Comparison of the Shear Bond Strength of Orthodontic Brackets in Bonding and Rebonding: Preparation with Laser Versus Conventional Acid Etch Technique

Abstract

Objective: The purpose of this study was to compare the bond strength of orthodontic brackets using laser versus acid etching. Background data: Debonding of brackets is a common problem in orthodontic treatments. Materials and methods: Eighty extracted premolar teeth were divided into two groups. The enamel of the teeth in group A and B were etched using CO₂ laser and phosphoric acid, respectively. The brackets were bonded to the teeth using Transbond XT and then debonded from the teeth by Instron machine. The remaining composite on the tooth surface was removed by a tungsten carbide polishing bur. Both groups were divided into two subgroups (A₁, A₂ and B₁, B₂). The teeth were prepared again with laser in A₁, B₁ subgroups and with acid in A₂, B₂ subgroups. At each stage, the shear bond strength and residual adhesive index were measured. One way ANOVA and χ² tests were used to analyze data. Results: The mean shear bond strength was significantly lower in group A and higher in group B compared with all other groups (p<0.05). Most of the bond failures were degree 0 and 1 in groups A, A₁, and B₁, and degree 2 and 3 in groups B, A₂, and B₂. Conclusions: Primary preparation with acid has a higher bond strength value than does CO₂ laser. Less adhesive residue remained on enamel after tooth preparation with laser following debonding. Secondary preparation of the enamel using laser has higher bond strength value than does primary preparation with laser, which can rationalize use of laser in rebonding of brackets.

Introduction

B racket debonding at the composite–tooth bonding interface indicates low bond strength, for various reasons such as weak adhesive bond to the tooth and/or enamel contamination before or after preparation.¹ Whereas only 2–4% of the initial bonding fails within 4–5 days, this rate has been reported to be 10–15% in bracket rebonding.^2,3 Bishara et al.³ reported that shear bond strength at the rebonding stage had a decrease of 33%.

Other than acid, laser, which is a source of electromagnetic radiation, is also used to etch the enamel surface. Hydroxyapatite has the maximum absorption of CO₂ laser when it is used in short time-pulsed radiation form. This type of laser can increase enamel resistance to acid and caries by eliminating the existing carbonate in the enamel structure.^4,5

Some studies concluded that laser offers acceptable tensile and shear bond strength, and is less time consuming for clinicians; therefore it can be good substitute for acid etch technique.^6,7 Also, CO₂ laser used for a few microseconds, will not cause any adverse effect on the pulp.⁸ On the contrary, some studies reported that surface prepared by laser has a lower bond strength than does the conventional acid etch technique, and they suggested that using laser is not a suitable method to increase the bond strength of brackets to teeth.^9

–12 Some studies have also stated that there is not any significant difference between shear bond strength of bonded brackets on the enamel surface prepared by laser or by acid.^13,14

The remaining resin tags in the enamel at the rebonding stage are the reasons for reduction in mechanical retention.¹⁵ The application of laser energy on the enamel will cause topical melting and ablation.¹⁶ This mechanism may be able to remove the remaining resin tags and help enhance mechanical retention. With the advanced CO₂ laser super pulse devices and companies testimonials to their performance in enamel preparation, a new viewpoint for using this type of laser in enamel preparation has been offered. None of the previous articles have studied the application of laser in the rebonding of brackets. As using laser reduces the risk of caries, and is less time consuming for the clinician,¹⁷ it is postulated that if good bond strength is provided by laser, it could be a good alternative for bonding the brackets at the rebonding stage.

The aim of this study was to compare shear bond strength of orthodontic brackets when two different methods of enamel preparation, CO₂ laser and conventional acid etch technique, were used in bonding and rebonding.

The null hypothesis of this study was that the shear bonding and rebonding strengths of brackets prepared by CO₂ laser and conventional acid etch technique were comparable.

Materials and Methods

In this study, 80 human premolar teeth that had been extracted for orthodontic treatments were used. The teeth were anatomically normal, without any caries, enamel crack, filling, or endodontic treatment. Prior to storage, teeth were thoroughly washed under tap water, and immersed in 1% Thymol solution for 24 h. Then each sample was stored in distilled water at room temperature. Collecting the entire samples took <6 months, a period in which the distilled water was changed weekly to avoid any unwanted bacterial growth.

Standard premolar brackets 0.022″ (0.022 Steel, American Orthodontics, USA) with a surface area of 12.09 mm² were employed. A light cure adhesive, Transbond XT (Transbond XT, code-712036, 3M Unitek, USA), which is one of the most common and widely used adhesives in the studies^14,18

–21 was employed in this study.

Primary tooth preparation and bonding

At this stage the teeth were divided into two groups. Group A included 40 teeth that were polished for 20 sec using a low speed handpiece, rubber cap (Tizkavan, Iran) and pumice (Golchay, Iran). Following the laser machine's operating manual (Smart US-20, DEKA, Hungary), laser beam coupled with water spray was directed perpendicular to the tooth surface.¹¹ The laser was used based on the predetermined parameters of the manufacturer and was irradiated from the joint articulated arm equipped with fiberoptic light over sample surface with following characteristics: power, 1.5 W; frequency, 100 Hz; distance from tooth surface, 12.5 mm; wavelength, 10600 nm; pulsating time, 0.2 msec.

Group B included 40 teeth that were polished like those in group A. Then the teeth were etched with 37% phosphoric acid gel (Unitek, 3M, USA) for a period of 30 sec, and then rinsed for 20 sec.

After enamel preparation in both groups, the brackets were bonded to the buccal surfaces of teeth using Transbond XT. Specimens were then light cured using halogen LED curing (Unit-Smartlife IQ2, Dentsply-Milford, USA) with a wavelength of 450 nm, from a distance of 3 mm for 40 sec. Specimens were then placed into the cylindrical autocured acrylic mold using surveyor jig (Jelenco, USA).²²

After bonding the brackets in both groups, all specimens were placed in distilled water at room temperature for 48 h. Afterwards, samples were located to encounter shear force in an Instron machine (Universal testing machine, Zwickroll, Germany) granted by a flat metal blade. The cross head speed was 1 mm/min.¹¹ The load was applied vertically to the surface between base and wing of the bracket.²² The magnitude of the forces (in Newtons) at the moment of bracket deboning were recorded by a computer linked to the machine, and these force levels were divided by the cross-sectional area of the bracket (12.09 mm²) so that the results were obtained in megaPascals (MPa).

The adhesive remnant index (ARI) was used to classify the failure modes by employing a stereomicroscope with a magnification of 10. Zero indicated that no adhesive remained on the enamel; 1 indicated that ≤50% of the adhesive remained on the enamel; 2 indicated that >50% of adhesive remained on the enamel; 3 indicated that all bonding resin remained on the enamel, along with a distinct impression of the bracket mesh.¹

Secondary tooth preparation and rebonding

The remaining composite on the tooth surface was removed by Tungsten carbide finishing bur (#1171 3M, Unitek, USA) with a low speed handpiece until the enamel surface showed a glossy appearance under dental unit light. Removing of the remaining composite was accomplished by one postgraduate orthodontic resident (M.M.N) and was confirmed by two expert orthodontists (M.O, H.Z.N).

The teeth in groups A and B were divided into two subgroups: A₁, A₂ and B₁, B₂ respectively. Each subgroup was composed of 20 specimens. At this stage, the teeth were prepared again with laser in the A₁, B₁ subgroups and with acid in the A₂, B₂ subgroups. In all subgroups, debonded brackets at the previous stage were discarded without any treatment on them, and new brackets were then bonded to the teeth with the same condition described previously.

The shear bond strength and residual adhesive index were again measured. The test was performed after 48 h storage of the samples in distilled water at room temperature. All conditions were similar to the primary shear bond test.

Statistical analysis

For statistical analysis, one-way ANOVA, Tukey, and the χ² tests were used. The level of significance used was p<0.05.

Results

Evaluation of the mean of all six groups showed that the highest shear bond strength values were recorded from group B, in which the primary tooth preparation was performed by using conventional acid etch technique. However, the lowest shear bond strength values were gained from group A, in which the primary tooth preparation was done by using laser beam. Shear bond strength test results for all six groups are shown in Table 1.

Table 1.

Mean and Standard Deviation of Shear Bond Strength and the Number of Samples in Each Adhesive Remnant Index (ARI) in Six Groups: A, B, A₁, A₂, B₁, and B₂

Group	n	Mean±SD of shear bond strength (MPa)	ARI 0	ARI 1	ARI 2	ARI 3
A_{(Laser bonding)}	40	40.61±18.06^a	14	25	0	1
B_{(Acid bonding)}	40	84.29±43.33^a	4	12	5	19
A₁ _{(Laser bonding, laser rebonding)}	20	61.37±20.29	1	15	4	0
A₂ _{(Laser bonding, acid rebonding)}	20	73.31±18.18	1	5	5	9
B₁ _{(Acid bonding, laser rebonding)}	20	57.52±22.73	3	14	3	0
B₂ _{(Acid bonding, acid rebonding)}	20	67.99±13.14	0	9	5	6

Significant difference.

In group B, for which the primary preparation was done by acid, secondary preparation with acid or laser did not show any significant difference. However, secondary preparation using acid had higher bond strength (p=0.224).

In group A, for which the primary preparation was done by laser, secondary preparation with acid or laser did not show any significant difference. However, secondary preparation using acid had higher bond strength (p=0.166).

Comparison of ARI among six groups indicated significant differences between groups illustrated in Table 1. In group A, 97% of the specimens had an ARI score of 0 and 1, whereas only 3% had an ARI score of 2 and 3. This means that after bracket debonding in this group, a small amount of adhesive remained on the enamel surface. In group B, 60% of the specimens had ARI score of 2 and 3, which indicates the presence of more adhesive on the enamel surface after bracket debonding. The amount of remaining adhesive on the enamel surface was greater in the teeth prepared by acid etch technique.

Discussion

In the present study, the shear bond strength of secondary preparation with acid was lower than that of primary preparation with acid. Also, other investigations have mostly shown a decrease in the bond strength compared with the primary bonding stage, because of the residual adhesive on the enamel surface.^2,3 However, the secondary preparation with laser, regardless of the primary preparation technique, resulted in higher bond strength than primary preparation with laser, and this could be assumed to be a confirmation of the theory that we postulated. This could be a positive indication for the employment of laser in the rebonding of brackets. The higher bond strength of laser in the secondary phase could be the result of the burning property of laser in the elimination of the residual adhesive on the tooth surface,²³ whereas the acid only affects hydroxyapatite crystals and is not effective on the restoration surfaces and resins.³ Secondary bond strength of laser in the group that had been prepared with laser in the primary stage was insignificantly higher than that of the group that had been prepared with acid in the primary stage.

According to Reynold, sufficient shear bond strength for orthodontic bonding is 5.7–8.7 MPa.²⁴ In this study, enamel preparation by acid, either primary or secondary, provided sufficient bond strength for brackets. However, primary and secondary enamel preparation by laser did not provide the minimum required bond strength. The mechanical retention in repeated etching with acid is affected by an aftermath known as “mushroom effect.”²⁵ This result may not exist in etching with laser, justifying the reduction in bond strength.

Obata et al.¹⁰ reported higher shear bond strength for CO₂ laser bonded brackets. The mean shear bond strength value of the group that had been prepared with laser was reported to be 5.7 MPa.¹⁰ However, the results of the present study showed the shear bond strength of the brackets in the laser group to be 3.3 MPa. The device power was set to be 1.5 W, and the radiation distance was 12.5 mm. The lower power and longer radiation distance in our study was assumed to be the reason for the lower mean shear bond strength in the present study compared with the mentioned one.¹⁰ It should be noted that in the present study, the laser was used based on the predetermined parameters of the manufacturer in order to reduce side effects while maintaining its competence.

Drummond et al.¹¹ also used CO₂ for enamel preparation prior to bonding the brackets. The mean shear bond strength in their study was measured to be 4 MPa which is slightly higher than 3.3 MPa in the present study. The higher cross head speed (2 mm/sec) and higher device power (3.5 W) in that study could explain the higher bond strength compared with the present study. It must be stated that this higher power might also have some side effects on the enamel.

Fuhrmann et al.⁶ considered the tensile bond strength obtained from etching by CO₂ laser (3.3 MPa) to be acceptable for orthodontic brackets, but it is not comparable with the shear bond strength in the present study.

In most of the previous studies,^{7,6,8,12
–14,18

–21} the shear bond strength in the group prepared by laser was less than that of acid-etched group. This is in agreement with our findings.

The surface by laser etched was acid resistant and, therefore. the occurrence of caries would probably be decreased in this type of enamel.²⁰ This can be considered to be an advantage in the bonding of orthodontic brackets, as the surrounding enamel of the bonded brackets is at high risk of caries and decalcification.

In the present study, the ARI was mostly 2 and 3 in the groups prepared with acid, and mostly 0 and 1 in the groups prepared with laser. This means that bond failure was mostly cohesive at the bracket–composite interface in the groups prepared by acid. On the contrary, bond failure in the groups prepared with laser was mostly adhesive, and at the composite–enamel interface. Therefore, the amount of remaining adhesive on the enamel after debonding would require less time to remove.

In Gokcelik et al.'s study,¹⁴ the mean ARI of the group prepared by Er:YAG laser was significantly higher than that of groups prepared by phosphoric acid and Er:YAG laser plus phosphoric acid. Whereas in the present study the mean ARI of the group prepared by CO₂ laser was significantly lower than that of the group prepared by acid. This might be attributed to the higher shear bond strength of the laser group in their study (10.3 MPa) compared with the present study (3.3 MPa). In addition, the type of laser and cross head speed used in their study was different. With high cross head speed the adhesives fail to expose their viscoelastic properties.²⁶ Therefore, bond failure would mostly happen cohesively and within enamel or composite, which causes an increase in ARI. The high cross head speed in the abovementioned study could be an explanation for high shear bond strength and probably for the greater remaining amount of adhesive on the enamel after debonding.

The results of Başaran et al.¹⁸ and also Ozer et al.¹⁹ showed the ARI to be lower in the groups prepared with laser than in the groups prepared by acid, which is in agreement with the present study.

Limitations

One of the limitations of this study was the different sizes of the specimens in the primary and secondary groups, which makes the comparison disputable. However, as the comparison between the primary groups was also desired and the specimens were randomly divided into the secondary groups, this comparison could be considered statistically correct.

Conclusions

Primary preparation with acid has a higher bond strength value than does CO₂ laser. Less adhesive residue remained on the enamel after tooth prepration with laser following debonding. Secondary preparation of the enamel using laser has higher bond strength value than does primary preparation with laser, which can provide a rationale for the use of laser in rebonding of brackets.

Footnotes

Acknowledgments

The authors thank the vice-chancellery of Shiraz University of Medical Sciences for supporting the research. This manuscript is relevant to the thesis of Dr. Naseri (May 2011- Thesis number: 1324). Also, the authors thank Dr. Hamedani for help with the English in the article.

Author Disclosure Statement

No competing financial interests exist.

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