Effect of green tea and sodium ascorbate on the shear bond strength of orthodontic brackets bonded on bleached enamel: An in-vitro study

Abstract

Objective

The primary objective was to assess and compare the shear bond strength of orthodontic brackets bonded on bleached enamel with and without using an anti-oxidizing agent.

The secondary objective was to compare the shear bond strength of brackets between the groups with anti-oxidant application along with immediate bonding and that of delayed bonding without any anti-oxidant treatment, on bleached teeth.

Materials & methods

75 premolars were collected and thereafter randomly distributed into the 5 groups. Group 1 to 4 had undergone bleaching with 10% carbamide peroxide and group 5 was control. Group 1 was bonded immediately after bleaching while bonding was delayed in group 4 for a period of 1 week. Groups 2 and 3 received the anti-oxidant treatment with 10% sodium ascorbate gel and 10% green tea gel respectively. Hounsfield universal testing machine was used for testing the shear bond strength.

Results

For group 1, the shear bond strength was significantly lower (mean 4.9 MPa) than group 5 (mean 12.5 MPa). The significant differences were observed on comparing the shear bond strength of antioxidant-treated and delayed bonding groups with control (p < 0.05). Mean shear bond strength in groups 2, 3 and 4 were 7.5 MPa, 9.6 MPa and 5.8 MPa respectively.

Conclusion

The bond strength of brackets reduces immediately after the bleaching procedure. The bond strength can be reversed either by delaying the bonding for a period of 1 week or by using the anti-oxidizing agents but it is not up to the level of unbleached enamel.

Graphical abstract

1. Introduction

Haywood and Heymann in 1989 gave their contribution in patient-applied, at-home bleaching by the application of carbamide peroxide.¹ Since that time, this method has been among the most sought after and performed esthetic treatments. However, the application of bleaching agents before bonding procedure significantly reduces the bond strength to tooth structures due to the presence of oxygen or peroxide remnants in tooth structures, which prevents polymerization of resin. This residual oxygen prevents adequate infiltration of the bonding agent into the tooth structure which inhibits its polymerization.² However, the reduced shear bond strength is temporary and the waiting period which is recommended for bonding after the bleaching procedure ranges from 24 hours to 4 weeks.³

Following the bleaching procedure, anti-oxidants helps in eliminating residual oxygen and hence can be used as an alternative to delayed bonding, especially for the patients in whom the orthodontic treatment has to be initiated immediately after bleaching.

The purpose of this study was to assess and compare the shear bond strength of orthodontic brackets bonded on bleached enamel with and without using an anti-oxidizing agent.

2. Material and methods

75 sound, non-carious extracted human premolar teeth with intact enamel surfaces were selected for the study. Teeth subjected to any pre-treatment with chemical agents, cracks or fracture, caries or restorations, fluorosis or other developmental anomalies were excluded.

These teeth were disinfected with 0.1% thymol and subjected to manual debridement with a periodontal curette to remove debris. The root portion of the teeth were embedded vertically (90° to the mold base) in blocks made of dental stone in the form of an arch (Fig. 1).

Fig. 1.

Extracted premolars embedded vertically 90° to the mold base.

Trays were fabricated for bleaching procedure and anti-oxidant application with 1 mm thick polyethylene sheets (COPYPLAST®), using the BIOSTAR® machine.

Subsequently, the specimens were divided into a control group and 4 experimental groups as depicted in Table 1. Control group neither received bleaching nor anti-oxidant treatment, and brackets were bonded to these specimens immediately after polishing. Specimens in groups 1–4 received the bleaching treatment that was performed to simulate at-home bleaching techniques using 10% carbamide peroxide. The bleaching agent was applied for 8 hours on the surfaces of the enamel per day and thereafter they were immersed in the artificial saliva for the rest of the day. These steps were continued for the period of 1 week. For group 1, bonding was done as soon as the bleaching procedure completed. Anti-oxidant was applied for groups 2 and 3. Anti-oxidant for group 2 was 10% sodium ascorbate gel and group 3 was 10% green tea gel. After the bleaching procedure for one week was completed, the respective anti-oxidants were applied for 60 minutes using customized trays following which etching and bonding was done. In group 4, no anti-oxidant was used after bleaching and bracket bonding was done after 1 week.

Table 1.

Treatment regimens before bonding.

GROUPS	BLEACHING	10% SODIUM ASCORBATE GEL	10% GREEN TEA GEL	BONDING
1	YES	NO	NO	Immediately
2	YES	YES	NO	Immediately
3	YES	NO	YES	Immediately
4	YES	NO	NO	1 Week
5 (CONTROL)	NO	NO	NO	Immediately

Pre-adjusted edgewise premolar brackets with 0.022-inch slot and MBT prescription were bonded on all the specimens in the control and experimental groups after cleaning and polishing. A conventional adhesive system (Transbond XT adhesive and primer, 3 M UNITEK) was used after etching with 37% phosphoric acid to bond the brackets. The adhesive was then light cured for 40 seconds (10 seconds each on the mesial, distal, gingival, and occlusal margins), following which, specimens were kept in artificial saliva for 24 hours to allow polymerization of the resin (Fig. 2).

Fig. 2.

Specimens in artificial saliva.

After 24 hours, each bonded bracket was positioned in a Hounsfield Universal Testing machine parallel to the direction of load application. The shear force was transmitted occlusally on the base of the bracket through the rectangular rod till the debonding of the brackets. The samples were stressed in an occluso-gingival direction with a crosshead speed of 1 mm/min. The direction of the shear force was parallel to the tooth-bracket interface (Fig. 3).

Fig. 3.

Rod transmitting shear force.

The force needed for debonding was recorded in kilograms force and then later converted into megapascals (MPa) by dividing Newtons by the surface area of the bracket base (1KgF = 9.806 N, 1Mpa = 1 N/mm²).

Shear bond strength of each sample was calculated using the formula:

Bond Strength = Breaking Load / Nominal area of bonding

3. Statistical analysis

Descriptive data was arranged as mean, standard deviation and range values. For intergroup comparison of shear bond strength one way ANOVA test was done. Tukey's Post hoc analysis was employed if any statistically significant difference exists between the groups under consideration. Results were considered statistically significant at p < 0.05.

4. Results

Shear bond strengths in MPa (mean ± SD) for the groups are shown in Table 2 and Graph 1. The results depicted the significant decrease in shear bond strength of brackets after bleaching with 10% carbamide peroxide (p < 0.05). Treating the bleached enamel surface with anti-oxidants 10% sodium ascorbate gel and 10% green tea gel or waiting for 1 week significantly reverses this reduction, although not up to the level of unbleached tooth but to the acceptable levels for bonding of orthodontic brackets (p < 0.05). 10% green tea gel was proved to be a more powerful anti-oxidant than 10% sodium ascorbate.

Table 2.

Multiple comparisons using Tukey's post Hoc test.

Dependent Variable	Group (I)	Group (J)	Mean Difference (I-J)	Std. Error	p value	95% Confidence Interval
						Lower Bound	Upper Bound
Kgf/MPa	1	2	2.66	0.59312	<0.001	−4.32	−1.00
		3	4.68	0.59312	<0.001	−6.34	−3.02
		4	−0.89	0.59312	0.566	−2.55	0.77
		5	7.57	0.59312	<0.001	−9.23	−5.91
	2	1	2.66	0.59312	<0.001	1.00	4.32
		3	−2.02	0.59312	0.009	−3.68	−0.36
		4	1.77	0.59312	0.031	0.11	3.43
		5	4.91	0.59312	<0.001	−6.57	−3.25
	3	1	4.68	0.59312	<0.001	3.02	6.34
		2	2.02	0.59312	0.009	0.36	3.68
		4	3.79	0.59312	<0.001	2.13	5.45
		5	2.88	0.59312	<0.001	−4.54	−1.22
	4	1	0.89	0.59312	0.566	−0.77	2.55
		2	1.77	0.59312	0.031	−3.43	−0.11
		3	3.79	0.59312	<0.001	−5.45	−2.13
		5	6.68	0.59312	<0.001	−8.34	−5.02
	5	1	7.57	0.59312	<0.001	5.91	9.23
		2	4.91	0.59312	<0.001	3.25	6.57
		3	2.88	0.59312	<0.001	1.22	4.54
		4	6.68	0.59312	<0.001	5.02	8.34

GRAPH 1.

Multiple comparison using Tukey's post Hoc test.

5. Discussion

Dental bleaching is a highly demanded procedure in esthetic dentistry, but this treatment is associated with few adverse effects like reduction in the bond strength of composite resin to the bleached surface, variations in tooth morphology, hypersensitivity of the dentin and alterations in the enzymatic activities of the pulp.

Titley et al. conducted a comparative study between the resin interfaces and bleaching surfaces of enamel with that of those established between resin and unbleached surfaces. In specimens where bleaching was done, resins unable to flow in the huge sections of the enamel and the tags formed were inadequately defined and their penetration depth was also less.⁴ Dishman M V, Covey DA, Baughan, L Wemphasized on the quality of a composite bond and stated that it was compromised due to the reduced number of resin tags.⁵

Additionally, it has been devised that the resin polymerization is inhibited by the residual oxygen that is left after the bleaching procedure and these antioxidants are useful for neutralizing this residual oxygen.

Bulut H, Turkun M, Kaya AD stated that the compromised bond strength of resin occurred due to extracoronal bleaching can be reversed by the use of anti-oxidizing agent and hence it can be used as an alternative to the delayed bonding method.^6,7

In recent years, green tea and its derivatives—such as epigallocatechin gallate has been widely studied in dentistry. GT gel is made from the leaves and buds of the shrub Camellia sinensis, which is rich in phenolic compounds and flavonoids. The antioxidant actions of such phenolic compounds have been examined.

According to various studies, Green tea is regarded as the more effective anti-oxidizing agent in comparison with that of Vitamin C and Vitamin E, as it effectively eliminates the residual oxygen following the bleaching and thereby preventing the inhibition of polymerization of resin, thus allowing the bonding immediately after the completion of bleaching procedure.⁸

This study showed that the bond strengths of the teeth that had been bonded immediately after bleaching was minimum while the delayed bonding group in which the bleached teeth were immersed in artificial saliva for a week regained the SBS of orthodontic brackets, but not up to the level of control.³ The groups 2 and 3 which got the anti-oxidant treatment with 10% sodium ascorbate gel and 10% green tea gel immediately after bleaching showed the improvement in shear bond strength of orthodontic brackets but not up to the level of the control group. The group which received the anti-oxidant treatment with 10% green tea gel was found to be more powerful in restoring the shear bond strength as compared to 10% sodium ascorbate gel.

Berger et al. compared the anti-oxidant effect of sodium ascorbate and green tea on bleached enamel and found that both Green tea and sodium ascorbate had significant effects on SBS after bleaching but 10% Green tea was a more powerful antioxidant as compared to latter. These findings are consistent with the present study.⁹

Contrary to the results of the present study, Uysal et al. reported that bleaching with 10% hydrogen carbamide or 35% HP did not affect the SBS of orthodontic brackets to enamel. The reduction of SBS after bleaching may be attributed to variations in the roughness and structure of enamel surfaces or the presence of residual oxygen from bleaching agent, which either interferes with the resin infiltration of bleached enamel or inhibits the resin polymerization. Thus, it is beneficial to postpone the bonding for 2–3 weeks.¹⁰

Consolaro et al. discussed the important factors for protecting both the mucosa and the cervical region, irrespective of the timing of performance of the bleaching procedure and its association with orthodontic treatment. They concluded that it is essential to be careful while performing the orthodontic treatment either before or after the dental bleaching procedures, although tooth enamel has adamantine fluid flowing within it, providing a specific metabolism which is peculiar and could scientifically explain and base the option of carrying out teeth whitening before and during orthodontic treatment.¹¹

Sasaki et al. compared the effects of sodium ascorbate and alpha-tocopherol and reported that the sodium ascorbate was unable to reverse the bond strength while alpha-tocopherol which was formulated in solution increased the bond strength of bleached enamel. These findings are contrary to the results of the present study in which sodium ascorbate was proved to be a successful anti-oxidant in regaining the shear bond strength of orthodontic brackets bonded on bleached enamel.¹²

6. Conclusions

• 1.The shear bond strength of composite resin to enamel was reduced when bonding is done immediately after the bleaching procedure using the 10% carbamide peroxide.
• 2.The reduced bond strength can be regained by delaying the bonding for a period of 1 week but is not up to the level of unbleached enamel.
• 3.In cases when the orthodontic treatment must be initiated immediately after the bleaching, 10% sodium ascorbate gel can be used as an anti-oxidizing agent that can reverse the reduced shear bond strength.
• 4.Treating the bleached enamel surface with 10% green tea gel satisfactorily restored the shear bond strength of bleached enamel and proved to be a more powerful anti-oxidant than 10% sodium ascorbate.

7. Scope for future research

The present study assessed the shear bond strength of orthodontic brackets bonded on sound non-carious teeth with normal intact enamel surface. Further research can be done to evaluate the efficacy of the anti-oxidizing agents on fluorosed and restored teeth after bleaching procedures as they offer a low bond strength values as compared to the sound teeth. As green tea can cause the alterations in the shade of the dentition, therefore in future, the clinical and laboratory studies can be conducted for assessing the effects of change in color of the teeth treated with green tea gel after the bleaching procedure.

Declaration of competing interest

The Author(s) declare(s) that there is no conflict of interest.