Evaluation and comparison of bond strength to 10% carbamide peroxide bleached enamel following the application of 10% and 25% sodium ascorbate and alpha-tocopherol solutions: An in vitro study.

AIM: To evaluate and compare composite bond strength to carbamide peroxide bleached enamel following the application of 10% and 25% sodium ascorbate and alpha-tocopherol solutions.
MATERIALS AND METHODS: Sixty premolars were divided into six groups. Groups I and VI served as unbleached and bleached controls respectively. Groups II, III, IV and V served as the experimental groups and were subjected to 10% carbamide peroxide bleaching followed by 10 min application of 10% and 25% sodium ascorbate and 10% and 25% alpha-tocopherol solutions, respectively. Following composite bonding, shear bond strength was determined and the results were analyzed using ANOVA and Tukey highest significant difference test.
RESULTS: Only Group IV showed significantly lower bond strength when compared to Group I (unbleached control). When compared to Group VI (bleached control), except Group IV, groups II, III and V showed significantly higher bond strength. However, there was no statistically significant difference between the experimental groups corresponding to 10% and 25% and similar concentrations of sodium ascorbate and alpha-tocopherol solutions.
CONCLUSION: Following 10% carbamide peroxide bleaching, except 10% alpha tocopherol, 10 min application of 10% and 25% sodium ascorbate and 25% alpha-tocopherol solutions significantly improves the shear bond strength of composite resin to enamel.

INTRODUCTION

Bleaching using carbamide peroxide has gained popularity as one of the esthetic treatment modality.[1] Occasionally, bleaching and composite resin restorations may be needed as a combined treatment to get the best esthetic outcome. In such instances, bleaching is usually indicated prior to composite resin restrations to attain desired shade.[2] However, studies have shown compromised bonding of composite restorations immediately carried out following bleaching.[3-5] This is attributed to the presence of residual oxygen and interference with resin penetration and resin polymerization.[6] Bleaching is also shown to cause physical alteration in enamel and reduced composite bond strength.[7]

To overcome this, various studies have suggested delaying composite restorations by 1 day to 3 weeks.[689] Clinically, this delay prevents carrying out composite restorations immediately following bleaching. Various methods have been proposed to avoid this delay. Among them, application of antioxidant agents such as sodium ascorbate (salt of vitamin C) and alpha-tocopherol (vitamin E) has shown encouraging results.[10-13] These agents restore the altered redox potential of the bleached surface and reverse the compromised bonding eliminating the need for delaying composite restorations following bleaching.[1415]

In this regard, a shorter clinical application time for these agents is an additional desirable feature. Ten percent sodium ascorbate, with an application time ranging from 10 min to 3 h, is commonly recommended to improve composite bond strength following bleaching. Ten percent sodium ascorbate in solution form is suggested for shorter application time.[10-1214-16] In the context of shorter application time, the use of higher concentration of sodium ascorbate solution has also been recommended.[17] However, the effect of this on the composite bond strength has not been widely evaluated.

Alpha-tocopherol, a powerful antioxidant in the lipid phase of the human body, has been recently suggested for improving composite bonding following bleaching. In a context similar to sodium ascorbate, solution form of alpha-tocopherol may be preferred for shorter application time. The beneficial effect of alpha-tocopherol solution is attributed, in addition to its antioxidant effect, to its alcoholic solvent.[12] However, studies are lacking on the effect of varying concentration and shorter application time of alpha-tocopherol solution on composite resin bond strength.

Since sodium ascorbate and alpha-tocopherol solutions have similar mode of action, studies comparing them would be clinically relevant. According to a previous study, 10% alpha-tocopherol solution when used for 2 h was found to be more effective than 10% sodium ascorbate.[12] However, further studies in the above context are of clinical importance.

Therefore, the present study was carried out to evaluate and compare the shear bond strength of composite resin to 10% carbamide peroxide bleached enamel following the 10 min application of 10% and 25% sodium ascorbate and alpha-tocopherol solutions.

MATERIALS AND METHODS

Sixty human premolars, extracted for orthodontic reasons, were randomly divided into six study groups as groups I, II, III, IV, V and VI, each consisting of ten samples (n = 10). Among the study groups, Group I served as the unbleached control and Group VI served as the bleached control. Groups II, III, IV and V served as the experimental groups. The study groups were treated, as shown in Table 1, in the following manner;

Table 1

Study design

Application of 10% carbamide peroxide bleaching gel

0.02 ml of 10% carbamide peroxide bleaching gel (Opalescence PF, Ultradent Products, Inc., South Jordan, UT, USA) was placed on the buccal surface of the samples using a calibrated syringe. The samples were kept partially immersed in a container containing distilled water at 37°C for 8 h a day. They were immersed in such a way that the enamel surface coated with the bleaching gel did not come in contact with the distilled water. The above procedure was repeated on a daily basis for one week.

Preparation and application of sodium ascorbate and alpha-tocopherol solutions

10 g and 25 g of sodium ascorbate powder (Sodium L (+) Ascorbate, LOBA CHEMIE Pvt. Ltd., Mumbai, India) were mixed separately in 100 ml of distilled water in a standard flask to prepare 10% and 25% sodium ascorbate solutions, respectively. Similarly, 10 g and 25 g of the alpha-tocopherol gel (DI-a-Tocopherol acetate, LOBA CHEMIE, Mumbai, India) were mixed in 100ml of ethyl alcohol to prepare 10% and 25% alpha-tocopherol solutions, respectively.

Following 10% carbamide peroxide bleaching, 10 ml of the 10% and 25% of sodium ascorbate solutions was applied on the buccal surface of groups II and III, respectively. Similarly, 10 ml of the 10% and 25% of alpha-tocopherol solutions was placed on the buccal surface of groups IV and V, respectively. The solutions were applied using a calibrated syringe with a flow rate of 1 ml/min. The applied solutions were continuously agitated using a sterile brush. After 10 min, the solutions were rinsed off the tooth surface for 30 seconds using distilled water and ethanol for sodium ascorbate and alpha-tocopherol solutions respectively and samples were air dried.

Composite resin bonding procedure

The samples were etched using 37% phosphoric acid gel (Scotchbond™ Multi-Purpose Etchant, 3M ESPE, St. Paul, MN, USA) followed by bonding agent (Adper™ Single Bond 2 Adhesive, 3M ESPE, St. Paul, MN 55144, USA) application as per manufacturer's instructions. Customized split metal casing was assembled around the samples to form a circular hole of dimensions 3 mm × 4 mm followed by placement of the resin composite (Z100™ Restorative, 3M ESPE, St. Paul, MN, USA) increments in the hole and curing as per manufacturer's directions. The metal casing was removed to obtain the samples with cylindrical extension of composite resin bonded to bleached enamel.

Shear bond strength evaluation

The samples were subjected to shear bond shear bond strength evaluation using Universal Testing Machine (Instron, model 3366, Instron Corp, Canton, MA, USA). A knife edge shearing rod and a crosshead speed of 0.5 mm per min were used. The load at failure was recorded by the software. The shear bond strength of the samples was calculated and expressed in megapascals (MPa).

Statistical analysis

The results were analyzed by using ANOVA (one-way analysis of variance) and Tukey highest significant difference (HSD) test.

RESULTS

The mean composite shear bond strength of the study groups tested is shown in Figure 1. One-way analysis of variance (ANOVA) showed statistically significant difference among the study groups (P < 0.00l).

Figure 1

Shear bond strength of the study groups

As shown in Table 2, according to Tukey HSD test, only Group IV (10% alpha-tocopherol solution) (P < 0.05) showed significantly lower composite shear bond strength among the experimental groups when compared to Group I (unbleached control).

Table 2

Multiple comparisons with group I (unbleached control) using Tukey HSD

As depicted in Table 3, except Group IV (10% alpha-tocopherol solution), Groups II (10% sodium ascorbate solution), III (25% sodium ascorbate solution) and V (25% alpha-tocopherol solution) (P < 0.05) showed significantly higher composite shear bond strength when compared to Group VI (bleached control).

Table 3

Multiple comparisons with group VI (bleached control) using Tukey HSD

However, as shown in Table 4, comparison between 10% and 25% concentrations and between similar concentrations of sodium ascorbate and alpha-tocopherol solutions showed no statistically significant difference in the shear bond strength between the corresponding groups among the experimental Groups I, II, III and IV.

Table 4

Multiple comparisons between 10% and 25% and similar concentrations of sodium ascorbate and alpha-tocopherol solutions using Tukey HSD

DISCUSSION

Delaying composite restorations, by 1 day to 3 weeks to overcome the adverse effect of bleaching on bonding, renders it impossible to carry out them as esthetic procedures soon after bleaching.[678] Application of sodium ascorbate and alpha-tocopherol has been proposed to immediately neutralize the effect of bleaching on composite bonding and improve bond strength following bleaching. Since both are biological antioxidants, their usage is expected not to cause any adverse biological effects.[11-131819] As antioxidants, both these agents are suggested to have similar mode of action regarding their influence on composite bonding following bleaching. It is said that sodium ascorbate and alpha-tocopherol allow free-radical polymerization of the adhesive resin to proceed without premature termination by restoring the altered redox potential of the oxidized bonding substrate and improve resin bonding.[1012]

In the present study, solution forms of 10% and 25% sodium ascorbate and alpha-tocopherol were evaluated. The solution form has got faster onset of action and is recommended for shorter clinical time of application.[15] Ten percent solution was employed in the study as it is suggested to be the minimum effective concentration and is most commonly employed in various studies, whereas 25% solution served to study the effect of higher concentration of sodium ascorbate and alpha-tocopherol on composite bond strength.

Lai et al.[10] in their study suggested that the antioxidant needs to be applied for no less than 1/3rd the bleaching time for it to completely reverse its effects. This corresponds to approximately 3 h, which is a long duration for carrying out restorative treatment in the same visit following bleaching. Therefore, shorter application time is clinically desirable. Application time of 10 min was chosen in the study, as this duration is considered to be adequate for clinical application of the antioxidant in solution form.[1420]

In the present study, results showed that except 10% alpha-tocopherol solution, 10 min application of 10% and 25% sodium ascorbate solutions and 25% alpha-tocopherol solution achieved significantly higher bond strength than the bleached control suggesting improved bonding following 10% carbamide peroxide bleaching. Nevertheless, the achieved bond strength was lower than the unbleached control. However, it was significantly lower only with respect to 10% alpha-tocopherol solution further denoting that the application of 10% and 25% sodium ascorbate solutions and 25% alpha-tocopherol solution improved bond strength closer to that of unbleached enamel.

The effectiveness of sodium ascorbate applied for a shorter time up to 10 min is in agreement with a study by Türkün et al.[14] However, there was no statistically significant difference in the bond strength between 10% and 25% sodium ascorbate solutions in the present study. This is in accordance with a study by Dabas et al.[21] where they showed no difference in bond strength with an increase in the concentration of sodium ascorbate.

On the other hand, with respect to alpha-tocopherol, 10 min application of only 25% alpha-tocopherol solution showed significantly higher bond strength to bleached enamel but not 10% alpha-tocopherol solution. Nevertheless, there was no statistically significant difference between them. In relation to 10% alpha-tocopherol solution, the finding in the present study is in contrast to a previous study. In a previous study, 10% alpha-tocopherol solution was found to be most effective.[12] This may be attributed to duration of application as 10% alpha-tocopherol solution was used for 2 h in the previous study, whereas in the present study it was used only for 10 min.

Unlike 10% sodium ascorbate solution in the present study, 10% alpha-tocopherol solution failed to significantly improve the bond strength to bleached enamel, though it is showed that the antioxidant activity percentage (AA%) of 10% alpha-tocopherol solution is comparable to 10% sodium ascorbate.[19]

Therefore, from the present study it can be inferred that for a shorter clinical application time up to 10 min, 10% alpha-tocopherol solution would be ineffective than either its higher or similar concentration sodium ascorbate solution in significantly improving composite bonding to carbamide peroxide bleached enamel.

Since comparing the bond strength between similar concentrations of sodium ascorbate and alpha-tocopherol solutions showed no statistically significant difference, from the present study it can be concluded that except 10% alpha-tocopherol solution, 10 min application of 10% and 25% sodium ascorbate solutions and 25% alpha-tocopherol solution significantly improves shear bond strength of composite resin to 10% carbamide peroxide bleached enamel. However, similar studies are required to evaluate the enamel and dentin bleached with different concentrations of carbamide peroxide and role of further higher concentration of sodium ascorbate and alpha-tocopherol solutions on bond strength to them and their clinical significance.

CONCLUSION

Following 10% carbamide peroxide bleaching, except 10% alpha-tocopherol solution, 10 min application of both 10% and 25% sodium ascorbate solutions and 25% alpha-tocopherol solution may be suggested to significantly improve shear bond strength of composite resin to enamel.