الفهرس 
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Abstract
This study aimed to evaluate and compare in vitro the fracture resistance and shear bond strength (SBS) of lithium disilicate occlusal veneers with two different dual cured resin luting cements and to determine mode of failure.
A total of 28 sound extracted human permanent upper molars were used in this study. It was inserted into self-curing acrylic resin using a custom made mold of 14mm diameter and 20 mm Length The 28 specimens were randomly divided according to tooth surface preparations into 2 main groups of 14 specimens each; group A represented anatomical occlusal tooth surface preparation in enamel for fracture resistance test and group B represented flattened occlusal tooth surface preparation with abrasive discs in enamel for Shear bond strength (SBS) test. Then each group was subdivided according to the type of adhesive resin cement used into 4 subgroups (group A1, A 2 and group B1, B2) of 7 specimens each.
For group A1 and B1, Panavia F 2.0 system self-etch resin cement (kurary, Noritake Dental Inc., Japan) While for group A2, B2 Variolink Esthetic system total etch resin cement was used (Ivoclar Vivadent, Schaan, Liechtenstein). All cements were dual cured. Fourteen occlusal veneers were constructed by scanning anatomically prepared specimens and wax_up designed by computer. Specimens of sub groups A 1, A 2 were cemented with Panavia F2.0 resin cement and Variolink esthetic resin cement. While, fourteen ceramic discs (4mm in diameter and 2 mm in height) were fabricated from (IPS e.max press, Ivoclar Vivadent) ceramic material and cemented to specimens of sub groups B 1 , B 2 with Panavia F2.0 resin cement and Variolink esthetic resin cement. The cementation was done under a constant load of 3.0 kgs for 5 minutes using a special static loading device. The bonded specimens were stored in distilled water at 37 °C for 24h and were then thermocycled 500 cycles between 5 C and 55 °C with a dwell time of 15 seconds at each temperature.
Specimens were secured to the universal testing machine and then fracture resistance of 0.8mm metal ball was used with speed of 0.5 mm/min at the central fossa of each occlusal veneer and the fracture resistance was recorded in N. All fractured occlusal veneers mode of failure determined either Mode (I,II,III). The shear test was a chisel blade applied at ceramic\tooth interface with speed of 0.5 mm/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).
The results were recorded, tabulated, and statistically analyzed.
The effect of the resin cement type on the lithium disilicate occlusal veneer fracture resistant specimens showed that there was statistically significant difference of the bond strength values between groups A 1 and A 2, total etch Variolink Esthetic resin cement (A2) provided higher fracture strength (2259.5 N) than self-etch Panavia F2.0 resin cement (A1) (1330.1 N). Also ,on comparing between the type of resin cement used in shear bond strength, there was statistically significant difference between groups B 1 and B 2 , total etch Variolink Esthetic resin cement (B2) provided higher bond strength (34.36 MPa) than self-etch Panavia F2.0 resin cement (B1) (25.60 MPa) (p ≤ 0.05). After fracture resistance test the mode of failure were Mode I failure of restoration in Variolink Esthetic resin; and only one Mode III, failure of restoration, enamel and dentin while all the specimens in Panavia F2.0 was Mode I failure of restoration .Through Stereomicroscope analysis it was observed that the most frequent pattern of failure in Variolink Esthetic and Panavia F2.0 resin were cohesive failure, Higher adhesive failures (28.6%) were found in Panavia F2.0 than in Variolink Esthetic (14.3%). However, those differences were insignificant.
It was concluded that the type of resin cement had a significant effect on the fracture resistance and shear bond strength.