Key words:
Ceramics - dental adhesives - shear bond strength
Introduction
Dental ceramics are among the main materials due to their well-recognized esthetic
properties, precision, biocompatibility, and strength.[1]-[3] Despite physical and mechanical properties, the marginal adaption after cementing
is still a concern. Acid-sensitive ceramics, such as glass-ceramics, require etching
with hydrofluoric acid and a silane-coupling agent, followed by an adhesive for bonding
between the resin cement and the ceramics.[4]-[6] Although the light activation of the adhesive before cementation is recommended,[7],[8] the fit of the restoration may be impaired if there is adhesive excess. To avoid
this problem, an alternative is applying the adhesive without light activating it
before applying the resin cement. However, the influence of this nonlight-activation
technique on the bond strength of resin-based cements to ceramics still needs evaluation
as the compatibility between cements and adhesives is important. The nonlight activation
could generate unwanted chemical reactions, depending on type of cement, pH, and hydrophilicity
of adhesives.[9]-[13]
Among adhesive systems, universal (multimode or multipurpose) share a wide versatility
and high performance due to their composition. The addition of silane-coupling agents
could promote adhesion between intrinsically different materials such as resin cements
and ceramics. However, the pH of universal adhesives (around 2.7-3.0) could lead to
lower compatibility with dual-cured cements when used without activators such as arylsulfinate
salts.
Considering differences between dual-cured and light-cured resin cements and the potential
incompatibility between simplified adhesives and dual-cured cements when adhesives
are not light activated before cementation, this study evaluated the influence of
light-activating simplified adhesives on the shear bond strength of resin cements
to a leucite-reinforced glass-ceramic. Null hypotheses were that there would be no
differences between shear bond strengths of (i) two simplified adhesives; (ii) two
resin cements; and (iii) two adhesive light-activation protocols.
Materials and Methods
Thirty-two 1-mm thick slices obtained from leucite-reinforced ceramic blocks (IPS
Empress CAD, Ivoclar Vivadent, Liechtenstein) were embedded in 3/4” Polyvinyl chloride
tubes with self-curing acrylic resin and then polished (Ecomet Buehler, Lake Bluff/IL,
USA) with sandpapers (granulometry #400 to #600) under water cooling.
Slices were etched for 60 s with 5% hydrofluoric acid (Condac Porcelana, FGM, Joinville/SC,
Brazil) and divided into eight groups according to the adhesive (two-step etch-and-rinse
[Single Bond 2, 3M ESPE, St. Paul/MN, EUA] or one-bottle universal adhesive [Single
Bond Universal, 3M ESPE]), resin cement (dual cured [AllCem, FGM] and light cured
[AllCem Veneer, FGM]) and light activation of the adhesive (light activation or nonlight
activation). Light activation was performed with a light-emitting diode (LED) device
(Valo Cordless, Ultradent, South Jordan/UT, USA) with irradiance of 1000 mW/cm2 for the manufacturer’s recommended time.
For Single Bond 2 groups, a silane-coupling agent (Prosil, FGM) was applied for 1
min, followed by 5-s air drying, before adhesive application. Single Bond Universal
was used without silane-coupling agent as follows: application and waiting for 10
s, 10-s solvent evaporation, and 10-s light activation (depending on the group). Materials
are described in [Tables 1].
Table 1:
Materials used in this study
|
Material
|
Brand
|
Composition
|
|
BisGMA: Bisphenol A diglycidyl ether dimethacrylate,
TEGDMA: Triethylene glycol dimethacrylate, BisBMA: Bisphenol
A glycol dimethacrylate, HEMA: Hydroxyethyl methacrylate
|
|
Universal adhesive
|
Scotchbond Universal (3M ESPE)
|
Hydroxyethyl methacrylate, BisGMA, decamethylene dimethacrylate, ethanol, silane-treated
silica, water, 2-propenoic acid, 2-methyl, 10-decanediol, phosphorous oxide, copolymer
of acrylic and itaconic acid, dimethylaminoethyl methacrylate, camphorquinone, dimethylaminobenzoate,
2-6-di-tert-butyl-p-cresol
|
|
Self-etching adhesive
|
Adper Single Bond 2 (3M ESPE)
|
Methacrylic monomers (BisGMA/HEMA), dimethacrylates, ethanol, water, methacrylate
functional copolymer of polyacrylic, polyitaconic acid
|
|
Dual-cured resin cement
|
AllCem (FGM)
|
Base paste: Methacrylic monomers (TEGDMA/BisEMA/BisGMA), camphorquinone, coinitiators,
barium, aluminum-silicate glass microparticles, silicone dioxide nanoparticles, inorganic
pigments Catalyst: Methacrylic monomers, dibenzoil, peroxide and stabilizers, aluminum
silicate glass microparticles
|
|
Light-activated resin cement
|
AllCem Methacrylic monomers
|
Veneer (FGM) camphorquinone, coinitiators, pigments, barium-aluminum-silicate silanized
glass particles and silicone dioxide
|
Resin cement cylinders were fabricated with surgical catheters (inner diameter of
1.40 mm and height of 1 mm). Four cylinders per ceramic substrate were made (16 cylinders
per group). Resin cements were inserted in the catheters and light activated for 20
s with the same LED device. After 10-min waiting, catheters were removed with #12
scalpel blades to expose cement cylinders. Samples were stored in 37°C deionized water
for 24 h and then submitted to the shear bond strength evaluation (Instron, High Wycombe,
United Kingdom) using a 0.2-mm wire loop and 0.5 mm/min crosshead speed. A stereomicroscope
was used to evaluate the type of failure: adhesive, cohesive in cement, cohesive in
ceramic, and mixed. Data were analyzed with three-way ANOVA and Tukey (α = 5%).
Results
[Tables 2] shows means and standard deviations. There were significant differences between
adhesives (P < 0.0001). Higher overall shear bond strength values were observed with Single Bond
Universal. There were no differences between resin cements (P = 0.0763) and light-activation protocols (P = 0.4385). Double and triple interactions were not significant (P > 0.05).
Table 2:
Shear bond strength means and standard deviations
|
Adhesive
|
Cement
|
Light-activation protocol for the adhesive layer
|
Shear bond strength (MPa)
|
|
Single Bond Universal
|
AllCem
|
Yes
|
18.53 (4.6)
|
|
Veneer
|
No
|
18.47 (5.6)
|
|
AllCem
|
Yes
|
19.06 (11.3)
|
|
No
|
22.06 (5.2)
|
|
Single Bond 2
|
AllCem
|
Yes
|
13.47 (3.9)
|
|
Veneer
|
No
|
14.01 (5.4)
|
|
AllCem
|
Yes
|
15.76 (5.34)
|
|
No
|
15.48 (4.17)
|
Discussion
The influence of scanning accuracy of computer-aided design-computer-aided manufacturing
systems, dental preparation, and polishing step on marginal and internal adaptation
of ceramic systems have been investigated.[14]-[17] However, the nonlight activation of adhesives during the cementation is an important
factor that is closely related to clinical procedures and certainly is a reason of
concern among clinicians, especially when universal adhesives are employed. The nonlight
activation of adhesives not only would allow reducing operative steps and easing the
adaption of prosthetic restorations but also could simplify the cementation by reducing
technique sensitivity. For this reason, the present study was designed to address
the bond strength using two different categories of simplified adhesives, including
a universal one, with light-cured and dual-cured resin cements. According to the results,
the only null hypothesis accepted was that there would be no differences between adhesives.
Thus, the chemical incompatibility between the evaluated simplified adhesives and
the dual-cured resin cement was not significant, which differs from the idea that
simplified adhesives are incompatible to dual-cured resin cements.[18],[19]
The present study shows that there was no direct relationship between shear bond strength
and resin cements (i.e., light cured and dual cured). The absence of difference between
cements could be explained by the fact that the light activation was performed directly
over the cement. Thus, it is assumed that most of the reaction happened at the expense
of the light activation for both cements because all specimens were light activated
immediately rather than waiting for chemical polymerization. It is noteworthy that
delaying light activation could impact properties of dual-cured resin cements.[20] Furthermore, in a clinical situation, the ceramics thickness may play a major role
on the light activation. Depending on the type, thickness, and color of the ceramic,
the radiant exposure may not be enough due to attenuation, compromising the polymerization
process.[21]-[24]
In the present study, factors such as thickness and composition of ceramic, light
transmittance through the ceramic material, characteristics of the lightcuring device,
radiant exposure, and concentration of hydrofluoridric acid were not evaluated. For
this reason, results of this study could be different not only if the ceramics were
interposed but also if many other factors were changed.
Regarding the bonding agents, results obtained are in accordance with other studies.[25]-[27] The present study found higher overall shear bond strength for the universal adhesive.
This may be explained by the idea that universal adhesives do not share the particularities
of other simplified etch-and-rinse systems.[26],[28] In the case of simplified self-etching adhesives, due to the poorly polymerized
oxygen-inhibited layer, the residual acidic resin monomers may react with basic catalytic
components such as tertiary amines by bimolecular redox reaction which involves an
electron transfer that prevents the generation of free radicals and compromises the
chemical reaction.[19]
The presence of 10-methacryloyloxydecyl dihydrogen phosphate monomer in Single Bond
Universal could explain the best results due to the fact that hydroxyl groups may
chemically react with the resin cement and the glass-ceramic phase. Moreover, the
incorporation of a silane-coupling agent in this material could improve the interaction
with silica through covalent bonds, avoiding the formation of contaminating layers
that could weaken bond strength.[29]
The shear bond strength method was used instead of a tensile one since it may predict
lateral forces and avoid pretesting failures since it is less technique sensitive
during specimen preparation. The use of wire loop may lead to a better stress distribution
at the bonding area, unlike the severe stress concentration at the load application
area by knife-edge chisels.[30] In addition, the small bonding area of cement cylinders decreases the likelihood
of a critical flaw being present. Regarding the bond durability and aging degradation,
no artificial aging process was performed. For this reason, the results of the present
study could be different if long-term water storage, thermocycling, or fatigue methods
were employed. Thus, further studies testing long-term durability of these combinations
of adhesives and dual-activated cements with nonlight-activated protocol are still
needed.
Conclusion
According to the results of this study, the non-light- activation protocol of simplified
adhesives did not influence the shear bond strength of resin cements to a leucite-reinforced
glass-ceramic. The universal adhesive presented better performance on bond strength
values when compared to the etch-and-rinse simplified adhesive.
Financial support and sponsorship
Nil.
