الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
In the history of dentistry, the goal of dental clinicians, prosthodontists, and manufacturers has been to achieve aesthetically and functionally perfect restorations. Marginal accuracy is one of the main factors for durability and clinical success in indirect restorations. Inadequate margins contribute to the accumulation of plaque, which increases the risk of carious lesions, and can cause microleakage, periodontal diseases and pulpal inflammation. Because of their good aesthetic and mechanical properties, heat-pressed LDS are used frequently nowadays, however because to their poor fracture resistance compared to other polycrystalline materials, they have certain limitations for clinical indications in the posterior area; therefore, they are only used as single restorations in this area. (1,2) New heat pressed LDS glass-ceramics materials were released into the market to overcome the earlier weakness of heat pressed LDS; the most recent was a material released by VITA, AMBRIA (ZLS). A tampering cycle (power firing) is suggested by the manufacturer to raise the Biaxial flexural strength from 400 MPa up to 550 MPa by pressing at temperature 880° C then expose the material to a tempering cycle at 800⁰C. A thermal tempering protocol has been proposed, which is based on the traditional manufacturer-recommended glaze firing initial pre-heating time, temperature, and rate of temperature increase with the exception that the longer glaze firing differs by slowly cooling until the temperature drops to 400°C in a closed furnace for a dwell time of 19 minutes. There was a question raised whether tempering the crowns as suggested by the manufacturer would improve edge stability over unexposed to tempering cycle. It was also unclear if tempering technique can be applied on LDS and other zirconia reinforced lithium silicates. A total of Fifty-Six specimens were fabricated from lithium disilicate heat pressed glass ceramic materials, divided into four groups according to type of heat pressed glass ceramic (n=14): group(L) GC initial LiSi Press, group(E) IPS e.max Press, group(C) Celtra Press, group(A) VITA Ambria). Each group was divided. Each group will be subdivided into two subgroups (n=7) according to suggested thermal tempering temperature. subgroup (T1) 9% and subgroup (T2) 5% below pressing temperature of each ceramic according to manufacturer’s recommendations. A Stereomicroscope was used for measuring marginal gaps for all specimens before and after tempering cycles. After that, all specimens were prepared for XRD, SEM, and EDX examination. Then the numerical data were collected, tabulated and explored for normality by checking the distribution of data and using tests of normality (Kolmogorov-Smirnov and Shapiro-Wilk tests). Data were presented as mean and standard deviation (SD) values. Bonferroni’s post-hoc test was used for pair-wise comparisons when ANOVA test is significant. The significance level was set at P ≤ 0.05. The microstructure of the glass-ceramic was determined by scanning electron microscopy (SEM). X-Ray Diffraction Analysis (XRD) investigates crystalline material structure. Energy Dispersive X-Ray Analysis (EDAX) was used to identify the elemental composition of materials. Within the limitations of this in vitro study, the addition of zirconia particles to lithium disilicate glass ceramics might have a negative effect in the marginal gap adaptation especially after tempering. Increasing the tempering temperature might lead to inadequate marginal adaptation of Lithium disilicate glass ceramics. Subjecting lithium disilicate and High Density Micronization (HDM) Lithium disilicate glass ceramic to tempering cycles has no negative effects in marginal adaptation.