# Evaluation of the Bond Strength and Cytotoxicity of Alkasite Restorative Material
## 1 Introduction  
Resin-based composite (RBC) materials can be used efficiently to restore large-size posterior cavities [1; 2]. Conventional RBC cannot be adequately light-cured in more than 2 mm increments [3]. Bulk-fill RBC (BF-RBC) materials allow the placement of posterior restorations in thick increments, making the restorative procedure less technique-sensitive [4]. Light-cured BF-RBC materials [5; 6] and restorations [7] have been extensively evaluated in-vitro, showing mechanical properties comparable to those of conventional RBCs [8]. Whether BF-RBCs or conventional RBCs are used, the marginal integrity in-vitro is similar in posterior restorations [7; 8]. Additionally, clinical studies haveproven that, over follow-up periods of one to six years, the clinical performance is also similar [9]. Using regular-viscosity BF-RBCs requires shorter placement times in restorations [10]. Compared to conventional RBCs, the primary advantage of BF-RBCs is their higher depth of cure, which allows clinicians to apply them in thicker increments. However, a recent systematic review showed that adequate light-curing at a 4-mm depth might not be achievable with all BF-RBCs [11]. Moreover, to achieve an adequate depth of cure at 4 mm, BF-RBCs should be light-cured at a high irradiance (\(\geq\)1000 mW/cm\({}^{2}\)) [5]. Thus, some clinicians doubt whether the depth of cure is sufficient [12]. Inadequ polymerization of RBCs would not only impair their mechanical properties, but could also reduce the degree of conversion and increase the residual monomer content [13], resulting in more adverse cytotoxic effects [14; 15]. Such cytotoxicity of BF-RBCs can be material dependent [16], and their chemical composition, thickness, and amount of ions released significantly influence their cytotoxicity [6].  
Cention N (CN; Ivoclar Vivadent, Schaan, Liechtenstein), advertised as an alkasite (a RBC with alkaline fillers), is a powder/liquid, self-cured BF-RBC, with a light-curing option. This material has an enhanced polymerization system and provides additional bioactive or ion releasing properties [12]. Its mechanical and micromechanical properties and wear behaviors support its use in posterior restorations [12; 17]. Compared to conventional RBCs, CN shows greater monomer conversion [18], and it can decrease dentin demineralization as a result of its bioactivity [19].  
Effective in-vitro evaluations of new restorative materials, including their biological and mechanical aspects, are required before clinical trials can be conducted [20]. The biological assessment of new dental materials is essential to predict their clinical performance [21]. In addition, bond strength testing could be a helpful tool for evaluating experimental variables [22].  
Previous studies have compared the resin-dentin shear bond strength (SBS) and micro-tensile bond strength (\(\upmu\)TBS) between CN and a conventional RBC [23] as well as between a self-adhesive RBC and a resin-modified glass ionomer cement (RMGIC) [24]. CN showed higher immediate SBS than conventional RBC [23] and superior \(\upmu\)TBS to self-adhesive RBC and resin-modified glass ionomer restorative [24]. However, the bond strength of CN has not been evaluated in comparison to light-cured BF-RBCs. Moreover, the cytotoxicity of CN has not yet been evaluated. Both the bond strength and the cytotoxicity of CN may affect the clinical performance of CN restorations. Therefore, the aim of this in-vitro study was to assess the resin-dentin \(\upmu\)TBS and cytotoxicity of CN in comparison it to those of a light-cured flowable BF-RBC. It was hypothesized that (1) the resin-dentin \(\upmu\)TBS of CN and of a flowable BF-RBC would not differ significantly, and (2) the cell viability for both materials would not differ significantly at thicknesses of 2 and 4 mm.