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Introduction

The new simplified universal adhesives can be used as 2-step etch-and-rinse [ER] or as 1-step self-etch [SE] adhesives. This dual strategy is accomplished by using 1-step SE adhesives associated or not with previous phosphoric acid (PA) etching, which gives dentists a more versatile and attractive adhesive system that can also be used in selective etching mode .

PA etching, the first step in the ER strategy, removes the smear layer and demineralizes a few micrometers of the dentin surface . After the removal of hydroxyapatite from the interfibrillar spaces, resin monomers infiltrate those spaces and impregnate collagen fibrils, leading to the formation of a thick hybrid layer of collagen and resin within the dentin substrate . Dentin hybridization in simplified ER adhesives is a sensitive technique due to the potential for discrepancy between the etching depth and the effective adhesive impregnation of the exposed collagen fibril network . SE adhesives, however, do no require preliminary PA etching, as dentin demineralization and priming may occur simultaneously . The simultaneous etching and infiltration of SE adhesives into dentin does occur for all SE adhesives .

The idea of using the same adhesive solution for either adhesive strategy is not new. In fact, the application of PA with earlier versions of SE adhesives has resulted in debatable results , which may be a consequence of specific adhesive composition and technical application of the same adhesive under ER and SE strategies .

The inclusion of hydrophilic monomers in the composition of simplified adhesives has increased substantially within recent years with the objective of making these adhesives compatible with the inherently wet dentin substrate. The degradation potential of resin–dentin interfaces formed with hydrophilic simplified adhesives has been shown to enhance due to residual solvents and water entrapped in the polymer. In fact, water droplets are formed in 1-step SE adhesives as a result of water absorption from dentin through osmosis . Water entrapment may hinder the formation of cross-linked polymers. This may occur via water-filled, nanometer-sized voids formed as a result of a decrease in the degree of conversion of the adhesive. A low degree of conversion affects the stability of the resin–dentin interface due to the elution on unreacted monomers and consequent formation of a porous hybrid layer .

One-step SE adhesives result in a thinner adhesive layer, being more susceptible to polymerization inhibition by oxygen . The suboptimal polymerization is a drawback related to the use of universal adhesives as 1-step SE adhesives. A clinical alternative is the application of an additional layer of a hydrophobic resin coating (HC) over the polymerized simplified adhesive . This extra resin coat aims at increasing the thickness and uniformity of the adhesive layer, as well as reducing the fluid flow across the adhesive interface . Excellent in vitro and clinical results have been reported after placement of a HC over 1-step SE adhesives .

Additionally, promising results in terms of bond strength, nanoleakage, and degree of conversion inside the hybrid layer, have been recently reported for universal adhesives . However, to the extent of our knowledge, studies on the longevity of this technique have not been carried out.

The aim of this study was to evaluate the immediate (24h) and 6-month (6m) microtensile bond strength (μTBS) and nanoleakage (NL) of universal adhesive systems used in the ER and SE approaches with or without an additional HC. The null hypotheses tested were that the use of an additional HC would not improve: (1) the immediate and 6-month bond strengths of universal adhesives used as ER or SE and; (2) the immediate and 6-month nanoleakage of resin–dentin interfaces formed with universal adhesives used as ER or SE.