الفهرس 
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Abstract
The purpose of this study was to evaluate the effect of different enamel regions and the direction of enamel sectioning on the microshear bond strength of a multi-mode adhesive used with either an etch-and-rinse adhesive protocol or a self-etch adhesive protocol.
A total of 34 recently extracted human upper premolars scheduled for extraction due to orthodontic treatment were selected for this study. The selected premolars were randomly divided according to the testing procedures. Thirty premolars were assigned for preparing 80 specimens for microshear bond strength testing. While 4 premolars were assigned for preparing specimens for ultra-morphological examination of enamel adhesive interface.
The microshear bond strength specimens (n=80) were divided into two main groups (n=40) according to the enamel region that was tested (A); cuspal enamel (A1), and mid coronal enamel (A2). Each group was divided into two subgroups (n=20) according to the direction of tooth sectioning (B); axial sectioning (B1), and tangential sectioning (B2). Each subgroup was divided into two classes (n=10) according to the applied adhesive protocol (C); where the multi-mode adhesive was applied with self-etch adhesion protocol (C1) while (C2) the multi-mode adhesive was applied with etch-and-rinse adhesion protocol.
Microshear bond strength was assessed by using universal testing machine at a cross head speed of 0.5 mm/min until failure. Resin-enamel interface was assessed under environmental scanning electron microscope.
Results of the microshear bond strength testing revealed that bonding using the multi-mode adhesive SingleBond universal to tangentially sectioned enamel recorded a statistically significant higher bond strength mean value than bonding to axially sectioned enamel. Also bonding using etch-and-rinse adhesion protocol recorded a statistically significant higher mean microshear bond strength value compared to bonding using the self-etch adhesion protocol. Moreover, bonding to cuspal enamel recorded non statistically significant higher mean bond strength value compared to bonding to mid coronal enamel.
Results of the interfacial analysis using environmental scanning electron microscope (ESEM) for specimens bonded using the multi-mode adhesive (SingleBond universal), revealed that in all specimens a hybrid layer with intimate adaptation between resin and enamel was observed. In tangentially sectioned enamel, the adhesive resin was bonded to enamel rod ends. Axially sectioned specimens showed adhesive resin bonded to enamel rod sides. Microtags were most prevalent in specimens bonded using the etch-and-rinse adhesion protocol.
Thus under the limitations of the current study, the following conclusions could be derived:
1. Application of multi-mode adhesive SingleBond universal in an etch-and-rinse adhesion protocol improves the resin-enamel bond strength.
2. The microshear bond strengths of the SingleBond universal adhesive systems is influenced by the direction of enamel rods.
3. The microshear bond strengths of the adhesive systems is not influenced by the enamel regions tested in this study.
Recommendations:
• Based on the findings of the current study, it is recommended to use SingleBond universal adhesive with etch-and rinse adhesion protocol when bonding to enamel.
• Also it is recommended to bond resin composite restorations to enamel rods ends whenever possible, to achieve higher bond strength for more successful restorations.
• Further studies are needed to assess the long term success of multi-mode adhesives used with self-etch adhesion protocol on enamel substrate.