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Acta Stomatologica Croatica logoLink to Acta Stomatologica Croatica
. 2022 Jun;56(2):120–131. doi: 10.15644/asc56/2/3

Effects of Different Over – the - Counter Whitening Products on the Microhardness, Surface Roughness, Color and Shear Bond Strength of Enamel

Esra Yildirim 1, Uzay Koc Vural 1,✉, Filiz Yalcin Cakir 1, Sevil Gurgan 1
PMCID: PMC9262110  PMID: 35821723

Abstract

Objective

The purpose of this in vitro study was to evaluate the effects of four over-the-counter (OTC) whitening products on the microhardness, surface roughness, color, shear bond strength (SBS) and surface charecteristics of human enamel compared with a product used for dentist-supervised home whitening.

Materials and methods

Seventy eight enamel specimens allocated into 6 groups (n=13): 1-Opalescence PF 10% (OP) dentist prescribed home whitening product, 2-Opalescence Go prefilled tray (PT), 3-Opalescence Whitening Toothpaste (WT), 4-Listerine Healthy White whitening mouth rinse (WMR), 5-Cavex Bite&White whitening pen (WP) and 6- no treatment (Con). The microhardness (VHN), surface roughness (Ra) and color of the specimens were measured (T0). The specimens were then subjected to whitening protocols for 14 days (T1.) followed by artificial saliva storage for 14 days (T2). The measurements were repeated at T1 and T2. The SBS test was done after the application of 35% phosphoric acid (Scotchbond Universal Etchant), followed by a universal adhesive (G-Premio Bond) and a micro hybrid/universal resin composite (Essentia) into a Teflon tube attached to the enamel surface (p<0.05). Surface morphologies of the enamel surfaces were examined by SEM. p value was set at 0.05

Results

Application of OP, PT and WP decrased the microhardness of enamel specimens (p<0.05) whereas, no significant changes were seen in the microhardness of enamel specimens treated with WT and WMR (p>0.05). Ra values of enamel specimens increased with the application of OP, PT and WT (p<0.05); whereas no changes were observed after the applications of WMR and WP (p>0.05). OP, PT, WMR, and WP changed the color of the enamel(p<0.05). There were not any significant differences among the SBSs groups, apart from OP applied enamel specimens. OP showed the least SBS values (p=0.001). SEM observations revealed smooth enamel surfaces.

Conclusions

The whitening products affected the microhardness, surface roughness, color of enamel differently. Only OP decreased the SBS of the enamel.

Keywords: MeSH Terms: Tooth Bleaching, Tooth Bleaching Agents, Dental Enamel, Hardness Tests, Surface Properties, Color, Author Keywords: Tooth Whitening, OTC Products, Microhardness, Surface Hardness, Shear Bond Strength

Introduction

Over the last decade, an increased interest has been observed in the field of tooth whitening since esthetic dentistry received more attention. Tooth whitening is gained growing popularity among the patients since it has been considered a conservative, safe, effective and minimally invasive method (1-4).

Today, tooth whitening methods range from professionally applied in-office whitening, professionally prescribed home whitening, nonprescription over-the-counter (OTC) whitening to the do-it-yourself (DIY) application (5-9).

Professionally prescribed home whitening is the most preferred whitening method (4). Dispensed to the patients and closely monitored by the dentists, this technique is done by using 10% carbamide peroxide (CP) in a tray that is worn for at least two weeks. This concentration is well accepted due to its safety, excellent esthetic results, low incidence of side effects and increased effectiveness of the whitening (9, 10).

The satisfactory results achieved with professionally prescribed home whitening systems have led the developments of OTC products. OTC products were first marketed by the year 2000, with their lower cost, availability, easy access and application (9, 11, 12).

Currently, there have been a huge number of OTC products available with capability of whitening within 1-4 weeks. These whitening products are in the form of gels, mouth washes, pens, gums, toothpastes, films or paint-ons with lower concentrations of hydrogen peroxide (HP), and are sold at pharmacies, supermarkets and over the Internet (9, 12-15). These products may have potential side effects (14-16). The influence of the whitening agent to oral tissue is important due to the oxidizing process which occurs during the whitening procedure. Several studies have reported an increase in the porosity, over-etched appearance, loss of prismatic structure and calcium and alterations in the organic content of enamel (3, 4, 6, 8, 16, 17).

However, there have not been a sufficient number of reports providing a scientific background for these whitening products. Since there has been a huge variety of new products and lack of evidence about their efficacy, the aim of this in vitro investigation was to evaluate the effects of four OTC products, that is, a prefilled tray, a whitening tooth paste, a whitening mouth rinse and a whitening pen, on the microhardness, surface roughness, color, shear bond strength (SBS), and surface charecteristics of human enamel as compared with a whitening gel (10% CP) used for dentist-supervised home whitening. The null hypothesis was that there would be no significant differences among the tested whitening products with regard to (1) microhardness, (2) surface roughness, (3) color, and (4) SBS to enamel.

Material and methods

The whitening products tested are shown in Table 1 and the experimental procedure is illustrated in Figure 1.

Table 1. Whitening products used in the study.

Whitening Product Manufacturer Composition Daily Use/Total
Number of Treatment Days
Opalescence PF 10%
(Dentist-supervised home whitening
(OP)		 Ultradent Products, South Jordan UT, USA 10% carbamide peroxide,
Polyacrylic acid, 0.3%
Sodium fluoride, 3%
Sodium hydroxide		 8 hours a day/14 days
Opalescence Go
(Prefilled Tray)
(PT)		 Ultradent Products, South Jordan UT, USA 6% Hydrogen Peroxide, sodium hydroxide, potassium nitrate, sodium fluoride 30 minutes a day/14 days
Opalescence
(Whitening Toothpaste)
WT)		 Ultradent Products, South Jordan UT, USA Sodium fluoride, glycerin, water (aqua), silica, sorbitol, xylitol, flavor, poloxamer, sodium lauryl sulfate, carbomer, FD&C Blue # 1 (Cl 42090), FD&C Yellow # 5 (Cl 19140), sodium benzoate, sodium hydroxide, Sparkle (CI 77019, CI 77891), sucralose, xanthan gum. 2 min twice daily/14 days
Cavex Bite&White
(Whitening Pen)
(WP)		 Cavex, Haarlem, Holland 6% Hydrogen Peroxide, polyethylene glycol, PVP peroxide, glycerin, Peppermint oil 30 minutes a day/14 days
Listerine Healty White
(Whitening Mouth Rinse)
(WMR)		 Johnson & Johnson Consumer Inc., New Jersey, USA Water, 8% alcohol, hydrogen peroxide 2%, sodium phosphate, poloxamer
407, sodium lauryl sulfate, sodium citrate, mint aroma, menthol, eucalyptol,
Sodium saccharin, sucralose		 2 min twice daily/14 days
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Figure 1.

Figure 1

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Study protocol

Ethical aspects

The study protocol was approved by the non- interventional Ethics Committee of the University (2020/08-41).

Sample size calculation:

One-way ANOVA-type power analyses were done to calculate the estimated sample size using G*Power package (version 3.1, Heinrich-Heine Dusseldorf University, Dusseldorf, Germany). The selected parameters were; 95% confidence interval, 80% power and 0.50 effect size. Twelve specimens per group were calculated.

Specimen Preparation

One hundred and twenty human permanent maxillary central incisors obtained from the pool of extracted teeth at Oral Surgery Department of the Dental School were examined by a stereomicroscope at 10 X magnification (American Optical, Buffalo, NY, USA) (18), and 78 teeth were selected for the study in line with the Human Tissue Act procedures. The teeth with similar shade, size and surface texture were employed. The roots were cut 1 mm below the cemento-enamel junction with a diamond saw attached to a sectioning machine (Isomet 1000 Precision Diamond Saw, Buehler Ltd, Illinois, USA). Debris was removed by a curette and air / water jet from the crowns and then stored in 0.1% thymol solution at 5OC. The crowns were embedded in acrylic resin molds (Integra, Ankara, Turkey) leaving the buccal surfaces exposed and then the enamel surfaces were polished by silicon carbide papers (600, 800, 1000, 1200 and 2000 grit) (English Abrasives, London, UK). Subsequently, the specimens were allocated into 6 groups (n = 13) randomly for whitening purposes as follows:

  • 1-Opalescence PF 10% (OP)/dentist prescribed at- home whitening product

  • 2-Opalescence Go (PT) / prefilled tray

  • 3-Opalescence (WT) /Whitening Toothpaste

  • 4-Listerine Healthy White (WMR)/ whitening mouth rinse

  • 5-Cavex Bite&White (WP)/whitening pen

  • 6- (Con) no treatment

Representative specimens (1; per group) were allocated for SEM. All the specimens were prepared for microhardness (VHN), surface roughness (Ra) and color testing before the application of whitening products.

Microhardness measurement

The baseline Vicker’s Hardness Number (VHN) of the specimens was measured with a Vicker’s microhardness tester (HMV-2, Shimadzu Corp., Kyoto, Japan) with a load of 980 g for 15s. Five indentations were taken at 100 mm intervals and averaged. Vicker’s microhardness tester was calibrated after each reading.

Surface roughness measurement

To measure the baseline surface roughness (Ra) of the specimens, a contact-type profilometer (Perthometer M2, Mahr GmbH, Gottingen, Germany) was used. Each specimen was placed on a custom-made jig to ensure its position, and then the needle of the device was inserted on the surface of the specimen. Five Ra measurements were recorded from the center of each specimen’s surface and averaged. The profilometer was calibrated after every 3 readings.

Color Evaluation

The baseline color evaluations were assessed by a spectrophotometer (CM-700d, Konica Minolta, Tokyo, Japan) with software (Spectra Magic NX, Konica Minolta). CIE L*a*b* recordings were obtained as L* is lightness, from white to black (100 - 0), a* is red – green and b* is yellow – blue chromatic coordinates. For each specimen, three readings were obtained and averaged. D65 standart light was used for irradiation, and a white reflectance standard (CM-A117, Konica Minolta) and a black box (CM-A182, Konica, Minolta) were used for calibration. Color change was calculated as follows: ΔE=[(L1-L0)2+(a1-a0)2+(b1-b0)2]1/2 and ΔE00 = [(ΔL´/KLSL) 2 + (ΔC´/KCSC) 2 + (ΔH´/KHSH) 2 +RT* (ΔC´/KCSC) * (ΔH´/KHSH)]1/2 (19-21).

After surface hardness, roughness and color evaluations, all goups were subjected to whitening protocols according to the manufacturer’s instructions.

Whitening protocols

Whitening agents were used with reference to the the manufacturer’s instructions. Between whitening procedures, artificial saliva at 4◦C (22) was used as storage media.

OP (n:12): Opalescence PF 10% (Ultradent, Inc., South Jordan UT, USA) was applied with a brush onto the enamel surfaces approximately 1 mm thick for 8 hours a day and the specimens were kept in humid enviroment at 37°C for 14 consecutive days. At the end of each whitening procedure, the whitening gel was removed and the specimens were thoroughly rinsed and dried. The specimens were then transferred into the artificial saliva which was renewed daily.

PT (n:12): After the prefilled whitening tray (Opalescence Go, Ultradent, Inc., South Jordan UT, USA) had been taken out from its packaging, the colored outer tray was removed leaving the white inner tray. Then, this layer was cut according to the size of enamel specimen and enamel surface was covered with the whitening strip for 30 min at 37°C for 14 days. After each whitening protocol, the specimens were thoroughly rinsed and dried, and then transferred into the artificial saliva.

WT (n:12): A soft electric toothbrush in daily mouth cleaning mode (Triumph 5000 D34; Oral B, Braun GmbH, Kronberg Germany) with a whitening toothpaste (Opalescence, Ultradent Inc., South Jordan, Utah,USA) which was dilued by distilled water at a 1:3 (w/v) ratio (23) was used for brushing purposes for 2 min twice/ a day during 14 days (24). Following the brushing procedure, the specimens were thoroughly rinsed and dried and then transferred into the artificial saliva.

WMR (n:12): The specimens in this group were immersed in whitening mouth rinse (Listerine Healthy White, Johnson & Johnson Consumer Inc., New Jersey, USA) for 2 min twice a day for 14 days in humid atmosphere at 37°C. The specimens were thoroughly rinsed and dried and then kept in artificial saliva following each immersion.

WP (n:12): After the cap of the whitening pen (Cavex Bite&White, Cavex, Haarlem, Nederland) had been taken off, the pen button was turned until a small droplet of whitening gel appeared on the tip brush. The enamel surface was covered with a thin layer of gel for 30 min/ a day for 14 days. At the end of each treatment, specimens were thoroughly rinsed and dried and then transfered into articial saliva.

After 14 days of whitening protocols, surface hardness, roughness and color measurements were repeated (T1) and then the specimens were kept in artificial saliva at 370C for another 14 days. The measurements were again repeated (T2).

SBS test

The specimens including the CON group, underwent a SBS test. After the application of 35% phosphoric acid (Scotchbond Universal Etchant, 3M ESPE, St. Paul, MN, USA) onto the enamel surfaces for 15s, rinsing with water for 5s and gentlty air-drying, a universal adhesive (G-Premio Bond, GC Corporation, Tokyo, Japan) was applied over the enamel surface according to the manufacturer’s instruction for 10 s by a micro brush, air blown for 5s and light-irradiated for 10s using an LED light curing device (440-465 nm, 1.400 mW/cm2, Starligt S, Mectron s.p.a., Carasco, Italy). Then, a Teflon tube (4mm diameter X 3mm height) was attached to the enamel surface and a micro hybrid/universal resin composite (Essentia, GC, Tokyo, Japan) was incrementally inserted, polymerized for 40s and then the tube was removed. Having been stored in distilled water at 37°C for 24h, the specimens were transferred to a universal testing machine (LR50K, Lloyd Instruments Ltd., Fareham, Hants, UK) in shear mode with a knife-edge testing apparatus at a crosshead speed of 1 mm/min. SBS was calculated as the ratio of fracture load and bonding area, expressed in megapascals (MPa) (25).

Scanning Electron Microscope (SEM) Analysis

Six specimens (1; per group) were gently air dried, dehydrated with alcohol, gold coated and then analyzed by SEM (JSM6400, Jeol, Tokyo, Japan) at X400 magnification (25).

Statistical Analysis

SPSS software 23.0 (The Statistical Package for The Social Sciences) was used to analyze the data. Surface microhardness, roughness, color change and SBS of groups were compared by Two-way ANOVA. Mean values were analyzed by ANOVA and Turkey’s test. The level of significance was set at p 0.05

Results

The average VHN values of enamel specimens and standard deviations (± SD) at T0, T1 and T2 are presented in Figure 2. No significant differences were found among VHN values of the enamel specimens before the application of whitening products (p>0.05). The application of OP, PT and WP decrased the microhardness of enamel specimens (p<0.001, p<0.001, p=0.010, respectively) whereas, there were no significant changes in the microhardness of enamel specimens treated with WT and WMR (p=0,058; p=0,052). After they had been stored in artificial saliva for 14 days, VHN values of the enamel specimens did not change (p>0.05).

Figure 2.

Figure 2

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Microhardness (VHN) values (mean ± SD) of the tested groups. Different lowercase letters indicate significant differences among times and different uppercase letters indicate significant differences among groups (p < 0.05).

The average Ra values and standard deviations (± SD) of enamel specimens at T0, T1 and T2 are shown in Figure 3. There were no significant differences among Ra values of the enamel specimens before the apllication of whitening products (p>0.05). Ra values of enamel specimens increased with the application of OP, PT and WT (p=0.002, p=0.033, p=0.002 respectively); whereas there were no changes after the applications of WMR and WP (p=0.747, p=0.174, respectively). After artificial saliva immersion for 14 days, no signicant changes were observed in the Ra of enamel specimens (p>0.05). Neither the application of WMR and WP nor the storage in artificial saliva for 14 days caused any changes in the Ra of the enamel specimens (p>0.05).

Figure 3.

Figure 3

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Surface roughness (Ra)(μm) values (mean ± SD) of the tested groups. Different lowercase letters indicate significant differences among times and different uppercase letters indicate significant differences among groups (p < 0.05).

Mean and standard deviation (± SD) of difference of CIELab parameters (ΔL, Δa and Δb) are shown in Table 2 and ΔE0 and ΔE000 (color difference between T0-T1), ΔE1 and ΔE100 (color difference between T1-T2) and ΔE2 and ΔE200 (color difference between T0-T2) values are shown in Table 3 and Table 4. There were significant differences in ΔE0 and ΔE2 of the enamel specimens treated with the whitening products (p<0.001), however, no differences were found among the groups in terms of ΔE1 (p=0.870). OP, PT, WMR and WP caused significant color differences (p<0.001, p=0.010, p=0.018, p<0.001 respectively); whereas no difference was observed with WT (p=0.221).

Table 2. ΔL, Δa and Δb values (mean ±SD) of the tested groups.

OP PT WT WMR WP
ΔL0 -1 0.47 ±2.86 -0.13 ±2.30 -0.38 ±1.58 -0.63 ±1.69 -0.01 ±2.52
ΔL1-2 -0.88 ±1.75 0.45 ±2.77 0.05 ±1.19 0.21 ±1.23 0.90 ±1.55
ΔL0-2 -0.42 ±2.03 0.33 ±2.63 -0.33 ±0.96 -0.43 ±1.59 0.89 ±3.16
Δa0-1 -0.83 ±0.69 -0.73 ±0.92 -0.35 ±0.57 -0.70 ±0.60 -0.58 ±1.06
Δa1-2 -0.11 ±0.48 -0.43 ±0.89 -0.39 ±0.52 -0.22 ±0.34 -0.22 ±0.64
Δa0-2 -0.94 ±0.78 -1.17 ±0.77 -0.74 ±0.46 -0.92 ±0.79 -0.80 ±0.96
Δb0-1 -5.53 ±4.26 -2.19 ±2.27 -0.69 ±1.68 -1.30 ±1.67 -2.84 ±2.39
Δb1-2 -1.07 ±1.74 -2.18 ±1.94 -1.47 ±1.13 -1.56 ±1.43 -0.84 ±1.93
Δb0-2 -6.60 ±4.23 -4.37 ±2.02 -2.16 ±1.30 -2.86 ±1.20 -3.68 ±1.80
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Table 3. ΔE0, ΔE1 and ΔE2 values (mean ± SD) of the tested groups.

Groups ΔE0 ΔE1 ΔE2 p+
OP 7.90 ±1.78Aa 2.56 ±1.13Ab 7.35 ±1.50Aa <0.001*
PT 5.37 ±1.59Ba 3.49 ±2.12Ab 3.89 ± 0.82Bb 0.010*
WT 2.67 ± 0.92Ca 1.95 ±1.15Aa 2.36 ± 0.61Ca 0.221
WMR 3.52 ±1.08Ca 2.14 ±1.20Ab 2.69 ±0.96Cb 0.018*
WP 5.11 ±1.44Ba 2.60 ±0.93Ab 4.49 ±0.63Ba <0.001*
p <0.001 0.870 <0.001
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Different letters, lowercase in rows for different times and uppercase in columns for different whitening products, indicate significant difference (p < 0.05).

Table 4. ΔE00, ΔE100 and ΔE200 values (Mean ±SD) of the tested groups.

Groups ΔE000 ΔE100 ΔE200 p+
OP 3.59 ±0.86aA 1.50 ±0.71bA 3.87 ±0.97aA <0.001
PT 2.27 ±0.54aB 2.20 ±1.25aA 3.08 ±0.80bA 0.035
WT 1.39 ±0.35aC 1.10 ±0.65aA 1.48 ±0.43aC 0.187
WMR 1.63 ±0.59aC 1.21 ±0.56aA 2.08 ±0.68bB 0.005
WP 2.58 ±0.44aB 1.54 ±0.63bA 2.98 ±0.81aAB <0.001
p <0.001++ 0.095+++ <0.001++
+: One-way repeated measures ANOVA results; ++: One-way ANOVA results, +++: Welch’s test results. Different letters, lowercase in rows for different times and uppercase in columns for different whitening products indicate significant difference (p < 0.05)
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Significant differences among the ΔE000 and ΔE200 of the enamel specimens treated with whitening products were found (p<0.001) but there were no differences among the groups in terms of ΔE100 (p=0.095). Significant differences were also found among the ΔE000, ΔE100 and ΔE200 of the groups except WT.

Mean and standard deviations (± SD) of SBS values are illustrated in Figure 4. There were no significant differences among the groups, except OP applied enamel specimens that showed the lowest SBS value (p=0.001).

Figure 4.

Figure 4

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Shear bond strength (SBS) values (MpA) (mean ± SD) of the tested groups. Different letters indicate differences among whitening products (p < 0.05).

SEM micrographs are illustrated in Figure 5. SEM observations revealed smooth enamel surfaces without any deleterious effects of whitening products. Only few scratches, due to the grinding procedure, were observed on the enamel surfaces.

Discussion

The results of studies evaluating enamel microhardness after whitening with OTC products are quite different. Zantner et al. (26) tested different OTC and home whitening products in terms of the changes they created on the surface of enamel, and reported that all materials used significantly affected the surface microhardness. Azer et al. (17) examined the microhardness of enamel specimens treated with 3 whitening tray and 2 whitening strip systems and observed that all products decreased the microhardness of the enamel. Greenwall-Cohen et al. (13) also reported that the tested non-HP OTC whitening products available in UK resulted in reduction in Vickers microhardness.

In the present study, OP, PT and WP decreased the microhardness of the enamel, while WMR and WT did not cause any changes in the microhardness of the enamel. Therefore, the first hypothesis was rejected. A shorter application time of WMR and WT compared to other tested materials could affect the results. The storage in artificial saliva 14 days after the whitening was completed did cause no change in the microhardness values of all specimens.

The change in enamel surface roughness has also been considered a problem in tooth whitening. Several studies have reported enamel surface roughness increases after whitening with high concentrations of HP or CP (27-29). However, Sasaki et al. (30) examined the surface morphology of the enamel specimens whitened with Colgate Platinum (10% CP) and Day White 2Z (7,5% HP) and reported some micro changes in the surfaces of enamel treated with both materials. On the other hand, Kwon et al. (27) have observed no changes in the enamel surface roughness after the application of professionally prescribed or OTC whitening agents. In the present study, a significant increase in the enamel surface roughness was observed in OP, PT and WT; whereas no changes were found in WMR and WP. Therefore, the second hypothesis was rejected. WMR had the lowest concentration of peroxide (2% HP) among products examined. Although WP had higher HP concentration (6%) than WMR, no change was observed on enamel surfaces roughness treated with this whitening product, either. The storage in artificial saliva 14 days after the whitening had been completed did not change the values of surface roughness of all specimens too.

In this study, the profilometer readings were also supported by the SEM evaluations. SEM investigations have been largely preferred for the evaluation of surface morphology of whitened teeth. However; the results of the SEM studies were different and conflicting. Most of them show little or no change of whitened enamel surfaces. (26, 27, 30-32) Auschill et al. (32) examined the changes in enamel morphology treated with two different whitening products; a tray based (5% HP) and a whitening strip (5.3% HP) by SEM and reported that both of the whitening products caused no changes on the enamel surface structure. In the present study, SEM examinations also revealed no changes of the enamel surfaces after applying whitening products.

The behavior of enamel regarding its color change (ΔE and ΔE00) was addressed as well. The clinical acceptance threshold considered for ΔE is 2.7 and for EΔ00 is 1.8 (19). Any color difference value higher than these thresholds can be distinguished by an unskilled individual and cannot be considered clinically acceptable (19). OP, PT, WMR and WP created significant ΔE and ΔE00 values but no difference was observed with WT. Therefore, the third hypothesis was also rejected. The highest ΔE value after 14 days was in OP followed by WP, PT and WMR. The lowest ΔE value was seen in WT. However, the highest ΔE00 value after 14days was in OP followed by PT, WP and WMR respectively. Additionally, the ΔE and ΔE00 values presented by WT were lower than 2.7 and 1.8, respectively. This may be due to the ingredients and and short application time of the product. Although WT contains silica, an abrasive which was considered to be a whitening agent, no color difference was seen after using this product.

It has been stated that whitening products with lower HP concentration show less whitening effect compared to products having higher peroxide concentrations (30, 31). However, other studies have reported that low or high concentrations do not make significant differences in terms of whitening when the application time is 1-2 weeks (30-32). Successful results were obtained with tray systems (30-32). Dietschi et al. (33) compared various OTC products and they obtained best whitening results with tray systems. Kielbassa et al. (34) examined 5 different OTC products including a tray system, a whitening strip and varnishes and concluded that whitening occured in the first few minutes and did not change with the extended time. In this study, the highest color change was also observed immediately after whitening, and color change decreased in all products after 14 days of bleaching. These results supported the recommendation of a delayed color determination (9).

It has been reported in several studies that whitening agents negatively affect the bond strength of resin composites (32, 35). When a subsequent esthetic resin composite restoration was planned after the whitening procedure, it has been recommended to wait for 2-3 weeks to ensure adequate bond strength (36, 37). Nevertheless, Zu et al. (25) reported that the amount of oxygen in the enamel either whitened or not did not differ and the reason for low bonding strength was not the residual oxygen, but structural micromorphological deterioration in the tooth tissues. A decrease in microhardness, calcium loss and organic structure changes have also been associated with weakening in bond strength (38). In the current study, a SBS test was done after the artificial saliva storage for 14 days. Only SBS values of OP applied specimens were different and lower than the specimens treated with other whitening products. This may be the due to the long application time of this product. The SBS values of the enamel surfaces treated with PT, WT, WMR and WP were not different from the values of the intact enamel surfaces. Hence, the fourth hypothesis was rejected.

Tooth surface undergoes a special interaction with the saliva that involves the interchange of various ions and regulates the re- and demineralization process in the oral cavity. Since it is not easy to imitate the actual oral conditions which could vary in each person, the in vitro changes of human enamel after whitening may not be relevant. Therefore, more clinical studies are required to understand the effects of whitening products, especially the effects of new products with different whitening mechanisms. Despite an increase in the variety of OTC products in the market, there is no long-term proof of safety and durability of whitening of these products. Therefore, it should be mandatory to observe the long-term results and their potential detrimental effects on the enamel.

Conclusions

The results of this in vitro study indicate that whitening for the 14 days with Opalescence PF 10%, dentist prescribed at-home whitening product (OP), Opalescence Go, prefilled tray (PT) and Cavex Bite&White, whitening pen (WP) decreased the microhardness of the enamel, whereas Opalescence, whitening toothpaste (WT) and Listerine Healthy White, whitening mouth rinse (WMR) did not make any changes.

The application of OP, PT and WT increased the surface roughness of the enamel, while WMR and WP did not cause any changes.

The most evident colour difference was found after application of OP, then PT, WMR and WP, respectively.

Only the application of OP decreased the shear bond strength of enamel to resin composite bonded with universal adhesive. Other teeth whitening products had no effects on shear bond strength.

The SEM analysis revealed that the tested whitening products cause no deleterious effects on tooth enamel.

Figure 5 (A-F).

Figure 5 (A-F)

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Representative SEM images of enamel surfaces subjected to the tested products; A- untreated, intact enamel, B-after application of OP, C- after application of PT, D- after application of WT, E- after application of WMR, F after application of WP (X400 magnifications)

Footnotes

Conflict of interest

Authors declare no conflict of interest

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Articles from Acta Stomatologica Croatica are provided here courtesy of University of Zagreb: School of Dental Medicine

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