Odontology DOI 10.1007/s10266-014-0163-4

ORIGINAL ARTICLE

Effect of various tooth whitening modalities on microhardness, surface roughness and surface morphology of the enamel So Ran Kwon • Steven R. Kurti • Udochukwu Oyoyo Yiming Li



Received: 15 January 2014 / Accepted: 8 June 2014 Ó The Society of The Nippon Dental University 2014

Abstract The purpose of this study was to evaluate the effect of four whitening modalities on surface enamel as assessed with microhardness tester, profilometer, and scanning electron microscopy (SEM). Whitening was performed according to manufacturer’s directions for over-the-counter (OTC), dentist dispensed for home use (HW) and in-office (OW) whitening. Do-it-yourself (DIY) whitening consisted of a strawberry and baking soda mix. Additionally, negative and positive controls were used. A total of 120 enamel specimens were used for microhardness testing at baseline and post-whitening. Following microhardness testing specimens were prepared for SEM observations. A total of 120 enamel specimens were used for surface roughness testing at baseline and post-whitening (n = 20 per group). Rank-based Analysis of Covariance was performed to compare microhardness and surface roughness changes. Tests of hypotheses were two-sided with a = 0.05. There was a significant difference in Knoop hardness changes (DKHN) among the groups (Kruskal–Wallis test, p \ 0.0001). Significant hardness reduction was observed in the positive control and DIY group (p \ 0.0001). Mean surface roughness changes (DRa) were significantly different among the groups (Kruskal–Wallis test, p \ 0.0001). Surface roughness increased in the OTC group (p = 0.03) and in the positive control (p \ 0.0001). The four whitening modalities—DIY, OTC, HW and OW induced minimal surface morphology changes when observed with SEM. It can be concluded that none of

S. R. Kwon (&) University of Iowa College of Dentistry and Dental Clinics, Iowa City, IA, USA e-mail: [email protected] S. R. Kurti  U. Oyoyo  Y. Li Loma Linda University, Loma Linda, CA, USA

the four whitening modalities adversely affected enamel surface morphology. However, caution should be advised when using a DIY regimen as it may affect enamel microhardness and an OTC product as it has the potential to increase surface roughness. Keywords Tooth whitening  Enamel  Microhardness  Surface roughness  Scanning electron microscope

Introduction The demand for tooth whitening is increasing and is reflected by a wide variety of whitening options available. Current tooth whitening products range from professionally applied in-office whitening (OW) and professionally supervised patient applied home whitening (HW) to nonsupervised over-the-counter products (OTC) and do-ityourself (DIY) regimens [1]. Several studies have investigated the effect of different whitening regimen on tooth whitening efficacy [2–4]. According to a clinical study that compared three different whitening techniques with respect to the whitening times required to achieve a defined level of whitening—the cycles required increased from office whitening, home whitening to OTC products [3]. Whereas a recent in vitro study showed that the whitening efficacy of Crest Whitestrips, an OTC product was comparable to professionally dispensed home whitening and professionally applied in office whitening for up to 3 months post whitening, while a DIY regimen composed of a strawberry mixture did not produce any significant color change [1]. The availability of OTC products and various DIY whitening regimens have provided better access to whitening for the public [5]. However, the ADA Council on

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Odontology Table 1 Active ingredient, pH and application regimen by group

CA citric acid, CHNaO3 Sodium bicarbonate, HP hydrogen peroxide

Group

Concentration and pH

NC: water of grade 3

pH 7.0

1 Application (60 min), at 35 °C

DIY: strawberry mix

CA, CHNaO3, pH 7.2

3 Applications (5 min each) at 5-day intervals

OTC: crest 3D intensive

9.5 % HP, pH 6.0

1 Daily application (2 h) for 7 days

HW: opalescence PF

10 % CP, pH 7.4

1 Daily application (6 h) for 14 days

OW: zoom whitespeed

25 % HP, pH 7.2

3 Applications (45 min each) at 5-day intervals

PC: citric acid

1.0 % CA, pH 3.9

1 Application (60 min), at 35 °C

Scientific Affairs has raised concerns regarding the longterm safety of unsupervised whitening procedures, due to possible undiagnosed or underlying oral health care problems [6–9]. There have been numerous studies evaluating the effect of home whitening and in-office whitening on surface morphology and microhardness changes [10–18]. However, the use of DIY whitening regimens have not been investigated in terms of effects on microhardness, surface roughness and surface morphology changes. The purpose of this study was to evaluate the effect of four different whitening modalities: in-office whitening, dentist dispensed home whitening, over-the-counter products and do-it-yourself whitening on surface enamel as assessed with microhardness tester, profilometer, and scanning electron microscopy following guidelines of International Standards Organization (ISO) 28399 [19]. The null hypotheses tested were that there would be no differences in the three tested parameters, microhardness, surface roughness and surface morphology, after whitening among the test groups.

Materials and methods Sample selection and preparation Extracted sound human third molars without identifiers (240) were collected and stored in 0.1 % buffered Thymol solution at 4 °C. Loma Linda University Institutional Review Board (IRB) approved the use of the extracted human teeth with no identifiers as a non-human subjects study. Teeth were cleaned of gross debris and placed in artificial saliva for 24 h at 37 °C prior to initiating the experiment. The artificial saliva was prepared according to the modified Fusayama solution and adjusted to pH 6.0 as described in ANSI/ADA Specification 41 [20]. Teeth were sectioned and the crowns embedded in self-curing polyacrylic cylinders (2 cm wide and 3 cm high) to expose a flat 4 9 6 mm window of enamel surface for microhardness, surface roughness measurements and SEM observations. The surface was ground using a sequence starting at P400 and sequentially increasing to P1200 silicone carbide paper under a constant flow of water. A slurry of aluminum oxide with a mean particle size of 0.3 lm was used for the final polishing.

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Application regimen

Experimental groups Table 1 shows the composition, pH, manufacturer, and regimen of the whitening protocols used in the study. Group NC was the negative control group, treated with water of grade 3 for 60 min at 35 °C. Group DIY received a puree of strawberry (15 gm) mixed with baking soda (2.5 gm) (Arm & Hammer Baking Soda, Church & Dwight Co., Inc, Princeton, NJ, USA) for 5 min followed by a final brush with a soft toothbrush (Colgate Oral Pharmaceuticals, Inc., New York, NY, USA) for 30 s. The procedure was repeated two more times at 5-day intervals [21]. Group OTC received daily applications of whitening strips (Crest 3D Intensive, Crest Pro Health, P&G, OH, USA) for 2 h for 7 days, according to manufacturer’s instructions. Group HW was treated daily with a 10 % carbamide peroxide gel (Opalescence, Ultradent Products Inc, UT, USA) placed in a custom fabricated tray for 6 h/day for 14 consecutive days. Group OW represents the professionally applied group and was treated with 25 % hydrogen peroxide (Philips Zoom White Speed, Philips Oral Healthcare, LA, CA, USA) for 45 min using light activation. The whitening material was replenished every 15 min according to manufacturer’s instructions. Group PC, the positive control group, was treated with 1.0 % citric acid for 60 min at 35 °C. Any remaining material of the treatment session was rinsed off with distilled water and specimens stored in artificial saliva at 37 °C throughout the experiment. Microhardness testing A total of 120 enamel specimens were used for microhardness testing (n = 20 per group). Measurements were taken with a microhardness tester (M-400-H1 Hardness Testing Machine, LECO, Joseph Charter Township, MI, USA) at baseline and 24 h after the last whitening session (post-treatment). Specimens were positioned perpendicular to the long axis of the diamond indentor to record the Knoop hardness (KHN). A load of 0.49 N was applied for 15 s. Three indentations were obtained from each specimen and averaged.

Odontology Table 2 Knoop hardness and surface roughness parameters (Mean/SD) by group at baseline and after the whitening treatment

* p \ 0.05, ** p \ 0.0001

Group

KHN

Ra

Baseline

After whitening

Baseline

After whitening

NC

333.13/27.51

334.34/27.26

0.045/0.017

0.046/0.016

DIY

324.45/32.23

275.06**/25.01

0.047/0.014

0.047/0.017

OTC

326.00/31.66

324.28/27.75

0.040/0.015

0.045*/0.018

HW

329.40/30.77

329.36/32.87

0.036/0.012

0.037/0.009

OW

332.80/34.80

330.77/31.03

0.045/0.013

0.048/0.015

PC

333.63/32.32

100.75**/12.33

0.042/0.011

0.076**/0.016

Surface roughness testing A total of 120 enamel specimens were used for surface roughness testing (n = 20 per group). A contact profilometer (Mitutoyo Surftest, SV-2000, Andover, UK) was used to measure the roughness at baseline and after the last whitening session (post-treatment). Surface roughness was measured along the X-axis with a preset evaluation length of 4 mm. Three scans were conducted from at least 100 z values across the scan as calculated by software and averaged (Ra).

summarized in Table 2 and illustrated in Figs. 1 and 2. There was no significant difference of the two measured parameters (KHN & Ra) among the six different groups at baseline. There was a significant difference in Knoop hardness changes (DKHN) among the groups (Kruskal– Wallis test, p \ 0.0001). There was a significant drop in hardness in the positive control and in the DIY group (p \ 0.0001) whereas the other groups showed no difference when compared to baseline values. The reduction in Knoop hardness after treatment was more than 10 % in the

Scanning electron microscopic (SEM) evaluation Enamel specimens used for the microhardness testing were removed from the acrylic block and prepared for observation with the scanning electron microscope (Hitachi High Technologies America, Inc, USA). Specimens were dried with a series of ethanol solutions and sputter coated with gold in a vacuum evaporator. Photomicrographs of representative areas were taken at 2,0009 magnifications. The enamel changes were classified as no alterations (0), mild or slight alteration as demonstrated by slight alterations in surface roughness and irregular patterns of conditioning (1), moderate when distinct etching patterns were observed demonstrating loss of superficial structure (2), and severe surface alterations including pitting of the enamel surface (3). Fig. 1 Boxplot of change in Knoop hardness (DKHN) by group

Statistical analysis Measurements of interest included Knoop hardness and surface roughness values. Rank-based Analysis of Covariance (ANCOVA) and the Kruskal–Wallis procedure were performed to compare microhardness and surface roughness changes among the different treatment groups. Tests of hypotheses were two-sided with an alpha level of 0.05. Analysis was conducted with SAS v 9.2 (SAS Institute, Cary, NC, USA).

Results Mean Knoop hardness (KHN) and surface roughness (Ra) by group at baseline and after whitening treatment are

Fig. 2 Boxplot of change in surface roughness (DRa) by group

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Fig. 3 Surface morphology as observed by scanning electron microscopy

Fig. 5 Representative SEM image exhibiting mild surface alterations

Fig. 4 Representative SEM image exhibiting no surface alterations

DIY group, thus failing to meet the requirements of ISO standard 28399. Mean surface roughness changes (DRa) were significantly different among the groups (Kruskal–Wallis test, p \ 0.0001). Mean surface roughness significantly increased in the OTC group (p = 0.03) and in the positive control (p \ 0.0001). However, it is noteworthy to point out that surface roughness changes in group OTC did not exceed three times the level of the positive control thus meeting the requirements based on ISO 28399. All other groups showed no significant change in surface roughness when compared to baseline values. Enamel surface morphology changes as observed by scanning electron microscopy are summarized in Fig. 3. No changes imply a surface that shows the scratching pattern associated with the polishing of surface enamel with polishing discs (Fig. 4). A few mild changes were seen in most groups exhibiting mild interprismatic dissolution on the surface (Fig. 5). The positive control

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Fig. 6 Representative SEM image exhibiting moderate surface alterations

exhibited mostly moderate changes in the surface morphology as observed by the presence of distinct type I and II etching patterns (Fig. 6). Severe surface alterations that are usually accompanied by pitting of the enamel surface could not be observed in this study.

Discussion The tooth surface interface exhibits a continuous dynamic ion exchange between the oral biofilm and the apatite crystals in both directions to maintain a proper mineral balance [22]. This physiologic process in the oral cavity

Odontology

makes it difficult to interpret the clinical relevance of laboratory measurements of changes in enamel hardness, roughness, and morphology. Thus within the limitations of this in vitro study the first null hypothesis was rejected. There was a significant difference in Knoop hardness changes among the four different whitening modalities with DIY exhibiting a decrease of more than 10 %. Based on the results the second null hypothesis was also rejected. Surface roughness significantly increased in the OTC group. However, the increase did not exceed three times the level of the positive control. The third null hypothesis was retained. All whitening modalities exhibited none or only a mild change in surface morphology as observed by scanning electron microscopy. Surface hardness measurement is a relatively simple method to determine the mechanical property of enamel and dentin to resist plastic deformation from a standard source, and is closely related to a loss or gain of mineral component [16]. However, despite its simplicity there is a great inconsistency in the outcome of studies evaluating the effect of tooth whitening on micro hardness changes. A review on studies applying microhardness testing for structural enamel defects after whitening showed that those studies, which simulated intraoral conditions closely by using human saliva and fluoride, and evaluation after a post-treatment phase, the risk of enamel microhardness drop seemed to be reduced as compared to the remaining studies [16]. Our study is in agreement with a study that compared the effect of strip, tray, and office bleaching systems on enamel hardness and showed that bleaching did not affect hardness values in vitro [23]. This may be attributed to the fact that artificial saliva was used throughout the study and a post-treatment phase of 24 h was allowed following the protocol of ISO standards. It is interesting to note the pH change throughout the preparation of the strawberry mixture in the DIY group. The fresh strawberry mixture when measured initially exhibited a pH of 3.4 and changed to neutral by adding baking soda into the mixture. The drop in Knoop hardness that was seen only in the DIY group may be explained by the fact that there were still small chunks of strawberry present in the mixture that contributed to a low pH in localized area and caused a drop in Knoop hardness. Profilometry is a widely used methodology that provides quantitative data on surface roughness and profile changes. The inconsistency reported in microhardness studies is unfortunately also seen in the outcome of studies evaluating the effect of tooth whitening on surface roughness. While several studies showed an increase in surface roughness after bleaching [24–26] others reported no effect of whitening on surface roughness [5, 23, 27, 28]. The erosive potential of acids is influenced by

numerous factors such as pH, titratable acidity, degree of saturation, kind of acid, chelating properties, and viscosity [29]. Among these variables pH is an important factor in the degree of tissue loss by erosion [30]. This is also reflected by our study that showed a slight increase in surface roughness in the OTC group, which had a measured pH of 6.0. However, the clinical relevance of this increase needs to be determined since the increase in roughness did not exceed three times the level of the positive control. Scanning electron microscopy has been widely used in analyzing the surface morphology of enamel and dentin following tooth whitening. Depending on the study design, type and concentration of peroxide compound, pH and exposure time, different results have been reported using SEM ranging from no changes in surface morphology [14, 15], mild surface pitting at localized areas and some enamel porosity to significant surface alterations [10, 11]. A recent study comparing neutral and acidic hydrogen peroxide (HP) on the surface morphology showed that neutral HP did not affect the surface morphology whereas acidic HP resulted in significant enamel surface changes [17]. Thus it has been suggested that studies which use whitening agents with relatively low pH are probably describing primarily demineralization effects caused by acidic erosion processes rather than adverse effects by peroxide per se [16]. Based on our SEM results all whitening modalities did not adversely affect the enamel surface that may be attributed to the neutral pH of whitening agents employed. It was interesting to observe that Group DIY that exhibited a significant decrease in Knoop hardness did not alter the surface morphology. Decrease in Knoop hardness reflects loss of mineral in the enamel but may have been not enough to induce visible changes in surface morphology. This is the first study that evaluated the effect of four different whitening modalities on surface enamel. Within the limitations of this study it can be concluded that, 1.

2.

None of the four whitening modalities induced adverse surface morphology changes when observed with SEM and can be considered safe However, caution should be advised to the general public when using a DIY regimen made of strawberry mixture as it may adversely affect enamel microhardness and an OTC product as it has the potential to increase surface roughness

Acknowledgments The authors would like to thank Ultradent Products Inc and Philips Oral Healthcare for kindly providing the bleaching materials used in this study. Conflict of interest of interest.

The authors declare that they have no conflict

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Effect of various tooth whitening modalities on microhardness, surface roughness and surface morphology of the enamel.

The purpose of this study was to evaluate the effect of four whitening modalities on surface enamel as assessed with microhardness tester, profilomete...
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