الفهرس 
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Abstract
This in vitro study was carried out to investigate the effect of mechanical loading on the microtensile bond strength of bioactive restorative materials in comparison to nano filled resin composite at different dentin levels.
The specimens were divided into 2 groups of 40 specimens each according to the tested restorative materials used (A), the first group (A1) bioactive restorative material was used. The second group (A2) Nano resin composite restorative material was used. Each group was subdivided into 2 subgroups of 20 specimens each according to dentin depth level (D), the first group (D1) represented superfacial dentin level and the second group (D2) represented deep dentin level. each subgroup was classified into 2 classes of 10 specimens each according to the number of received load cycling (T); the first class (T1) that represent 24 h and the second class (T2) 3 month.
The prepared dentin surfaces were thoroughly rinsed with water to remove debris and dried. Acid etching for bio active restoration, etching prepared tooth surface with 37% phosphoric acid etching gel for 15 seconds, rinsed off with water for 10 seconds and air dried for 5 seconds.
Tofflemire matrix system was used to give bioactive composite its shape during activa bioactive composite application .Syringe that containing Activa restorative material was inserted into ACTIVA-SPENSER and snap into place using firm pressure . Material was dispensed using gentle pressure to obtain an even mix of base and catalyst that was applied directly on the tooth surface. After a delay of 10 seconds to allow for the acid component to react with the tooth surface. The outer surface of each levels was then filled of 2mm depth and light curing was done for 20 seconds according to manufacturer instruction.(lead light 1,470 mW/cm2 intensity) . After removal of the matrix, all the restorations were light cured 40 s.
Single bond universal adhesive was applied on dentin by brush, agitated solvent to evaporate and dried by high air flow for 5 seconds .The bonding procedure was done according to the manufacturer’s instructions, then light cured for 20 seconds using (lead light 1,470 mW/cm2 intensity).
Nano-hybrid resin composite (Filtek Z250 XT) was used for buildup. Tofflemire matrix system was used to give the restoration its shape during resin composite packing. Two increments of two millimeters each was packed using gold plated composite applicator and light cured for 40 seconds according to manufacturer’s instructors. The specimens were kept in distilled water at 37C; half of the specimens were load cycled equivalent to 3 month before microtensile testing and the other half were subjected to microtensile test after 24 h. The load application was associated with thermocycling procedure includeing the immersion in cold\hot water bath with temperature variation 5 C\55 C and dwell time 60 seconds.
Each tooth was sectioned to obtain beams by using an isomet cutting machine to produce stick shaped specimens 1mm in diameter. For each tested class 30 beams were selected. Microtensile bond strengrh test was done; data were recorded using computer software(Bluehill Lite software, Instron, MA,USA). Microtensile bond strength value tabulated and statistically analyzed.
The analysis of variance ANOVA test revealed that:
1-There was no statistical significant difference in the mean value of microtensile bond strength in Nanohybrid composite at superficial and deep dentin level after 24hrs and after 3months cyclic loading .
2-There was statistical significant difference on microtensile bond strength in Activa composite at superficial and deep level after 24hrs and after cyclic loading 3months. Decreased in the mean value of microtensile bond strength was shown in groups load cycled for 3 months.
3-There was no statistical significant difference on microtensile bond strength in Nanohybrid composite at superficial and deep dentin levels .
4-There was statistical significant difference on microtensile bond strength in Activa composite at superficial and deep dentin levels. Decreased in the mean value of microtensile bond strength was shown in groups deep dentin level than superficial dentin level.