Table of Contents  
Year : 2020  |  Volume : 12  |  Issue : 1  |  Page : 24-29

Shear Bond Strength at the Resin/Bracket Interface of Sandblasted Brackets with Different Aluminum Oxide Particle Size

1 Division of Orthodontics, School of Dentistry, Universidad Científica del Sur, Lima, Perú
2 Department of Orthodontics. Bauru Dental School, University of São Paulo, Brazil
3 Division of Orthodontics and Division of Oral and Maxillofacial Radiology, School of Dentistry, Universidad Científica del Sur, Lima, Perú

Date of Submission26-Dec-2019
Date of Acceptance09-Mar-2020
Date of Web Publication12-Jun-2020

Correspondence Address:
Luis Ernesto Arriola-Guillén
Division of Orthodontics, Faculty of Dentistry, Universidad Científica del Sur, Lima
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jofs.jofs_159_19

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Introduction: The aim of this study was to compare the shear bond strength at the resin/bracket interface of metal brackets sandblasted with aluminum oxide particles of 25 µm, 50 µm and 110 µm. Materials and Methods: Sixty metal brackets were recycled and randomly assigned into four groups according to the aluminum oxide (Al2O3) particle size (µm) used during sandblasting. Brackets of the first three groups (Group 1, n = 15; Group 2, n = 15 and Group 3, n = 15) were sandblasted with 25µm, 50µm, and 110µm Al2O3 particle size, respectively. The control group (Group 4, n = 15) included brackets without sandblasting. Shear bond strength was evaluated before and after sandblasting. Brackets with some variation in shape or structure were excluded. Intragroup and intergroup comparisons were performed with paired t-test and one-way analysis of variance (ANOVA) followed by Scheffé test, respectively. Results: The recycled sandblasted brackets showed greater shear bond strength approximately 4 to 6 Mpa more than those that did not receive sandblasting. There were no statistically significant differences between the sandblasted groups (P > 0.05). However, Group 3 (110µm) showed a numerically greater mean value of shear bond strength (9.34 ± 4.18 Mpa). Conclusion: Similar share bond strength at the resin/bracket interface can be expected after bracket sandblasting with 25µm, 50µm, and 110µm Al2O3 particle size. Independently of the particle size used, the sandblasted brackets showed greater shear bond strength than brackets without sandblasting.

Keywords: Bracket, orthodontics, Shear bond strength

How to cite this article:
Salcedo-Alcaychahua AB, Aliaga-Del Castillo A, Arriola-Guillén LE. Shear Bond Strength at the Resin/Bracket Interface of Sandblasted Brackets with Different Aluminum Oxide Particle Size. J Orofac Sci 2020;12:24-9

How to cite this URL:
Salcedo-Alcaychahua AB, Aliaga-Del Castillo A, Arriola-Guillén LE. Shear Bond Strength at the Resin/Bracket Interface of Sandblasted Brackets with Different Aluminum Oxide Particle Size. J Orofac Sci [serial online] 2020 [cited 2023 Jun 8];12:24-9. Available from:

  Introduction Top

Bracket recycling consists in removing the resin from its base without weakening the structure or distorting the dimensions of the slot. It can be done in the dental office and also through supply companies.[1] Several methods to achieve this procedure have been described such as thermal, chemical and physical.[1],[2] Among them, sandblasting with aluminum oxide (Al2O3) particles is one of the most effective methods for this purpose. There are few studies reporting different Al2O3 particle size and there is no consensus on which one should be used for sandblasting.[3],[4],[5],[6],[7] The bond strength values reported after Al2O3 sandblasting with different particle size are variable and depends on the substrate used for testing (artificial, animal or human teeth).[8],[9]

Brackets re-bonding is a frequent practice for clinicians during orthodontic treatment. This could occur due to various factors as bonding errors, occlusal interference or intentionally, to improve the orthodontic mechanics. The recycling method used should avoid delays in treatment or inconvenience for the patient; The ideal alternative is to use a new bracket, but it implies an additional cost.[4],[10],[11],[12]

Some shear bond strength evaluation has been questioned because they do not usually include the two interfaces involved in adhesion, the resin/bracket and the resin/tooth interface. This last one could be affected by many variables such as substrate, physicochemical properties and sample preparation.[13],[14],[15],[16] In addition, the greatest number of failures are presented in the resin/bracket interface. This could be evidenced by several studies that have tried to improve the design of the meshes. Therefore, the effectiveness of bracket recycling methods analyzing this interface should be evaluated.[17],[18],[19],[20],[21] In this regard, the efficiency of a new procedure to evaluate the bond strength in the resin/bracket interface, has been reported.[22] Some studies[2],[23] found that sandblasting with aluminum oxide was the most effective method for bracket recycling, but in these studies a single particle size was evaluated. Contrary to one study,[24] a shear bond strength decrease has been reported to be associated with the increase of aluminum oxide particles size.[3] Nevertheless, they did not evaluate the resin-bracket interface. Since brackets recycling is a common practice between clinicians, more studies evaluating the share bond strength at the resin/bracket interface after sandblasting procedure should be performed. Therefore, the aim of the present study was to compare the shear bond strength at the resin/bracket interface of sandblasted brackets with different aluminum oxide particle size.

  Materials and Methods Top

Ethical approval for this in vitro study (Ethical Committee N°: 232-2018-POS8) was provided by the Ethics in Research Committee of the School of Dentistry of the Científica del Sur University, Lima, Peru, on 03 February 2018. The sample size was calculated using a significance level of 5% and a test power of 80%, to detect a mean difference of 2.2 Megapascals (MPa) between groups, with a standard deviation of 2.24 MPa in the shear bond strength, obtained from a previous pilot study. Then, the determined sample size was 13 per group, but to avoid sample loss, a total of 60 brackets were divided into four groups (n = 15) using simple randomization.

Upper central incisor brackets with micro-etched meshes with horizontal and vertical configuration (Orthoclassic Edgewise prescription, Orthoclassic Inc, McMinnville, Ore, USA) were used. Brackets that had shape and structure alterations were excluded from the study.

Sample Preparation

A millimeter paper was used to make guides into the center of the brackets and paste the mesh of the brackets on a double-contact adhesive tapes, which were glued to the 3/4inch polyvinyl chloride (PVC) tubes. Each tube was 16 mm high, so the brackets were positioned in the center of the tubes. A self-curing acrylic was used to fixed the brackets into the tubes. Once the polymerization process was completed, the adhesive tapes were removed, thus the braid meshes were visualized in the center of the smooth surface of each tube.[2]

Bracket meshes were cleaned with pumice and water using a prophylaxis brush using a low-speed device at 500 rpm for one minute. Then, they were rinsed and dried for 30 seconds. In addition, absorbent paper was used to complete this procedure.[2]

An acetate matrix (1 mm thickness) with the same diameter of the tube and with a central opening with equal dimension to the size and shape of the bracket́s base, was customized. Thereby, when the matrix was placed on the tube, the only thing that should be noted was the bracket’s mesh. This acetate matrix was used to standardize the thickness of the orthodontic adhesive in the bracket’s mesh.

Application of the orthodontic adhesive

The Transbond™ XT adhesive (3M, Unitek, Monrovia, CA) was placed in the bracket́s base in small portions until the matrix opening was evenly filled. The adhesive was light cured with a LED unit (Woodpecker, Guilin, China) at a distance of 3 mm during 10 seconds with an intensity set at 1000 mW/cm2 and a wavelength of 480 nm (ISO 10650-2018); measured by two radiometers for LED lamps, one digital (Woodpecker, Guilin, China) and the other analog (Gnatus, São Paulo, Brazil). Then, the matrix was carefully removed from the sample tube [Figure 1].
Figure 1 (A) Sample with the bracket positioned in the center of the PVC tube. (B) Sample with the bonding agent located on the bracket mesh.

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Shear bond strength test

A digital universal test machine (LianGong, Model CMT-5L 7419 series, China) was used to perform the tests, each sample was adjusted with screws to keep them fixed in the machine [Figure 2]. The load 0.5 KN was applied with a bevel-shaped active tip on the orthodontic adhesive at 0.5 mm/min. Once the adhesive of all the 60 brackets were removed, they were randomly distributed into 4 groups: three experimental groups sandblasted with aluminum oxide particle size of 25 µm, 50 µm and 110 µm (Zeta Sand; Zhermack Dental, Polesine, Italy) for the recycling of the brackets, and a control group that did not receive sandblasting. A sandblasting device (Microblaster; Bio-Art, São Carlos, SP, Brazil) connected to a dental equipment at a pressure of 80 lib/in and a distance of 10 mm from the sandblasted mesh was used during 30 seconds.[2] The sandblasted samples were washed again and dried as in the previous procedure. For the new test, the orthodontic adhesive was placed again to the brackets mesh using the acetate matrix. Then, light-curing was performed for all 4 groups.
Figure 2 (A) Shear bond strength test at the universal testing machine (speed 0.5 mm/min). (B) Sandblasting with Al2O3 on debonded sample inside the sandblasting box.

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Shear bond strength was evaluated again on all groups. The shear bond strength values were originally obtained in Kilonewtons and according to the area of their brackets mesh, they were transformed into megapascals (MPa).

Statistical analyses

All statistical tests were performed using the SPSS software for Windows (Version 24; IBM, Armonk, NY). The normal distribution of the data was confirmed with Shapiro Wilk tests. Intragroup comparisons were performed with dependent t tests and intergroup comparison was performed with one-way analysis of variance (ANOVA) followed by post-hoc Scheffé tests. Results were considered significant at P < 0.05.

  Results Top

Intragroup comparisons showed that the sandblasted groups (Groups 1, 2 and 3) showed statistically significant greater shear bond strength after sandblasting with aluminum oxide (approximately 4 to 6 Mpa more than those that did not received sandblasting) ([Table 1]).
Table 1 Shear bond strength before and after the sandblasting procedure − Intragroup comparisons (dependent t test)

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Intergroup comparisons showed statistically significant greater shear bond strength in the sandblasted groups (Groups 1, 2 and 3) when compared to the group without sandblasting (Control group) (P = 0.002). However, no statistically significant difference was found between the groups sandblasted with different aluminum oxide particle size. The numerically greatest shear bond strength mean value was observed in the sandblasted group with 100 um aluminum oxide size (9.34 ± 4.18 Mpa) [Table 2] Figure 3].
Table 2 Shear bond strength after the sandblasting procedure − Intergroup comparison (one-way ANOVA followed by Scheffé tests)

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Figure 3 (A) Macro-photography images (focus stacking) of a new bracket, (B) without sandblasting bracket, and sandblasted brackets using different particle size of (C) 25 µm, (D) 50 µm and (E) 110 µm.

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  Discussion Top

The sandblasting procedure using aluminum oxide particles (Al2O3) can be considered as an effective and simple method to recycle debonded brackets. This procedure can be easily performed in the dental office.[2],[3],[23] In this way, the objective of the present study was to compare the shear bond strength at the resin/bracket interface, between sandblasted brackets with different sizes of aluminum oxide particles.

The shear bond strength is defined as the applied force divided by the area of the adhered interface and it is expressed in Mpa (kN /mm2).[1],[2],[3],[7],[17],[23] Because the bonded brackets must counteract the masticatory forces; the bond strength values are a matter of discussion. Some authors claimed to be between 5 and 10 Mpa (19), other between 6 and 12 Mpa (24), and even smaller values have been reported by other authors (5-8 Mpa).[2],[23],[25] In this study, only the sandblasted brackets showed values within this range. It has been reported that the minimum and maximum adhesion resistance required by a bracket against the masticatory forces are 2.86 Mpa and around 13.5 Mpa, respectively.[2],[19] The values obtained in the present study are within the safe range.

It has been suggested that the conditioning of the enamel, the type of cement used and the design of the bracket’s base are important factors for the bonded brackets resistance.[26] In the present research the conditioning of the enamel was not involved because the study aimed to assess the resistance at the resin/bracket interface. In the same way, we use the same type of adhesive for all brackets, eliminating some factors that could interfere with the shear bond strength evaluation. The bracket’s base configuration is important when evaluating the shear bond strength. Different types of designs could lead to different results in the shear bond strength.[16],[26] In the present study, all brackets had the same base configuration, avoiding this problem.

In our investigation, sandblasting with Al2O3 particles increased shear bond resistance, finding a significant difference between the groups that received sandblasting compared to the group without sandblasting. Similar results were observed in other studies.[6],[24],[26],[27] However, other authors found greater values in shear bond strength in new brackets compared to recycled brackets and some even recommend the combination with other methods of recycling to improve its resistance.[1],[3],[5],[7],[17] These differences may also be influenced by the procedure used when measuring the resistance in the resin/bracket interface[17]. Mondenelli et al.[22] obtained an average of 3.47 MPa in their control group. This value was similar to the results obtained in our study for the control group. Nevertheless, when the resistance in the sandblasted brackets was compared, they found lower mean values, which could be due to the 10-second sandblasting time they used on their study; contrary to our study, where 30 seconds was used.[2],[23] Another previous study, that used the same type of brackets showed that it takes 27.5 seconds to completely clean the resin that remains in the mesh.[26] Longer or smaller time should be needed to achieve complete cleaning of the brackets bases, depending on their configuration.[26] In the present study, the bracket was sandblasted for 30 seconds and it was observed that the resin was completely removed from the mesh. Logically, the base could present modifications but without altering or damaging their structure. Then, it could be suggested that the recycling time and the type of brackets are important factors to consider in the recycling procedure.[26] In addition, Montero et al.[3] showed that when consecutive sandblasting is not properly performed on the bracket, its shear bond resistance decreases.

Some authors have mentioned that sandblasting with 50 µm Al2O3 particles is the most recommended option[6],[26] but there are few studies where they compare them with other particle size and there are no studies comparing the resistance in the resin/bracket interface. There are only similar studies in clinical simulated models[3] that used sandblasting groups similar to those used in the present study. Opposite to our findings, they found that the group of 25 µm Al2O3 particle size obtained greater mean values when compared to larger particles (50 µm and 110 µm) but without statistically significant differences between them, suggesting that small particles allow better cleaning of the retentive areas along the mesh.[3] Similarly, Robles et al.[8] found higher resistance in the sandblasting of dental enamel with smaller particles. Contrarily, our investigation found that the group with the largest particle size of Al2O3 (110 µm) showed, numerically, the greatest value in relation to the other two, but with no significant differences between them, as expected.[24] One possible explanation is that with larger particles, the micro-density in the bracket mesh could increase, improving the values of shear bond strength. Similar results are observed when sandblasting with Al2O3 was performed on other surfaces such as artificial teeth (acrylic resin).[9] Smaller particles are likely to provide a polishing effect of the brackets mesh and smaller micro-retentions compared to larger particles.[28]

The present research was carried out in order to guide the clinician in choosing the appropriate size of the aluminum oxide particles to perform the sandblasting protocol, when necessary.[23] Although it was found that the 110 µm particles presented, numerically, the greatest shear bond strength mean value, no significant differences were found with the other particle sizes.In the clinical practice, it is common to expect debonded brackets forcing the clinician to bond them again. This sandblasting conditioning should be optimal to avoid a new debonding which may lead to increases in the time of orthodontic treatment, patient discomfort and additional costs. The other alternative is to replace it with a new bracket that implies and additional cost and environmental consequences. It should be pointed out that brackets recycling has been reported as an option to help sustainability in orthodontics.[29],[30]

Regarding the ethical aspect, it is important to mention that recycling should be performed only for the same patient and thus avoid possible legal consequences of unethical practices.[3],[10],[23]

Because it is an in vitro study, it is not possible completely to extrapolate the results of what would happen in the oral cavity. In addition, our values are not comparable with the results from other studies that used clinical models. The shear bond strength in vivo is usually smaller than those recorded in vitro because there are many confounding variables.[31] Therefore, the findings presented in this investigation should be interpreted carefully and more studies that support the results of this research should be carried out.

  Conclusions Top

  1. Shear bond strength, at the resin/bracket interface, increased after the sandblasting procedure.
  2. Similar share bond strength can be expected after bracket sandblasting with 25µm, 50µm, and 110µm Al2O3 particle size.
  3. Independently of the particle size used, the sandblasted brackets showed greater shear bond strength than brackets without sandblasting.

Financial support and sponsorship

This research didn’t have sponsors.

Conflicts of interest

The authors don’t have conflicts of interest.

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Montero M, Vicente A, Alfonso-Hernández N, Jiménez-López M, Bravo-González L. Comparison of shear bond strength of brackets recycled using micro sandblasting and industrial methods. Angle Orthod 2015;85:461-7.  Back to cited text no. 3
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  [Figure 1], [Figure 2], [Figure 3]

  [Table 1], [Table 2]


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