K etchant gel

Before abutment build-up, 40% phosphoric acid (Ketchant GEL, Kuraray Noritake Dental, Tokyo, Japan) was applied to each FRC post (1.6 mm in diameter, Fiber post, GC, Tokyo, Japan); these were washed with water, dried, then silane coupling agent (CERAMIC PRIMER II, GC) was applied. A bonding agent (Clearfil ® Universal Bond, Kuraray Noritake Dental) was applied to the root canal walls, aired for 10 s with mild air, and then irradiated for 10 s with an LED irradiator. An abutment was built up via direct method using the FRC post and resin (Clearfil ® DC Core Automix ® ONE, Kuraray Noritake Dental). The height of the build-up was 3 mm from the margin. Each specimen was light irradiated for 40 s from 5 directions. The abutment tooth was prepared at an axial inclination of 6° with deep chamfer preparation using a surveyor and diamond bur (101 LRD, Shofu).

The material compositions used in this study are listed in Table 1. KATANA AVENCIA (KATANA AVENCIA BLOCK, Kuraray Noritake Dental, Tokyo, Japan) is a CRB and PANAVIA V5 (PV5, Kuraray Noritake Dental) is a resin cement with its tooth primer. Forty percent phosphoric acid (K-etchant gel, Kuraray Noritake Dental) was used to clean up the CRB surface. Clearfil Ceramic Primer (Ceramic Primer, Kuraray Noritake Dental) was used as a silane coupling agents for the CRB surfaces.

A total of 24 caries-free human teeth stored in distilled water after extraction were prepared as in experiment 1 (n = 8 in each group). After water storage for 24 h, the coronal and apical sides were removed so that microleakage was not disturbed by the resin core at the coronal side or the gutta-percha points at the apical side. The abraded surface was treated with 40% phosphoric acid (K etchant gel, Kuraray Noritake Dental) for 3 min to remove the smear layer [17] . Then, a nail varnish was carefully applied to the root surface to avoid contact with the adhesive interface between the dentin and resin.

Three human non-caries molars were stored in HBSS and used within 6 months after extraction. Mid-coronal dentin surfaces were obtained by removing the occlusal third of the molar crowns using the low-speed diamond saw under water cooling. A standard smear layer was prepared using 600 grit SiC paper (Fig. 1a). The dentin surface was divided into four sections and each surface was treated as follows:
1. No treatment (Cont).
2. Etching: Etch the dentin surface for 15 s using 37% phosphoric acid (K-Etchant gel, Kuraray Noritake Dental), rinse, and air dry. 3. NC: Etch the dentin surface using phosphoric acid, rinse, air dry, and apply 10% NaClO gel (AD gel, Kuraray Noritake Dental) for 60 s. 4. NC+AC: Etch the dentin surface using phosphoric acid, apply NaClO gel and sulfinic acid sodium salt (Accel, Sun Medical) for 15 s, and air dry (Fig. 1g). Specimens were then observed by SEM (Fig. 1h).

The glass fiber posts (Clearfil Fiber Post #4, Kuraray Noritake Dental) were cut 3.0 mm above the finish line of the crown. The glass fiber posts were etched with 40% phosphoric acid gel (K-etchant GEL, Kuraray Noritake Dental) for 5 s and cleansed with distilled water. Subsequently, glass fiber posts were air-dried and coated with a ceramic primer (Clearfil Ceramic Primer Plus, Kuraray Noritake Dental). An automix composite resin was injected into the post spaces of the surface-treated experimental roots, and the posts were inserted into these spaces. The abutment was built and adjusted as described above.

Three types of core build-up systems were used (Fig. 3): composite resin core (RC), composite resin core with fiber posts (FC), and composite resin core with a prefabricated zirconia tube (ZC). Each group comprised eight specimens. The surfaces of the experimental roots were subjected to 40% phosphoric acid gel (K-etchant GEL, Kuraray Noritake Dental) for 5 s, cleaned with water, and dried with air and paper points. Subsequently, the surface of the coronal tooth structure and post spaces were applied with a ceramic primer (Clearfil Ceramic Primer Plus, Kuraray Noritake Dental) and dried with air and paper points. The surface of the coronal tooth structure and the post spaces of the specimens were applied with a bonding agent (Clearfil Universal Bond Quick, Kuraray Noritake Dental) for 10 s. Mild air was applied to the post spaces to spread a thin layer of the bonding agent, and the surplus bonding agent of the post spaces was removed using paper points. The surfaces of the experimental roots were cured using curing light with an intensity of 650 mW/cm 2 for 20 s using a light-curing unit (Blueshot, Shofu, Kyoto, Japan). The mold was fabricated using a transparent high-strength addition-cure-type silicon impression material (KE-1603-A/B, Shin-Etsu Chemical Co., Ltd.) to maintain the shape of the core.