Absolute digimatic

The specimens were attached to the arm of a low-speed machine (Isomet, Buehler, Lake Bluff, IL, USA) with diamond saws and sectioned perpendicularly to the long axis under water cooling to obtain nine 1-mm-thick specimens out of each root: three coronal, three medial, and three apical specimens. The thickness of the slices was measured with a digital caliper (Absolute Digimatic, Mitutoyo, Tokyo, Japan). Each slice was separately identified in Eppendorf containers containing 1 mL of deionized water. The tests were performed in a universal testing machine (MTS, Material Test System 810, Systems Corporation, Eden Prairil, MN, USA) at a cross-head speed of 0.5 mm/min in the apical-coronal direction. Each slice was positioned on the base in such a way that the coronal surface of the slice faced the device, and the post was centered over the opening in the jig. The post-segments were loaded with the punch pin (Ø 0.9-1.1 mm) centered on the post-segment, with no contact with the surrounding dentin surface. The force of post dislocation was registered at the moment of displacement of the post fragment from the canal. The maximum failure load was recorded in Kgf and converted into MPa. The bonding surface area was calculated for each slice using the following formula (1):
where Π=3.14, R2=fragment coronal radius, R1=fragment apical radius, and h=slice thickness.

The dimensional change was measured by using the method recommended by ISO 6876/2012. Each material was placed into a cylindrical silicon mold with an internal diameter of 6 mm and a height of 12 mm (n = 5). After setting, we measured the distance between the flat ends (M1) to an accuracy of 10 μm by using a digital caliper (Absolute Digimatic, Mitutoyo, Kawasaki, Japan). The materials were then stored in distilled water at 37 ± 1 °C. After 7, 14, and 21 days, the distance (M₂) was re-measured to an accuracy of 10 μm. The test was carried out three times, and the mean change in length was recorded as the dimensional change (D) using the following formula: D = (M₂ – M₁)/M₁ × 100.

For the biaxial flexural strength test, 4 YSZ discs (Ø = 15 mm, 1 mm thickness) were obtained, polished, submitted to the same grinding protocol, and sintered, as previously described [5 (link),28 (link),30 ]. All specimens were cleaned and distributed among the different groups, followed by the aforementioned surface treatments (Table 2).
The resin composite was applied with incremental technique on the zirconia discs through a polyvinyl siloxane matrix (0.4 mm thickness). After application, a smooth glass plate was pressed against the resin composite, and the specimens were light-cured at 1200 mW/cm² (Radii-cal LED curing light, SDI, Bayswater, Australia) for 60 s. The top surface of the resin composite was subsequently polished with SiC sandpaper #600- and # 1200-grit until a surface free of defects was obtained in the desired final thickness (0.4 mm – resin composite, 1 mm - ceramic; total thickness = 1.4 mm). The thickness was meticulously checked with a digital caliper (Absolute digimatic, Mitutoyo, Kawasaki, Japan), being any specimen with thickness out of the range of 1.4 mm ± 0.02 was replaced by a new one to avoid the effect of thickness factor on the stress concentration during the mechanical test. The specimens were stored in distilled water for at least 24 hours at 37 °C before testing.