الفهرس 
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Abstract
Materials used for crown and bridge applications include metals, ceramics, and metal-ceramics. Although metals have excellent mechanical properties, full metal crowns are not generally preferred due to poor esthetics. Dental porcelains play an important role in the fabrication of the most esthetic fixed restorations. Translucency, light transmission, and biocompatibility give dental ceramics highly desirable esthetic properties. All-ceramic restorations are now used for single crowns and possibly short span bridges because of their improved strength and esthetics.
Marginal fit is considered as one of the most important criteria for the long term success of all - ceramic crowns. Many factors affect the marginal adaptation of a crown such as the preparation dimensions, type of finish line, type and cement viscosity, and location of cement application.
The type of luting cement is as well considered an important factor affecting both the marginal gap and microleakage, depending on the viscosity which ease the escape of excess cement and hence reduce the marginal gap due to a better seating of the crown on the prepared tooth, and the ability of the cement to adhere to the tooth structure as in the case of GIC cement and resin based cements which reduces cement dissolution and microleakage.
Microleakage is defined as the movement of fluids carrying bacteria and other molecules and ions at the boundary between a restoration and a tooth. It is considered to be another major factor influencing the longevity of dental restorations since it may lead to staining at the marginal areas, recurrent caries at the tooth/restoration interface, hypersensitivity of restored teeth, or development of pulpal pathology.
The aim of this study was to compare the effect of chamfer and shoulder finish lines on the marginal gap and microleakage of IPS e-max press ceramic crowns when cemented by two different luting cements.
Twenty eight freshly extracted maxillary first premolars of similar size have been selected. The selected teeth were prepared using a milling machine in order to standardize the preparations. Fourteen teeth were prepared with chamfer finish line (group 1) and fourteen teeth with shoulder finish line
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(group 2). Impressions were taken using addition polyvinyl polysiloxane impression material in a sectional tray to mimic clinical steps and were poured using type IV stone die material producing twenty eight stone dies.
In order to eliminate technician variability of the adaptation of the IPS e.max press ceramic coping on its respective die, injection molding technique was used after milling out plastic copings from plastic blocks using Cercon Degudent CAD/CAM system. IPS e.max Press ingots were then heat pressed to form the core which was then veneered by IPS e.max Ceram veneering porcelain.
Each group was further divided into two sub-groups (n=7) according to the type of cement used. The fabricated crowns will then be cemented onto their respective teeth using either self-adhesive resin cement (RelyX Unicem) (sub-group A) or conventional GIC cement (Ketac Cem) (sub-group B). A static load of 5 kg was applied using a special apparatus to ensure complete seating of the crowns during cementation. Two layers or nail varnish were applied to the entire tooth except for 1 mm below the finish line and 1 mm above it.
The specimens were then subjected to 500 thermal cycles between 5oC and 55oC for 20 seconds each with 15 seconds interval in between and then stored in distilled water for 1week at room temperature and then immersed in basic fuschin dye solution for 24 hours and were then removed and allowed to dry.
Specimens were embedded in epoxy resin blocks and were left to completely set for 12 hours; finally the specimens were sectioned in mid-bucco-lingual plane with a low speed diamond saw parallel to the long axis of the crown. The cut sections were then viewed under stereomicroscope to evaluate microleakage and measure the marginal gap of each crown.
Chamfer finish line produced significantly better marginal fit than shoulder finish line, while it did not significantly effect microleakage.
GIC cemented crowns had better marginal fit than crowns cemented by resin cement, but the difference was not significant.
Self-adhesive resin cemented crowns were found to be more resistant to microleakage than those cemented by GIC cement.