*2.2. Specimen Preparation*

Eighty sound human maxillary central incisors, extracted for periodontal reasons, were selected for this study. The exclusion criteria included the presence of caries, restoration, root canal treatment (RCT), crack/s, attrition, very long or very short teeth, and/or severe root curve. Teeth were thoroughly cleaned with a brush after extraction, and a scalpel and a periodontal curette were used to remove any remaining hard and soft tissues from the root surfaces. Teeth were subsequently stored in 0.2% sodium azide (Merck KGaA, Frankfurter Str. 250, Darmstadt, Germany) prior to specimen preparation (ISO 28399;

2011) [28]. Each tooth was randomly assigned a number (from 1 to 80) and allocated into one of 4 groups (*n* = 20 each). Group 1 teeth were restored with custom-cast non-precious post and cores (CCNPPCs); Group 2 teeth were restored with custom-milled titanium post and cores (CMTPCs); Group 3 teeth were restored with custom-printed titanium post and cores (CPTPCs), and Group 4 teeth were restored with custom-milled zirconia post and cores (CMZPCs).


**Table 1.** Commercial names and details of materials used in the study.

2.2.1. Mounting Teeth in Acrylic-Resin Blocks

Teeth were individually mounted in a special specimen holder using epoxy resin (Exakto-form; Bredent, Derbyshire, UK) (with an elastic modulus of 12 GPa, which is similar to the elastic modulus of human bone (18 GPa)) [29] with the use of a test mount former of 2 cm3 in dimension 2–3 mm below the cementoenamel junction (CEJ). A prefabricated jig was used to position each tooth in the test mount former during the immersion of the tooth in acrylic resin to standardize the tooth position to be centralized within the test mounts. The test mount jig was used to standardize the tooth position while performing tooth preparation and RCT (Figure 1). To avoid the dehydration of natural teeth due to the heat generated during the polymerization of acrylic resin, the resin block was cooled in water [30]. The teeth were prepared to have a 2 mm ferrule and a 1 mm shoulder finish line (Figure 2) [31–33].

**Figure 1.** Teeth were mounted in acrylic-resin blocks using a test mount former.

**Figure 2.** Preparation of tooth to have a 2 mm ferrule and a 1 mm shoulder finish line. (**A**) Occlusal view. (**B**) Lateral view.

## 2.2.2. Root-Canal Preparation and Obturation

Access-cavity preparation was performed using a size 2 diamond round bur on a high-speed handpiece with copious water. The working length was established as 1 mm shorter than the root apex. Root-canal preparation for each tooth was performed up to size 40/0.06 with a Vortex Blue rotary file (Dentsply Sirona, Tulsa, OK, USA) in a crowndown fashion. Canals were irrigated with NaOCl 5.25% and EDTA 17% (chelating agent) (Ultradent Products Inc., South Jordan, UT, USA) to remove organic and inorganic debris and smear layers [34]. Upon completion of the cleaning and shaping and before obturation, the prepared root canals were dried with sterile paper points (Sure-endo, Gyeonngi-do, Republic of Korea). A matching size 40/0.06 gutta-percha master cone (Dentsply Sirona, Tulsa, OK, USA) coated with AH Plus sealer (Dentsply Sirona GmbH, De-Trey-Straße, Konstanz, Germany) was used for obturation [34]. Root canals were obturated using the warm vertical compaction technique with System-B and Obtura (Kerr/Sybron Endo Corp., Brea, CA, USA).

## 2.2.3. Post-Space Preparation

To obtain a standardized length for the posts, the coronal portion of gutta-percha was removed with System-B (Kerr corporation, Brea, CA, USA) until an adequate length (11 mm) was achieved [35–38]. Definitive post length and width were prepared and established with the use of Peeso reamers (Maillefer S.A., Ballaigues, Switzerland) up to size 3. Each canal was then cleaned using an air/water spray and EDTA to remove debris and then dried with paper points.
