# Evaluation of the Bond Strength and Cytotoxicity of Alkasite Restorative Material
## 4 Discussion  
This study was designed to evaluate the resin-dentin \(\upmu\)TBS and cytotoxicity of CN compared to a flowable BF-RBC, given that this type of BF-RBC could provide superior curing compared to BF-RBCs of regular viscosity [11]. The curing of BF-RBCs might have a material-dependent effect on resin-dentin \(\upmu\)TBS [27], and could affect their cytotoxicity [28]. Despite its technique sensitivity, the \(\upmu\)TBS test was applied in this study because it is a versatile and effective method for evaluating the resin-dentin bond strength [29; 30; 31]. Compared to SBS tests, \(\upmu\)TBS data may correlate better with clinical outcomes of RBC restorations [32]. Bonded samples were subjected to thermocycling to simulate intra-oral thermal conditions [33]. To focus on the impact of the restorative material on the resin-dentin bond strength, a single adhesive was used with both CN and the flowable BF-RBC. The \(\upmu\)TBS results showed no statistically significant difference between CN and the BF-RBC. Therefore, the first hypothesis was confirmed. Several all-in-one adhesives reportedly have chemical incompatibilities with dual- or self-cured RBCs [34; 35]. The primary cause could be the residual acidic monomers in universal (self-etch) adhesives of less than pH 3, whereby the polymerization of dual- or self-cured RBCs can be inhibited [36; 37; 38; 35], decreasing the bond strength. For this reason, some manufacturers provide a self-curing activator for their universal (self-etch) adhesives. Despite the positive effects of these activators, their influence might be material dependent [39]. Although the adhesive used in this study was of pH (2.5-3), the bond strength was not affected. This could be attributed to the chemical compatibility between this adhesive and the CN, and to the efficacy of the CN polymerization system. The liquid component of CN contains two Norrish type 1 photo-initiators (i.e., a dibenzoyl germanium derivative and an acyl phosphine oxide), which might be more reactive than camphorquinone, the most widely used photo-initiator in restorative materials [40]. Additional light-curing can accelerate the polymerization kinetics of CN [12] and can improve CN mechanical properties, such as wear resistance [41]. Another factor that might affected the bond strength of CN is its high mechanical properties, which might be similar to those of RBCs of regular viscosity [12]. This can be explained by the filler content of CN, which accounts for 78.4% of the product by weight. The filler content is directly related to the mechanical properties of RBC materials [42]. The most predominant failure modes in both groups were CR and M failures. This can be explained by the high stress levels in the dentin and the RBC materials [43] in addition to the relatively high bond strengths obtained in both groups. Aging by thermocycling can influence the resin-dentin bond strength [44] because of stresses created as a result of contraction and expansion [45]. [PERSON] and [PERSON] recently found that thermal changes in samples can be significantly influenced by the size of the samples and by dwell time during thermocycling [46]. They also found that dwell times of 20 to 30 s, applied in many thermocycling protocols, could be insufficient for large size samples [46] such as the bonded samples in this study. Samples thermocycling using a short dwell time before sectioning likely limits the effect of thermal changes on the bonded interface, and the mechanical properties of the restorative materials. Accordingly, the frequencies of CR and M failures in restorative materials might be explained by the \(\upmu\)TBS achieved, which might not have been negatively affected by the thermocycling protocol applied in this study. Moreover, hand-mixing could be associated with the presence of voids within the restorative materials, which might be a cause of cohesive failure in restorative materials. This could be supported by the presence of voids, detected during our failure mode assessment (Figure 3b).