2.2.4. Post-and-Core Fabrication

Eighty custom post-and-core patterns were fabricated with the use of auto-polymerizing acrylic resin (Pattern Resin LS; GC America, Alsip, IL, USA) and serrated plastic posts. Each serrated plastic post was relined with acrylic resin and then inserted into the root canal until the canal space was bound to the walls of the prepared teeth. Then, the core was built up using a GC pattern and prepared using a diamond bur (ISO No. 010; Brasseler USA Dental, Georgia) to achieve a core with a 4 mm height (Figure 3). A hole was made in the coronal part of the GC pattern core and was used to attach the post and core to an Instron machine (Instron; Model 5585H; Instron Corp., Norwood, MA, USA) during the pull-out test.

**Figure 3.** Pattern resin post-and-core build-up.

Specimens with a post-and-core pattern were randomly divided into 4 groups. Twenty acrylic-resin pattern posts were cast with a non-precious alloy using the conventional lost-wax casting technique. Sixty acrylic-resin pattern posts were scanned with a desktop scanner (3Shape D900L; 3Shape Dental System, Copenhagen, Denmark). The STL files for each scanned post and core were sent to Core 3D and Renovis for the fabrication of 20 milled titanium (Grade 5) (Ti6AL4V), 20 printed titanium (Grade 5) (Ti6AL4V), and 20 milled zirconia post and cores (Figure 4). The prepared specimens were stored with 100% humidity at room temperature to simulate the in vivo humidity until they were returned for testing [39] (Figure 5).

**Figure 4.** Sample of STL file of the proposed design of the specimen.

**Figure 5.** (**A**) Custom-cast non-precious post and core. (**B**) Custom-milled titanium post and core. (**C**) Custom-printed titanium post and core. (**D**) Custom-milled zirconia post and core.

## 2.2.5. Surface Treatment of Post and Core

According to the manufacturer's recommendation, Group 1 (CCNPPCs), Group 2 (CMTPCs), and Group 3 (CPTPCs) post and cores were first treated using ultrasonic cleaning solution in 96% isopropyl (3 min) and were then airborne-particle abraded (50 μm Al2O3 at 2.8 bar for 5 s). Group 4 (CMZPCs) post and cores were treated using ultrasonic cleaning in 96% isopropyl (3 min) and Rocatec soft (30 μm airborne-particle abrasion at 2.8 bar for 12 s) over the entire zirconia surface; finally, silane coupling agent (Espe-Sil; 3M ESPE; Seefeld, Germany) was applied [40].
