This study was approved by the Local Research and Ethics Committee of the Federal University of Pelotas (150/2010). Single rooted caries-free human teeth were obtained and examined. Roots with open apices and resorptive defects were excluded and seventy-five teeth were randomly divided into five groups (n = 15), according to the endodontic substance used.
Teeth were mechanically cleaned with a curette to remove soft tissue remnants from the root surfaces. The teeth were decoronated at the cementoenamel junction (CEJ) with a double-sided diamond disk (Isomet, Buehler, Lake Bluff, IL, USA) in low-speed rotary instrument under constant water-cooling, in order to leave 15 mm of root length. A size 10 K-file (Dentsply Maillefer, Ballaigues, Switzerland) was used to create an apical stop with a step-back preparation. Working length was determined by placing the instrument into the canal until visible at the apical foramen and subtracting 1 mm from this length. The cervical portion of the root was prepared with Gattes-Gliden drills up to 11 mm. Next, the root canals were instrumented with K-flex endodontic files (Maillefer/Dentsply) in sequence up to K = 50 associated to the previously determined substance for each group, as follows: 0.9% NaCl (control), 2% CHX gel, 2% CHX gel with final irrigation with 17% EDTA, 5.25% NaOCl and 5.25% NaOCl with final irrigation with 17% EDTA. The working time was standardized for each file (1 min), as well as the volume of the substance used (30 mL of 0.9% NaCl, 5.25% NaOCl and 17% EDTA; 3 mL of 2% CHX gel followed by 30 mL of 0.9% NaCl).
When the endodontic preparation was complete, the root canals were prepared for fiber post cementation (#0.5, White Post DC, FGM, Brazil) using the drill of the post system. The canals were thoroughly washed with saline solution and kept moist until use. Absorbent paper points were used to remove the excess of saline solution from the canals. Finally, the fiber post was previously prepared by cleaning with 70% ethanol, air-dried and silanized for 1 min and gently air dried for 5 s (Silano, Angelus, Brazil). The self-adhesive cement was prepared according to manufacturer’s instructions and the resin cement was inserted into the root canal (RelyX Unicem, 3 M ESPE, St Paul, MN, USA). The fiber post was immediately placed to seat, excess of the cement was removed with a microbrush and the luting composite was light-cured for 60 s (Radii-Cal, SDI, Australia).
Specimens were then stored at 100% humidity and 37°C for 24 h to allow complete polymerization. The specimens were fixed on acrylic plates and then sectioned transversally into 1.5 mm slices containing cross sections of the fiber postsunder water-cooling with a precision machine (Buehler Isomet, USA). The cervical and apical diameter of the canal and the thickness of all of the slices were measured with a digital caliper (Mitutoyo Digimatic Caliper, France).
Push-out strength test
Each section was marked on its apical side and positioned on a base, with a central hole, in a universal testing machine (DL2000, EMIC, São José dos Pinhais, PR, Brazil). The push-out test was performed by applying a compressive load to the apical side of each slice by using a 0.7-mm-diameter cylindrical plunger attached to theupper portion of the testing machine. A crosshead speed of 0.5 mm/min was applied until bond failure occurred. The load upon failure was recorded in Newton (N). The force required for dislocation of the post (bond strength) was calculated as follows: F = R/A, where F = force of displacement of the post (N), and A = bond area (mm2). Calculation of the bond area: A = π.g. (R1 + R2); where π = 3.14; g = taper of the root [g = (h2 + (R2-R1) 2) 1 / 2]; R1 represents root opening in the apex and R2 represents the root opening in the cervical. R1 and R2 were measured in Image-J (Wayne Rasband, National Institutes of Health, Bethesda, MA, USA). Thickness of the slices (h) was measured with a digital caliper. The bond strength values of the tooth slices of each group were averaged for statistical analysis.
The data were analyzed using a statistical software package (R, Version 0.98.490 – © 2009–2013 RStudio, Inc. USA). For comparison among chemical substances, one-way ANOVA was performed followed by a multiple comparison Tukey post hoc test. Statistical significance was considered as α = 5%.