Keywords
bracket - bonding - dental adhesives - double application - universal adhesive
Introduction
The adequate bonding between teeth and brackets is one of the factors influencing
the success of the fixed orthodontic treatment, which forms an important part of orthodontic
practice.[1]
[2]
[3] Adequate bond strength is very important to make a successful orthodontic treatment.[4]
[5] The bond strength of the bracket depends on different factors, such as the bracket
base, the adhesive material, and the preparation of tooth surface.[1] The Transbond XT (3M Unitek, St. Paul, Minnesota, United States) etch-and-rinse
adhesive system is one of the standard adhesive systems frequently used in orthodontic
treatments.[2] This system provides adequate bond strength to resist masticatory and other forces
in the oral environment.[2]
[6]
The developments in adhesive dentistry aim to ease bonding procedures through decreasing
application steps, abridging clinical application time, and reducing technique sensitivity.[7] Clinicians want to use only one adhesive for all cases and shorten the application
time. The latest generation of adhesives is so-called universal or multimode adhesives
that may use in any bonding strategy, including etch-and-rinse, self-etch, and selective
enamel etching.[8] The manufacturers state that universal adhesives can be employed for the placement
of both direct and indirect restorations, including metals, zirconia, porcelain, and
composite. A previous study concluded that the low shear bond strength was obtained
for orthodontic brackets, which were bonded by universal adhesives in self-etch mode.[3] However, it has been stated that the shear bond strength of orthodontic brackets
bonded with a universal adhesive could be increased by supplement, an initial acid
etching or laser conditioning step.[6] Moreover, it has been shown that the universal adhesives could provide sufficient
bond strength for orthodontic bracket bonding to composite, ceramic, zirconia, and
porcelain surfaces.[9]
[10]
[11]
[12] There is not enough result regarding the efficacy of universal adhesives in the
bonding of orthodontic brackets.
The universal adhesives had a rather thin film thickness because high solvent content
requires more evaporation by air blowing/thinning, thus causing low bond strength.[13]
[14] It has been revealed that the double application of universal adhesives was effective
in improving the microtensile bond strength of dentin[13]
[15] and shear bond strength of enamel.[16] The enhanced bond strength by the double application has been attributed to an increase
in adhesive layer thickness.[17]
[18]
[19] The thicker adhesive layer might absorb stresses cumbered on the adhesive interface,
including polymerization shrinkage stresses and enhance stress distribution during
testing.[20]
[21] Nevertheless, the effect of the double application of universal adhesives on the
shear bond strength of orthodontic brackets has not also been tested so far.
Wherefore, the purpose of the present study was to evaluate the effects of the double
application of universal adhesives on the shear bond strength of orthodontic brackets.
The null hypotheses to be tested were (1) that there would not be significant differences
in shear bond strength between the universal adhesives and Transbond XT, and (2) that
the double application of universal adhesives would not improve the shear bond strength
of orthodontic brackets.
Materials and Methods
Seventy-five human premolars which were extracted for orthodontic purposes were employed
following ethical approval (ref no: 2019/327). The teeth had no endodontic treatment,
carious lesions, restorations, enamel defects, and visible cracks. The teeth were
kept in 0.5% chloramine-T solution at 4°C and employed within 3 months following extraction.
The teeth were checked for the absence of cracks and defects under a stereomicroscope
(S4E; Leica Microsystems, Wetzlar, Germany). After the cleaning of the buccal surfaces
of teeth using a rubber cap and slurry of nonfluoridated pumice, they were fixed on
acrylic resin blocks. The buccal enamel surface of each tooth was treated with 37%
orthophosphoric acid (Transbond XT Etching Gel; 3M Unitek) for 15 seconds, water rinsed,
and air-dried. The teeth were randomly assigned into five groups based on the adhesive
procedures (n = 15).
The two universal adhesive systems were tested: Scotchbond Universal Adhesive (3M
Oral Care, St. Paul, Minnesota, United States) and Prime&Bond Universal (Dentsply
DeTrey GmbH, Konstanz, Germany). As control, the orthodontic adhesive system Transbond
XT primer (3M Unitek) was used. The adhesives were employed based on the manufacturer’s
recommendations and polymerized using a LED light-curing unit (Valo; Ultradent, South
Jordan, Utah, United States) with a light output of 1,000 mW/cm ([Table 1]). In double application groups, one coat adhesive was applied and light-cured, then
the second layer was applied similarly. After adhesive application, a light-cured
orthodontic adhesive composite resin (Transbond XT; 3M Unitek) was applied to the
base of stainless steel premolar brackets (MBT 0.22 slot diamond, Miniseries 2000
Ormco, United States) with a bracket base area of 10.29 mm2, and then the bracket was bonded at the center of the clinical crown by pressing
tightly onto the buccal enamel surface. The surplus composite resin was distracted
from bracket margin using a scaler. The light-curing was performed for 40 seconds
(10 seconds from each side) at a distance of 1 to 2 mm of light tip from the bracket
margins using the LED light-curing unit. The teeth were kept in distilled water for
24 hours at 37°C before testing.
Table 1
Adhesive systems used, chemical composition, and application procedure
|
Adhesive systems
|
Composition
|
Application procedure
|
|
Abbreviations: 10-MDP, 10-methacryloyloxydecyl dihydrogen phosphate; Bis-GMA, bisphenol-glycidyl
methacrylate; HEMA, hydroxyethylmethacrylate; PENTA, dipentaerythritol penta acrylate
monophosphate; TEGDMA, triethylene glycol dimethacrylate; UDMA, urethane dimethacrylate.
Note: Composition as provided by the manufacturers.
|
|
Scotchbond Universal (3M Oral Care, St. Paul, Minnesota, United States) Lot no: 602724
|
10-MDP phosphate monomer, dimethacrylate resins, HEMA, methacrylate-modified polyalkenoic
acid copolymer, filler, ethanol, water, initiators, silane
|
1. Apply the adhesive to the entire preparation with a microbrush and rub it in for
20 s
2. Direct a gentle stream of air over the liquid for about 5 s until it no longer
moves and the solvent is evaporated completely
3. Light-cure for 10 s
|
|
Prime&Bond Universal
(Dentsply DeTrey GmbH, Konstanz, Germany)
Lot no: 1802000551
|
HEMA, 2-hydroxy-3 acryloyloxypropyl methacrylate, UDMA, trimethylolpropane trimethacrylate,
PENTA, diketone, organic phosphine oxide, stabilizers, cetylamine hydrofluoride, acetone,
water
|
1. Apply the adhesive to air-dried enamel/dentin surface with rubbing for 20 s
2. Gentle stream of air applied over the liquid for at least 5 s
3. Light-cure for 10 s
|
|
Transbond XT
(3M Unitek, St. Paul, Minnesota, United States) Lot no: N884766
|
Bis-GMA, TEGDMA, 4-(dimetylamino)-benzene ethanol, DL-camphorquinone, hydroquinone
|
1. Apply Transbond XT etching gel to tooth surface for 15 s
2. Rinse thoroughly with water to ensure total removal of etchant
3. Apply thin uniform coat of Transbond XT primer
|
Each specimen was placed on mounting jig in a universal testing machine (Autograph
AGS-X; Shimadzu, Kyoto, Japan) with bracket base parallel to the shear load. A shear
force for debonding was applied to bracket base in occluso-gingival direction at a
crosshead speed of 1 mm/minute. The maximum force required to debonding of the bracket
was recorded as Newton and calculated in MPa by dividing the imposed force (in Newton)
at the time of fracture by the bracket base area (in mm). After debonding, the enamel
surfaces were examined with a stereomicroscope at ×20 magnification to check site
of bond failure and remaining adhesive on tooth using adhesive remnant index (ARI)
as described by Årtun and Bergland.[22] This index uses four scores—(0) no adhesive residue in bonding area on tooth, (1)
less than 50% of the adhesive remaining in bonding area on the tooth, (2) more than
50% of the adhesive remaining in the bonding area on the tooth, and (3) all the adhesive
remaining on the tooth in the bonding area.
Data of shear bond strengths were statistically evaluated with the SPSS Program, version
20.0 (Statistical Package for the Social Sciences; SPSS, Chicago, Illinois, United
States). The normal distribution of data was confirmed by Kolmogorov–Smirnov test.
The data were analyzed using one-way analysis of variance (ANOVA), followed by post
hoc least significant difference (LSD) tests to compare the means between groups.
The data of ARI scores were submitted to chi-squared test. p-Value of less than 0.05 was considered statistically significant for all statistical
analyses.
Results
The one-way ANOVA disclosed statistically significant differences between the experimental
groups ([Table 2]). The mean shear bond strengths and standard deviations are shown in [Table 3] and [Fig. 1] including the results of multiple comparisons by LSD post hoc test. The highest
shear bond strengths were acquired by Scotchbond Universal (p < 0.05). The double application of Scotchbond Universal did not affect the shear
bond strength (p > 0.005). The lowest shear bond strength was found in the Prime&Bond Universal group
(p < 0.05). The double application of Prime&Bond Universal increased the shear bond
strength (p < 0.05).
Table 2
One-way ANOVA results for shear bond strength test
|
Sum of squares
|
df
|
Mean square
|
F
|
p
|
|
Abbreviations: ANOVA, analysis of variance; df, degrees of freedom.
aStatistically significant differences (p < 0.05).
|
|
Between groups
|
501.253
|
4
|
125.313
|
14.928
|
0.000a
|
|
Within groups
|
587.634
|
70
|
8.395
|
|
|
|
Total
|
1088.887
|
74
|
|
|
|
Table 3
Mean shear bond strengths of the different experimental groups
|
Experimental groups
|
MPa ± SD
|
|
Abbreviation: SD, standard deviation.
Note: Same superscript small letter indicates no statistically significant difference
in the columns.
|
|
Scotchbond Universal
|
11.38 ± 3.25a
|
|
Scotchbond Universal-Double application
|
11.92 ± 3.88a
|
|
Prime&Bond Universal
|
4.92 ± 1.24b
|
|
Prime&Bond Universal-Double application
|
8.25 ± 2.89c
|
|
Transbond XT
|
7.52 ± 2.55c
|
Fig. 1 Shear bond strengths (MPa ± standard deviation [SD]) of the different experimental
groups. Means and standard deviations are represented inside the bars. Means with
the same superscript are not significantly different from each other (DA, double application).
The distribution of ARI scores is shown in [Table 4] and [Fig. 2]. The evaluation of the ARI scores by chi-squared test revealed no statistically
significant difference in the distribution of scores between groups (p > 0.05).
Table 4
Distribution of ARI scores
|
Experimental groups
|
0
|
1
|
2
|
3
|
p
|
|
Abbreviation: ARI, adhesive remnant index.
aStatistically not significant differences (p < 0.05).
|
|
Scotchbond Universal
|
5
|
4
|
2
|
4
|
|
|
Scotchbond Universal-Double application
|
5
|
3
|
3
|
4
|
|
|
Prime&Bond Universal
|
9
|
4
|
2
|
0
|
0.868a
|
|
Prime&Bond Universal-Double application
|
6
|
4
|
3
|
2
|
|
|
Transbond XT
|
6
|
3
|
3
|
3
|
|
Fig. 2 The frequencies of adhesive remnant index (ARI) scores (%) observed using light microscopy
(DA, double application).
Discussion
The clinicians have been very successful in the “direct bonding method” for nearly
40 years since the 1970s. With the development of the direct bonding technique, the
time spent in the clinic for bracket bonding has shortened, thus obtaining a more
aesthetic and hygienic orthodontic treatment.[23] Nowadays, the direct bonding technique with light-curing adhesives for bonding orthodontic
brackets is widely used. The bonding strength of adhesives must be able to withstand
the bite forces, the tension of the arch wires, and the patient’s harmful mouth habits.
According to Reynolds,[24] the bonding strength is sufficient between 5.9 and 7.8 MPa for orthodontic treatment.
The shear bond strength of the bracket to enamel must be high enough for preventing
bracket debonding during treatment, but it must not cause any enamel damage during
debonding.[2]
[5] It has also been reported that the enamel fractures might be formed when the shear
bond strength was higher than 14 MPa.[25]
Several generations of adhesive materials have been developed for restorative purposes
and later been used for also orthodontic treatments.[23] Dental adhesives are currently classified as “etch-and-rinse” and “self-etch” systems.[7] In etch-and-rinse adhesives, the adhesion mechanism is mainly mechanical interlocking
between demineralized dental hard tissue and a cured adhesive resin layer. Before
the application of an etch-and-rinse adhesive, the phosphoric acid effectively demineralizes
the dental hard tissues and contributes to adhesion via the penetration of resin monomers
into dentinal tubules and the formation of a hybrid layer.[7] The universal adhesives are the latest developed adhesives.[8] Although the universal adhesives may be employed in both etch-and-rinse and self-etch
modes, in this study, the universal adhesives were tested in etch-and-rinse mode.
It has been previously reported that the application of an etching step prior to universal
adhesives improved their bonding performance to enamel.[8] It has also concluded that an acid etching for 15 seconds before universal adhesives
increased bracket shear bond strength.[6] In this study, the acid etching was performed for 15 seconds in all experimental
groups. If the etching time prolongs, the frequency of enamel fracture might increase.[5]
[6]
In the present study, all shear bond strengths were lower than 14 MPa. Scotchbond
Universal provided the higher shear bond strength than Transbond XT. The lower shear
bond strength was obtained with Prime&Bond Universal than Transbond XT. Therefore,
the null hypothesis, that there would not be significant differences in shear bond
strength between the universal adhesives and Transbond XT was rejected. The difference
in bond strengths could be due to the functional monomers of the universal adhesives,
as they are different. The performance of adhesives that have self-etching ability
is related to their functional monomer content.[26] The universal adhesives are single-component and one-step adhesives, which involve
functional resin monomers that can promote chemical and micromechanical adhesion to
the dental hard tissues.[8] Scotchbond Universal includes 10-methacryloyloxydecyl dihydrogen phosphate (MDP)
as the acidic functional monomer. 10-MDP is considered the most effective acidic functional
monomer because it etches dentin, ionically bonds to calcium in hydroxyapatite, and
forms stable nanolayered calcium salts.[7]
[26] Scotchbond Universal also involves a polyalkenoic acid co-polymer, which can also
bond chemically to hydroxyapatite.[27] Nevertheless, it has been stated that the polyalkenoic acid co-polymer potentially
contests with the 10-MDP functional monomer for calcium-bonding areas in hydroxyapatite,
and may also inhibit monomer polymerization due to its high molecular weight.[27] In agreement with this study, it has been concluded that Scotchbond Universal showed
higher bond strength than Transbond XT, and this resulted from the 10-MDP monomer.[6] The main functional monomer of Prime&Bond Universal is PENTA. In a previous study,
Prime&Bond Universal presented similar bond strengths when compared with Scotchbond
Universal.[28] But, in this study, Scotchbond Universal provided higher shear bond strength than
Prime&Bond Universal. Furthermore, Scotchbond Universal includes filler particles
which Prime&Bond Universal did not have. The filler particles might contribute to
high bonding performance,[29] additionally high shear bond strength for orthodontic brackets.[6]
[30]
The thin adhesive layer thickness of universal adhesives might cause a decrease in
the bond strength of universal adhesives.[13] The enhanced bond strength of universal adhesives by the double application of universal
adhesives and the application of an extra adhesive layer with these adhesives has
been reported.[13]
[14]
[15]
[16] This improved bond strength has been attributed to the formation of a thicker adhesive
layer by the double application. The thicker adhesive layer has enhanced mechanical
properties, thus reducing polymerization stresses and achieving stress-distribution
during testing.[15]
[18]
[21]
[29] However, it may be more difficult to volatilize the solvent from a thicker adhesive
layer before light curing.[14] The solvent in adhesive formulation must completely be evaporated because the residual
solvent weakens the adhesive interface and reduces the bond strength.[7] This problem could be overcome by light-curing the adhesive layer before the application
of second layer.[14] It has been reported that when the first layer is light-cured, the thickness of
the adhesive layer could be increased by the application of second layer. Nevertheless,
there is still no consensus about what the optimum thickness for the adhesive layer
of each adhesive. Moreover, the double application might enhance the hardness of the
adhesive layer, doing so improve the bond strength.[15] The double application may also create a more uniform adhesive layer by compensating
for probable application defects.[16]
In this study, the double application of Prime&Bond Universal increased the shear
bond strength, but the double application of Scotchbond Universal did not impact the
shear bond strength. Therefore, the null hypothesis that the double application of
universal adhesives would not improve the shear bond strength of orthodontic brackets
was partially rejected. The improved bond strength of Prime&Bond Universal could be
due to the increasing of adhesive layer thickness by double application. Scotchbond
Universal provided adequate bond strength by a single application, and the bond strength
was not affected after double application. It could result from that Scotchbond Universal
contains filler particles. It has been stated that the filler containing adhesives
might form a sufficient thick adhesive layer by a single application, and the double
application could not affect the bond strength of these adhesives.[17]
[29]
The high shear strength values of orthodontic brackets might be associated with high
amounts of adhesive remaining on the enamel surface.[1]
[6] However, in the present study, there was no statistically significant difference
among the ARI values of the groups. The high or low ARI score may depend not only
on the shear bond strength but also on many factors, such as the content of the adhesive,
the base design of the brackets, and the properties of the prepared enamel,[6] therefore the ARI values may not exactly represent bond strength.[6] The tested adhesives revealed more an ARI value of 0, which indicates that there
is no residual adhesive or too little on the tooth surfaces so that the residual adhesive
may easily be removed without sacrificing dental enamel. Furthermore, it can also
be said that the tested adhesives are safe for clinical use because no enamel cracks
or fractures were detected on the tooth surfaces.
Conclusion
Under the limitations of this in vitro study, the higher bond strength was obtained
with Scotchbond Universal than Transbond XT. The double application of Prime&Bond
Universal provided a similar bond strength with Transbond XT. The universal adhesives
might be an alternative for the bonding of orthodontic brackets. The use of universal
adhesives in orthodontic treatment may be beneficial decreasing the number of required
adhesive systems in dental clinics. Nonetheless, further laboratory studies must be
conducted, and clinical studies are necessary to confirm the results.
