Permeability of dental adhesives IntroducIno 430 Adhesive dentistry has increasingly sought simplification of procedures. Conventional multi- step adhesive systems have been replaced by “single-step” systems that are, apparently, sim- pler and faster to use. However, simplifying con- temporary dental adhesives has occurred at the expense of an increasing incorporation of hydro- philic monomers (2-hydroxyethyl methacrylate - HEMA; 10-methacryloyloxydecyl-dihydrogen phosphate - MDP; Biphenyl dimethacrylates - BPDM; Methacryloyloxydecamethlene malonic acid - MAC-10), and organic solvents with variable volatility (acetone, ethanol and water).1 Not unex- pectedly, research has shown that these materials have drawn water from hydrated dentin through an apparently intact, polymerized adhesive layer.2,3 Long-term clinical evaluations have shown that restorations bonded with simplified adhesives exhibit an unsatisfactory performance, especially in terms of marginal discoloration4 and loss of retention.5 Recent studies revealed that simpli- fied dental bonding agents behave as permeable membranes after polymerization,2 allowing water to flow through the adhesive layer.6,7 In fact, once unreacted components of adhesives are leached out due to the water diffusion through polymer chains,8 minute water-filled voids are produced within the polymerized matrix, forming a perme- able adhesive interface. Multi-step dental adhesives are characterized by the application of a separate solvent-free, hy- drophobic resin that is less permeable to water.8 This procedure is thought to improve the connec- tion between the dentin hybridized with hydro- philic monomers and the hydrophobic and viscous resin composite.9-11 However, the bonding agent of multi-step adhesives needs to infiltrate, mix, and co-polymerize sufficiently well with the primed dentin in order to guarantee the mechanical and chemical stability of the bonded interface; other- wise, the primed dentin that is hybridized with hy- drophilic resins would be prone to permeation by fluids originated from hydrated dentin via dentinal tubules. Nanoleakage evaluations have been used to identify permeability zones in bonded interfac- es.7,12,13 This methodology relies on the storage of the resin-bonded specimen in a silver nitrate so- lution, which allows the silver ions to be carried European Journal of Dentistry by water through structural nanoporosities, where these metallic ions may precipitate. Backscat- tering scanning electron microscopy (SEM) and/ or transmission electron microscopy (TEM) al- lows the identification of these nanoporosities by examining the silver precipitate deposited in the structural defects of the bonded interfaces. Silver nitrate, especially in its basic ammoniac form, is considered to be a tracer of hydrophilic and acidic groups. The ammonium groups of the diamine-sil- ver ions are capable of replacing water from active hydrogen-bonding sites, while they may addition- ally react with acidic groups.14 Thus, nanoleakage expression in adhesive layers may predict the per- meability features of adhesives. However, as the bonding procedure varies as a function of many parameters (e.g., the adhesive’s composition, the efficiency of solvent evaporation, the nature and depth of dentin, the amount of water present in the substrate, among others),15-18 it is difficult to verify whether the permeability of resin-dentin bonds is more closely related to the adhesive features or to the bonding technique. Hence, this study evalu- ates silver infiltration in dental adhesives without observing their application on dentin. Considering that adhesive degradation in- creases with its permeability,14,19,20 the aim is to evaluate the permeability of different commercial adhesive systems that are applied on dentin in sin- gle or multiple steps. The hypothesis tested was that there would not be a difference in the perme- ability to the silver nitrate solution among multi- step and simplified adhesives. MAtErIALs And MEtHods Specimen preparation The bonding resins of seven adhesive systems were used in this study. These systems comprise four approaches of bonding to dental hard tissues: one three-step etch-and-rinse system (Adper Scotchbond Multi-Purpose - MP); three two-step etch-and-rinse adhesive systems (Adper Single Bond 2 – SB; Excite – EX; One-Step – OS); one two- step self-etch primer system (Clearfil SE Bond – SE); and two single-step self-etch adhesives (Adper Prompt – AP; One-Up Bond F – OU). In ad- dition, one primer+bond group (SE-PB), consisting of specimens obtained after mixing and polym- erization of the primer and bond components of Clearfil SE Bond systems, was tested. The com-