Comparative Evaluation of Outcome of Natural Antioxidants on Shear Bond Strength of Composite Bonded to Bleached Enamel: An In vitro Study.

Background: The depletion of bond strength after the bleaching procedure has resulted in the usage of several antioxidants to get rid of discharging oxygen from residual peroxides before any adhesive restorations. Aim: The in vitro study aimed to compare and evaluate the shear bond strength of composite after application of three over-the-counter natural antioxidants on bleached enamel. Materials and
Methods: Thirty-six extracted human anterior teeth were decoronated at the level of cementoenamel junction and implanted into self-cure acrylic resin exposing the labial surface alone. The embedded specimens were categorized into six groups of six teeth each, and the groups were as follows: Group I: unbleached + composite bonding, Group II: bleaching + delayed composite bonding, Groups III, IV, and V: bleaching + antioxidants - olive oil/Vitamin E oil/propolis + immediate composite bonding, and Group VI: bleaching + immediate composite bonding. The shear bond strength analysis was performed with the help of the universal testing machine, and the values obtained were statistically analyzed using IBM SPSS for Windows, v. 21.0.
Results: The results obtained from the study reveal that all the three experimental groups showed an increase in the shear bond strength in comparison to Group VI (positive control) and the difference in the bond strength between the experimental groups and positive control is also statistically significant.
Conclusion: Within the limitations of this study, although the bond strength of composite resin after application of the three natural over-the-counter antioxidants falls very closely, the usage of oil-based antioxidants had been found to perform less effectively in improving the shear bond strength. Copyright:

INTRODUCTION

With the rising concern and evolving demand for esthetic dentistry, the management of discolored teeth has become one of the frequently performed treatments in the clinical practice. The treatment options available for rectifying tooth discoloration include veneering, bleaching, micro/macroabrasion, and placement of porcelain crowns. Among the various options available, the bleaching technique has gained popularity in the recent decade as it is simple, relatively safe, effective, economical, and a conservative approach to manage the discoloration.[1]

The bleaching agents generally used for performing the procedure include hydrogen peroxide, carbamide peroxide, and sodium perborate. The bleaching technique eventually relies on the complex oxidation–reduction mechanism that involves oxygen-free radical liberation from the bleaching by-products which infiltrate through the microporosities of prismatic enamel structures to the dentin, possibly because of the low molecular weight of the substances (about 30 g/mol).[23]

More often, the clinicians were made to utilize adhesive restorations instantly after bleaching in case scenarios such as esthetic recontouring, diastema closures, or as a part of shade enhancement procedures.[4]

One of the essential difficulties after the immediate bleaching procedure was decreased bond strength obtained between restored composite and enamel following an adhesive restoration.[5] This reduction in bond strength was owing to the existence of residual oxygen particles that inhibit the composite resin polymerization process. The trapping of leftover bleach by-products into the dentinal tubules and collagen matrix along with the alteration in the bleached enamel surfaces following acid etching also results in the reduction of calcium content and microhardness with loss of enamel prisms which ultimately culminated in bond strength depletion.[67]

In general, delaying the adhesive restorative procedures for the period of 24 h–4 weeks was found to resolve the scenario.[8] Since the bleaching procedures are commonly performed at the esthetic zone (in the anterior region), a waiting period may not always be practicable due to the profound esthetic concern.

Several techniques have been suggested to counteract the compromised bond strength postbleaching, which include eliminating the superficial outermost layer of enamel, exposing the bleached tooth surface to alcohol before adhesive restoration, treating the bleached surface specimens with artificial saliva/saline/water, incorporating adhesives containing organic solvents, and usage of antioxidants.[910]

Among the wide variety of former-mentioned practices, the utilization of antioxidants has been suggested to be an effective and relatively safe modality of increasing the bond strength during the postbleaching period. Both the natural and synthetic antioxidants employed for this purpose efficiently impede oxidation process and have been effectively helped to improve the bond strength.[11]

Although several studies done in the past have utilized antioxidants such as sodium ascorbate, pine bark extract, grape seed extract, Aloe vera, green tea extract, and pomegranate peel,[1213] the lack of ease in commercial availability of such agents and the necessity of antioxidant preparation makes the whole procedure laborious.

Hence, the present study aimed to evaluate and compare the shear bond strength after the application of three natural over-the-counter antioxidants – olive oil, Vitamin E oil, and propolis on the resin composite bonded after the bleached enamel.

The null hypothesis of this study was that the use of any over-the-counter antioxidant agent whether oil based/water based before the resin composite bonding would reverse the reduced bond strength.

MATERIALS AND METHODS

Specimen selection and preparation

This study was approved by the Institutional Review Board prior to the commencement of the research (2021/IRB/047). Thirty-six intact, sound human anterior teeth indicated for extraction due to periodontal reasons were obtained for the study. The specimen for the study was chosen after a thorough examination of the presence of caries, cracks, developmental defects, and surface irregularities. The specimen collected was methodically rinsed, cleaned free of any surface debris, and calculus using scalers. The crowns were decoronated at the level of cementoenamel junction with the help of a diamond disc under water coolant. The crowns were then implanted exposing the labial surface alone with the help of self-cure acrylic resin. After crown embedment, the labial portion of the crowns was made into a flat surface and then polished with extrafine diamond abrasive to facilitate surface treatment and bonding to the enamel.

Group specifications

The 36 embedded crown specimens were categorized into six groups of six teeth each, and the specimen distribution along with the materials used is summarized in Table 1.

Table 1

The distribution of specimens and study groups (n=6)

Groups	Bleaching agent	Antioxidant used	Source of antioxidant	Composite buildup
Group I	No bleaching treatment	No antioxidant	-	Done immediately
Group II	35% hydrogen peroxide	No antioxidant	-	Done 3 weeks after bleaching
Group III	35% hydrogen peroxide	Olive oil	Cold-pressed pure olive oil from Urban Botanics Pvt. Ltd., India	Done immediately
Group IV	35% hydrogen peroxide	Vitamin E oil	Evion 400 mg capsules (tocopherol acetate IP) manufactured by Artcollectibles India	Done immediately
Group V	35% hydrogen peroxide	Propolis	Hawaiian herbal bee natural propolis liquid drops, Hawaiian Herbal Pvt. Ltd., India	Done immediately
Group VI	35% hydrogen peroxide	No antioxidant	-	Done immediately

IP: Indian Pharmacopoeia

Bleaching protocol

The bleaching procedure was carried out on all the experimental groups except Group I (unbleached group) using 35% hydrogen peroxide (Pola office, SDI Limited). The specimens were treated with the bleaching gel with three applications of 8 min each. Following the application of the bleaching agent, the specimens were rigorously rinsed with water from a three-way syringe for 1 min and air-dried.

Experimental procedure

Group I (negative control group): The unbleached group was directly bonded with composite resin restoration

Group II (bleaching and delayed bonding without antioxidant): After receiving the bleaching treatment, the specimens of this group were stored in distilled water at room temperature (37°C) for 3 weeks and subsequently received composite restoration

Group III/IV (bleaching + olive oil/Vitamin E + composite bonding): These groups received the bleaching treatment followed by the application of group-specific antioxidants – olive oil/Vitamin E oil, respectively, for 10 min. This was followed by rinsing with distilled water for 30 s and then bonded with composite resin immediately

Group V (bleaching + propolis + composite bonding): This group received the bleaching treatment followed by the application of group-specific antioxidant – propolis, respectively, for 10 min. This was followed by rinsing with distilled water for 30 s and then bonded with composite resin immediately

Group VI (positive control group): This group received the bleaching treatment followed by composite resin bonding immediately.

Composite bonding

After the bleaching regimens and antioxidant application procedures, pumice prophylaxis was performed on all specimens before undergoing composite bonding. The specimen surfaces were then etched with the help of 37% phosphoric acid (D-Tech Etchant) for a period of 15 s, rinsed off with water for 20 s, and bonding agent Te-Econom Bond (Ivoclar Vivadent) was applied with an applicator tip, air thinned, and cured for 20 s as per manufacturer's instruction. Following this, the specimen received a 2-mm increment of composite resin (Tetric N-Ceram, Ivoclar Vivadent, India) on the prepared labial surface and was light cured for 40 s using the LED light-curing unit (Ivoclar Bluephase N MC).

The resin-restored specimens were then stored in distilled water at room temperature (37°C) for 24 h before the evaluation of shear bond strength.

Shear bond strength analysis

Each restored specimen was positioned in between the jigs of universal testing machine (Shimadzu AG-X plus 50KN) for the analysis of shear bond strength. A needle-like shearing rod was located at the junction of resin tooth surface, and the experiment was carried out. The specimen was subjected to static loading at a rate of 0.5 mm/min until the restoration fractured. The universal testing machine was interfaced with computer software with which the operation was managed and the shear bond strength analysis was made.

Statistical analysis

The mean shear bond strength values obtained were statistically analyzed using IBM SPSS (IBM corporation, Armonk, New York, United States) for Windows, v. 21.0. One-way analysis of variance and post hoc Tukey's test were performed to analyze the data. Statistical significance was set at P < 0.05.

RESULTS

The shear bond strength was inferred to be highest in Group I, which was statistically significant in comparison with other groups (P < 0.05). Group VI was found to have the least shear bond strength values and performed statistically significantly among other groups (P < 0.05). Although the experimental groups (Groups III, IV, and V) were not statistically significant among each other, the mean shear bond strength value reveals that Group V performed superior to that of Groups III and IV. The P values associated with each comparison along with the mean shear bond strength values are summarized in Table 2.

Table 2

Comparison of mean shear bond strength (Mpa) values between the different groups included in the study

Groups	Group description	Mean shear bond strength (mean±SD)	Group comparison	P
I	Negative control (Unbleached group)	14.07±1.21	G-1 versus G-2, G-3, G-4, G-5, G-6	0.042*
II	Bleached and delayed bonding after 3 weeks	10.96±0.996	G-2 versus G-6	0.048*
III	Bleaching+olive oil+immediate bonding	11.23±0.993	G-3 versus G-6	0.048*
IV	Bleaching+Vitamin E oil+immediate bonding	12.02±1.081	G-4 versus G-6	0.048*
V	Bleaching+propolis+immediate bonding	12.78±1.093	G-5 versus G-6	0.048*
VI	Positive control (bleached and immediately bonded)	6.15±0.981

*Statistically significant (P<0.05). SD: Standard deviation

DISCUSSION

All the three natural over-the-counter antioxidants used in the current study irrespective of whether oil/water based were capable enough to reverse the deteriorated bond strength after bleaching that affirmed the hypothesis of the study.

The extracoronal bleaching technique is the most frequently employed, effective, and conservative treatment option to manage discolored teeth.[14] In this current study, the same extracoronal bleaching procedure was performed utilizing 35% hydrogen peroxide and the study inferred that a significant depletion in bond strength values in comparison with the unbleached group.

The bleaching products liberate free radicals such as perhydroxyl ions and nascent oxygen when they are administered on the discolored tooth structure. Free radical is a molecule that consists of an unpaired electron resulting in high reactivity.[11]

The free radicals liberated from the bleaching products penetrate through interprismatic regions into the enamel surface and react with the organic enamel along with the organic pigmented molecules. The elimination of organic constituent ultimately results in surface irregularities and morphologic alterations. Apart from the favorable outcomes obtained from the bleaching technique, it also culminates in surface alterations, decreased microhardness, and bond strength as reported by various studies.[15]

The proposed theory behind the association of bleaching agents and bonding was that the presence of peroxides and their by-products existing inside the tooth structure interferes with the adhesive resin polymerization.[16]

In the bleaching treatment with hydrogen peroxide, the hydroxyl radicals in the apatite lattice are replaced by peroxide ions, developing in the generation of peroxide-apatite. With the 2-week waiting period, these peroxide ions may disintegrate and the hydroxyl ions occupy the apatite lattice, which results in removing morphologic or structural changes that occurred by the deposition of peroxide ions.[17] Hence, the results from the present study obtained revealed that a waiting period of 2 weeks produced a shear bond value closer to that of the unbleached group.

Moreover, from a study done by Sharafeddin et al., it was reported that following bleaching, dentinal fluid and dentin also can potentially act as an oxygen and peroxide reservoir.[18]

Bee products such as bee wax, propolis, royal jelly, and honey are contemplated to be a potent source of naturally occurring antioxidants able to curb the consequences of oxidative stresses behind the pathogenesis of several diseases.

Raw bee propolis is primarily composed of 50%–60% of balms and resins (including phenolic compounds), 30%–40% of fatty acids, and waxes, 5%–10% of essential oils, 5% of pollen, and others including micronutrients, amino acids, and vitamins. From the literature, it is also evident that almost nearly 300 constituents belonging to polyphenols, amino acids, sugars, terpenoids, steroids, and others have been found in propolis.[19] Even though several compositional variations do exist, the antioxidant capacity of bee propolis extract remains constant. The two main constituents responsible for the free radical scavenging property in bee propolis include phenolic acids and polyphenols.[2021] The aqueous solutions of propolis were also proven to exhibit antioxidant potential in various animal studies and cell cultures.[2223] The better performance of propolis to reverse the compromised bonding in the present study may be due to the usage of aqueous-based solution in comparison to the other oil-based agents used.

Alpha-tocopherol stands to be the most dynamic component of Vitamin E complex and is the most potent lipid-based antioxidant. The product due to its ability to prevent the free radical generation was found to be used on the enamel and dentin structures for better bonding results, and studies had also proven its performance similar to that of ascorbic acid.[1024]

The term Vitamin E collectively includes the tocotrienols and tocopherols, among which the alpha-tocopherol contains the maximum biological activity. Furthermore, owing to its property of hydrophobicity, the vitamin exhibits increased stability and oxidizing potential in comparison with the ascorbate. The alcohol added along with the alpha-tocopherol in a few studies was also one of the prime reasons for improved bond strength.[11] However, in the present study, no such additives were added to over-the-counter Vitamin E oil and hence the improved bond strength in such a scenario should be noted.

Olive oil is a natural derivative whose constituents differ widely depending on the manufacturing process, the storage conditions, and the types of olives present. In general, the extra virgin olive oil consists of a fairly higher number of phenolic antioxidants such as tyrosol, hydroxytyrosol, and oleuropein.[25] Hydroxytyrosol obtained from oleuropein by the process of enzymatic hydrolysis is found to be the most active component in comparison to various olive polyphenols and is also a powerful antioxidant.

Several studies done in the past have demonstrated the antioxidant potential of polyphenols present in olive oil (Visioli, Poli, and Gall et al., 2002, and Servili et al., 2009). When performed oxygen radical antioxidant capacity assay, authors (Zullo and Ciafardini et al., 2008) have concluded that hydroxytyrosol contains the maximum antioxidant potential in comparison to the other olive oil polyphenols. Hence, the study compared the over-the-counter antioxidants consisting of polyphenols (propolis and olive oil) with alpha-tocopherol (Vitamin E oil).

Although there were several previous studies on improvement of shear bond strength with the usage of antioxidants following bleaching, the present study differs from them by the usage of over-the-counter antioxidant formulations. Since the study evaluates newer formulations to improve the bond strength, the sample size was kept minimum. These formulations also show a statistically significant increase in shear bond values in comparison to the bleached and immediately bonded group (Group VI). The use of over-the-counter antioxidant formulations would make the composite adhesion more simple, easy, and nonarduous procedure following bleaching with effective elimination of an additional antioxidant preparation time. Since the present study is an in vitro study, further clinical trials in this regard for validating these observations are felt necessary.

CONCLUSION

Within the limitations of the present study, the conclusions drawn from this study were as follows:

Bonding the composite immediately after the bleaching procedure deteriorated the shear bond strength to a great extent

Usage of over-the-counter antioxidants though improved the shear bond strength, the shear bond values after antioxidant application were found lesser to that of the unbleached tooth

Over-the-counter antioxidants although established to have free radical scavenging activity, the type whether oil based or water based also plays a pivotal role with regard to the application before composite resin bonding. In this current study, although both Vitamin E and olive oil are considered to be good sources of antioxidants, the water-based/aqueous-based antioxidant propolis performed better in improving the bond strength.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.