الفهرس 
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Abstract
Dental restorations success rate is controlled largely by their marginal adaptation, fracture resistance and aesthetic value. IPS E.max was launched in 2006 with improved physical properties. Zirconia reinforced lithium silicate ceramic (ZLS) is reinforced with tetragonal zirconia fillers (about 10% by weight) allowing higher strength values than Lithium disilicate glass ceramics. Recently a new pressable zirconia reinforced lithium disilicate ceramic material (VITA Ambria) is introduced with improved strength in addition to its high esthetics.
This in-vitro study aimed to investigate the effect of two thermal tempering temperatures on fracture resistance of 4 types of heat pressed glass ceramics. One of the thermal tempering temperatures used in this study was calculated according to the manufacturer’s recommendation of Vita Ambria which was 9% below pressing temperature and another intermediate thermal tempering temperature (5% below pressing temperature) was also selected.
Four heat pressed glass ceramic materials were used in this study: Lithium disilicate (IPS E.max press), High Density Micronization (HDM) Lithium disilicate (GC initial LiSi Press), Zirconia reinforced lithium silicate (Celtra Press) and Zirconia reinforced lithium disilicate (VITA Ambria).
A lower first molar acrylic prototype was mounted in an acrylic resin base using the surveyor. A computerized numerical control (C.N.C) lathe-cut milling machine was used to prepare the prototype following the manufacturer’s recommendation to receive a ceramic posterior crown. Duplication material was used for making impressions of the master model. After setting, the molds were poured with chemical cured epoxy resin.
A 5-axis milling machine was used for indirect fabrication of wax patterns. The milling order was given to mill 84 wax pattern crowns. The resulting molds were hot pressed by ingots of four different lithium disilicate materials.
After heat pressed procedure of all crowns. Each group was subdivided into three subgroups (n=7) according to thermal tempering protocol applied. Subgroups were: not subjected to thermal tempering, subjected to thermal tempering at a temperature 9% below pressing temperature and subjected to thermal tempering at a temperature 5% below pressing temperature. The internal surfaces of the crowns were surface treated. All crowns were cemented using a specially designed loading device.
All samples were individually mounted on a computer controlled universal testing machine for fracture resistance testing. Failure modes were observed by the same operator, classified and tabulated. The microstructural feature of each material was tested using Xray diffraction (XRD), Energy Dispersive x-ray analysis (EDAX) and Scanning electron microscopy (SEM).
The effect of ceramic type on fracture resistance regardless of thermal tempering: LiSi press showed the statistically significantly highest mean fracture resistance value (1937.8 N). There was no statistically significant difference between E.max and Celtra (1772.4 N); both showed statistically significantly lower mean values (1772.4 N) and (1762.2 N) respectively. VITA Ambria showed the statistically significantly lowest mean fracture resistance value (1622.6 N).
The effect of thermal tempering on fracture resistance Regardless of ceramic type: there was no statistically significant difference between thermal tempering at 5% and 9% below pressing temperature; both showed statistically significantly higher mean fracture resistance than non-tempered group. Thermal tempering at 5% below pressing temperature showed statistically significantly higher mean fracture resistance than 9% below pressing temperature.
The effect of non-tempering (control) on fracture resistance: E.max showed the statistically significantly highest mean fracture resistance value (1611.1 N) while, there was no statistically significant difference between LiSi, Celtra and Ambria.
Comparison between thermal tempering temperatures: Whether with LiSi, Celtra as well as Ambria; there was no statistically significant difference between at 5% and 9% below pressing temperature; both showed statistically significantly higher mean fracture resistance than non-tempered group. While with E.max; there was no statistically significant difference between non-tempered group and 9% below pressing temperature; both showed statistically significantly lower mean fracture resistance than 5% below pressing temperature.
The SEM image observations of E.max press thermally tempered at 5% below pressing temperature shows shorter, broader crystals and more fused together than control and 9% below pressing temperature samples. At magnification (20.000x): The measured length of lithium disilicate crystals was 1.305-1.758 µm, while the measured width was 695.0-871.2nm. The SEM image observations of LiSi press thermally tempered at 5% below pressing temperature shows elongated crystals and highly interlocking microstructure. At magnification (20.000x): The measured length of crystals was 2.299-2.403 µm in length while the measured width was 497.6-985.6 nm. The SEM image of Celtra press thermally tempered at 5% below pressing temperature shows shorter crystals and more fused together than non-tempered and 9% below pressing temperature samples. At magnification (20.000x): The measured length of crystals was 1.305-1.758 µm, while the measured width was 510.4-640.4 nm. The SEM image observations of VITA Ambria thermally tempered at 5% below pressing temperature shows nono-clusters in shape, well aggregated with each other and more fused together. At magnification (20.000x): The measured width was 274.1-283.9 nm.