## 3 Results
### Failure Modes  
The distributions of the failure modes according to immediate and aged groups are summarized in Figure 3.  
For the immediate groups, OptiBond eXTRA Universal groups (Groups 1 and 2) mainly failed at the adhesive interface (63.3% and 60.0%, respectively) rather than exhibiting a mixed failure (36.7% and 40.0%, respectively), and specimens remained intact. OptiBond Solo Plus groups (Groups 5 and 6) mainly failed at the adhesive interface (100% and 96.7%, respectively). In contrast, OptiBond Universal groups (Groups 3 and 4) displayed mainly mixed failure (60.0% and 69.0%, respectively) rather than failure at the adhesive interface (40.0% and 31.0%, respectively).  
For aged groups, all six groups mainly failed at the adhesive interface (96.7%, 90.0%, 75.9%, 82.1%, 83.3%, and 96.6%, respectively) rather than a mixed failure (3.3%, 10.0%, 24.1%, 14.3%, 16.7%, and 3.3%, respectively). Only for one specimen in Group 4 did we observe a cohesive failure in the CAD/CAM RBC block.  
Representative SEM images of the fractured surfaces after \(\mu\)TBS testing of immediate groups are shown in Figure 4. Figure 4A,B are examples of Type A. In Figure 4A, fine undulations are observed on almost the entire fracture surface, while in Figure 4B, scratches formed by SiC paper can be seen on part of the fracture surface. Figure 4C shows the fracture surface on the RBC CAD/CAM block side. Most of the fractured surface has fine undulations similar to those in Figure 4A. However, an obvious scratch is observed in the upper right part of the fracture surface, and a layer of what appears to be adhesive is observed between fine undulations and the scratch. In Figure 4D, scratches are observed on more than half of the fracture surface, and a characteristic image of a broken CAD/CAM block can be seen in the lower right part of the fracture surface.  
Figure 3: Fracture mode distribution in percentage (%) for immediate and aged groups: Type A: complete adhesive failure between the post-and-core RBC and CAD/CAM RBC block, Type C-1: cohesive failure in the post-and-core RBC, Type C-2: cohesive failure in the CAD/CAM RBC block, Type M: mixture of adhesive and cohesive failures.  
Representative SEM images of the fractured surfaces after \(\upmu\)TBS of aged groups are shown in Figure 5. In Figure 5A,B,D,F, fine undulations can be seen on most of the fracture surfaces. In Figure 5C, scratches formed by SiC paper are evident on almost the entire fracture surface. In Figure 5E, scratches can be seen on the upper left and lower right sides of the fracture surface.  
Figure 4: Representative scanning electron microscope images of the fractured surfaces after microcensile bond strength (\(\upmu\)TBS) testing in the immediate 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, CAD/CAM-block side. Obvious scratches (double-sided arrows) and failure within the cement layer (asterisk) are observed; (**D**) OptiBond Universal without silane, build-up material side. Obvious scratches (double-sided arrows) can be seen; (**E**) OptiBond Solo Plus with silane, build-up material side. Fine undulations are evident; (**F**) OptiBond Solo Plus without silane, build-up material side. Fine undulations and failure within the cement layer (asterisk) can be seen.