الفهرس 
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Abstract
The aim of this in vitro study was to evaluate shear bond strength for immediate and delayed repair of composite with microhybrid and nanohybrid resins using different bonding agents.
One hundred sixty nanohybrid composite discs was prepared using custom made split Teflon mold with dimensions 7 mm in diameter and 2 mm in height. The discs was divided into 16 experimental groups (10 each) according to the 3 experimental factors evaluated in this study: Factor 1: Repair time (2 groups), Factor 2: Repair protocol (4 groups) and Factor 3: Repair composite material (8 groups). For immediate repair group (I), discs (n=80) was divided into 4 subgroups 20 discs each.
Each subgroup was divided into 2 groups (a & b) according to repair material as follows group 1 (a): no conditioning with nanohybrid composite, group 1 (b): no conditioning with z250 microhybrid composite, group 2 (a) Adhese Universal bond with Tetric evoceram nanohybrid composite, group 2 (b) Adhese Universal bond with z250 microhybrid composite ,group 3 (a) All-Bond Universal with Tetric evoceram nano-hybrid composite , group 3 (b) All-Bond Universal with microhybrid composite, group 4 (a) Scotchbond™ Universal with tetric evoceram nanohybrid composite, and group 4 (b) Scotchbond™ Universal with z250 microhybrid composite. As for delayed repair group (II), the discs (n = 80) was prepared and stored in artificial saliva at 37ºc for three months. After three months, the discs were divided and repaired similar to the immediately repaired groups.
All the specimens of group I & II were subjected to thermocycling (1000 thermal cycles), then subjected to shear bond strength test using universal testing machine. Specimens were
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secured to the universal testing machine and the shear load was applied at a constant cross-head speed of 0.5mm/min, then shear bond strength was calculated in MPa.
All fractured de-bonded surface samples were examined under a stereomicroscope to identify the failure mode (Adhesive, cohesive, or Mixed), while representative specimens of each group were chosen to be further analyzed by scanning electron microscope to determine the surface topography of the material.
The results were recorded, tabulated, and statistically analyzed. The effect of time on bond strength of repaired composite showed that immediately repaired composite had higher bond strength (21.97 ± 1.19 MPa -17.66 ± 2.12 MPa) than delayed repair groups.
On comparing between the bonding agents and repair materials used, it was found that scotch bond with z250 groups (I4b, II4b) had the highest bond strength in both repair times (21.97 ± 1.19 MPa- 18.71 ± 1.49 MPa).
Through stereomicroscope and SEM analysis, it was observed that the most frequent pattern of failure was adhesive failure between prepolymerized substrate and repair material.
It was concluded that repair time, bonding agent and repair material used had a significant effect on the bond strength of repaired composite.