Supervisor : Rana Sherif, Mostafa Hussein. Includes Arabic Summary.Mansour, Nariman Hazem,2023-12-282023-12-282023EG-CaMIUDNT Ths497 M.Sc. 2023https://iorep.miuegypt.edu.eg/handle/20.500.13071/206DISSERTATION NOTE-Degree type M.Sc.DISSERTATION NOTE-Name of granting institution Misr International University, Faculty of Oral and Dental MedicineIncludes bibliographic references.Nowadays, the increasing demand of people for esthetics and natural looking smile became a great concern for manufactures and clinicians. Clinical applications of zirconia ceramics are increasing worldwide due to their favorable esthetics, optimal biocompatibility, dimensional, wear resistance, and high fracture toughness. Conventional zirconia (3Y) has already proved its strength in function and longevity but unfortunately with compromised esthetics. The introduction of highly translucent zirconia (5Y) solved the problem of esthetics but on the expense of its strength which was diminished. Therefore, gradient zirconia was introduced combining 5Y in occlusal or incisal area and 3Y in dentine area in one blank to fit the requirements of both esthetics and function. The oral cavity has humid nature, temperature fluctuations and bacteria causing aging of dental restorations. Aging has an impact on surface roughness causing bacterial adherence, which may increase the incidence of periodontal diseases and secondary caries. Therefore, the aim of this study was to assess the surface roughness and bacterial colonization of cubic and gradient zirconia ( BruxZir Anterior and IPS e.max ZirCAD Prime) before and after accelerated hydrothermal aging. The study included two equal groups: group B (n=13) cubic zirconia and group I (n=13) gradient zirconia with standardized dimensions of 1.2 mm thickness and 12 mm diameter, the disc samples were designed and milled using CAD/CAM. Sintering, finishing, polishing and glazing of the discs were done according to manufacturer’s instructions for both materials. Surface roughness was measured for all samples using 3D optical non contact profilometer. Artificial saliva and streptococcus mutants reference strain were prepared. Samples then were placed in artificial saliva and Streptococcus Mutans for two hours at 37oC. For accelerated hydrothermal aging samples were placed in a steam autoclave at temperature 134oC and 2 bar pressure for 5 hours according to the international organization of standardization (ISO 13356:2015). After aging surface roughness and bacterial colonization for all samples were measured. Data was recorded, tabulated, and statistically analyzed using one-way ANOVA test.176 pages : illustrations, photo ; 29 cm + 1 CD-ROM (4 3/4 in.)Fixed prosthodontics