الفهرس 
Review of LiteratureOnly 14 pages are availabe for public view
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Abstract
Though prosthodontic treatment had been previously advocated to restore mainly the lost function, the massive demand for optimum esthetics- dictated by social pressure and patients’ needs- had changed all perspectives of dental treatment (1). Approaching the end of the last century, a huge wave of unacceptance for the use of metal alloys in the oral cavity had emerged and implicated the development of new all-ceramic materials for prosthetic rehabilitations. Trying to fulfill the increased demand for safe, biocompatible and esthetically pleasing materials, both dental materials and dental technologies have progressed remarkably (1-3). What has to be considered and well-established is which material should be selected, not only for its good esthetics and optical properties, but also for its favourable mechanical properties, adequate clinical function and longevity (2). Recently and over the last decade, zirconia has propelled a rapid development of metal-free dentistry. It represents a versatile material that holds a unique place among oxide ceramics owing to its excellent mechanical properties, biocompatibility and relatively enhanced esthetic properties (4). Mechanical properties of zirconia are considered the highest-ever reported-for any dental ceramic owing mainly to its capability of transformation toughening, but like the other side of the coin, the mechanism is also responsible for its low temperature degradation (LTD). LTD is regarded as one of the major limiting factors in the extensive use of zirconia where it has multiple consequences that include surface degradation, grain pullout, micro-cracking and strength degradation (4,5). Despite the unsurpassed mechanical properties of zirconia and its highest degree of relative translucency compared to metal, it is relatively opaque and monochromatic in color. This limits its use solely without an esthetic veneering ceramic. The eye is able to easily distinguish between a natural tooth and an artificial one, even when there are minute differences in color and translucency, where translucency and color are highly correlated properties of a ceramic crown (4-6). Since translucency is affected by crystal content within the material, chemical nature and particle size, technical and industrial attempts aiming at controlling grain size, crystalline content, difference in refractive indices between crystals and matrix and controlling the amount of porosity of Y-TZP were all behind the evolution of nano-structured polycrystalline zirconia trying to add an esthetic value to the mechanical supremacy (7). Although nanocrystalline zirconia allows the fabrication of monolithic crowns and fixed partial dentures in the posterior region of the oral cavity, yet the build-up of the final anatomic shape and color characterization by manual veneering is necessary for restorations that require a high degree of individual coloring and esthetics (8). Owing to the inertness and surface stability of Y-TZP, establishing a durable chemical or mechanical bond between zirconia core and veneering porcelain was proven to be a difficult task . According to clinical studies, delamination or minor chipping of the veneering porcelain was described as the most frequent reason for the failures of zirconia fixed partial dentures (10). Accordingly, to ensure structural integrity of such layered restorations under functional loads without chipping or delamination of the veneering porcelain, the core-veneer bond strength has to be of a certain minimal value. Different surface treatment methods of the zirconia core have been investigated trying to improve the bond strength between the zirconia core and veneering ceramic, but up to date no conclusion has been reached regarding the exact bonding mechanism (11). Though it’s quite uncommon for zirconia-veneered restorations to undergo bulk fracture of the zirconia framework itself, where it had been reported in a 4 –year clinical study that zirconia framework failure rate was only 1%, yet it’s still uncertain whether the enhanced mechanical properties of the core material would improve the clinical performance of the ceramic-veneered restoration .