With the rise of the conservative dentistry concept, new types of restorations and bioactive materials are being developed every day to be able to preserve dental tissues on one hand, and to combat caries and enhance remineralization on the other [1], [2], [3].

Indeed, the contemporary therapeutic gradient aims at preserving dental tissues as much as possible in order to increase their lifespan and facilitate the capacity for re-intervention [4]. Currently, composite resins are the most aesthetic restorative materials used in direct techniques [5]. It has been shown however that some of these materials allowed plaque formation and bacterial proliferation, and thus would result in the recurrent caries phenomenon leading to eventual failure of the restoration [6].

In this context, a group of new materials were developed with active ion release properties and remineralization ability, these materials have been considered as potential replacements for conventional restorative materials [7]. These new restoratives also offer the advantage of being bulk filled, and thus avoid the challenges and inconvenience of incremental filling including interlayer voids, time-consuming application and contamination [8].

Several studies have reviewed the benefits associated with pH rise and ion release properties on remineralization and the prevention of dental caries [9], [10]. The type of ions released and their rate of release were considered more important for the material antibacterial properties than the pH change [7]. The pattern of discharge and the rate of fluoride release after the initial burst were similarly investigated to assess the prolonged bioactive effect of these materials inside the oral cavity [11], [12]. They were estimated to inhibit dentin and enamel demineralization and decrease the potential for secondary caries [8].

Some materials have a double setting reaction that seems to negatively affect their mechanical resistance properties [13], [14]. The extent to which these properties can be affected depends on several factors including the basic composition of the material, the type of setting reaction, the percentage of filler and the type of bond between the filler particles and the matrix. In addition to the specific microstructure of each material [13], [14].

Restorative materials intended to replace tooth structure should have comparable mechanical properties to the tissue being replaced [8]. Thus, not all laboratory tests are considered relevant or correlate well with the clinical outcome, yet some of these show an obvious correlation with material longevity in the oral cavity [5], [15]. More precisely, a positive correlation was found between fracture toughness and clinical fractures of posterior resin composite based restorations, which was also moderately correlated to flexural strength, especially after ageing [16]. Other studies have shown correlations between fracture toughness and marginal wear of composites [17], [18].

The adhesive joint or the interface with dental tissues represent another point of weakness for most restorations. Insufficient adhesion to tooth structure or leakage at the interface could lead to discoloration percolation, bacterial infiltration and consequently failure of the restoration [19]. The self-adhesive restorative materials avoid the problems related to meticulous and time-consuming adhesive application protocols. Cieplik et al. clinically investigated a self-adhesive composite (SABF; 3 M) over a period of 3 years and found acceptable results [20]. Microstructural differences, however, can play an important role, therefore, each material has to be studied independently. Moreover, several studies investigated the bond strength and microleakage of self-adhesive composites and found these to be inferior to results obtained by conventional composite adhesive bonding [21], [22], [23].

Cention® Forte (CF, Ivoclar-Vivadent AG, Schaan, Liechtenstein, launched in 2016) and Surefil one™ (SO, Dentsply-Sirona, Konstanz, Germany, launched in 2019), are new materials that have been designated as viable replacements to conventional restoratives, and as bioactive composites. These materials are intended to combine the mechanical properties of composites with the characteristic bioactivity and ionic release properties of glass ionomer cements to overcome the disadvantages of both families.

A variety of studies evaluated their physical, biological and mechanical properties. Iftikhar et al. compared several restorative materials and found the compressive strength of CF to be close to that of composite and significantly higher than that of glass ionomer cements [24]. Lohbauer and Belli examined the mechanical properties of SO as compared to a composite and glass ionomer cements, they considered its properties to be sufficient for restoring posterior teeth [25].

The light cured versions of these materials were shown to outperform the self-cured materials in numerous respects [26], [27], [28]. Nevertheless, the presence of light polymerizing resin in these materials makes us assume that some volumetric shrinkage takes place upon setting, it is thus important to investigate the polymerization kinetics of these materials, and assess the resulting polymerization shrinkage or shrinkage stress. These would eventually affect the interface of the material with the tooth structure and could have a negative effect on the adhesive joint.

Garoushi et al. studied fluoride-releasing restorative materials, and found that the ion release phenomenon was accompanied by filler leakage and microcrack formation around filler particles [12]. Therefore, the assessment of the resistance of these materials to degradation and their mass loss over time should not be neglected. Several methods have been proposed, for example, Nascimento et al. used thermogravimetric analysis (TGA) to evaluate degradation and thermal stability considering the extrapolated onset temperature, mass loss, and the weight percent loading of particles [29].

The aim of this study was, therefore, to study the mechanical properties of these materials, their thermal stability, their leakage, as well as their polymerization shrinkage stress. Moreover, remineralization potential was assessed by quantifying the Fluoride ion release profile and the pH change over time.

The suggested null hypotheses were: (i) that mechanical properties would not differ according to the material type, (ii) that there would be no difference in ion release profile between tested materials, and (iii) that there would be no differences in the degradation and thermal stability between tested materials.