| Objective: The purpose of the present study was to evaluate the effect of two primers (γ-MPS,VTD) on the shear bond resistance between a light-cured resin and three metal alloys (Ni-Cr,Ti,SP-2.0 ) through mechanical test for clinical application.Methods: Three groups of porcelain alloys (Ni-Cr,Ti,SP-2.0) were cast from disk-shaped wax patterns with the following dimensions: 2 mm in thickness and 8 mm in diameter by Wax-lost Casting Craft. After removing investment materials, 54 specimens were chosen without blowholes and casting defects, 18 of each . All disks were sanded with a series of silicon-carbide abrasive papers and finished with 600-grit paper. Then all castings were airborne-particle abraded using 50μm Al2O3 particles. Airborne-particle abrasion was performed for 30 seconds at 0.2MPa ,with a 90-degree angle at a 5mm nozzle-metal surface distance. All specimens were cleaned in an ultrasonic bath for 5 minutes and dried using an oil-free air stream. The following primers were applied on 6 discs of every group treated with airborne-particle abrasion:γ-MPS, VTD and none. According to these steps , specimens could be divided into nine groups with six each. Disks in group A were Ni-Cr alloys, as for blank control group, Disks in group B were Ni-Cr alloys withγ-MPS were applied, Disks in group C were Ni-Cr alloys with VTD were applied, Disks in group D were Ti ,as for blank control group, Disks in group E were Ti withγ-MPS were applied, Disks in group F were Ti with VTD were applied, Disks in group G were SP-2.0 ,as for blank control group, Disks in group H were SP-2.0 withγ-MPS were applied, Disks in group I were SP-2.0 with VTD were applied. After natural drying for 1 minute, A thin layer of singlebond2 was painted with a sable brush, then the surface should be polymerized for 10 seconds using a light-polymerizing device. For the specimens to be bonded, an adhesive tape with a central hole measuring 5mm in diameter was positioned over the airborne-particle-abraded surface to determine the bonding area. Filtek Z350 was used for the veneering of the these alloy surfaces. Two layer of resin opaque OA3 were applied on the metal surface, Each layer was polymerized for 10 seconds using a light-polymerizing device, according to the manufacturer's instructions. A transparent plastic cylindrical tube with an internal diameter of 5 mm was positioned perpendicular to the treated alloy surface to add the veneering portion of the specimen. Two layers of dentin veneering resin, with a total thickness of 2 mm, were applied and polymerized following the same procedures. The samples were immersed in water of 37℃for 24 hours[1]. Then all the specimens were mounted in the universal test machine for shear bond resistance test at a crosshead speed of 0.5mm/min while the load angle was parallel with bond-interface until failure. The shear bond strength was measured and analysed.The metal-resin specimens were prepared for shear test, scanning electron microscope(SEM) and energy spectrum analysis Shear bond strength, F/A (force per unit area), was calculated from the recorded failure loads, with an adherence area ofπ(2.5mm)2 for all specimens.Results: Group E has the highest shear bond resistance with the value of 14.95±0.38MPa while group G has the lowest fracture resistance with the value of 6.75±0.41MPa. There were statistically significant differences(p<0.05) between these groups. Factorial analysis was used to test significant differences of the shear bond resistance loads among all groups. The results showed the differences were significant with material factor(p<0.05), and the differences were significant with primer factor (p<0.05).There was an interactive relationship between these two factors. Different alloy materials had significant difference in shear bond resistances, Different primers had significant difference in shear bond resistances, Ni-Cr alloys applied withγ-MPS had the highest shear bond resistance, followed by sp-2.0 alloys applied with VTD, and the sp-2.0 alloys as blank control had the lowest shear bond resistance. The modes of failure were adhesive for all the test groups.Conclusions: The shear bond resistances of resin to alloys were different for different coupling agents. In clinical application, coupling agent should be carefully chosen according to alloy types to obtain ideal bonding strength. The modes of failure were adhesive for all the test groups, neither resinous cohesive fracture nor compound. It suggested that the bonding strengths between alloys and resins were lower than resinous cohesive strength. Physical or chemical method still remains further study to improve the shear bond resistance.
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