## 4 Discussion
However, the values tended to be slightly lower than those of OptiBond eXTRa Universal and OptiBond Solo Plus. OptiBond Universal showed less adhesive failure than the other two products, while scratches that may have been formed by the SiC paper were clearly observed in the SEM images. OptiBond Universal contains water, acetone, and ethanol as solvents. If the volatilization of these solvents is insufficient, the adhesive itself cannot be cured even after light-curing [41]. Furthermore, sufficient air-drying to volatilize the solvents would cause a thin adhesive layer [44], and the greater standard deviation compared with OptiBond eXTRa Universal also suggests that adhesion is unstable. OptiBond Solo Plus, classified as a two-step etch-and-rinse adhesive, has the longest history among the adhesives used in this study. In general, OptiBond FL, a three-step etch-and-rinse adhesive, and Clearfil SE Bond, a two-step self-etch adhesive, are known as gold standard adhesive systems [40]. OptiBond Solo Plus has been reported to exhibit high immediate adhesion to dentin, almost equal to these adhesives [45]. Furthermore, it has been used as an adjunctive adhesive in indirect restorations using Nexus 2 (Kerr), the previous generation of NX3 luting cement [46, 47]. This adhesive shows high bond strength not only to dentin but also to enamel because of the presence of phosphoric acid [46]. In the present study, a very high initial bond strength was obtained without additional treatment with Silane Primer, suggesting that good adhesion to enamel, dentin, and RBC post-and-core build-up material of the abutment teeth can be obtained by treating them in the same manner. In the present study, the aging of I-beam-shaped adhesive specimens by immersion in water for an additional 6 months significantly reduced the uTBS of all groups. It is generally believed that the decrease in bond strength is caused by hydrolysis resulting from inadequate chemical bonding at the adhesive interface. However, one of the main causes of the decrease in bond strength in this study was assumed to be the degradation of the luting cement itself due to aging [48]. NX3 is a dual-cured composite cement, so mixing the two pastes results in a certain degree of curing without light-curing. However, its polymerization has been noted to be limited [49]. A 9 mm thick RBC CAD/CAM block was used in this study, and light-curing was performed for 20 s in each of four directions immediately after luting, which may have resulted in a higher polymerization rate of the luting cement at the edges. However, this light causes reflection and scattering inside the block, so very little light can penetrate [50,51]. The degradation of the luting cement may have been accelerated because all I-beam-shaped microdensile bond specimens were made after 1 week of immersion in water and were then aged again in water. Another possible cause is the deterioration of adhesion between the RBC CAD/CAM blocks and the luting cement. [PERSON] et al. [29] investigated the pre-treatment of NX3 using LAVA Ultimate (3M Oral Care), an RBC CAD/CAM block. They reported that there was no significant difference between the results using Silane Primer and OptiBond XTR Adhesive before bonding to NX3 [29]. [PERSON] et al. [36] also investigated the pre-treatment of NX3 using RBC CAD/CAM blocks. They reported that the use of both Silane Primer and OptiBond XTR after alumina airborne particle abrasion resulted in significantly higher bond strength than Silane Primer alone or OptiBond XTR alone [36]. Based on these previous studies, this study used Silane Primer followed by OptiBond eXTRa Universal after alumina airborne particle abrasion. However, [PERSON] et al. [52] reported that the bond strength of RBC CAD/CAM blocks with silane-containing surface treatment decreased after long-term immersion in water. In fact, the fine undulations observed with SEM in this study were similar to the delamination between RBC CAD/CAM blocks and luting cement reported by [PERSON] et al. [29].