الفهرس 
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Abstract
Objectives : To evaluate and compare the microtensile bond strength (μTBS) of four different types of adhesives to dentin, one of them containing the bioactive hydroxyapatite-nanorod. In time, the adhesive/dentin interface was investigated with a scan electron microscope immediately after 24h and 6m storage in artificial saliva and thermocycling (2000 cycles). Materials and Methods : Eighty freshly extracted caries-free human permanent molars were selected. After cleaning and disinfection, they were assigned randomly into four equal groups (n=20). group 1 ; receive one-step self-etch adhesive (NMA), group 2; receive modified-bioactive adhesive (self-etch adhesive containing bioactive hydroxyapatite nanorods) (MA), group 3; receive universal adhesive (UA), group 4; receive resin modified glass ionomer- based (RMGI) adhesive (RA). After the selected molars were fixed in an acrylic resin block, the occlusal enamel and superficial dentin were removed for each tooth, exposing the mid-dentin area. Then all adhesives were applied and cured with a light emitted diode (LED) unit according to the manufacturer’s recommendations. A nanohybrid composite restorative material (Filtek Z250 XT, 3M-ESPE, St. Paul, MN, USA) was placed incrementally in 4-5 mm height to form a crown segment, each increment with 1mm thickness was light-cured for 20s. The intensity of the LED unit was monitored via a radiometer. Half of the restored teeth (n = 40) were preserved in an incubator for 6months in artificial saliva at 37°C and then they were subjected to thermocycling (2000 cycles).For microtensile bond strength test (n=56), the restored specimens for each subgroup were prepared and tested according to the aging time. After that, the resorted specimens were receiving a series of cutting perpendicular to the longitudinal axis in X and Y directions using a low-speed automated saw to obtain composite/dentin beams (1±0.1 mm2). The study was conducted with a total of 280 specimens (35 beams of each subgroup). During microtensile testing, the beams were placed in a universal testing machine and the load was applied at cross-head speed of 0.5 mm/minute until the bond through the beams failed. The value obtained in newtons (N) was divided into a cross-sectional area (mm2) to convert it into Mega Pascal (MPa). After deboning, the fractured sites of all the beams were observed under a stereomicroscope to identify the mode of failure that occurred at the tooth/ adhesive system/restoration interface. The μTBS data were statistically analyzed using two-way ANOVA and multiple comparison post-hoc tests. Three specimens from each subgroup were used for micromorphological observation of adhesive/dentin interface using scanning electron microscope (SEM) (n=24). The restored specimens for each subgroup were prepared and tested according to the aging time. Results : For the μTBS test, the two-way ANOVA test showed that the ””adhesive type”” and ””time”” had significant differences on the µTBS results. Tukey post hoc multiple comparison test showed that there was no significant difference in μTBS between non-modified and modified groups (p>0.05). However, there were significant differences in μTBS between the universal group (delayed) and the non-modified group (immediate). Also, there is a high significance of all tested groups with the delayed RMGI-based adhesive group (p<0.05). The hybrid layers created by resin-based adhesives; non-modified (NMA), modified (MA), universal adhesive (UA) showed high resin tags infiltration. While the glass ionomer-based adhesive showed a typical micromorphological pattern of the hybrid component of this material. Conclusions: The present study is based on the outcome of μTBS test supported by analysis of failure mode and micromorphological analysis of adhesive\dentin interface, the incorporation of bioactive HAP-NR to one-step self-etch adhesive improves the bond strength. Aging did not affect negatively the bond strength between groups except Riva and the resin modified glass ionomer-based adhesive provided inferior bond strength compared to resin -based adhesives after aging.