الفهرس 
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Abstract
This in-vitro study was designed to evaluate the efficiency of Er:YAG laser in the debonding of two glass ceramic materials with two different thicknesses 0.3mm &0.7mm of lithium disilicate (IPS e.max CAD) and fully stabilized Cubic zirconia(Katana Ultra translucent multilayered zirconia) using one type of resin cement,light cured resin cement.
Fifty-six freshly extracted maxillary premolars were collected, examined and those with cracks or structural defects were discarded. The premolars were stored in saline during the study. The extracted teeth were embedded in acrylic resin blocks for fixation. The fifty-six samples (n=56) were randomly divided into two main groups:
• group (A) Er:YAG (N=28)
• group (B) control group (no Laser application) (N=28)
Each group was subdivided into two subgroups according to material used:
• Subgroup (i) Emax (N=14)
• Subgroup (ii) Cubic Zirconia (N=14)
Each subgroup was further divided into two divisions according to material thickness: Division (X): 0.3mm (N=7) and Division (Y): 0.7mm (N=7)
Using IsometTM 4000, IPS e.max CAD and cubic zirconia specimens were sectioned with dimensions 4x4 mm and thicknesses 0.3 mm and 0.7mm under copious cooling. Cubic zirconia Samples were prepared so as to be 25% larger than the final required size.
Dimensions and thickness of ceramic specimens were confirmed after sectioning by caliper. IPS e.max CAD Crystal/Glaze paste**was applied evenly on one side of the veneer Veneer samples were crystallized and glazed using the designated compatible glaze. For Zirconia samples they were sintered and glazed in high temperature furnace according to the manufacturer’s recommendations.
On the other Hand tooth preparation was done using A three wheel depth cutter diamond stone (909/018) was used to place a number of ‘2’ 0.5 mm depth grooves to guide the amount of preparation, Buccal surface was prepared using blue coded round end tapered diamond stone (856 014 F FG) to have a flat surface for the veneer specimens to fit without rocking. Surface was then finished by red coded finishing stone (369/025) and yellow coded finishing stone (368/023).
Surface treatment was performed for bonding surfaces of the veneer and tooth. Ceramic veneers of E.max groups were etched using 9.5% hydrofluoric acid for 20 seconds to roughen the surface and increase surface area for bonding then rinsed thoroughly with water and dried followed by application of silane coupling agent for one minute. While for zirconia specimens the bonding areas were sandblasted with 50μm aluminum oxide at a pressure of 2 bars at a distance of 10mm for 10 seconds. Then a universal adhesive containing MDP was applied to the pretreated surfaces of the Zirconia samples using a micro-brush and left to react for 60 seconds then dried with a stream of oil-free air as recommended by the manufacturer.
Teeth were etched using 37% phosphoric acid for 30 seconds then rinsed with air/water jet for 60 seconds and dried to remove the excess water, then Universal bonding agent was applied and activated for 20 seconds then light cured for 10 seconds.
Each sample was then loaded with 1 Kg using loading device to ensure uniform cement space for all specimens. The samples were subjected to 5000 thermal cycling between 5 and 55 °C in deionized water with a dwell time of 30 s and transfer time of 20 s to simulate about six months of thermal changes in an oral environment.
Following thermal cycling, Er:YAG laser beam with wavelength 2,940 nm was applied to half of the samples with 20 % water cooling and 80% air at a power of 5 W. Laser was applied by scanning method through the surface for 9 seconds with horizontal movements perpendicular to the surface. Control groups and the veneers of both ceramic groups that did not debond after the time selected in the study of laser application were further tested for shear bond strength by Instron Universal testing machine to determine the change in bond strength. Data were collected, tabulated and statistically analyzed.
Results showed that Debonding of ceramic veneer samples
Regarding effect of treatment on shear bond strength within e.max samples: It was found that Control samples (10.71±1.47) had statistically significant higher shear bond strength values than laser treated samples (4.61±1.15) (p<0.001)
Regarding effect of treatment on shear bond strength within Cubic zirconia samples: It was found that Control samples (5.51±1.55) had statistically significant higher shear bond strength values than laser treated samples (0.571±1.68) (p<0.001).
Regarding the Effect of ceramic thickness on shear bond strength within e.max samples:
It was found that there was there was significant difference between both groups (p=0.031).
Regarding the Effect of ceramic thickness on shear bond strength within cubic zirconia samples:
It was found that Regardless of treatment, there was no significant difference between both groups (p=0.052).
Regarding the Effect of ceramic material on shear bond strength within control samples:
Regardless of the ceramic thickness, Emax samples (10.714±1.47) had statistically significant higher shear bond strength values than Cubic zirconia samples (5.51±1.55) (p<0.001) in control group.
Regarding the Effect of ceramic material on shear bond strength within Er:YAG laser treated samples:
Regardless of the ceramic thickness, Emax samples (4.614±1.15) had statistically significant higher shear bond strength values than cubic zirconia samples (0.571±1.68) (p<0.001) in laser treated group.