Eighty-eight freshly extracted bovine incisors with anatomically similar root segments were used. Sixty-five teeth were prepared for the push-out bond strength (POBS) test and scanning electron microscopy (SEM) while the remaining 23 teeth were prepared for surface energy and components analyses.
The solutions used in the present study were prepared by the same manufacturer and used after preparation.
Sixty-five teeth were selected. Each tooth was decoronated below the cementoenamel junction perpendicularly to the longitudinal axis with a rotary diamond disk at low speed. The apical segment was also removed. Roots from the cervical and middle thirds were cut to a uniform length of 7 mm.
Pulp tissue was removed, and the root canals were enlarged and standardized by using a 3-mm cylindrical drill attached to a bench drill. The roots were randomly divided into five groups (n=13).
In the control group, the roots were rinsed with 10 mL of distilled water (DW) and dried with paper points.
In the NaOCl group, the roots were immersed in 5.25% NaOCl for 20 minutes to simulate the contact time of irrigant during chemomechanical preparation . The roots were then rinsed and dried as described for the control group.
In the plasma group, after the treatment described for the NaOCl group, a non-thermal atmospheric pressure plasma jet was used. Premixed helium and oxygen gas (98% He and 2% O2) were employed. The gas pressure was 6 bar and a voltage of approximately 1000 V was applied to generate plasma. During the treatment, the distance between the tip of the plasma jet and the sample was 5 mm. The teeth were exposed to the plasma for 1 minute. After the treatments, the root canals were dried with paper points
In the thiosulfate and ascorbate groups, after immersion in NaOCl for 20 minutes, the samples were rinsed with 10 mL of DW and immersed in 5% sodium thiosulfate or 10% sodium ascorbate for 1 minute, respectively. Finally, the samples were rinsed with 10 mL of DW and dried with paper points.
Fiberglass post cementation
Post cementation was performed in 10 samples from each group.
For post treatment, the fiberglass posts were submitted to an ultrasonic bath in 70% alcohol for 10 minutes so that any superficial contaminant could be removed . A thin layer of silane was then applied homogeneously on the fiber post surface using a microapplicator. After 1 minute, the surface was dried with air jet.
Since a self-adhesive resin cement was used, dentin treatment was not necessary. The cement was manipulated and placed into the canal using 20-gauge Accudose Needle Tubes. The posts were placed in position immediately after cement application. Light polymerization was performed for 40 seconds with an irradiance of 500 mW/cm2.
Push-out test and failure mode
Each root was horizontally sectioned with a slow-speed, water-cooled diamond saw to produce approximately 1-mm thick slices. The first (coronal) and last (apical) slices were discarded. Three slices from each root canal were evaluated.
Bond strength was analyzed through the push-out test by using a universal testing machine at a crosshead speed of 0.5 mm/min and a cylindrical post measuring 1.2 mm in diameter and 20 mm in length with a 200-kgf load cell.
To express the bond strength in Megapascals (MPa), the load value recorded in Newtons (N) was divided by the area (mm2) of the bonded interface. It was calculated as follows : A= 2prh, where p is equal to 3.14, r is 1.5 mm (the canal was standardized to a diameter of 3 mm) and h is the thickness of the slice in millimeters. The thickness of each slice was measured using a digital caliper.
All fractured specimens were observed under a stereoscope at 10X magnification to determine the failure mode. The failure mode was classified into four types : (I) Adhesive between the cement and dentin: fracture occurs predominantly at the cement/dentin interface; (II) Mixed cohesive dentin: some fracture occurs within the dentin; (III) Mixed cohesive post: fracture occurs at the post; (IV) Mixed cohesive post/dentin: fracture occurs at the post and dentin.
In each group, three samples were analyzed qualitatively by scanning electron microscopy to verify the effect of antioxidant agents and non-thermal plasma on the dentin surface.
Two longitudinal grooves were prepared on both buccal and lingual surfaces by using a diamond disk without penetrating the canal. The roots were then split into two halves with a hammer and chisel.
The samples were coated with gold and analyzed under a scanning electron microscope. The most central area of the canal was selected and three predetermined areas (1 mm from the cervical side, center of the sample, and 1 mm from the apical side) were selected. Images were recorded at 500X magnification.
Surface tension analysis
The surface tension of the irrigants was measured using the Du Noüy ring method. The TensioCad tensiometer was used at a constant room temperature of 23 °C. The principle of the instrument depends on the fact that the force necessary to detach a platinum-iridium ring immersed at the surface is proportional to the surface tension . The ring dimensions were: mean diameter: 19.09 mm and toric section diameter: 0.37 mm.
Before measurements, distilled water was used for zero calibrations. 50 mL of each tested solution was placed into a 3-cm glass dish and the dish was elevated by a precision screw until contact with the platinum ring was established. Torsion was then applied to the screw until the platinum ring became detached and readings on the scale were recorded in dyne/cm. Ten readings were made for each irrigant.
Surface free energy analysis
Another 23 bovine teeth were selected. The standardized roots (7 mm), obtained as described later, were cleaved towards the long axis of the tooth, resulting in two segments, totaling 46 samples. The samples were planned with the help of a sandpaper (100 grit) under water cooling in a sanding machine to create a standard smear layer. Forty-five dentin surfaces were analyzed.
The contact angle analysis was done with a Ramé-Hart goniometer (Ramé-Hart Instrument). The contact angle between the dentin surface and the solutions (water (polar), ethylene glycol (polar), and diiodomethane (apolar) was measured .
Nine samples from each group, three in contact with each solution, were evaluated. Sixty measurements were made for each drop of the above-mentioned solutions. Based on the data collected, the Ramé-Hart software measured the surface free energy, as well as the polar and dispersed components of the different groups.
Both Kolmogorov-Smirnov and Levene’s tests were used to evaluate normality and heteroscedasticity in the data. The bond strength data were then analyzed using ANOVA followed by Tukey’s post hoc test for comparisons (α = 0.05). Surface free energy and components were analyzed using the nonparametric tests, Kruskal-Wallis, and Mann-Whitney (α = 0.05).