This research was funded by the Science and Technology Committee of Fengxian District, Shanghai, China (Grant number FK20221402), the National Clinical Research Center for Oral Diseases, Shanghai, China (Grant number NCRCO202332), and the Shanghai Natural Science Foundation (Grant number 24ZR1443600).

Data availability

All data generated or analysed during this study are included in this published article.

Declarations

Ethics approval and consent to participate

This research project (Ref. number: SH9H-2023-T342-2) was approved by the research ethics committee of the Shanghai Ninth Hospital in accordance with the ethical guidelines of the Helsinki Declaration as revised in 2000. All participants signed the informed consent form.

Consent for publication

All participants signed the informed consent form.

Competing interests

The authors declare no competing interests.