الفهرس 
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Abstract
This study evaluated the effect of ceramic materials, cement types and their interactions on the marginal and internal gaps of dental restorations.
Forty full contoured anatomical crowns were constructed from two different CAD/CAM ceramic materials (Zirconzahn and IPS e.max CAD). Specimens were divided into two groups (twenty crowns of each) according to the used ceramic material. Each group was then subdivided into 2 subgroups according to the type of the luting cement used (self-adhesive resin cement and resin modified glass ionomer cement).
Sound extracted maxillary premolar teeth were collected. The teeth were checked to assure that they are free of caries, cracks, fracture and attrition. Only one tooth was selected for preparation and duplication into forty standardized epoxy resin dies. This tooth was selected with crown size 8mm according to its mesio-distal diameter. selected tooth was ultrasonically cleaned (Coltene Whaledent, Biosonic UC50 Ultrasonic, USA) and stored in sterile saline solution for one week at room temperature to prevent microbial growth.
Preparation of Sound extracted maxillary premolar tooth was performed with diamond cutting tools (Diamond bur, Neytech, USA) to produce a uniform coping preparation with exact taper and fit by the same operator and under the same condition with the guide of silicone indecies to determine the thickness of reduction. Forty impressions of prepared tooth were made using a polyvinyl siloxane (Zhermack, Italy) by mixing of heavy paste and catalyst according to manufacturer’s instructions. Forty epoxy dies were made from kemapoxy 150 (CMB, Egypt) by mixing One size of epoxy paste and 1.5 size of activator in vibrator (Vibromaster Bego Bremer, GmBA, Germany) for two minutes then leaved ten minutes before impression pouring to become homogenous mixture. All crowns (lithium disilicate & zirconia) were fabricated with full anatomical contour according to the directions of manufacturing companies for each fabrication system. Each crown luted to its corresponding prepared epoxy die with the selected cement type under a load 5 kg for 10 minutes by using specially designed loading device to standardize the cementation procedure of all specimens in addition to ensuring complete seat of all crowns. Specimens were stored in distilled water for 24 hours and then subjected to 5000 cycles of thermocycling between water temperatures of 5°C and 55°C with a dwell time of 30 seconds and transfer time of 20 seconds.
Marginal gap of each crown was measured by direct viewing under computer-aided stereomicroscope at 45× magnification (Eclipse ME600, Nikon, Tokyo, Japan). Measurements of marginal gap were made at six equidistant points (P1, P2, P3, P4, P5 and P6) on each of the four axial walls (mesial, buccal, palatal and distal). Each crown was then vertically sectioned mesiodistally into two halves by ISOMET 4000 micro saw (Buehler, USA). The surface of each section was digitally photographed under computer aided stereomicroscope at 9 reference points.
Marginal and internal gap values were collected, tabulated and statistically analyzed.
Conclusions:
Within the limitation of this study, it was concluded that ceramic material, cement type and interaction of them had significant effects on marginal accuracy of dental restorations.
• Lithium disilicate ceramic recorded higher marginal and internal gap than zirconia ceramic.
• Self-adhesive resin cement recorded higher marginal and internal gap than resin modified glass ionomer cement.
• Lithium disilicate crowns luted with self-adhesive resin cement recorded the highest marginal and internal gap, followed by zirconia crowns luted with resin modified glass ionomer cement, then lithium disilicate crowns luted with resin modified glass ionomer cement, while zirconia crowns luted with self-adhesive resin cement recorded the lowest marginal and internal gap.