Effect of Nd:YAG Laser Bleaching and Antioxidizing Agents on the Shear Bond Strength of Brackets

Abstract

Objective: The aims of this study were to evaluate the effect of hydrogen peroxide bleaching agents, both nonactivated and activated by a neodymium:yttrium-aluminum-garnet (Nd:YAG) laser, and of antioxidant treatment on the shear bond strength (SBS) of orthodontic brackets. Background data: Nd:YAG laser activation is expected to accelerate the bleaching therapy without decrease shear bond strength. Materials and methods: Ninety extracted maxillary central incisors were divided into two experimental groups and a control group. Group I was the control group, Group II was bleached with 35% hydrogen peroxide and had no photoactivation, and Group III was bleached with 35% hydrogen peroxide with activation by a Nd:YAG laser (4.0 W, 60 Hz frequency, 1 mm distance, 20 sec). Each group was divided into two subgroups: Subgroup A was immersed in artificial saliva for 2 weeks, and then bonded using the total etch system, whereas subgroup B was treated with an antioxidant agent (10% sodium ascorbate) and then bonded using the same system. The samples were stored in water for 24 h at 37°C, and thermocycled. The SBS in megapascals (MPa) was determined by a shear test with 1 mm/min crosshead speed, and failure types were classified with modified adhesive remnant index scores. The data were analyzed with two way analyses of variance, Tukey, and χ² tests at the α=0.05 level. Results: In both Groups II and III, the SBSs of brackets bonded after bleaching (Group II 15.16, Group III 17.50 MPa) were significantly lower than those of brackets in the bonded unbleached group (Group I 22.13 MPa); however, sodium ascorbate treatment significantly increased the SBSs of brackets in the bleached groups (Group II 21.52, Group III 22.43 MPa), but had an insignificant effect on the SBS of the control group (Group I 23.66MPa). Conclusions: Hydrogen peroxide bleaching agents reduce the SBSs both with and without Nd:YAG laser activation; however, treatment of the bleached enamel surface with 10% sodium ascorbate prior to bonding negates the effect.

Introduction

O ne of the great esthetic problems in dentistry is tooth discoloration. Therefore, the demand for esthetic treatments has increased in recent years.^1,2 Tooth discoloration has many complex etiologic factors that are usually classified as being intrinsic, extrinsic, or internalized in nature.³ Tooth bleaching is a more conservative and simple method of addressing discoloration than are invasive therapies, such as composite fillings, laminates, crowning, or the placement of veneers.⁴

Recent reviews have highlighted that enamel bleaching with hydrogen peroxide (HP) bleaching gel reduces the shear bond strength (SBS) of orthodontic brackets, whereas delaying the bonding procedure could restore the decreased bond strength values.^5
–7 However, although there is no consensus on how much time should pass after enamel bleaching therapies before bonding, many clinicians recommend to wait after bleaching.^8,9 Most studies propose that the reduced bond strength values increase to normal value within 3 weeks after bleaching treatment, with residual bleaching radical release.^6,10 It has been proposed that residual oxygen from the HP gel remains in the enamel tissue and inhibits resin polymerization.

Oxygen-free radicals, which originate from the decomposition of HP, are stabilized by capturing an electron from surrounding molecules. This process bleaches the teeth by decomposing pigments.¹¹ Light, heat, or laser can be used to accelerate the activation of the chemical agents involved in the whitening process.¹² Photosensitive agents absorb light, and the resultant absorbed energy is transferred to the peroxide; this process accelerates the oxidation-reduction reaction.^13,14 A neodymium:yttrium-aluminum-garnet (Nd:YAG) laser is recommended by many studies to accelerate the bleaching process.^15,16

Several preventative methods, such as removal of the superficial layer of enamel,¹⁷ pretreatment of the bleached enamel with alcohol,¹⁸ or the use of adhesives containing organic solvents,¹⁹ have been suggested to enhance the SBS immediately after bleaching. In the last few years, a new approach to enhance SBS after bleaching by removing the residual oxygen has been proposed. If the residual oxygen decreases the bond strength of brackets on bleached enamel, the application of biocompatible and neutral antioxidants before the resin composite may restore the bond strength. In an in vitro study by Bulut et al.²⁰ who investigated the effect of antioxidant treatment on the SBS of metal brackets bonded after bleaching, it was found that treating the bleached enamel surface with 10% sodium ascorbate eliminated the reduction in bond strength.

In addition, so far, to our knowledge, no studies have investigated the effect of Nd:YAG laser photoactivated bleaching on the SBS values of brackets. Therefore, the aims of this study were to evaluate the effect of HP both nonactivated and activated by a Nd:YAG laser and antioxidant treatment after bleaching on the SBS of metal brackets bonded to human enamel.

Materials and Methods

Selection and preparation of teeth

Ninety noncarious, freshly extracted, sound human maxillary central incisors, which were extracted in our facility because of periodontal problems, were collected and placed in a solution of 0.1% thymol at room temperature (25°C). Teeth with hypoplastic areas, cracks, restoration, or gross irregularities were excluded. The criteria for tooth selection dictated no pretreatment with a chemical agent such as alcohol, formalin, or hydrogen peroxide, or any other form of bleaching. All residual tissue tags were cleaned from the tooth surface under running tap water. All teeth were mounted vertically in self-cure orthodontic acrylic until two thirds of the root was embedded. The labial surfaces of the teeth were cleaned and polished with oil and fluoride-free fine pumice and water using a brush and a slow-speed handpiece, then rinsed with a water spray for 20 sec and dried with oil-free compressed air for 10 sec.

This study was approved by the Regional Ethical Committee on Research of the Selcuk University in Turkey. The power analysis was established by G*Power version 3.0.10 software (Franz Faul Universitat, Kiel, Germany). Based on the 1:1 ratio between groups, a sample size of 15 teeth per group imparts > 80% power to detect significant differences with a 0.40 effect size and at the α=0.05 significance level.

The specimens were randomly divided into a control group (Group I) and two experimental groups that included Group II, which was bleached with 35% HP bleaching agent, and Group III, which was bleached with the same agent and activated with a Nd:YAG laser during bleaching. All groups were divided into two subgroups: Subgroup A, which was not treated with an antioxidant, and subgroup B, which was treated with an antioxidant (10% sodium ascorbate) just before bonding (Table 1).

Table 1.

Information About Experimental Groups and Their Treatment Methods

Groups	Subgroups	Treatment methods
Group I	Subgroup A	No antioxidant treatment	Not bleached (control)
	Subgroup B	10% sodium ascorbate	Not bleached (control)
Group II	Subgroup A	No antioxidant treatment	35% hydrogen peroxide (HP) without photoactivation for 15 min
	Subgroup B	10% sodium ascorbate	35% HP without photoactivation for 15 min
Group III	Subgroup A	No antioxidant treatment	35% HP with Nd-YAG activation
	Subgroup B	10% sodium ascorbate	35% HP with Nd-YAG activation

The procedure for Group II was as follows: A commercial, 35% HP office bleaching agent (Whiteness HP, FGM Produtos Odontológicos Ltda, Joinvile, PR, Brazil) was applied on the enamel surfaces of the embedded central incisors for 15 min without photoactivation at room temperature of 25°C. After the reaction time for each protocol, the bleaching gel was aspirated, and the tooth surface was washed with distilled water for 30 sec. This cycle was repeated three times per bleaching procedure, as described by manufacturer instructions.

The procedure for Group III was as follows: The same bleaching agent used to treat Group II was applied and activated using a Nd:YAG laser (noncontact bleaching handpiece, output power 4.0 W, pulse repetition rate 60 Hz, pulse length 320 μs, energy density 0.23 J/cm²) for 20 sec at room temperature of 25°C. The bleaching gel was left on for 9 min and 40 sec after photoactivation, and then the teeth were cleaned according to the procedure used for Group II. This cycle was repeated three times per bleaching procedure, as described by manufacturer instructions.¹¹

After bleaching, the specimens in subgroup A were immersed in 500 mL of artificial saliva at 37°C and held for 2 weeks. The artificial saliva, which has an electrolyte composition similar to that of human saliva, was prepared from 0.103 g CaCl₂H₂O; 0.04 g MgCl₂6H₂O; 0.544 g KH₂PO₄; 2 g N₃Na; 2.24 g KCl; 4.77 g HEPES buffer, and sufficient KOH to achieve pH 7.0 as described by Donmez et al.²¹ The artificial saliva was changed twice daily. Immediately after bleaching and before bonding, the specimens in subgroup B were treated with an antioxidant, that is, 10% sodium ascorbate, for 10 min at a flow rate of 1 mL/min with continuous agitation. In both subgroups, the enamel surfaces were thoroughly rinsed with distilled water for 30 sec before the brackets were bonded to the prepared enamel surfaces.

Bracket bonding procedure

Before applying the brackets, the enamel surface was etched with 37% orthophosphoric acid for 30 sec. The etching liquid and demineralized tooth particles were removed with an air-water syringe, which was applied for 10 sec. The teeth were then dried for 10 sec with oil-free compressed air. After surface preparation, liquid Transbond XT primer was applied to the etched surfaces and the brackets were bonded onto the maxillary central incisors using Transbond XT (3M Unitek, Monrovia, CA). Any excess resin around the brackets was removed using an explorer. A light-emitting-diode (LED) curing light (Elipar Freelight-2, 3M ESPE, Seefeld, Germany) was then applied for 20 sec to cure the adhesive resin.

The specimens in all groups were stored in distilled water at 37°C for 24 h and thermocycled for 5000 cycles between 5 and 55°C with a dwell time of 30 sec at each temperature. A knife-edge–shaped apparatus was placed between the joint of the enamel surface and the resin material. The SBS was evaluated using a universal testing machine (TSTM 02500, Elista Ltd. Sti, Istanbul, Turkey) with a crosshead speed of 1 mm/min. The value of the maximum load required to debond the bracket was recorded. The SBS was calculated as the ratio of the fracture load to the bonding area and expressed in megapascals (1 MPa=1 N/mm²).

After the SBS test, all teeth and brackets were observed using a stereomicroscope at 40× magnification (CX41, Olympus, Tokyo, Japan) to identify the mode of fracture by one operator (T.Y.) who was blinded to the group allocations. Any adhesive remaining after bracket removal was assessed using the modified adhesive remnant index (ARI),^{5

–22} and scored according to the amount of resin adhering to the enamel surface.

Statistical analysis

All statistics were performed using SPSS version 17.0 (SPSS Inc, Chicago, IL). The Shapiro–Wilk test for normality and Levene's variance homogeneity test were applied to the data. The data were found to be normally distributed, and there was homogeneity of variance among the groups. Therefore, a statistical evaluation of the SBS values among the test groups was performed using parametric tests.

Descriptive statistics, including the mean, standard deviation, and minimum and maximum values, were calculated for all groups. The mean values of SBS were statistically analyzed using two way analysis of variance (ANOVA) and Tukey's multiple comparison test at p<0.05. For the ARI scores, the χ² test was used to identify any significant differences among the groups.

Results

The results of two way ANOVA indicate that the differences between the groups (F=33.731, p<0.001) and between the subgroups (F=13.119, p<0.001) were significant, and that there was interaction between the groups and subgroups (F=3.790, p<0.05) (Table 2). The descriptive statistics and comparisons of the six groups in this study are shown in Table 3. Tukey's test showed that the SBSs of brackets that were bonded after bleaching both with and without activation (Group II: 15.16±2.36, Group III: 17.50±2.83) were significantly lower than those of brackets bonded to unbleached teeth (Group I: 22.13±4.31) (p<0.01); however, sodium ascorbate treatment significantly increased the SBSs of the brackets in both bleaching groups (Group II: 21.52±3.70, Group III: 22.43±3.78) (p<0.05), but did not significantly affect the SBSs of the brackets in the control group (Group I: 23.66±3.61) (p>0.05). No statistically significant differences were found among the three antioxidant-treated groups and the control group and between the bleached groups.

Table 2.

Results of Two-Way ANOVA

Source	df	F	p
Bleaching methods	1	33.731	0.000
Antioxidant treatment	2	13.119	0.000
Intercept	2	3073.61	0.000
Bleaching methods ^aantioxidant treatment	1	3.79	0.027

Described interaction between two sections.

Table 3.

Descriptive Statistics and Results of ANOVA and Tukey's Test Comparing the SBS of the Experimental Groups in MPa

Groups	Subgroups	n	Mean	SD	Min	Max	Sign (Tukey)
Group I (control)	Subgroup A (no antioxidant)	15	22.13	4.31	14.04	28.62	A
	Subgroup B (10% sodium ascorbate)	15	23.66	3.61	17.94	29.21	A
Group II (no photo)	Subgroup A (no antioxidant)	15	15.16	2.36	10.92	18.84	B
	Subgroup B (10% sodium ascorbate)	15	21.51	3.69	15.09	28.07	A
Group III (Nd:YAG)	Subgroup A (no antioxidant)	15	17.49	2.83	13.71	23.44	B
	Subgroup B (10% sodium ascorbate)	15	22.43	3.78	16.86	28.83	A

SBS, shear bond strength; SD, standard deviation; n, sample size; Sign: significance; A and B capital letters in Sign section describe the statistically significant differences between groups.

To assess the amount of resin left on the enamel surfaces after debonding, the ARI was used. The ARI scores for the various groups tested are listed in Table 4. The results of the χ² comparisons indicated that there were no significant differences among the six groups (χ²=6.245, p>0.05).

Table 4.

Frequency of Distributions and Comparison of Adhesive Remnant Index (ARI) Scores

Groups	Subgroups	n	1	2	3	4	5
Group I (control)	Subgroup A (no antioxidant)	20	4 (27%)	6 (40%)	5 (33%)	0	0
	Subgroup B (10% sodium ascorbate)	20	5 (33%)	5 (33%)	5 (33%)	0	0
Group II (no photo)	Subgroup A (no antioxidant)	20	0	6 (40%)	5 (33%)	2 (13%)	2
	Subgroup B (10% sodium ascorbate)	20	3 (20%)	7 (47%)	4 (27%)	1 (7%)	0
Group III (Nd:YAG)	Subgroup A (no antioxidant)	20	0	7 (47%)	5 (33%)	1 (7%)	2 (13%)
	Subgroup B (10% sodium ascorbate)	20	4 (27%)	7 (47%)	4 (27%)	0	0

ARI scores, 1: all of composite, with impression of bracket base, remained on tooth; 2: > 90% of composite remained; 3: > 10% but < 90% of composite remained on tooth; 4: < 10% of composite remained on tooth surface; 5: no composite. remained on enamel.

p>0.05, χ²=6.245.

Stereomicroscope images of the nonphotoactivated and photoactivated bleached enamel surfaces are presented in Fig. 1. The teeth treated with each type of light-curing unit exhibited similar morphologies, excluding those treated with the Nd:YAG and diode lasers (i.e., Groups II and III); in addition, some scratches were evident in these two groups. There were no morphologic alterations to the bleached enamel surfaces that were activated either with the Nd:YAG laser or not photoactivated (i.e., Groups II and III).

FIG. 1.

Stereomicroscope images of the enamel (A) nonphotoactivated (B) and photoactivated (C) bleached enamel surfaces.

Discussion

This study was performed to evaluate the effects of two different bleaching procedures and antioxidant treatment on the SBS of orthodontic brackets bonded to an enamel surface. According to the results of the present study, it is evident that both photoactivated and nonphotoactivated HP adversely affect the SBS when the bonding procedure is performed 2 weeks after bleaching. However, antioxidant treatment restored the SBS in both bleaching groups. The maximum mean SBS value was obtained in the control group after antioxidant treatment, followed by the group that was bleached with Nd:YAG laser activation and treated with antioxidants, the control group, and the group that was bleached without activation and treated with antioxidants. However, no statistically significant difference was found among these four groups.

Many bleaching systems, which use different concentrations and various forms of hydrogen or carbamide peroxide as the active bleaching agents, are available now. It is known that the use of carbamide or HP for vital tooth bleaching is the most conservative and safe procedure for treating tooth discoloration.^5,23 Enamel can be bleached by simple chemical activation of these agents or by additional photoactivation via LED or plasma light sources, KTP, diode, or Nd:YAG (or Er:YAG) laser.

The effect of bleaching on the SBS of resin composite to enamel has been extensively investigated;^18–24–28 these studies generally found that bleaching agents adversely affected the bond strength of adhesive materials.^6,28,29 There have been a number of reports regarding the relationship between the bleaching agents and the decreased SBS of orthodontic brackets, in addition to changes in the chemical or morphologic structure of the enamel because of the direct contact of the bleaching agent with the enamel surface of the vital teeth for a lengthy period.^{6,18,25

–28}

To provide an explanation for the decreased bond strength that results from bleaching,⁴ several studies have determined that bleaching agents decrease the SBS of brackets by physical alterations of the teeth by bleaching treatments. Some authors believe that bleaching changes the enamel surface structure, increases porosity, results in a loss of minerals especially calcium, decreases the microhardness, and alters the organic substances.^27,30 The results of present study are consistent with these suggested explanations, and demonstrate that the reduction in the SBS of brackets 2 weeks after bleaching was significant when compared with a control group. These results are in agreement with those of Cavalli et al.,⁶ Turkun and Kaya,⁴ Bulut et al.,²⁰ and Mullins et al.;³¹ however, they do not agree with those of Uysal et al.³²

Uysal et al.³² and Uysal and Sisman⁵ investigated the effect of the time period between bleaching and bonding procedures by storing bleached samples in artificial saliva; they suggested that 2–3 weeks between bleaching and bonding might be sufficient to avoid the decreased SBS caused by the bleaching agent. However, the results of the present study did not support these results.^5,8,9,32,33

As a possible explanation for the decreased bond strength after bleaching with HP, it has been proposed that residual free chemical ions, especially residual oxygen from the bleaching agent, inhibit resin polymerization.²⁶ Therefore, it is believed that removing the chemical ions on enamel surfaces may restore the SBS to the original value. Montalvan et al.³⁴ evaluated the SBS of acetone- and/or ethanol-based bonding agents to enamel that was bleached with 35% HP 24 h prior, and found that the bond strength values were significantly lower for the bleached teeth. They also found no differences between the bond strength values of acetone- and/or ethanol-based adhesives. On the other hand, ascorbic acid and its salts are well-known antioxidants that can reduce various oxidative compounds, especially free radicals.³⁵ Bulut et al.²⁰ found that treating the bleached enamel surface with an antioxidant agent before bonding appeared to restore the reduced SBS of brackets to teeth that were treated with 10% carbamide peroxide. Therefore, they suggested that the antioxidant treatment could be an alternative to delaying the bonding procedure. In recent studies,^4,20 10 min of antioxidant treatment was found to be sufficient. Consistent with these studies, an ascorbic acid salt, sodium ascorbate, was applied for 10 min in order to restore the SBS.

The aim of photoactivation during the bleaching procedure is to accelerate the decomposition of HP, and, hence, tooth lightening.³⁶ A survey of published literature indicates that no investigations have been conducted on the effect of Nd:YAG laser-activated bleaching on the SBS values of composites. However, Goharkhay et al.³⁷ found that the Nd:YAG laser-activated bleaching had less of an effect on the enamel surface morphology. In contrast, in the present study, there were no significant differences in the SBS values of brackets bonded to teeth that underwent nonactivated and Nd:YAG laser-activated bleaching.

To determine the effect of antioxidant treatment after bleaching with carbamide peroxide, Bulut et al.²⁰ applied 10% sodium ascorbate before bonding, thereby negating the compromised bond strength of the composite. Therefore, they suggest that brackets could be bonded immediately after bleaching with the additional antioxidant treatment. In the present study, the antioxidant treatment eliminated the reduction in the SBS values after using HP bleaching agent.

Reynolds³⁸ suggested that the minimum bond strength value that is adequate for most clinical orthodontic needs and routine clinical use is 5.9–7.8 MPa. In this present study, all groups exhibited SBS values above the minimum bond strength values, which indicates that the SBS of all groups were sufficient for clinical use. However, the values suggested by Reynolds³⁸ are based on tensile strength, whereas, in the present study, SBS values were evaluated. However, clinical conditions and in vitro conditions are different. As the present study is an in vitro study, the conditions do not completely duplicate the rigors of the oral environment.

The results of the ARI score comparisons in the present study indicate that there are no significant differences among the six groups tested. However, in the groups that were bleached and not treated with antioxidant, there was a higher frequency of ARI scores in the range of 1–4, which indicates cohesive and adhesive failures. The failures of the other groups were cohesive. In the literature, adhesive failure types are more commonly seen in bleaching groups than are cohesive failures.^5,8

Conclusions

The following conclusions were drawn within the limitations of this in vitro study.

1. Significantly lower SBS values were recorded for the groups that underwent bleaching.

2. There were no differences between the groups that underwent photoactivated and those that did not.

3. Antioxidant treatment before the bonding procedure could negate the reduction in the SBS.

4. Delaying the bonding procedure to restore the SBS was not reliable.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

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