EFFECT OF DIFFERENT ANTIOXIDANTS ON REVERSING COMPROMISED RESIN BOND STRENGTH AFTER ENAMEL BLEACHING: AN IN VITRO STUDY

ABSTRACT

Over years, one of the most frequent complaints that patients present with is that of stained teeth. With the advent of straightforward and fast procedures the craze for smile makeovers has risen. Bleaching is the most conservative treatment for discolored teeth compared with other treatments, such as veneers or crowns. Among the various undesirable effects of bleaching is the post treatment compromised enamel bond strength. The waiting period for bonding procedure after bleaching has been reported to vary from 24hrs to 4weeks. To overcome this delay in bonding, several antioxidant agents like 10% sodium ascorbatewith different application times after the bleaching procedure have been used. Hence, the purpose of this in vitro study was to evaluate and compare the effects and efficacy of 10% sodium ascorbate solution, 10%α-Tocopherol solution, 10% grape seed extract solution and 10% guava seed extract solution on the bond strength between composite resin and bleached enamel after 10 minutes and 120 minutes of application.

INTRODUCTION

Discoloredteeth is a common problem affecting people of various ages. Etiology is multifactorial and different surfaces of tooth can take different stains. With the advent in esthetic dentistry, simple fast in-office and at home bleaching procedures have become effective and popular.¹To state in layman’s words the term ‘in office bleaching’means bleaching techniques that are carried out in office. The demand of this technique has increased because of various advantages like: reduction in total treatment timeandgreater potential for immediate results that may enhance patient satisfaction and motivation. It utilizes a gel or liquid of hydrogen peroxide (35-38%) or carbamide peroxide(35%).²

The disadvantages associated with hydrogen peroxide as a bleaching agent include hypersensitivity and gingival irritation, reduced enamel microhardness andmicromorphological defects due to demineralization.^4,5In addition, hydrogen and carbamide peroxide effect the bonding of enamel and dentin tissues when restorations are applied immediately after bleaching treatment.⁸

Various techniques have been proposed to resolve the clinical problems related to post bleaching compromised bond strength.Barghi and Godwin¹⁰ treated bleached enamel with alcohol before restoration,Cvitko and others¹¹ proposed removal of the superficial layer of enamel, Sung etal¹² suggested the use of the adhesives containing organic solvents. However, the general approach is to postpone any bonding procedure for a period from 4 daysto 4 weeks.¹³

To overcome this delay in bonding, application of various antioxidant agents like 10% sodium ascorbate, Vitamin E and Proanthocyanidins after the bleaching procedure have been proposed.¹⁴Vitamin E(α-Tocopherol), acts as peroxyl radical scavenger and prevents the propagation of free radicals in tissues by reacting with them to form a tocopherol radical. This radical is then oxidized by a hydrogen donor and thus returned to its reduced state.¹⁵OligomericProanthocyanidins Complexes (OPC) are high molecular weight molecules consisting of monomeric flavan-3-ol cathechin and epicathechin. They are found in high concentrations in natural sources such as grapes, guavas, lemon tree bark and cranberries. It is a naturally occurringmetabolite and has been proven to be safe as an antioxidant in various clinical and dietary supplements. Although studies have shown the efficacy of sodium ascorbate⁹ and grape seed extract¹⁶ in reversal of reduced bond strength to bleached enamel, there is still a paucity of research onvitaminE(α-Tocopherol) and proanthocyanidines derived from other fruits like guava as viable alternatives to sodium ascorbate.

Moreover, studies have reported incongruous findings with concerns to absolute reversal of bond strength to base line levels after 10 minutes of application.^17,18Therefore, increase in time of application and varying the concentration of antioxidants to neutralize the residual oxygen is warranted. Hence, the purpose of this in vitro study was to evaluate and compare the effects and efficacy of 10% sodium ascorbate solution, 10%α-Tocopherol solution, 10% grape seed extract solution and 10% guava seed extract solution on the bond strength between composite resin and bleached enamel after 10 minutes and 120 minutes.

The null hypothesis tested was that there is no difference in bond strength reduction reversal after application of different antioxidants for different application times after bleaching.

MATERIALS AND METHOD

Eighty freshly extracted human permanent maxillary central incisors extracted for periodontal reasons were collected and rinsed in 0.9% unbuffered saline. Labial surfaces of these teeth were flattened with 600 grit silicon carbide paper & roots were embedded in acrylic resin block so that only the coronal portion was exposed. Individual moulds of tooth colouredself cure resin were prepared over the specimens. Modelling wax was used as a separator, so as to block the undercuts and also to provide space for antioxidant solution on the labial surface of the specimens. A customised cylindrical silver mould of 3mm diameter and 5mm height was also fabricated.

For preparation of 10% antioxidant solutions, 10 gmsodium ascorbate powder(sd fine cHEM Limited, Mumbai)and 10 gmgrape seed extract powder(Biovea, USA) were dissolved in100ml of distilled water.For preparation of 10% alpha tocopherol, 10 ml alpha tocopherol(sd finecHEM Limited, Mumbai) was dissolved in 100 ml of ethyl alcohol.

Soxhlet extraction method was used for the preparation of10% guava seed extract.40 gm of guava seedswere placed inside a thimble made from thick filter paper, which was loaded intothe main chamber of the Soxhlet extractor. The Soxhlet extractor was placed onto a flask containing 40 ml of ethyl alcohol as extraction solvent. The Soxhlet was then equippedwith a condenser. The solvent was heated to reflux. The chamber containing the solid material was slowly filled with warm solvent. When the Soxhlet chamber was almost full, the chamber was automaticallyemptied by a siphon side arm, with the solvent running back down to thedistillation flask. This cycle was repeated many times, over 5 hours.During each cycle, a portion of the non-volatile compound dissolved in thesolvent. After many cycles the desired compound was concentrated in thedistillation flask. After extraction, the solvent was removed, by means of a rotaryevaporator, 10 ml of this extract was then dissolved in 100 ml of distilled water to make 10 % guava seed extract solution.

Labial surfaces of seventy two specimens were bleached with Pola office one patient kit (SDI, Victoria, Autralia) according to manufacturer instructions. The gel was then completely rinsed off with water. These specimens were divided into five experimental groups and control group:

Group I(n=16)treated with 10% sodium ascorbate solution

Group II(n=16)treated with10% alpha tocopherol solution

Group III(n=16)treated with10% grape seed extract solution

Group IV(n=16)treatedwith 10% guava seed extract solution

Group V(n=8)treated with no antioxidant solution

Group VI(n=8)Control (unbleached specimens)

Based on application period of antioxidants, groups I-IV were further subdivided as:

Subgroup A(n=8)Antioxidant treatmentfor 10 minutesimmediately afterbleaching

Subgroup B(n=8)Antioxidant treatment for 120 minutesimmediately afterbleaching

The antioxidant solutions were refreshed after every 10 minutes in subgroup B. Specimens were then rinsed with water. Groups V & VI did not receive any antioxidant treatment.

Labial surfaces of all specimens of Subgroup A and B (Group I-IV), Group V and Group VI were etched with 37% phosphoric acid for 15 seconds, followed by rinsing with water for 20 seconds. Bonding agent (Adper single bond) was then applied and light cured for 20 seconds.Customised cylindrical silver mould was placed on the labial surface of each specimen and composite resin (Filtek Z 350 XT) was then placed in three increments so as to have a final build up of 3mm diameter and 5mm height.

All specimens after composite build up were stored in distilled water for 24 hours and shear bond strength (SBS) testing was done using Universal Testing Machine (Banbros Engineering Pvt Ltd) at across head speed of 1mm per minute. Data was tabulated and subjected to statistical analysis.

RESULTS

Table 1: Mean Shear bond strength (MPa) in samples of Group I- IV

The mean value for samples after bleaching and without application of any antioxidant agent (Group V) was 43.63 MPa. The mean value for samples in Group VI was 77.75 MPa.

Bond strength of Subgroup IB (74.13+2.95) was found to be significantly higher (p<0.001) than that of Subgroup IA (54.88+2.75).

Bond strength of Subgroup IIB (75.88+3.04 units) was found to be higher than that of Subgroup IIA (55.13+1.81 units) and this difference was found to be statistically significant (p<0.001).

Bond strength of Subgroup IIB (76.75+1.75 units) was found to be higher than Subgroup IIA (64.75+2.12 units) and this difference was found to be statistically significant (p<0.001).

Though bond strength of Subgroup IVB (77.00+2.67 units) was found to be higher than that of Subgroup IVA (74.63+2.45 units) but this difference was not found to be statistically significant.

Table 2: Analysis of variance in subgroup A

Analysis of variance and box plot thereafter revealed statistically significant intergroup differences (p<0.001). Values in Group V were of lower order while values in Groups IVA and VI were of higher order. Values in groups IA, IIA and IIIA were of middle order.

Table 3: Analysis of variance in subgroup B

Analysis of variance and box plot thereafter revealed statistically significant intergroup differences (p<0.001). Values in Group V were of lower order while values in all the other group/subgroups were of higher order.

Graph 1: Intra group Bond strength of different groups

DISCUSSION

Bleaching treatment focuses on the use of certain types of oxidizing agents, such as hydrogen peroxide, carbamide peroxide and sodium perborate, which, when decompose into free radicals initiate the bleaching reaction on dental tissues. The free radicals released, oxidize the pigments and coloring matters impregnated in the dental tissue; breaking the large aromatic chains of the darker coloring pigments and transforming into slightly lighter unsaturated linear chains. As the oxidizing process continues, these chains are converted into even lighter saturated linear chains, thus making the teeth whiter.⁵²

However bleaching affects the tooth at structural and functional level.⁵³ Previous studies have shown that the use of hydrogen peroxide or carbamide peroxide containing bleaching agents adversely affects the bond strength of resin composites to acid etched enamel when bonding is performed immediately after bleaching procedure.^54,55,56

This problem is more significant when the desired results or esthetics after bleaching might not be achieved and the patient may wish for additional esthetic options like direct and indirect veneers or full coverage restorations.However studies also point out that this reduction in bond strength is temporary and the original bond strength values are achieved within 1-2 weeks in oral environment.^19,22,56,57

Various methods have been proposed for reversal of reduced bond strength^54,55,56,12immediately after the bleaching procedure.Kaya et al⁵⁶, Han et al⁵⁰ andThapa et al⁵⁸ found that compromised bonding to acid etched bleached enamel and dentin can be reversed with sodium ascorbate.^50,54,56 Ascorbic acid and its sodium salts are potent antioxidants that are capable of quenching reactive free radicals in biological systems.¹⁰

Alpha tocopherolallow free-radical polymerization of the adhesive resin to proceed without premature termination by restoring the altered redox potential of the oxidized bonding substrate and improves resin bonding.¹⁷ Proanthocyanidins are high-molecular-weight polymers comprising of monomeric flavan-3-ol (+)catechin and (−)epicathechins. In vitro studies have confirmed that OPCs are 50 times more effective than vitamin E and 20 times more powerful than vitamin C.⁴⁸

To our knowledge, guava seed extract has not been studied as antioxidant to reverse the reduced bond strength after bleaching. Moreover,there is no specific recommendation with regards to application time of antioxidants. As the role of application time in complete reversal of reduced bond strength has not been extensively studied, the current study was carried out with the aim of evaluating and comparing the effects of guava seed extract,grape seed extract, sodium ascorbate and alpha tocopoherolin reversal of bond strength between bleached enamel and composite resin after 10 minutes and 120 minutes of application.

Since, the protocol tested in the current study compared the antioxidant preparations in solution form, individual resin moulds were prepared for adequate contact of antioxidants with labial surface of the specimens. In clinical scenario, mould can be substituted by customized trays which can be worn by the patient in the desired area for appropriate time.

To standardize the concentration of the antioxidants, 10% concentration was used in the present study, and also the previous studies have suggested that the application of 10% sodium ascorbate waseffective in reversing the compromised bonding to theoxidized enamel and dentin.^61,64,33

Sodium ascorbate and alpha tocopherol solutions were prepared from the powder supplied but guava seed extract was prepared from seeds of white guava using soxhlet extraction method. The advantage of soxhlet extraction is that instead of manyportions of warm solvent being passed through the sample,just one batch of solvent is recycled and after extraction, the solvent is removed by means of a rotaryevaporator, yielding the extracted compound. The non-soluble portion of the extracted solid remains in the thimble, and is discarded.³⁶

Kaya AD14 and Dabaset al62 determined the most effective time duration for the application ofsodium ascorbate to be 10 minutes. The authors also concluded that as the application period of the antioxidant increased, the bond strength of the composite on enamel tissue also increasedtill 120 minutes.¹⁴ In order to determine the fruition time for reversing the reduced bond strength, two application time periods for antioxidants that is 10 minutes and 120 minutes were choosen for the current study. Subgroup A samples received antioxidant treatment for 10 minutes and subgroup B samples for 120 minutes. The antioxidants were refreshed after every 10 minutes when applied for 120 minutes.

The most frequently ignored guideline in the test protocol is following the ISO/TS 11405 specification (2003) i.e.‘a limitation of the bonding area is important’⁶⁶,so to standardize the dimensions of composite cylinders which were fabricated over the labial surface of the specimens after the antioxidant treatment, preformed mold of silver with 3mm internal diameter and 5mm height was made. This silver mold was then placed over the individual specimens to build up the composite cylinders. The shear bond strength was evaluated using universal testing machine at a cross head speed of 1 mm/min as described in previous studies.^15,17,19

The mean shear bond strength of Group VI (control) was 77.75 MPa and of Group Vwas 43.63MPa. This observation is in accordance with the results of other studies in the literature.^68,69Many reasons have been proposed for reduction in bond strength after bleaching.⁷¹Lai et al⁵⁴ stated that reduction in resin enamel bond strength to bleached etched enamel is likely to be caused by a delayed release of oxygen that affects the polymerization of resin components. Also Rueggeberg and Margeson⁷⁰ proposed that release of oxygen, could either interfere with resin infiltration into etched enamel or inhibit polymerization of resins that cure via a free radical mechanism.Titleyet al⁵⁶ stated that hydrogen peroxide causes denaturation of proteins in the organic components of the dentin and enamel and thus causing the alteration in organic and inorganic ratio with an increase in inorganic component.Titley et al,⁵⁶Kalili et al,²⁰Dishman et al⁸ stated that most acceptable version might be presence of the residual oxygen from the bleaching agent which inhibits resin polymerization.

In the present study, after the application of respective antioxidants for 10 minutes, mean shear bond strengths of different groups were evaluated as 54.88MPa, 55.13 MPa, 64.75 and 74.63 MPa for group IA, II A, III A and IV A respectively. Except for Group IV A (guava extract solution), none of the antioxidants (Group IA, IIA, IIIA) showed complete reversal in bond strength when applied for 10 minutes. This corroborates with the findings of Thapaet al¹⁸, Sasaki et al¹⁷, Torres et al²⁹.

In the present study, after the application of respective antioxidants for 120 minutes, mean shear bond strengths of different groups were evaluated as 74.13 MPa, 75.86 MPa, 76.75 MPa and 77 MPa for group IB, II B, III B and IV B respectively. Thus complete reversal of reduced bond strength was achieved in all the experimental groups when applied for 120 minutes and is in accordance with the previous studies,^17,18,29 which stated that bond strength increases with increase in antioxidant application time till 120 minutes.^17,18

The results of the study demonstrate that guava seed extract have highest antioxidant activity both for 10 minutes and 120 minutes of application among all the antioxidants used in the present study. Further studies are required using higher concentrations of antioxidants and reducing their time of application so as to achieve complete reversal of reduced bond strength after enamel bleaching.

CONCLUSION

Within the limitation of the present study it could be concluded that:

• Immediate bonding of resin composite to bleached enamel results in reduced shear bond strength.
• Treatment of bleached enamel surface with 10 minutes application of 10% sodium ascorbate, 10% alpha tocopherol, 10% grape seed extract and 10% guava seed extract improves the reduced bond strength significantly immediately after bleaching, but only the application of 10%guava seed extract for 10 minutes reverses it to baseline levels.
• All the antioxidants after 120 minutes of application completely restored the reduced bond strength.