Sixty extracted, erupted, noncarious, nonrestored, uncracked human lower third molars were gathered by the Non-Invasive Clinical Research Ethics Committee of Cukurova University Faculty of Medicine, dated 13.03.2020, numbered 42. Informed consent forms were signed by all patients. The procedures used in this study adhere to the tenets of the Declaration of Helsinki. Furthermore, teeth of nearly equal size and similar occlusal anatomy were selected.
Teeth were embedded in self-curing acrylic resin, and standardized uniform box-shaped Class I cavities (4 mm × 4 mm × 4 mm) were prepared with a high-speed handpiece under air-water spray using cylindrical flat-end diamond burs. The burs were replaced after every five preparations. Teeth were randomly divided into 5 groups according to the removal protocol (n = 10 per group). The remaining 10 teeth were used for scanning electron microscopy (SEM) analysis. The cavity of the control group (G1) was not contaminated with canal sealer. The teeth in the experimental groups were dried with an air stream. Approximately 1-mm-thick layer of calcium silicate-based Endosequence BC Sealer (Brasseler, Savannah, GA, USA) was injected on the surface of the dentin at the base of the cavity and left for 5 minutes. According to the removal protocol, uncured sealer remnants were removed from cavities as follows: G2- A dry cotton pallet, 2×size nr. 2, was used to scrub the access cavity walls for 5 seconds, the same process was repeated by new a dry cotton pallet, G3- A wet cotton pallet, size nr. 2, was used to scrub the access cavity walls for 5 seconds, the same process was repeated by new a wet cotton pallet, G4- Access cavity walls were rinsed with the air/water spray from a dental unit for 5 seconds, and G5- by passively applying aqueous ultrasonic energy without any contact with the canal walls for 5 seconds with ultrasonic scaler (PiezonMaster 400, EMS, Nyon, Switzerland).
After the removal of the canal sealer, the cavity surface was restored with a one-bottle universal adhesive (G-Premio BOND, GC, Tokyo, Japan) and in two 2-mm-thick vertical layer composite resins (G-ænial posterior, GC, Tokyo, Japan) according to the manufacturer's instructions. Each layer was cured for 20 seconds (Elipar Freelight, 3M-ESPE, Minn, USA). All the restored specimens were stored for 7 days at 37°C in distilled water to allow for complete sealer setting and were then subjected to both thermocycling and mechanical loading in a chewing simulator. Mechanical loading was performed with a 50 N load for 240,000 cycles. The load was vertically applied on the central fossa of the molar with a steel ball (6 mm in diameter) at a frequency of 1.6 Hz. A 0.5-mm sliding movement was also applied during loading. Thermocycling was performed for 1000 thermal cycles in deionized water from 5°C to 55°C, with a 60-second dwelling time and a 10-second transfer between temperature baths.
After simulating aging, the restored teeth were sectioned into three or four 1-mm-thick slices perpendicular to the bonded surface using a slowspeed diamond saw (Diamond cutt-off Wheel MOD15, Struers, Rødovre, Denmark) under water irrigation (Accutom 10, Struers, Rødovre, Denmark). The tooth was then rotated 90 degrees and sectioned again to obtain 1-mm2 sticks. The cross-sectional area of each specimen was measured with a digital calliper (Shinwa, Osaka, Japan) and recorded to calculate the bond strength. Specimens that failed during sectioning were excluded. The beams were fixed to the jig by cyanoacrylate glue (Pattex, Henkel, Duesseldorf, Germany) and tested in a universal testing machine (MOD Dental MIC-101, Esetron Smart Robotechnologies, Ankara, Turkey) at a cross-head speed of 0.5 mm/min. The tensile strength was measured in Newtons. The µTBS values were calculated in megapascals (MPa) by dividing the fracture force by the cross-sectional surface area. Fractured specimens were then examined under a 40× magnification stereomicroscope (SOIF optical instruments, Stereo microscope ST6024-B2, İstanbul, Turkey) to analyze fracture mode.
For SEM analysis, 2 teeth in each experimental group were cut in half longitudinally in a buccolingual direction from the center of the restoration with a water-cooled diamond disc (Struers, Diamond cutt-off Wheel MOD15, Rødovre, Denmark). The specimens were dehydrated for 24 hours at room temperature and mounted on aluminum stubs, desiccated, sputter coated with gold/palladium, and examined under a scanning electron microscope (FEI, Quanta 650 FEG, Oregon, USA).
The Shapiro-Wilk test was used to evaluate the normality of the data. The normally distributed data were analyzed by one-way analysis of variance (ANOVA) and the Tukey HSD (IBM SPSS Statistics V21 Armonk, USA).