Push-out bond strength
Two brands of glass fiber posts were used: Reblida post (RP; size # Ø 1.5, VOCO, Cuxhaven, Germany) and RelyX post (RX; size # 2, 3M ESPE, St. Paul, MN, USA). Each type of post (n = 40) was divided into four groups (n = 10/group) and exposed to the surface treatment as follows:
Group 1
control (C); no treatment.
Group 2
sandblasting (SB); the specimens were treated with a tribochemical silica-coated (CoJet system; 3M ESPE) with 30 µm aluminum oxide particles at 2.5 bar pressure for 15 s.
Group 3
hydrofluoric acid (HF); the specimens were treated with 9% HF (Ultradent Porcelain Etch, Ultradent Products, South Jordan, UT, USA) for 1 min.
Group 4
TiF4 4 wt/v% (Aldrich Chemical Company, Milwaukee, WI, USA); the specimens were immersed in TiF4 4 wt/v% solution for 4 min [23].
The specimens that were treated with HF and TiF4 4 wt/v% were ultrasonically cleaned in distilled water for 1 min and then air-dried. Each group was further subdivided into two subgroups based on the type of CSCs used as follows:
Subgroup A
BD (Septodont).
Subgroup B
WMTA (PD MTA White; Produits Dentaires SA, Vevey, Switzerland).
A sticky wax was used to position the post perpendicularly on a square plastic plate. Then, the post was surrounded by a cylindrical plastic matrix (10 mm diameter) [22, 24]. The cylinder was filled with the CSCs using a MAP system (MAP One, Produits Dentaires SA, Vevey, Switzerland). The specimens were stored at 37 °C and 100% humidity for 48 h. The specimens were exposed to 10,000 thermocycles in distilled water between 5 and 55 °C with 5-s transfer time and 30-s dwell time [4, 7].
After that, each CSCs-post system was sectioned using a low-speed diamond saw (Isomet 1000, Beuhler Ltd., Lake Bluff, IL, USA) that resulted in 5 discs (n = 50/group). The push-out bond strength of each disc was tested using a universal testing machine (Model TT-B, Instron Co., Canton, MA, USA) and loaded with a cylindrical plunger (1 mm diameter) at 0.05 mm/min cross-head speed. The push-out bond strength (MPa) was calculated by dividing the load at failure (Newtons) by the bonding area (mm2). The bonding area was calculated using the following equation [22]:
A = 2r × π × h
Where r is the post radius, h is the thickness of each post section, and π is the constant 3.14.
The debonded specimens were observed by stereomicroscope (ZEISS, Stemi 2000-C, Oberkochen, Germany) at 50 × for analyzing the failure pattern. Failure mode was classified as adhesive, cohesive or mixed.
A total of 20 specimens of each type of post (n = 5/group) were prepared and grouped as mentioned before. The specimens were sputter-coated with gold (Sputter Coater S 150A; Edwards, Crawley, England). A scanning electron microscope (JSM 5600 Lv JEOL, Tokyo, Japan) was used to characterize the surface topography of the specimens at magnifications of 500×.
Statistical analysis
Data of push-out bond strength were statistically analyzed (SPSS 22.0 software; IBM Corp., Armonk, NY, USA) using three-way ANOVA based on three factors (the type of post, type of treatment, and type of CSCs) and their interactions. Multiple comparisons were conducted by the Tukey’s test. The level of statistical significance was set at P < 0.05. A Weibull analysis (SuperSMITH software; Fulton Findings, Torrance, CA, USA) was performed on the push-out bond strength data.