## 4 Discussion  
The purpose of this study was to investigate the effect of additional silane treatment applied to RBC post-and-core build-up material on the immediate and aged uTBS of RBC CAD/CAM blocks. The results revealed that no benefit was evident from additional silane treatment for OptiBond eXTRa Universal, OptiBond Universal, or OptiBond Solo Plus. Additional silane treatment prior to the application of OptiBond eXTRa Universal reduced the microdensile bond strength in the aged group. Therefore, the null hypothesis could not be rejected regardless of the duration of the immersion in water. NX3 is an RBC luting material and does not contain adhesive monomers or amines. Therefore, it is reported to have relatively less water absorption [34] and less long-term  
Figure 5: Representative scanning electron microscope images of the fractured surfaces after microdensile bond strength (uTBS) in the aged groups: (**A**) OptiBond eXTRa Universal with silane, build-up material side. Fine undulations can be seen; (**B**) OptiBond eXTRa Universal with OptiBond eXTRa Universal Primer (without silane), build-up material side. Fine undulations are evident; (**C**) OptiBond Universal with silane, build-up material side. Obvious scratches (double-sided arrows) are visible; (**D**) OptiBond Universal without silane, build-up material side. Fine undulations are evident; (**E**) OptiBond Solo Plus with silane, build-up material side. Fine undulations are visible, and obvious scratches can be seen (double-sided arrows); (**F**) OptiBond Solo Plus without silane, build-up material side. Fine undulations are evident. color change [35] compared with common adhesive resin cements and self-adhesive resin cements. However, its application requires a combination with an adhesive system [36], and the OptiBond series is the recommended adhesive system. The OptiBond series includes the three-step FL, the two-step self-etching eXTRa Universal, the two-step etch-and-rinse Solo Plus, and the one-step Universal. Unfortunately, OptiBond FL is not distributed in Japan and is difficult to obtain. This study therefore tested and compared the other three products. OptiBond eXTRa Universal, classified as a two-step self-etch adhesive, is the successor to OptiBond XTR. OptiBond XTR has been reported to have high adhesive strength to both enamel and dentin, similar to Clearfil SE Bond (Kuraray Noritake Dental) [37, 38, 39], one of the gold standard dental adhesives [40]. Unlike Clearfil SE Bond, OptiBond XTR contains ethanol as a solvent, which causes insufficient polymerization due to residual ethanol if thorough air-drying is not performed after adhesive application [41]. However, because it does not contain HEMA, it has very low water absorbency [41]. There are few studies on the adhesive properties of its successor, OptiBond eXTRa Universal [42, 43], which probably shows high adhesion to enamel and dentin because it has the same basic composition as OptiBond XTR [42]. Our results showed that OptiBond eXTRa Universal exhibited high uTBS after 1 week of immersion in water, whether pre-treated with Silane Primer or OptiBond eXTRa Universal Primer. The results revealed that the RBC post-and-core build-up material also exhibited high adhesion without silane treatment. OptiBond Universal, classified as a one-step one-bottle self-etch adhesive, is the successor to OptiBond All-in-One. [PERSON] et al. [44] showed that the uTBS of CAD/CAM RBC blocks bonded with NX3 after the application of OptiBond All-in-One to bovine dentin was significantly lower than that of Clearfil Universal Bond Quick and Panavia V5 (both from Kuraray Noritake Dental) or Scotchbonol Universal Adhesive and Rely X Ultimate (both from 3M Oral Care). They speculated that this was due to the shorter and more hydrophilic spacer chains of GPDM, the adhesive monomer in OptiBond All-in-One, compared with MDP, the adhesive monomer in Clearfil Universal Bond Quick [44]. Our results showed that uTBS was higher than 50 MPa in the immediate group regardless of prior pre-treatment with Silane Primer.