Specimen preparation
Feldspathic specimens (N = 48) were fabricated by slicing CAD-CAM ceramic blocks (CEREC Blocs C, S2-M, Size 14, Sirona Dental Systems GmbH, Germany) with diamond discs in a slow-speed precision cutter (Isomet; Buchler, Ltd., Lake Bluff, USA) into 12 mm × 10 mm slices of approximately 1,55 mm thickness. All the specimens were finished, polished and taken to the final thickness of 1.5 mm with waterproof abrasive papers (400 to 1200 grit), respectively (Mecatech Z34, Presi, France). During this process, the thicknesses of specimens were repeatedly checked with a micrometer (C-master; Mitutoyo, Tokyo, Japan) to ensure a final 1.5 ± 0.05 mm thickness. Then, one side of the specimens were glazed according to the manufacturers’ recommendations and cleaned in distilled water by a ultrasonic cleaner (Eurosonic Energy, Euronda SpA, Vicenza, Italy) for 10 min and dried with oil-free air for 30 s before the surface conditioning procedures. The specimens were randomly divided into 4 subgroups (n = 12 per group) to be conditioned with one of the following methods:
Surface conditioning
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Group C: No surface treatment.
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Group AA: Specimens were air-abraded with 25 μm AI2O3 particles (Korox, Bego, Bremen, Germany) with a microetcher (Airsonic Mini Sandblaster, Hager & Werken, Duisburg, Germany) for 15 s from a distance of approximately 10 mm (perpendicular to the treated surface) at 2.8 bar.
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Group HF: Specimens were etched with 9.6 % hydrofluoric acid (Bisco Inc., Schaumburg, USA) for 20 s, rinsed with distilled water for 20 s and dried with oil-free air for 30 s.
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Group L: Specimens were irradiated with Er:YAG laser (Fotona, At Fidelis, Ljubljana, Slovenia) with a contact hand-piece (R 14; 1.3 mm in diameter) under water-cooling for 20 s, 1 mm perpendicular to the surface with following parameters 500 mJ; 10 W; MSP mode; 20 Hz, 37,68 J/cm2 [11].
Light transmittance
The incident light power given by the light emitting diode (LED) curing light device (Elipar S10, 3 M ESPE, Seefeld, Germany) and the transmitted light power through each sample was measured using a digital LED radiometer (SDI, Victoria, Australia). The LED radiometer is designed to measure the energy between 400 and 525 nm, and gives readings from 0 to 2100 mW/cm2. Light transmittance of specimens were measured by placing the specimen over the radiometer sensor (8 mm in diameter) and activating LED curing light device for 10 s. The tip of LED curing light device was placed in contact with glazed surface of specimen and maintained in a fixed position with a holding device (Fig. 1). During irradiation, highest and lowest values were recorded. For each sample, measurements were made in this way three times, and averages were recorded as mW/cm2.
Surface roughness
Surface roughness were measured by a profilometer (Dektak 8, Veeco Ins, Plainview, NY, USA) with a 0.25-mm cutoff value, 0.01 mm resolution with the transverse length of 4.0 mm, stylus diameter of 5 μm. For determining the average roughness value, 0.5 mm/s measuring speed was used. On each specimen, six measurements were made with equal distances (1.5 mm) and the reading direction was always perpendicular to the surface of the porcelain specimens. The measured roughness parameter was Ra (arithmetical average value of all absolute distances of the roughness profile).
Three dimesional (3D) optical profilometer
Specimens from each group was evaluated with a 3D optical surface profilometer (NanoMap-500LS, Aep Technology, Santa Clara, CA,USA) under the beam of the white light interferometry which is usually made up of the He-Ne, 633 nm [12]. The specimens were analysed by software (Digital Surf, TalyMap Platinum software, Leicester, England. Version no. 6.1.6001).
Scanning electron microscopy (SEM)
Additional four specimens were prepared as described previously to evaluate effects of surface conditioning. Specimens were gold coated with a sputter coater (S150B; Edwards, Crawley, UK) and examined at 15 kV using a scanning electron microscope (JSM-6335 F; JEOL, Tokyo, Japan). For visual inspection, SEM photomicrographs were represented with x 500 magnification.
Statistical analysis
The sample size was calculated using a power analysis software programme (G*Power Version 3.1.9, Dusseldorf, Germany) considering α equal to 5 %, effect size equal to 0.50 and power of 80 % according to One-Way ANAOVA test. Based on the calculations 12 specimens per group yielded to 80 % power. Data were analyzed using a statistical software programme (SPSS Software V.22, Chicago, IL, USA).
The Kolmogorov-Simirnov test showed that the data were normally distributed, One-way analysis of variance (ANOVA) and post-hoc Tukey’s HSD test were applied to analyze the data where the values of transmitted light and surface roughness were the dependent variables and surface conditioning methods (4 levels) as independent variables. P < 0.05 was considered to be statistically significant in all tests.