Evaluation of the effect of concentration and duration of application of sodium ascorbate hydrogel on the bond strength of composite resin to bleached enamel.

AIM: The effect of different concentrations of hydrogel of sodium ascorbate on bond strength of bleached enamel for varying periods of time and the mode of failure was evaluated.
MATERIALS AND METHODS: Seventy enamel surfaces were obtained from 35 human extracted premolars. Specimens were divided into four groups: no bleaching (control), bleaching with carbamide peroxide gel, bleaching and application of 10% / 20% sodium ascorbate hydrogel for 30, 60, 120 min. Surfaces were bonded with a total etch bonding system and composite resin. Specimens were tested for shear bond strength. Mode of failure was determined by stereomicroscope. Data were analyzed using a two-way analysis of variance, and Scheffe's post hoc test.
RESULTS: Sodium ascorbate hydrogel application following bleaching increased the resin-enamel bond strength and was directly proportional to its duration of application. However, there was no difference in bond strength with an increase in the concentration of sodium ascorbate hydrogel.
CONCLUSION: Immediate bonding of composite resin to bleached enamel is possible after treatment with antioxidant sodium ascorbate hydrogel.

INTRODUCTION

Vital tooth bleaching using carbamide peroxide is a safe, well accepted and an increasingly popular procedure for the treatment of surface and intrinsic staining of teeth.[1-3] Clinicians should be aware of the outcome of the bleaching treatment and the interactions with further dental treatments, especially additional adhesive aesthetic interventions such as composite bonding, laminate veneers, or orthodontic treatment to restore the esthetic deficiencies.[4]

A number of studies have shown that the bond strengths of adhesive restorations to tooth structures are reduced when the tooth has been bleached.[5-7] It has also been reported that the weakening of the bond occurred both superficially and internally.[89] This could be attributed to the presence of residual peroxide, which interferes with the resin tag formation and inhibits the resin polymerization.[5] Removal of the superficial layer of enamel, treating the bleached enamel with alcohol before restoration and the use of adhesives containing organic solvents are some of the techniques that have been suggested to solve the clinical problems related to the postbleached compromised bond strength.[10-13] However, the general approach is to postpone any bonding procedure for a period varying from 24 h to 4 weeks after bleaching since the reduction of bond strength has been shown to be temporary.[51415]

Compromised sealing ability of composite restorations can be reversed through application of 10% sodium ascorbate solution as an antioxidant before restoration.[16] In previous investigations, sodium ascorbate has been used in its solution form.[16-18] However, the gel form of sodium ascorbate is easy to apply (because of its higher viscosity and better control) and its application is less expensive for patients compared to the application of sodium ascorbate solution by the dental practitioner due to the shorter chair time needed.[19]

However, there are limited data at present to support the effect of different concentrations and duration of application of sodium ascorbate hydrogel on bond strength of bleached enamel. Hence, the aim of our study was to evaluate the effect of different concentrations of hydrogel of sodium ascorbate on bond strength of bleached enamel for varying periods of time and to determine the mode of failure at the enamel-resin interface.

MATERIALS AND METHODS

Thirty-five intact extracted human premolars were selected for the study. Teeth free of caries, cracks, or developmental enamel defects were included. Teeth were stored in 0.1 % thymol solution as it is an antifungal and bacteriostatic. High-speed diamond burs were used to remove roots. Each crown was sectioned to obtain two non-occlusal enamel surfaces and a total of 70 enamel specimens were obtained. A putty silicon mould was made and specimens were mounted in acrylic such that only the enamel surfaces were exposed. Specimens were polished with wet 320-grit silicon carbide paper to prepare standardized flat enamel surfaces for treatment and bonding. The specimens were stored in an incubator at 37°C in 100% relative humidity throughout the experiment.

The specimens were then randomly divided into following groups:

Group A (5): untreated (control group).

Group B (5): only bleaching with 17% carbamide peroxide.

Group C (30): bleaching with 17% carbamide peroxide followed by application of 10% sodium ascorbate hydrogel.

Group D (30): bleaching with 17% carbamide peroxide followed by application of 20% sodium ascorbate hydrogel.

In groups C and D, the samples were further subdivided into three subgroups of ten teeth each depending on the duration of application of sodium ascorbate hydrogel.

C1/D1: 30 min

C2/D2: 60 min

C3/D3: 120 min

: Group A: the specimens were left untreated

: Groups B, C and D

Bleaching procedure

The specimens in groups B, C and D were treated with 17% carbamide peroxide (Perfect Bleach, Voco, Germany) for 8 h a day for 5 days. After completion of daily bleaching procedure, the specimens were thoroughly rinsed with an air–water spray for 30 s and air dried.

Preparation of sodium ascorbate hydrogel (antioxidant gel)

Carbopol 974P was dispersed in water by constant stirring in an ultrasonic water bath. After the Carbopol 974P was dispersed, ascorbic acid was added and dissolved. Then, sodium hydroxide was added as a neutralizer to thicken the gel. The gels were stored in small airtight bottles.

Application of sodium ascorbate hydrogel

After the bleaching procedure, antioxidant gel was placed on enamel surfaces of embedded teeth for 30, 60 and 120 min in groups C1 and D1, C2 and D2, C3 and D3, respectively. However, the antioxidant gel was not applied on specimens in group B. The specimens were then rinsed and immersed in distilled water for 10 min to dissolve sodium ascorbate crystals that were deposited on the enamel surface

Bonding procedure

After etching with 35% phosphoric acid (Ultra-etch, Ultradent, South Jordan, Uttah, USA), a total etch adhesive (Adper Single Bond 2, 3M ESPE, St. Paul, MN) was applied to the flattened surface of specimens according to manufacturer`s instructions. A plastic tube with an internal diameter of 2 mm and 2 mm height was applied to bonded specimen prior to curing the adhesive and then light cured for 30 s. The plastic tube was filled with resin composite (Filtek Z350, 3M ESPE, St. Paul, MN) and light cured for 40 s. The tube was left in place throughout the testing process.

Shear bond strength testing

The specimens were mounted and stressed in shear at a rate of 1 mm/min using a Universal testing machine that used a knife-edged loading head. The maximum load at failure was recorded and converted to megapascals (Mpa). Shear bond strength (Mpa) = F (N)/π r2

Microscopic evaluation

Samples were examined under a stereomicroscope at 10× magnification to evaluate the mode of failure. Type of failure was classified into following categories: adhesive failure (between enamel and bond/ composite and bond); cohesive failures (within the enamel/resin composite) and mixed type.

Statistical analysis

The data obtained were analyzed using a two-way analysis of variance (ANOVA), and Scheffe's post-hoc test. The significance level was set at P< 0.05

RESULTS

Mean shear bond strengths and standard deviations (SD) recorded in the control and experimental groups are displayed in Table 1. Bleaching with carbamide peroxide produced a significant decrease in the bond strength in group B as compared to the control group (A).

Table 1

Means and standard deviation of shear bond strength values (in Mpa) of all the groups

Comparison between the various groups is summarized in Table 2. Comparison within the groups C and D are displayed in Table 3. Application of sodium ascorbate hydrogel improved the bond strength of composite resin to bleached enamel. There was statistically significant difference between groups A and C1, C3, D1, D3; B and C2, C3, D2, D3. However, no significant difference was observed between groups A and C2, D2; B and C1, D1. The longer the duration of application, the higher the bond strength. At the 30 min application, the bond strengths were significantly lower than the unbleached group (A). However, there was no difference in bond strengths between the sodium ascorbate groups (C1, D1) and the bleached group (B). But at the 60 min application, the bond strengths were higher than the bleached groups (B, C1, D1) and similar to the unbleached group (A). At 120 min application, the bond strengths (C3, D3) were higher than all the other groups.

Table 2

ANOVA for comparison between the groups

Table 3

Scheffe's post hoc test for comparison within the groups

There was no difference; however, with regards to the concentration of sodium ascorbate hydrogel (10%, 20%) at all the time intervals (C, D).

All three types of failures (adhesive, cohesive and mixed) were observed in various groups that are summarized in Table 4. More of adhesive failures were seen in groups B, C1, D1. However, number of adhesive failures reduced in groups C2, D2, C3, and D3.

Table 4

Different modes of failure in various groups

DISCUSSION

Premolars extracted for orthodontic purpose were selected so as to have four surfaces for bonding for four groups from same tooth to eliminate bias.

In this study, bleaching with carbamide peroxide produced a significant reduction in bond strength in groups (B, C, D) treated with it [Table 1]. Carbamide peroxide (10%, 16%, and 20%) reduces the shear bond strength of composite resin to bleached bovine enamel. In addition, the higher the concentration of carbamide peroxide, the greater the reduction.[20] This is likely to be caused by delayed release of oxygen that could interfere with resin infiltration into etched enamel or inhibit polymerization of resins that cure via free radical polymerization.[2122]

Some researchers have suggested physical alterations in enamel after bleaching with carbamide peroxide. They found an increased porosity of enamel as manifested by an over-etched appearance with loss of prismatic structure. Also, loss of calcium, decrease in microhardness and alterations in the organic substance, might be factors that contribute to the decrease in the bond strengths.[2324]

The interfaces between resin and bleached enamel as seen on SEM examination are substantially different from those formed between resin and unbleached enamel. In the bleached specimens, large areas of the enamel surface were free of resin. When tags were present, they were fragmented and poorly defined; also, they had penetrated to a lesser depth than in the unbleached controls.[5] The quality of a composite bond is compromised by a smaller number of resin tags.[15] SEM observations suggested an association between a high concentration of voids in the bond area and low mean bond strengths.[25] In another study, SEM examinations of interfaces between resin and bleached enamel displayed a granular and porous aspect with a bubbly appearance. It has been suggested that these might be due to gaseous bubbling, possibly the result of oxidization of peroxide trapped in the subsurface layer of the enamel.[2026] We share the opinion that the bubbly appearance of the resin might be caused by residual oxygen in the enamel structure.

Presently, emphasis is on neutralizing the oxygen by application of an antioxidant agent. Ascorbic acid and its salts are well-known antioxidants that can reduce various oxidative compounds, especially free radicals.[27] Previous in vivo studies have demonstrated the protective effect of ascorbic acid against hydrogen peroxide-induced damage in biologic systems.[2829] In our study, treatment of the bleached enamel with sodium ascorbate hydrogel (both 10% and 20%) before bonding appeared to restore the reduced shear bond strength of composite resin to enamel. The results concur with the previous studies,[182030] where similar reversal of bond strength was observed by antioxidant treatment. The possible explanation for this could be the antioxidant ability of sodium ascorbate that helps to neutralize and reverse the oxidizing effects of bleaching agents. Also, sodium ascorbate allows for the free radical polymerization of the adhesive to proceed without premature termination by restoring the altered redox potential of the oxidized bonding substrate.[192831] However, with regards to the concentration of the hydrogel (either 10% or 20%), there was no significant difference observed in the bond strengths between the groups. Kimayi and Valizadeh observed similar findings and suggested that 10% sodium ascorbate hydrogel might be as effective as 20% sodium ascorbate hydrogel in neutralizing the oxidizing agents and increasing the bond strengths.[19]

Regarding the duration of application of the hydrogel, this study suggests that bond strengths of composite resin to bleached enamel increased with an increase in the duration of application of sodium ascorbate. Lai et al.,[18] found reversal of bond strengths after the bleached specimens were immersed in 10% sodium ascorbate solution for 3 h. They considered this duration as one-third of the time of application of the bleaching agent. Nevertheless, in some studies[2030] the duration of antioxidant treatment was 10 min, which was considered a reasonable time period for clinical application. However, the findings in our study differ. The application of sodium ascorbate hydrogel for 30 min (C1, D1) increased the bond strength of composite resin to bleached enamel, but it was still less than in the untreated group. The bond strengths in groups C2, D2 (60 min) were similar to that of untreated group. There was significant increase in bond strengths in groups C3, D3 (120 min) when compared to all other groups. Thus, this study suggested that it takes at least 60 min for bond strength to return to pretreatment levels [Tables 2 and 3].

It has been demonstrated that peroxide ions take the place of the hydroxyl radicals in the apatite lattice and produce peroxide apatite. In addition, when peroxide ions decompose, substituted hydroxyl radicals re-enter the apatite lattice, resulting in the elimination of the structural aberrations caused by incorporation of the peroxide ions.[32] The incorporation process of peroxide ions might also be reversed by an antioxidant as has been explained earlier.[18] Syprides et al. demonstrated that adhesives containing ethanol and water reversed the decrease in the bond strength in 10% carbamide peroxide treated dentin.[33] In another study that used an alcohol based adhesive, there was no difference between bleached and unbleached enamel in terms of bond strength and using alcohol-based bonding agent made the resin composite application possible immediately after the bleaching process. Ethanol is known to increase the bond strength of enamel by decreasing surface water.[34] Considering these, an ethanol-based bonding agent adper single bond 2 was used in this study. However, we observed that it failed to reverse the effect of the bleaching agent on enamel bond strength.

When the modes of failure were evaluated, all three types of failure, that is, adhesive, cohesive, and mixed were observed. More of adhesive failure's were seen with Group B, C1, D1 (60%) indicating the reduction of bond strengths between the tooth and the restorative material. However, the number of adhesive failures reduced after the application of sodium ascorbate hydrogel (C2-30%, D2-20%). Also, similar reduction in failures was seen with the increase in the duration of application of the hydrogel (C3-20%, D3-20%) [Table 4].

Sodium ascorbate was used in gel form instead of solution since gel form is more acceptable clinically. Patients can apply gel in the same tray used for bleaching, thus reducing the chair time.[1819] Also, manipulation of solution is more difficult than gel. Solution should be used several times prior to bonding procedure. Also, the efficiency of sodium ascorbate solution is swift and short-term.[35] In this study, ascorbic acid was converted to gel form, its efficiency remained as long as the container was not opened. Since Vitamin C (ascorbic acid) is nontoxic and is widely used in the food industry, it is unlikely that it will cause any adverse biological effect or clinical hazard, sodium salt of ascorbate was used to prevent the acidic effect of ascorbic acid.[17]

CONCLUSIONS

Within the limitations of this study, it can be concluded that the use of sodium ascorbate hydrogel following bleaching, increased the composite resin-enamel bond strength and the shear bond strength was directly proportional to the duration of application of the sodium ascorbate hydrogel. However, there was no difference in bond strengths on increasing the concentration of sodium ascorbate hydrogel. Hence, immediate bonding of composite resin to bleached enamel is possible after treatment with antioxidant sodium ascorbate hydrogel.