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rev port estomatol med dent cir maxilofac . 2019;60(3):96-103 101

Several studies indicated a beneficial effect on dentin bond year of water aging, although nano-layering features were
strength with the application of an extra hydrophobic adhesive identified in the dentin interface where 10-MDP and primer
layer over Scotchbond TM Universal or other adhesive sys- were applied, bond strength decreased significantly, and sug-
tems. 17,19,20,30-32 A recent paper highlighted that the overall ef- gested that binding between calcium salts and the dentin sur-
fect of using an extra bonding layer on the immediate and aged face had become weaker. In resin composites, inorganic fillers
dentin bonding strength of the more recently introduced uni- are silanized with methacryloxy silanes to allow their bond to
versal adhesives was dependent on the specific adhesive and the methacrylate resin matrix. In the case of MDP-calcium
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its application mode. The application of an extra hydrophobic salts, the inward-facing of the methacrylate groups of two 10-
resin layer has been suggested to improve the adhesion of uni- MDP molecules may drastically reduce the number of freely
versal adhesives to dentin since it adds unsolvated hydropho- available methacryloxy functionalities for coupling to the res-
bic monomers to the adhesive interface, which decreases the in matrix, and this can explain the deterioration of the chem-
relative concentration of retained solvents and unreacted ical interaction. Furthermore, the presence of polyalkenoic
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monomers in the adhesive layer. Furthermore, the increased acid copolymer in the composition of SBU may compete with
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thickness of this layer and the improved monomer conversion the 10-MDP by binding to the calcium of the hydroxyapatite.
may also contribute to an increase in the dentin microtensile Apart from impairing the bonding of this functional monomer
bond strength of the adhesive interface, due to the formation to dentine, the polyalkenoic acid copolymer may also have
of a more densely packed hybrid layer, which renders it less prevented monomer approximation during polymerization
permeable to water and less prone to degradation effects over due to its high molecular weight, by diminishing the degree of
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time. 17,20,31,34 Ermis et al. measured dentin microtensile bond conversion and the hybridization quality. 28
strength of universal adhesives immediately and after 6 On the other hand, Scotchbond TM Universal contains the
months of water aging, with and without an additional hydro- monofunctional monomer hydroxyethyl methacrylate (HEMA),
phobic layer applied over a separately light-cured or non-light- a low molecular weight water-soluble monomer that behaves
cured universal adhesive. They found that if the universal ad- as a solvent by improving the miscibility and solubility of
hesive was used as a primer and was not light-cured before the monomers with different polarity features. Besides, HEMA can
application of the extra hydrophobic resin layer, no significant also stabilize the collagen fibril network, improving the den-
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increase in bond strength was recorded, thus proving that this tinal permeability, wettability and monomer diffusion. How-
thicker adhesive layer impaired solvent volatilization, negative- ever, the presence of HEMA further increases the hydrophilic
ly affecting bond strength. Therefore, the bond durability of nature of self-etch adhesives as it retains water and diminish-
universal systems would only benefit from the application of es its polymerization efficiency. In fact, HEMA promotes water
an extra hydrophobic layer if the universal adhesive were sep- uptake from the underlying dentin through osmosis, thus in-
arately light-cured. However, this polymerization strategy was ducing the presence of water droplets on the surface of the
employed in the present study and caused no significant im- adhesive layer that behaves as a semi-permeable mem-
provement on bond strength. brane. 39,40 This high hydrophilicity weakens the polymer me-
Several studies focusing on long-term bonding efficiency chanical strength, making it more prone to degradation.
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showed that adhesives suffer from bond degradation upon Furthermore, HEMA has been documented to reduce nano-lay-
artificial and accelerated aging. 6,19,26,33,35 Those findings are in ering of 10-MDP at adhesive interfaces. 42
line with the results of the present study, meaning the second The combination of all the above-mentioned factors may
null hypothesis is rejected. After 4 years of water storage, a explain the loss of bonding efficiency and integrity of adhesive
significant drop in bond-strength values was observed both in dentin interfaces over time, after water aging. Therefore, the
the SBU (26.0±6.50 MPa) and the SBU+HL groups (32.5±5.72 contribution of chemical bonding to the overall bonding dura-
MPa). Most studies evaluate long-term bonding performance bility requires further investigation.
of universal adhesives after at most one year of water storage, The determination of the most favorable application mode
while this study focuses on a 4-year aging period. Considering for universal adhesives remains debatable. Randomized clin-
that specimens were directly exposed to water and accelerat- ical trials evidenced that using an etch-and-rinse or a self-etch
ed hydrolysis was expected, relatively acceptable bond- approach to bond resin composite restorations in non-carious
strength values were still found for both adhesive strategies cervical lesions with universal adhesives did not significantly
in this experimental worst-case scenario. influence their clinical performance regarding marginal adap-
Scotchbond TM Universal contains 10-MDP as its acidic tation, marginal discoloration, post-operative sensitivity and
functional monomer and offers a low demineralization capac- secondary caries. Nevertheless, improved survival rates are
ity. Its interaction with dentin is either micromechanical, linked to the use of the etch-and-rinse or the selective enam-
through shallow but adequate dentin hybridization, 23,27 or el etching procedure. 43,44 A recent clinical trial indicated that
chemical, through the establishment of a strong ionic bond Scotchbond Universal applied with the self-etch mode induced
between 10-MDP and calcium atoms within hydroxyapatite a relatively high level of marginal discoloration. 45
crystals, forming a highly organized layer of stable and low-sol- Further in vitro and, mainly, in vivo experiments should
uble calcium-phosphate salts. 12,13 Nevertheless, recent find- be carried out to understand better the degradation mecha-
ings demonstrate that the hybrid layer stability of resin-dentin nisms of universal adhesives on dentin using differential and
bonds in 10-MDP-based adhesives has been wrongly attribut- complementary aging methods, and to evaluate the long-
ed to the presence of these nanolayered structures with insol- term clinical performance of this newest category of adhesive
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uble MDP-calcium salts. Tian et al. evidenced that, after 1 systems.