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20 rev port estomatol med dent cir maxilofac. 2021;62(1):16-22

Table 2. Two -way analysis of variance for variables surface treatment and adhesive system.
Degrees of Sum of squares Mean square F -value P -value
freedom
Surface treatment 2 2043.57 1021.79 702.99 0.0001

Adhesive system 1 0.06 0.06 0.04 0.8415
Surface treatment * adhesive system 2 5.51 2.76 1.90 0.1566
Residual 84 122.09 1.45

Table 3. Post -hoc tests by the Student -Newman -Keuls method for surface treatment, with a statistical significance of 5%.
Effect: Surface treatment Mean difference Critical difference Statistical significance
No treatment vs. Tungsten bur 7.50 0.62 Yes
No treatment vs. Sandblasting Al O 3 11.49 0.74 Yes
2
Tungsten bur vs. Sandblasting Al O 3.99 0.62 Yes
2 3

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adhesion. Additionally, all samples were thermocycled,
which is another factor that reduces bond strength. 6,27 Both
these procedures simulate what happens in the oral envi-
ronment when temporary crowns have been cemented for
some time. Although 500 is the number of cycles estimated
to occur in less than 2 months in the mouth, in this study, it
was chosen following the 1994 ISO/TR 11405, since there is
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still no standardized protocol in the literature for thermal
cycling. 29
By bonding brackets to PMMA using a Bis -GMA resin, such
as Transbond XT, materials with different chemical compo-
sitions are used, and consequently, adhesion forces are ex-
pected to diminish. Since these experimental conditions
were the most adverse from the adhesion point of view, bond
strength was expected to be significantly lower than in the
control group. Mechanical surface treatment was the only
Figure 3. Adhesive remnant index (ARI) scores factor that significantly influenced the adhesion of brackets
distribution by experimental groups. [NT = No treatment;
TB = tungsten bur; ASB = aluminum sandblasting; SU = to PMMA. This result agrees with other studies, which have
Scotchbond Universal; MMA= methylmethacrylate shown that surface roughening, by increasing the area avail-
monomer; TXTP = Transbond XT Primer] able for retention, increases the adhesion of acrylic resins to
other materials. 30,31
In this study, ASB was the most effective surface treatment,
being significantly superior to TB. No study evaluating bond
Discussion strength after PMMA abrasion with a TB was found. However,
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a previous report found that aluminum oxide blasting of
The present study arose from the need to investigate ortho- polycarbonate crowns before bracket bonding yielded higher
dontic brackets’ adhesion to PMMA since previous research adhesion values than those obtained when diamond -bur abra-
and clinical practice point to a lower than clinically accept- sion was performed. Scanning electron microscopy studies 6,31
able adhesive resistance. 2,14 Although there is no determined have also shown that the apparent roughness of diamond -bur
value for clinical bracket bond failure, many studies cite or green -stone abrasion surfaces is only due to the presence
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Reynolds’s suggestion, which considered tensile strengths of shallow ridges. By contrast, ASB creates thousands of mi-
between 5.9 and 7.8 MPa as adequate for orthodontic needs. croscopic retentions, resulting in a significantly more retentive
However, like the most recent studies, the present investiga- surface. 6
tion evaluated shear rather than tensile strengths, as suggest- It is important to notice that the adhesion values in PMMA
ed by Reynolds. specimens submitted to ASB appear to have reached the ma-
Aging PMMA discs in water stored at 37ºC for 55 days terial’s cohesive limit since most blasted discs presented co-
reduces the amount of free residual monomer, complicating hesive substrate failures. No specimens from the remaining