The Open Dentistry Journal




ISSN: 1874-2106 ― Volume 14, 2020
RESEARCH ARTICLE

Comparative Study of Adhesion of Brackets with Metal Injection Molding (MIM) Technology and Welded bases: In vitro Study



Luis A. V. Izquierdo1, Francyle S. H. Sanches1, Francisco Molina1, Rafael P. Henriques2, Emerson F. Cruz2, Karina M. S. Freitas3, *
1 Department of Orthodontics, University of Hemispheres, Quito, Ecuador, FACOP, Bauru, Brazil
2 Department of Health Sciences, FACOP, Bauru, Brazil
3 Department of Orthodontics, Uninga University Center, Maringa, Brazil

Abstract

Introduction:

Brackets bonded to enamel surface depend on the adhesion material and the quality of the bracket base.

Objective:

The aim of this study was to compare the shear bond strength of metallic brackets with Metal Injection Molding (MIM) technology base or welded base.

Materials and Methods:

Forty mandibular extracted premolars mounted in acrylic resin blocks were divided randomly into two groups, both bonded with Transbond XT. In Group 1, brackets with MIM technology bases (Masel) were used, and in group 2, brackets with a welded base (Morelli) were used. After 24 hours, all brackets were tested for shear bond strength in a universal testing machine. Intergroup comparison was performed with an independent t test.

Results:

MIM base brackets showed a mean maximum load registered of 107.55 N, a mean shear bond strength of 9.58 MPa with a standard deviation of 5.80 MPa and the welded base brackets showed a mean maximum load of 167.37 N, a mean shear bond strength of 13.28 MPa with a standard deviation of 2.58 MPa. The difference between the two groups was statistically significant, indicating a higher shear bond strength of the welded base brackets.

Conclusion:

It was concluded that the brackets with welded bases presented a significantly higher shear bond strength than the brackets with MIM bases.

Keywords: Shear bond strength, Bracket base design, Bonding, Orthodontics, MIM technology, Dental enamel.


Article Information


Identifiers and Pagination:

Year: 2020
Volume: 14
First Page: 240
Last Page: 246
Publisher Id: TODENTJ-14-240
DOI: 10.2174/1874210602014010240

Article History:

Received Date: 02/01/2020
Revision Received Date: 05/03/2020
Acceptance Date: 11/03/2020
Electronic publication date: 23/05/2020
Collection year: 2020

© 2020 V. Izquierdo et al.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: (https://creativecommons.org/licenses/by/4.0/legalcode). This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Correspondence: Address correspondence to this author at the Department of Orthodontics, Uninga University Center, Maringa, Brazil; Tel: 55 14 991026446; E-mail: kmsf@uol.com.br





1. INTRODUCTION

Dental enamel is one of the strongest structures in the human body, composed of various minerals that provide it with hardness and elastic properties, making it resistant to wear and fractures while being friable [1Katchburian E, Arana V. Oral histology and embryology: text, atlas, clinical correlations 3rd ed. 1999.].

Since the introduction of phosphoric acid in dentistry, long-term effects have been achieved for bonding orthodontic appliances to the enamel surface of the teeth allowing for increased adhesion force between the enamel surface and the bonding system, and providing mechanical imbrication of the resin and microporosities produced by the acid treatment of the enamel surface [2Buonocore MG, Matsui A, Gwinnett AJ. Penetration of resin dental materials into enamel surfaces with reference to bonding. Arch Oral Biol 1968; 13(1): 61-70.
[http://dx.doi.org/10.1016/0003-9969(68)90037-X] [PMID: 5237555]
].

Thus, bracket bonding improved and minimized the replacement of orthodontic fixtures, achieving a significant improvement in orthodontic therapy [3Newman GV, Sun BC, Ozsoylu SA, Newman RA. Update on bonding brackets: an in vitro survey. J Clin Orthod 1994; 28(7): 396-402.
[PMID: 8613518]
]. Similarly, a variety of adhesive systems and different brackets have been developed for direct bonding with various base types to improve adhesive imbrication between the tooth and orthodontic accessories [4Keizer S, ten Cate JM, Arends J. Direct bonding of orthodontic brackets. Am J Orthod 1976; 69(3): 318-27.
[http://dx.doi.org/10.1016/0002-9416(76)90079-8] [PMID: 766645]
-8Zachrisson BU, Brobakken BO. Clinical comparison of direct versus indirect bonding with different bracket types and adhesives. Am J Orthod 1978; 74(1): 62-78.
[http://dx.doi.org/10.1016/0002-9416(78)90046-5] [PMID: 356618]
].

Failure of bracket bonding during the course of orthodontic therapy is not an uncommon occurrence [9Read MJF, O’Brien KD. A clinical trial of an indirect bonding technique with a visible light-cured adhesive. Am J Orthod Dentofacial Orthop 1990; 98(3): 259-62.
[http://dx.doi.org/10.1016/S0889-5406(05)81603-8] [PMID: 2206041]
], therefore, the bracket bases have been modified to improve the anatomy and to achieve better mechanical retention by perforations, meshes, grooves, spherical designs, and different metal particles [7Smith NR, Reynolds IR. A comparison of three bracket bases: an in vitro study. Br J Orthod 1991; 18(1): 29-35.
[http://dx.doi.org/10.1179/bjo.18.1.29] [PMID: 1827348]
].

Several authors have suggested that the microscopic characteristics of the bracket base, particularly the mesh designs, may increase or decrease the effectiveness of mechanical interlocking with the adhesive. These features include rough or smooth mesh wires, mesh size, wire diameter, weld points, the integrity of the enamel surface, the conditioning procedure, and the type of adhesive system used [10Dickinson PT, Powers JM. Evaluation of fourteen direct-bonding orthodontic bases. Am J Orthod 1980; 78(6): 630-9.
[http://dx.doi.org/10.1016/0002-9416(80)90202-X] [PMID: 7006404]
-12Maijer R, Smith DC. Variables influencing the bond strength of metal orthodontic bracket bases. Am J Orthod 1981; 79(1): 20-34.
[http://dx.doi.org/10.1016/0002-9416(81)90098-1] [PMID: 6450539]
].

Several researches have been conducted to determine the in vitro adhesion forces for each type of metal bracket base [6Silverstone LM, Dogon IL. Proceedings of an International Symposium on the Acid Etch Technique: North Central Publishing Company 1975., 7Smith NR, Reynolds IR. A comparison of three bracket bases: an in vitro study. Br J Orthod 1991; 18(1): 29-35.
[http://dx.doi.org/10.1179/bjo.18.1.29] [PMID: 1827348]
, 13Lopez JI. Retentive shear strengths of various bonding attachment bases. Am J Orthod 1980; 77(6): 669-78.
[http://dx.doi.org/10.1016/0002-9416(80)90158-X] [PMID: 6992590]
-15Reynolds IR, von Fraunhofer JA. Direct bonding of orthodontic brackets--a comparative study of adhesives. Br J Orthod 1976; 3(3): 143-6.
[http://dx.doi.org/10.1179/bjo.3.3.143] [PMID: 788775]
], other factors highlighted and the weld between the base and the metal mesh reducing the available retention surface, which can be significant in view of the tendency to reduce bracket base sizes [4Keizer S, ten Cate JM, Arends J. Direct bonding of orthodontic brackets. Am J Orthod 1976; 69(3): 318-27.
[http://dx.doi.org/10.1016/0002-9416(76)90079-8] [PMID: 766645]
, 8Zachrisson BU, Brobakken BO. Clinical comparison of direct versus indirect bonding with different bracket types and adhesives. Am J Orthod 1978; 74(1): 62-78.
[http://dx.doi.org/10.1016/0002-9416(78)90046-5] [PMID: 356618]
, 10Dickinson PT, Powers JM. Evaluation of fourteen direct-bonding orthodontic bases. Am J Orthod 1980; 78(6): 630-9.
[http://dx.doi.org/10.1016/0002-9416(80)90202-X] [PMID: 7006404]
, 13Lopez JI. Retentive shear strengths of various bonding attachment bases. Am J Orthod 1980; 77(6): 669-78.
[http://dx.doi.org/10.1016/0002-9416(80)90158-X] [PMID: 6992590]
-16Newman GV. Current status of bonding attachments. J Clin Orthod 1973; 7(7): 425-434, passim.
[PMID: 4577529]
].

Among the main methods of manufacture of metal brackets are: casting, molding, and injection. A wide range of methods have been employed for the manufacture of orthodontic accessories, including a various kinds of metals and stainless-steel alloys [17Eliades T, Zinelis S, Bourauel C, Eliades G. Manufacturing of orthodontic brackets: A review of metallurgical perspectives and applications. Recent Pat Mater Sci 2008; 1: 135-9.
[http://dx.doi.org/10.2174/1874464810801020135]
].

The objective of this study is to compare the shear bond strength of metal brackets with metal injection molding (MIM) technology or welded bases. The tested null hypothesis was that there is no difference in the shear bond strength of metal brackets with metal injection molding (MIM) technology and welded bases.

2. MATERIALS AND METHODS

In order to establish the number of specimens required to indicate statistical significance, between the experimental conditions, a pilot test with 10 specimens with a minimum difference of 3.18 MPa, an alpha error of 5% and a beta error of 20%, indicated that 40 teeth were needed this research, 20 in each group.

Inclusion criteria for the selected teeth were: first premolars extracted for orthodontic purposes; absence of fractures caused by forceps; absence of caries lesions and absence of restorations. All teeth were examined with 10x magnification. The premolars used in this study were donated by educational institutions.

For the 40-specimen preparation, a prophylaxis with pumice paste (without fluoride) was performed with a Robson brush on the buccal surfaces of the teeth. The teeth were inserted in blocks of acrylic resin (20mm x 20mm) until the cement-enamel junction, leaving the crown fully exposed (Fig. 1). The bonding procedures of both groups started with acid conditioning with 37% phosphoric acid for 30 seconds [6Silverstone LM, Dogon IL. Proceedings of an International Symposium on the Acid Etch Technique: North Central Publishing Company 1975., 18Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J Dent Res 1955; 34(6): 849-53.
[http://dx.doi.org/10.1177/00220345550340060801] [PMID: 13271655]
] and shortly thereafter, were rinsed with abundant water jet and dried with an oil and a water-free air source.

Soon after, the teeth were randomly divided into two groups of 20 teeth, and two types of brackets were bonded.

Group 1: 20 MIM (metal injection molding) technology stainless steel brackets for first mandibular premolars (Roth 0.022” x 0.028”, Masel Enterprise, Bistrol, USA).

Fig. (1)
Specimens.


Group 2: 20 welded bases stainless steel orthodontic brackets for first mandibular premolars (Roth 0.022” x 0.028”, Morelli, Sorocaba, Brazil).

Brackets of both groups had the same linear area of their bases of 12.5 mm2.

The adhesive system used was Transbond XT (3M, Unitek, Monrovia, USA). A thin layer of the primer was applied for 10 seconds and light-cured for 30 seconds with an 1800 mW Woodpecker Led-F light curing light, wavelength 420-480 mm2. After a uniform layer of the resin was applied to the base of the brackets, and were positioned in the center of the clinical crown exerting pressure toward the tooth, excess of resin was removed from the bracket margins and light-cured for 60 seconds (Fig. 2). All specimens were prepared and brackets were bonded by the same operator (L.A.V.I).

Fig. (2)
Light curing after bracket positioning.


After 48 hours of bracket bonding, they were placed in a device fixed to the bottom base of an automated Universal Testing Machine (Tinius Olsen Super L120 1980 Testing Machine, Houston, USA) (Fig. 3), which performed shear strength testing at a speed of 0.5 mm per minute.

Fig. (3)
Universal testing machine used for the shear test.


The tooth blocks were held in an upright position, and a beveled stainless-steel tip 1mm high and having a 10mm wide active edge was attached to the load cell #2 of the testing machine, positioned and rested on the resin plateau to test the bonding interphase between the bracket base and the resin (Fig. 4). The machine was then driven at a speed of 0.5 mm per minute in the direction of compression, thus developing a shear stress at the bracket-resin interphase until the fracture or rupture of the adhesive union. The values were recorded directly in kgf/cm2 on the digital monitor of the testing machine.

The shear bond strength was measured in Megapascals (MPa) and the maximum load registered was measured in Newtons (N).

2.1. Statistical Analysis

The normality of the data was checked with Shapiro-Wilk test. Data presented normal distribution and then parametric tests were applied.

Intergroup comparisons of the shear bond strength and maximum load registered were performed with independent t-tests. The level of significance was 5% (P<0.05). All statistical analyses were performed with SPSS version 22 for Windows (SPSS Inc., Chicago, IL, USA).

3. RESULTS

The shear bond strength of MIM base brackets had a maximum shear bond strength of 24.43 MPa and a minimum strength of 3.22 MPa, an average of 9.58 MPa, and a standard deviation of 5.80 MPa, with a maximum load registered of 274.13 N and a minimum load of 17.70 N.

Fig. (4)
Shear test of the brackets performed with chisel.


The strength of the welded base brackets had a maximum shear bond strength of 20.18 MPa and a minimum strength of 9.72 MPa, an average of 13.28 MPa and a standard deviation of 2.58 MPa, with a maximum load registered of 222.11 N and a minimum load of 160.99 N. When the two groups were compared, there was a statistically significant difference in the shear bond strength and in the maximum load registered, indicating that the welded brackets presented a significantly higher shear bond strength and maximum load registered than the MIM base brackets (Table 1, Figs. 5 and 6).

Table 1
Results of intergroup comparison of shear bond strength (MPa) and maximum load registered (N) (independent t tests).


Fig. (5)
Shear bond strength in MIM and welded bases.


Fig. (6)
Maximum load registered in MIM and welded bases.


4. DISCUSSION

Aiming at improving the adhesion strength of the materials, various bracket bases with different designs were developed, trying to improve the interaction of the materials and the enamel structure, and introduce modifications in the base designs, including base screen [10Dickinson PT, Powers JM. Evaluation of fourteen direct-bonding orthodontic bases. Am J Orthod 1980; 78(6): 630-9.
[http://dx.doi.org/10.1016/0002-9416(80)90202-X] [PMID: 7006404]
-12Maijer R, Smith DC. Variables influencing the bond strength of metal orthodontic bracket bases. Am J Orthod 1981; 79(1): 20-34.
[http://dx.doi.org/10.1016/0002-9416(81)90098-1] [PMID: 6450539]
, 19Alexandre P, Young J, Sandrik JL, Bowman D. Bond strength of three orthodontic adhesives. Am J Orthod 1981; 79(6): 653-60.
[http://dx.doi.org/10.1016/0002-9416(81)90357-2] [PMID: 6453532]
-21Kechagia A, Zinelis S, Pandis N, Athanasiou AE, Eliades T. The effect of orthodontic adhesive and bracket-base design in adhesive remnant index on enamel. JWFO 2015; 4: 18-22.
[http://dx.doi.org/10.1016/j.ejwf.2014.12.002]
], grooves [11Ferguson JW, Read MJF, Watts DC. Bond strengths of an integral bracket-base combination: an in vitro study. Eur J Orthod 1984; 6(4): 267-76.
[http://dx.doi.org/10.1093/ejo/6.4.267] [PMID: 6391941]
, 22Thanos CE, Munholland T, Caputo AA. Adhesion of mesh-base direct-bonding brackets. Am J Orthod 1979; 75(4): 421-30.
[http://dx.doi.org/10.1016/0002-9416(79)90164-7] [PMID: 285618]
], or circular or rectangular perforated design bases [8Zachrisson BU, Brobakken BO. Clinical comparison of direct versus indirect bonding with different bracket types and adhesives. Am J Orthod 1978; 74(1): 62-78.
[http://dx.doi.org/10.1016/0002-9416(78)90046-5] [PMID: 356618]
, 10Dickinson PT, Powers JM. Evaluation of fourteen direct-bonding orthodontic bases. Am J Orthod 1980; 78(6): 630-9.
[http://dx.doi.org/10.1016/0002-9416(80)90202-X] [PMID: 7006404]
, 22Thanos CE, Munholland T, Caputo AA. Adhesion of mesh-base direct-bonding brackets. Am J Orthod 1979; 75(4): 421-30.
[http://dx.doi.org/10.1016/0002-9416(79)90164-7] [PMID: 285618]
].

The purpose of this study was to compare the bond strength of two types of bracket bases, using brackets with a mesh base and base injected in human teeth, as was also done in other studies [5Mizrahi E, Smith DC. Direct cementation of orthodontic brackets to dental enamel. An investigation using a zinc polycarboxylate cement. Br Dent J 1969; 127(8): 371-5.
[PMID: 5260557]
, 10Dickinson PT, Powers JM. Evaluation of fourteen direct-bonding orthodontic bases. Am J Orthod 1980; 78(6): 630-9.
[http://dx.doi.org/10.1016/0002-9416(80)90202-X] [PMID: 7006404]
-12Maijer R, Smith DC. Variables influencing the bond strength of metal orthodontic bracket bases. Am J Orthod 1981; 79(1): 20-34.
[http://dx.doi.org/10.1016/0002-9416(81)90098-1] [PMID: 6450539]
, 14Reynolds IR, von Fraunhofer JA. Direct bonding of orthodontic attachments to teeth: the relation of adhesive bond strength to gauze mesh size. Br J Orthod 1976; 3(2): 91-5.
[http://dx.doi.org/10.1179/bjo.3.2.91] [PMID: 779822]
, 20Buzzitta VAJ, Hallgren SE, Powers JM. Bond strength of orthodontic direct-bonding cement-bracket systems as studied in vitro. Am J Orthod 1982; 81(2): 87-92.
[http://dx.doi.org/10.1016/0002-9416(82)90031-8] [PMID: 6758593]
, 21Kechagia A, Zinelis S, Pandis N, Athanasiou AE, Eliades T. The effect of orthodontic adhesive and bracket-base design in adhesive remnant index on enamel. JWFO 2015; 4: 18-22.
[http://dx.doi.org/10.1016/j.ejwf.2014.12.002]
, 23Bishara SE, Soliman MM, Oonsombat C, Laffoon JF, Ajlouni R. The effect of variation in mesh-base design on the shear bond strength of orthodontic brackets. Angle Orthod 2004; 74(3): 400-4.
[PMID: 15264654]
, 24Ianni Filho D, Silva TBC, Simplício AHM, Loffredo LCM, Ribeiro RP. In vitro evaluation of the bond strength of bonding materials in orthodontics: mechanical shear tests. Rev Dent Press Ortodon Ortop Facial 2004; 9: 39-48.]. Some studies used recently extracted bovine teeth [4Keizer S, ten Cate JM, Arends J. Direct bonding of orthodontic brackets. Am J Orthod 1976; 69(3): 318-27.
[http://dx.doi.org/10.1016/0002-9416(76)90079-8] [PMID: 766645]
, 7Smith NR, Reynolds IR. A comparison of three bracket bases: an in vitro study. Br J Orthod 1991; 18(1): 29-35.
[http://dx.doi.org/10.1179/bjo.18.1.29] [PMID: 1827348]
, 13Lopez JI. Retentive shear strengths of various bonding attachment bases. Am J Orthod 1980; 77(6): 669-78.
[http://dx.doi.org/10.1016/0002-9416(80)90158-X] [PMID: 6992590]
, 25MacColl GA, Rossouw PE, Titley KC, Yamin C. The relationship between bond strength and orthodontic bracket base surface area with conventional and microetched foil-mesh bases. Am J Orthod Dentofacial Orthop 1998; 113(3): 276-81.
[http://dx.doi.org/10.1016/S0889-5406(98)70297-5] [PMID: 9517718]
], which is well accepted in the literature due to the similar behavior of both enamel surfaces [13Lopez JI. Retentive shear strengths of various bonding attachment bases. Am J Orthod 1980; 77(6): 669-78.
[http://dx.doi.org/10.1016/0002-9416(80)90158-X] [PMID: 6992590]
, 26Teruel JdeD, Alcolea A, Hernández A, Ruiz AJ. Comparison of chemical composition of enamel and dentine in human, bovine, porcine and ovine teeth. Arch Oral Biol 2015; 60(5): 768-75.
[http://dx.doi.org/10.1016/j.archoralbio.2015.01.014] [PMID: 25766469]
, 27Oesterle LJ, Shellhart WC, Belanger GK. The use of bovine enamel in bonding studies. Am J Orthod Dentofacial Orthop 1998; 114(5): 514-9.
[http://dx.doi.org/10.1016/S0889-5406(98)70171-4] [PMID: 9810047]
].

The methodology of the present study was standardized; the specimens were well adapted to the universal testing machine used; torques of the brackets were similar; the pressure during bracket positioning was standardized, so that the adhesive thickness was thin and uniform and adhesive excesses were removed without displacing the bracket [28Fleishmann LA, Sobral MC, Santos GC Junior, Habib F. Comparative study of six types of orthodontic brackets for adhesion strength. Rev Dent Press Ortodon Ortop Facial 2008; 13: 107-16., 29Littlewood SJ, Redhead A. Use of jigs to standardise orthodontic bond testing. J Dent 1998; 26(5-6): 539-45.
[http://dx.doi.org/10.1016/S0300-5712(98)00016-5] [PMID: 9699449]
]

The strength of direct bonding orthodontic accessories is provided by several factors such as the design of the bracket base, the quality and type of substrate to be bonded on, enamel surface contaminants, and the adhesive system used [12Maijer R, Smith DC. Variables influencing the bond strength of metal orthodontic bracket bases. Am J Orthod 1981; 79(1): 20-34.
[http://dx.doi.org/10.1016/0002-9416(81)90098-1] [PMID: 6450539]
, 30Abu Alhaija ES, Abu AlReesh IA, AlWahadni AM. Factors affecting the shear bond strength of metal and ceramic brackets bonded to different ceramic surfaces. Eur J Orthod 2010; 32(3): 274-80.
[http://dx.doi.org/10.1093/ejo/cjp098] [PMID: 19903729]
]. Among these variables, the adhesive system used in this research was Transbond XT adhesive system (primer and resin)(3M Unitek, Monrovia, USA), considered as the gold standard adhesion system [31Akhavan A, Sodagar A, Mojtahedzadeh F, Sodagar K. Investigating the effect of incorporating nanosilver/nanohydroxyapatite particles on the shear bond strength of orthodontic adhesives. Acta Odontol Scand 2013; 71(5): 1038-42.
[http://dx.doi.org/10.3109/00016357.2012.741699] [PMID: 23294142]
, 32Minick GT, Oesterle LJ, Newman SM, Shellhart WC. Bracket bond strengths of new adhesive systems. Am J Orthod Dentofacial Orthop 2009; 135(6): 771-6.
[http://dx.doi.org/10.1016/j.ajodo.2007.06.021] [PMID: 19524837]
].

The shear tests were performed in the occlusal-gingival direction due to the larger mesiodistal dimension of the bracket in relation to the occlusal-gingival dimension, as performed in other studies [3Newman GV, Sun BC, Ozsoylu SA, Newman RA. Update on bonding brackets: an in vitro survey. J Clin Orthod 1994; 28(7): 396-402.
[PMID: 8613518]
, 23Bishara SE, Soliman MM, Oonsombat C, Laffoon JF, Ajlouni R. The effect of variation in mesh-base design on the shear bond strength of orthodontic brackets. Angle Orthod 2004; 74(3): 400-4.
[PMID: 15264654]
, 24Ianni Filho D, Silva TBC, Simplício AHM, Loffredo LCM, Ribeiro RP. In vitro evaluation of the bond strength of bonding materials in orthodontics: mechanical shear tests. Rev Dent Press Ortodon Ortop Facial 2004; 9: 39-48., 33Romano FL, Ruellas ACO. Comparative study in vitro of the shear bond strength and resin remnant between the concise and superbond composites. Rev Dent Press Ortodon Ortop Facial 2003; 8: 69-75.], offering better resistance to its removal when force is applied in this direction, and also standardizing the methodology employed.

Profit [34Proffit WR, Fields HW, Nixon WL. Occlusal forces in normal- and long-face adults. J Dent Res 1983; 62(5): 566-70.
[http://dx.doi.org/10.1177/00220345830620051201] [PMID: 6573373]
] reports that the maximum force during chewing by a child with a normal height between 6 and 11 years old is 5.01 kg and in adults, it is 13.5 Kg, these results are similar to other studies [35Koolstra JH, van Eijden TMGJ, Weijs WA, Naeije M. A three-dimensional mathematical model of the human masticatory system predicting maximum possible bite forces. J Biomech 1988; 21(7): 563-76.
[http://dx.doi.org/10.1016/0021-9290(88)90219-9] [PMID: 3410859]
, 36Commisso MS, Martínez-Reina J, Ojeda J, Mayo J. Finite element analysis of the human mastication cycle. J Mech Behav Biomed Mater 2015; 41: 23-35.
[http://dx.doi.org/10.1016/j.jmbbm.2014.09.022] [PMID: 25460400]
]. Thus, it would be reasonable to infer, from these studies, that the release forces of brackets can vary from 5 to 13 kg. The results found in the present research demonstrate that using standardized bonding protocols and adhesive techniques, it was observed that the adhesion strength of the welded base brackets presented an average shear bond strength of 13.28 MPa (13.54 Kg) when compared to the MIM technology base brackets with 9.58 MPa (9.76 Kg), and this difference was statistically significant (Table 1). This difference is possibly attributed to the design of the bracket base that has a larger mesh spacing allowing more air to escape and better resin penetration, resulting in greater adhesion strength, as also reported in other researches [12Maijer R, Smith DC. Variables influencing the bond strength of metal orthodontic bracket bases. Am J Orthod 1981; 79(1): 20-34.
[http://dx.doi.org/10.1016/0002-9416(81)90098-1] [PMID: 6450539]
, 37Wang WN, Li CH, Chou TH, Wang DDH, Lin LH, Lin CT. Bond strength of various bracket base designs. Am J Orthod Dentofacial Orthop 2004; 125(1): 65-70.
[http://dx.doi.org/10.1016/j.ajodo.2003.01.003] [PMID: 14718881]
].

Molina et al. [38Molina F, Freitas KMS, Ordóñez MCRB, Cruz EF, Henriques RP, Aguirre Balseca GM. Comparison of Shear Bond Strength of MIM Technology Brackets with Conventional and Rail-Shaped Mesh Bases: An in vitro Study. Open Dent J 2019; 13: 255-60.
[http://dx.doi.org/10.2174/1874210601913010255]
] compared the shear bond strength of MIM technology brackets with two types of meshes, conventional or rail-shaped and found no difference in shear bond strength, with values close to ours for the MIM conventional mesh base brackets.

Wang et al. [37Wang WN, Li CH, Chou TH, Wang DDH, Lin LH, Lin CT. Bond strength of various bracket base designs. Am J Orthod Dentofacial Orthop 2004; 125(1): 65-70.
[http://dx.doi.org/10.1016/j.ajodo.2003.01.003] [PMID: 14718881]
] tested several welded mesh-based brackets with different base sizes and found that the brackets with the larger mesh size produced greater bond strength than the brackets with smaller mesh sizes. In the present study, both brackets tested presented the same size of the bases.

The amount of resin that remained in the enamel surface was not compared between the groups in the present study, but most of the resin remained in the tooth surface after the shear test. For bracket debonding it is preferable that the bracket/adhesive interface is predominant, leaving much of the remaining resin adhered to the tooth enamel, minimizing enamel fractures [28Fleishmann LA, Sobral MC, Santos GC Junior, Habib F. Comparative study of six types of orthodontic brackets for adhesion strength. Rev Dent Press Ortodon Ortop Facial 2008; 13: 107-16.].Therefore, excessive adhesion is not desirable as it can cause damage to tooth enamel. The mean force of shear bond strength of the MIM base brackets was compatible with clinical use and similar to several previous studies [28Fleishmann LA, Sobral MC, Santos GC Junior, Habib F. Comparative study of six types of orthodontic brackets for adhesion strength. Rev Dent Press Ortodon Ortop Facial 2008; 13: 107-16., 37Wang WN, Li CH, Chou TH, Wang DDH, Lin LH, Lin CT. Bond strength of various bracket base designs. Am J Orthod Dentofacial Orthop 2004; 125(1): 65-70.
[http://dx.doi.org/10.1016/j.ajodo.2003.01.003] [PMID: 14718881]
, 38Molina F, Freitas KMS, Ordóñez MCRB, Cruz EF, Henriques RP, Aguirre Balseca GM. Comparison of Shear Bond Strength of MIM Technology Brackets with Conventional and Rail-Shaped Mesh Bases: An in vitro Study. Open Dent J 2019; 13: 255-60.
[http://dx.doi.org/10.2174/1874210601913010255]
]. However, the force of shear bond strength of the welded base brackets was higher than previous studies [28Fleishmann LA, Sobral MC, Santos GC Junior, Habib F. Comparative study of six types of orthodontic brackets for adhesion strength. Rev Dent Press Ortodon Ortop Facial 2008; 13: 107-16., 37Wang WN, Li CH, Chou TH, Wang DDH, Lin LH, Lin CT. Bond strength of various bracket base designs. Am J Orthod Dentofacial Orthop 2004; 125(1): 65-70.
[http://dx.doi.org/10.1016/j.ajodo.2003.01.003] [PMID: 14718881]
, 38Molina F, Freitas KMS, Ordóñez MCRB, Cruz EF, Henriques RP, Aguirre Balseca GM. Comparison of Shear Bond Strength of MIM Technology Brackets with Conventional and Rail-Shaped Mesh Bases: An in vitro Study. Open Dent J 2019; 13: 255-60.
[http://dx.doi.org/10.2174/1874210601913010255]
], and could be slightly above what is necessary for clinical use, which can cause damage to the enamel at debonding, so it should be done more carefully.

Besides the results of the present study, it has to be considered that the force of adhesion is also related to other factors, such as the type of adhesive material used [39Bishara SE, Gordan VV, VonWald L, Jakobsen JR. Shear bond strength of composite, glass ionomer, and acidic primer adhesive systems. Am J Orthod Dentofacial Orthop 1999; 115(1): 24-8.
[http://dx.doi.org/10.1016/S0889-5406(99)70312-4] [PMID: 9878954]
, 40Fernandez SL, Calero JMP, Ibarra JG, Lozano MB, Pérez HE. Fuerza de retención al esmalte con adhesivos usados en ortodoncia, utilizando dos tipos de base de brackets (estudio comparativo in vitro). Rev Odontol Mex 2004; 8: 122-6.], the curvature and size of the bracket base [41Park DM, Romano FL, Santos-Pinto A, Martins LP, Nouer DF. Analysis of the adhesion quality of different metallic bracket bases. Rev Dent Press Ortodon Ortop Facial 2005; 10: 88-93.
[http://dx.doi.org/10.1590/S1415-54192005000100010]
, 42Floriano H, Mori AT, Maltagliati AMA, Lino AP. Studies of the tensile strength of bonded metal brackets in relation to some types of base. Rev Paul Odontol 2001; 23: 20-3.], and the quality of the dental enamel, among other factors [38Molina F, Freitas KMS, Ordóñez MCRB, Cruz EF, Henriques RP, Aguirre Balseca GM. Comparison of Shear Bond Strength of MIM Technology Brackets with Conventional and Rail-Shaped Mesh Bases: An in vitro Study. Open Dent J 2019; 13: 255-60.
[http://dx.doi.org/10.2174/1874210601913010255]
].

CONCLUSION

The null hypothesis tested was rejected since brackets with welded bases presented a significantly higher shear bond strength than brackets with MIM bases. The results of the present study show clinical relevance by indicating that the type of bracket base influences their adhesion to tooth enamel.

ETHICS APPROVAL AND CONSENT TO PARTICIPATE

Not applicable.

HUMAN AND ANIMAL RIGHTS

Not applicable.

CONSENT FOR PUBLICATION

Not applicable.

AVAILABILITY OF DATA AND MATERIALS

The data sets analyzed during the current study are available from the corresponding author [K.F] on request.

FUNDING

None.

CONFLICT OF INTEREST

The authors declare no conflict of interest, financial or otherwise.

ACKNOWLEDGEMENTS

Declared none.

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