Curing efficiency of four self-etching, self-adhesive resin cements

Abstract

Objectives

To evaluate the degree of cure (%DC) of four self-etching, self-adhesive resin cements, and one conventional resin cement, in their self- and dual-curing mode.

Methods

The self-etching, self-adhesive resin cements studied were RelyX™ Unicem (3M™ ESPE™ AG), Maxcem™ (Kerr Corporation), Biscem™ (Bisco, Inc.) and Multilink^® Sprint (Ivoclar Vivadent^® AG) and the classic resin cement was Multilink^® Automix (Ivoclar Vivadent^® AG). Twelve specimens of each material (1.8 mm × 4 mm × 4 mm) were prepared in room temperature (23 ± 1) °C following the manufacturers’ instructions. Six of them were treated as dual-cured, thus irradiated for 20 s with a halogen light curing unit and left undisturbed for 5 min. The other six were treated as self-cured and were not irradiated, but left in dark and dry conditions for 10 min. The assessment of the %DC was made using micro-ATR FTIR spectrometry.

Results

The %DC in their self-curing mode was very low (10.82–24.93%), with Multilink Sprint exhibiting the highest values among the five. In the dual-curing mode the values obtained were also low (26.40–41.52%), with the exception of Multilink Automix (61.36%). Maxcem was found to have the lowest DC.

Significance

The low %DC found raises questions as to whether these materials can be successfully used in clinical applications, where light attenuation takes place. Increased irradiation times could potentially lead to higher %DC, in applications where light is not completely blocked by the overlying restoration.

Introduction

Resin cements have an increasing application in the cementation of fixed prostheses [1], since they exhibit enhanced mechanical, physical and adhesive properties, compared to conventional luting agents [2]. Further, they provide adequate stability [3], [4] and increased fracture resistance of overlying all-ceramic restorations [5], [6], together with an optimal esthetic result. However, the fact that they are technique-sensitive materials [7] complicates clinical procedures and makes the cementation time-consuming and susceptible to manipulation errors.

In an attempt to simplify procedures, a new group of resin cements, the self-etching, self-adhesive resin cements, have been introduced. According to their manufacturers, these products are self-adhesive, including acidic and hydrophilic monomers in their composition, which simultaneously demineralize and infiltrate enamel and dentin, resulting in strong bonding. Therefore, they require no conditioning or priming pretreatments of tooth substrate [8], [9], [10], [11]. Furthermore, in the case of one of those materials (RelyX™ Unicem), the phosphate groups of the functionalized monomers included in the monomer mixture are claimed to react with the hydroxyapatite of the tooth substrate, resulting in additional retention through chemical bonding. Indeed, a chemical interaction between the cement and the Ca⁺ of hydroxyapatite has been reported [12]. Moreover, these acidic monomers are claimed to interact chemically with the basic inorganic fillers of the material, leading to an additional acid–base setting reaction, apart from the free radical polymerization of the material [10]. Information for this type of chemical reactions is not provided for the other materials of this group, which are currently available on the market.

The literature regarding self-etching, self-adhesive cements, concerns mainly RelyX™ Unicem, since it is the first material of this group which has been launched on the market. The majority of the studies conducted, concern the bonding effectiveness of RelyX™ Unicem, which has been found to have low demineralization capacity, since it seems to interact only superficially with dentin, failing the formation of a hybrid layer or resin tags [13], [14], [15]. Nevertheless, its bond strength to dentin has been found to be comparable to other widely used resin systems [13], [16], [17], [18]. When bonding to enamel [13], [17], [18], [19], [20], [21] and to root dentin [22], [23], [24], [25], however, RelyX™ Unicem has been found to be less effective. Maxcem™, another material of this group, has been found to have a relatively poor bonding ability, irrespective of the tooth substrate [26], [27]. No literature is available regarding other materials of this group.

The literature concerning other properties of self-etching, self-adhesive resin cements, such as the curing efficiency of these materials [28], [29], is limited. It has been reported that low degrees of conversion (%DC) result in inferior clinical performance [30], in terms of ultimate hardness [31], fracture toughness [32], wear resistance [33], elastic modulus [34], solubility and hydrolytic degradation [35], [36], as well as biocompatibility [37], [38].

Therefore, the purpose of this study was to evaluate the %DC of four self-etching, self-adhesive resin cements, and one conventional resin cement, in their self- and dual-curing mode. The testing hypothesis was that there are no significant differences in the %DC, among the materials tested.