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Introduction

Ideally, in dentin the adhesive associates intimately with surface and creates a layer which can withstand the mechanical and chemical stress imposed upon it in the oral environment. The hybrid layer where collagen and adhesive intermingle is also considered the weak link, prone to degradation over time by host-derived proteinases and hydrolysis . Failure of fillings to perform is often due to failure in this interface .

The wetting ability of the adhesive is essential to the formation of a quality bond . The difference in the free energies of the adhesive and solid surface dictates the manner of interaction. High energy surface is easily wetted, while low energy surface causes the adhesive to bead up on the surface. For optimal wetting solid surface energy should be maximized by surface treatments, which are usually able to expose charged groups or other high energy structures on the surface being treated .

In both self-etch and etch-and-rinse techniques, dentin surface is modified to enhance adhesive performance. The acid-treated dentin surface includes both crystalline apatite and collagen type I. It is a heterogeneous surface with the ability to absorb liquid and change in dimension when treated with different solvents . The etched dentin presents a unique challenge to adhesive to interact with crystalline structures and organic components. Expansion of the collagen network is essential for allowing resin infiltration within and among collagen fibrils and for the formation of a good quality hybrid layer .

Manufacturers have tried to compose their adhesives in such a manner that their wetting ability on dentin surface is maximized. This has led to adhesives that are very hydrophilic . Etched dentin modification with 2-Hydroxyethyl Methacrylate (HEMA) increases wettability . However, increase of hydrophilic monomer concentration leads to a hybrid layer with high water sorption which is prone to hydrolytic degradation .

Dimethyl sulfoxide (DMSO) possesses a high dielectric constant combined with low surface energy and a capacity to solvate polymers and adhesives . DMSO is an aprotic solvent which possesses the polarity needed to break down water’s self-associative tendencies and to form stable complexes with water, especially at 25–35 vol% concentration, resulting with e.g. the lowest freezing point and viscosity . DMSO is an ideal solvent to facilitate radical polymerization reactions such as are used in dental adhesion. Pretreatment of acid-etched dentin with DMSO may improve long-term bond preservation by inhibition of collagenolytic enzymes . It may even increase the immediate bond strength with etch-and-rinse adhesive which may be due to improved penetration of adhesive into the exposed collagen matrix related to the DMSO-induced changes in dentin surface energy. To date, DMSO pretreatment has been shown to have positive effects on dentin bonds strength and its durability with very low (0.004%) and relatively high (50%) concentrations. Using all possible DMSO concentrations with conventional dentin bond testing methods, such as microtensile bond strength, would be extremely laborious and time-consuming. Therefore, we aimed to look at the effect of different DMSO concentrations on demineralized dentin behavior in search for the potential explanations that hopefully would guide the further adhesive bonding studies. The null hypotheses tested were that (1) DMSO has no effect on demineralized dentin surface energy, and that (2) DMSO has no effect on the expansion of demineralized dentin.