الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
The purpose of this study was to evaluate the in vitro micro shear bond
strength of three universal bonding agents; Tetric N bond universal (ivoclar
vivadent), All bond universal (Bisco) and Single bond universal (3MSPE) in self and
total-etch modes and to analyze the fractured composite-dentin surfaces
fracographically using SEM.
A total of 60 specimens were used in this study. The occlusal surface of the
teeth were ground flat to remove enamel and create a flat dentin surface. They were
mounted in rubber molds filled with acrylic resin. They were randomly divided into
two main groups: (group A: self-etch mode and group B: total-etch mode ) each
consisting of 30 specimens which were subdivided into three subgroups according
to bonding agent used (Tetric N-bond universal, All bond universal and Single bond
universal) with each subgroup containing 10 specimens. A resin composite was built
up using a 1 mm diameter polyethylene tube; according to manufacture instructions.
After 1 week storage in artificial saliva at 37ºC, the specimens were thermocycled
500 cycles.
The dentin specimens were secured to the universal testing machine. A
shearing load was applied at a cross‑head speed of 0.5 mm/min until bonding failure
occurs. The micro shear bond strength values (expressed in MPa) were calculated
from the Maximum load at failure divided by the bonded surface area. Based on the
percentage of substrate area (adhesive, resin composite and dentin) observed on the
dentin bonding sites by a stereomicroscope, the types of bond failure were recorded
as: adhesive failure; if failure occurred in the interface between dentin and resin
composite or between the dentin and adhesive, cohesive failure; if the fracture
occurred exclusively within the composite or dentine or both, Mixed failure;
partially adhesive and partially cohesive. Nineteen fractured dentin specimens were
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selected from all groups representing different modes of failure, further air-dried,
gold-sputtered and then observed by SEM. The results were recorded, tabulated, and
statistically analyzed. Statistical analysis was set at a significance level of 0.05.
The total-etch approach significantly improved the micro shear bond strength
of Single bond universal, while Tetric N bond universal and All bond universal
showed almost no difference in both etching modes. Single bond universal showed
the highest mean bond strength values. Stereomicroscopic observations also
revealed that higher bond strength values were generally associated with mixed
failures and lower bond strength values with adhesive failures.
It is evident from the SEM fractographs that the specimens showed a wide
variety of surface features. The increase in micro shear bond strength values was
related to slow fracture rates and accordingly the decrease in micro shear bond
strength to rapid fracture rates. Non-uniform stresses generated within the shear zone
have a significant effect on the mode of failure. It was shown that complex stresses
occur at the adhesive surface and result in unstable crack propagation; leading to
mixed failures.
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Within the limitations of this study, the following can be concluded:
1. In self-etch mode, the three tested universal adhesives performed similarly in
relation to their micro shear bond strengths.
2. An etching step prior to the application of the universal adhesives, didn’t
significantly improve their micro shear bond strength, except for Single bond
universal.
3. Adhesive and mixed types of failure were the predominant failure modes in
the fractured specimens. Higher bond strength values were generally
associated with slow progression of mixed failures, meanwhile lower bond
strength values were mainly associated with rapid progression of adhesive
failures.
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Within the scope of this study, it is recommended that:
1) For bonding resin composite to dentin, universal adhesives can be applied using
either the total-etch or the self-etch approaches.
2) It is recommended to etch dentin prior to the use of Single bond universal
adhesive.
3) More research in fractographical analysis is needed to improve our understanding
of the fracture process, which is the most common cause of failure of dental
restorations.
4) Further studies in association with in vivo tests are needed to assess the clinical
longevity of universal adhesives in different etching modes when bonded to deep
dentin under moist and dry conditions.