A comparative evaluation of the three different antioxidant treatments on the bond strength of composite resin to bleached enamel: An in vitro study.

CONTEXT: Natural antioxidants have capacity to neutralize the action of free radicles, therefore, effective in reversal of compromised bond strength. AIMS: The aim of this study was to investigate the effect of three different antioxidant treatments on the bond strength of composite resin to bleached enamel.
MATERIALS AND METHODS: Ninety extracted human anterior teeth were divided into five groups: Group I (n = 10) - control group no bleaching, Group II (n = 20) - bleaching with 35% carbamide peroxide gel, Group III (n = 20): bleaching with application of 10% sodium ascorbate, Group IV (n = 20): bleaching with application of 6.5% proanthocyanidin, and Group V (n = 20) - bleaching with application of 50% Aloe vera. These groups were further subdivided into two subgroups based on whether composite buildup was done immediately (Subgroup A) or after a delay of 2 weeks (Subgroup B). Shear bond strength of the specimens was tested under universal testing machine. STATISTICAL ANALYSIS USED: Data were statistically analyzed using Student's unpaired t-test, ANOVA test, and Tukey's Kramer multiple comparison test.
RESULTS: The mean bond strength of Group IVA was significantly higher (4.27 ± 1.86) which indicated that immediate application of proanthocyanidin showed higher bond strength after bleaching (P < 0.05).
CONCLUSION: Within the limitations of this study, it can be concluded that immediate application of antioxidants on bleached enamel before composite restoration reversed the bond strength of composite restoration. The immediate application of proanthocyanidin proved superior followed by A. vera.

INTRODUCTION

With an increase in demand for a “white smile,” immense concern and emphasis has been placed on tooth discoloration.[1] Clinician should be aware of the outcome of bleaching treatment and its interactions with other dental treatments, especially additional adhesive esthetic procedures such as composite bonding, laminate veneers, or orthodontic treatment to restore the esthetic deficiencies.[2]

In this era of esthetic dentistry, vital bleaching is considered as a safe, popular, conservative, and well-accepted treatment procedure for discolored teeth.[3] Bleaching agents in varying concentrations such as hydrogen peroxide and carbamide peroxide have been used successfully for many years to achieve rapid esthetic results.[4]

An important complication following bleaching procedure is the decreased bond strength of composite resin to enamel. This may be due to the presence of oxygen ions that interfere with resin polymerization. This decreased bond strength of composite resin can be improved by delaying its placement for 1–3 weeks following the bleaching procedure.[4]

To avoid this delay, several methods have been proposed to reverse the compromised bond strength such as subjecting the bleached enamel to alcohol treatment before the restoration, removing the outermost layer of enamel, and use of organic solvent-containing adhesives.[5]

Antioxidants are substances that have potential to neutralize the free radicals and are able to react at the different stages of free radical formation.[6] Various antioxidant agents have been introduced such as sodium ascorbate, ascorbic acid, catalase, ethanol, acetone, glutathione, alpha-tocopherol, and sodium bicarbonate.[7]

Sodium ascorbate, a derivative of ascorbic acid (Neutral pH), has been reported to be a potent antioxidant. It neutralizes the effect of residual oxygen layer and allows free radical polymerization of resin materials to proceed without premature termination by restoring the altered redox potential of oxidized bonding substrate, thus reversing the compromised bonding.[5]

The interest in natural antioxidants of plant origin has greatly increased in recent years. Grape seed extract is a natural antioxidant that contains 98% oligomeric proanthocyanidin complexes (OPCs).[8] Proanthocyanidin is found in high concentrations from natural sources such as grape seed extract, cocoa beans, pine bark extract, cranberries, lemon tree bark, and hazelnut tree leaves. Polyamides (Pas) are high-molecular-weight polymers comprising the monomeric flavan-3-ol(+) catechin and (−) epicatechin.[9] It possesses antibacterial, antiviral, anticarcinogenic, anti-inflammatory, and anti-allergic properties.[8] OPC contains multiple electron donor sites (hydroxyl sites) that allow it to bind to free radicals by donating its hydrogen atoms. The presence of gallic acid also increases the free radical scavenging activity.[9]

Aloe vera (Aloe barbadensis) is a medicinal plant with antibacterial, anti-inflammatory, antiviral, and antioxidant properties. It contains polyphenolic compounds such as aloin A and B. Polyphenols constitute a large class of molecules containing a number of phenolic hydroxyl groups attached to ring structures allowing for their antioxidant activities.[10] These compounds are multifunctional and act as reducing agents, hydrogen-donating antioxidant, and singlet oxygen quenchers.[11]

Considering the above-mentioned factors, this study was undertaken to compare and evaluate the effect of treatment of sodium ascorbate, proanthocyanidin, and A. vera on the bond strength of composite resin to bleached enamel. The null hypothesis tested was that there was no difference in the reversal of reduced bond strength after the application of 10% sodium ascorbate, 6.5% of proanthocyanidin, and 50% A. vera immediately and after 2 weeks.

MATERIALS AND METHODS

Ninety freshly extracted human maxillary and mandibular anterior teeth were obtained from the Department of Oral and Maxillofacial Surgery. Teeth extracted for periodontal reasons were selected. The teeth were cleaned, rinsed with water, and stored in saline.

Preparation of specimens

The roots were embedded in self-cure acrylic resin (DPI-RR Cold Cure) block till cementoenamel junction [Figure 1]. Labial surfaces were flattened with 600-grit silicon carbide paper (234Q Wetordry ™, Raipur, India). The samples were divided into five groups, of which 10 teeth served as control group. The remaining 80 specimens were bleached with 35% carbamide peroxide gel (Ultra White, Ammdent) for 30 min according to the manufacturer's instructions. The bleach was rinsed off with water. Then, the specimens were divided into four groups of 20 teeth each, depending on the type of antioxidant used.

Figure 1

Resin composite placed on sample

Group I (n = 10): No bleaching (control group)

Group II (n = 20): Bleaching only (no antioxidants)

Group III (n = 20): Bleaching followed by treatment with sodium ascorbate for 10 min

Group IV (n = 20): Bleaching followed by treatment with proanthocyanidin for 10 min

Group V (n = 20): Bleaching followed by treatment with A. vera for 10 min.

Preparation of the antioxidant solution

Ten grams of sodium ascorbate (Molychem, Mumbai, India) powder was dissolved in 100 ml of distilled water to obtain 10% sodium ascorbate. About 6.5 g of grape seed extract (Best Source Nutrition, India) powder was collected from capsule and dissolved in 100 ml of distilled water to obtain 6.5% proanthocyanidin solution. An A. vera leaf was obtained and sterilized with ethanol. The outer, green dermis of the leaf was peeled off using a sterile blade. The inner flesh (gel) was cut into small pieces and weighed, and 25 g of this was taken. Fifty milliliters of distilled water was added to it. This mixture was then boiled for 10 min. After cooling to room temperature, it was filtered using Whatman filter paper no. 1 and obtained solution (50%) was used.

The Groups II–V were further subdivided into subgroup based on the application period before composite buildup.

Subgroup A: Immediate (n = 10)

Subgroup B: Delay of 2 weeks (n = 10).

The samples were etched with 37% phosphoric acid (3M ESPE Scotchbond ™ multipurpose etchant) for 15 s, rinsed with water for 20 s, and bonded, followed by composite (3M ESPE, Filtek ™ Z350XT) buildup. All the samples were placed in an incubator (Teknik universal incubator model-JEQ-4) at 37°C till testing procedure.

Testing procedure

Shear bond strengths were measured using universal testing machine (Instron, National Institute of Technology, Raipur, Chhattisgarh). The knife edge of indent of universal testing machine was loaded at the interface between the composite and enamel surface [Figure 2]. The bond strength was measured at a crosshead speed of 0.5 mm/min until fracture occurred. The bond strength of all groups was recorded in Newton (N) and converted into megapascal (MPa) using formula shear bond strength (Mpa) = F (N)/πr2.

Figure 2

Sample under universal testing machine

Statistical analysis

Data were statistically analyzed using Student's unpaired t-test, ANOVA test, and Tukey's Kramer multiple comparison test. SPSS 17.0 software (SPSS Inc., Chicago, IL, USA) was used for the statistical analysis, and P < 0.05 was considered as the level of statistical significance.

RESULTS

As shown in Table 1, there was a statistically significant difference between the control and the experimental groups (P = 0.0001). The mean bond strength of Group IVA (proanthocyanidin) was seen to be significantly higher (4.27 ± 1.86) as compared to other experimental groups. As shown in Table 2, Group IIB where restorations were delayed for 2 weeks after bleaching showed higher bond strength (3.75 ± 0.64). However, among the experimental groups, Group VB (A. vera) had a higher bond strength (2.95 ± 0.35). Student's unpaired t-test showed statistically significant difference between bond strengths of Group IIA and IIB (t = 8.06, P = 0.0001), Group IIIA and IIIB (t = 4.01, P = 0.001), and Group IVA and IVB (t = 2.42, P = 0.02), while there was no statistically significant difference in the bond strength values between Group VA and VB (t = 0.51, P = 0.61).

Table 1

Mean comparison of bond strength of immediate groups with control group

Group	n	Mean	SD	SE	95% CI for mean		Minimum	Maximum
					Lower bound	Upper bound
Control (no bleaching)	10	3.34	0.89	0.28	2.71	3.98	1.59	4.14
II A (bleaching only, immediate restoration)	10	1.91	0.31	0.09	1.68	2.13	1.59	2.38
III A (sodium ascorbate, immediate application)	10	2.52	0.32	0.10	2.28	2.75	2.00	3.18
IV A (proanthocyanidin, immediate application)	10	4.27	1.86	0.58	2.93	5.60	2.00	6.52
V A (Aloe vera, immediate application)	10	3.03	0.34	0.10	2.79	3.28	2.38	3.58

CI: Confidence interval, SD: Standard deviation, SE: Standard error

Table 2

Comparison of bond strength of delayed groups with control group

Group	n	Mean	SD	SE	95% CI for mean		Minimum	Maximum
					Lower bound	Upper bound
Control	10	3.34	0.89	0.28	2.71	3.98	1.59	4.14
II B (bleaching only, delayed)	10	3.75	0.64	0.20	3.29	4.22	2.23	4.70
III B (sodium ascorbate)	10	1.93	0.32	0.10	1.69	2.16	1.59	2.38
IV B (proanthocyanidin)	10	2.72	0.76	0.24	2.17	3.27	1.83	4.00
V B (Aloe vera)	10	2.95	0.35	0.11	2.70	3.21	2.38	3.34

SD: Standard deviation, SE: Standard error, CI: Confidence interval

DISCUSSION

In this era of conservative dentistry, the concept now is the conservation of the tooth structure. It has become very common to combine direct composite restorations and tooth whitening procedures to achieve esthetic clinical result.[12]

A study by Arumugam et al. demonstrated that 10%–35% carbamide or hydrogen peroxide adversely affects the bond strength of composite to acid-etched enamel immediately after bleaching.[4] This may be due to free radicals that are substituted by peroxide ions, resulting in the formation of peroxide apatite. These free radicals are responsible for inhibiting resin polymerization and interfering in resin penetration into the enamel.[4] Our study confirms that immediate bonding to bleached enamel results in a statistically significant difference in bond strength (1.91 ± 0.31) when compared to control group (3.34 ± 0.89). These findings are similar to those of a study conducted by Khamverdi et al.[13]

Studies by Khoroushi and Aghelinejad and Danesh-Sani and Esmaili suggested a 2-week storage period during which the peroxide ions decompose and the substituted hydroxyl radicals re-enter the apatite lattice, resulting in elimination of structural changes caused by incorporation of peroxide ions.[1415] Hence, it was recommended to delay the bonding for 1–3 weeks following the bleaching procedure.[4]

In the present study, sodium ascorbate and grape seed extract solutions were prepared from the powder form. The grape seed extract used was water extracted, and there was no alcohol residue or other solvents such as acetone that could hamper the hypothesis. Studies by Paul et al., Thapa et al., and Subramonian et al.[51617] have demonstrated that 10-min application of 10% sodium ascorbate on bleached enamel surface is effective in reversing of bond strength of composite resin to bleached enamel. A statistically significant difference was observed between Group IIIA and IIIB (P = 0.001) where Group IIIA showed a higher bond strength.

It was reported that grape seed extract in concentration of 6.5% of proanthocyanidin applied for 10 min increased the bond strength to enamel, but it was not able to reverse it completely.[18] This may be related to the fact that the molecular weight of the antioxidant should be less than 500 g/mol for the efficient elimination of free radicals. The Proanthocyanidines (Pas) have molecular weight of 500–3000 g/mol, and it is a highly hydroxylated structure that can form insoluble complexes with proteins and carbohydrates hindering its action on the residual oxygen.[19]

However, in this study, a statistically significant difference was observed between the immediate application (IVA) and delayed group (IVB), confirming that delaying the restoration in 2 weeks following the antioxidant application was of no clinical significance. Furthermore, on immediate application of 6.5% grape extract in Group IV, a significantly higher bond strength (4.27 ± 1.86) was observed when compared to the other immediate bonded groups (P = 0.0001). This was similar to a study by Vidhya et al.[18]

In this study, it was seen that although the immediate application of 50% A. vera in Group V reversed the bond strength of bleached enamel, there was a statistically significant difference between Group IVA and VA (P = 0.046). However, there was no statistically significant difference between Group IIIA and Group VA. This was in accordance to a study done by Kadiyala et al.[11] The antioxidant activity of A. vera may be due to the presence of phenolic contents, ascorbic acid along with tocopherol, and vitamin A. Anthraquinones present in A. vera was also reported to exert antioxidant property.[11]

The results of our study indicated that the immediate application of all antioxidants was equally effective in reversing the compromised bond strength. Further, studies should be conducted to evaluate the effect of natural antioxidants on the reversal of bond strength after bleaching, and clinical trials remain necessary to validate the laboratory observations. Thus, the null hypothesis that there was no difference in reversal of reduced bond strength after the application of 10% sodium ascorbate, 6.5% proanthocyanidin, and 50% A. vera immediately and after 2 weeks was rejected.

CONCLUSION

Within the limitation of this in vitro study, the following conclusions were drawn:

On immediate application for 10 min on bleached enamel, 6.5% grape seed extracts were superior to 10% solution of sodium ascorbate and 50% A. vera in restoring the bond strength to the basic strength

Observing a period of 2-week delay between the antioxidant application and composite restoration played no significant role in reversing the bond strength.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.