الفهرس 
MATERIALS AND METHODSOnly 14 pages are availabe for public view
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Abstract
Pressable ceramics is a category of dental ceramics which is supplied in a form of ingots.
These ingots are melted at high temperatures then pressed into a mold constructed by the lostwax technique (hot pressing technique). Lithium disilicate glass ceramics are based on SiO2-
Li2O system. This glass ceramic has high mechanical properties with a flexural strength of 350–
450 MPa and a fracture toughness approximately three times higher than other glass ceramics
(leucite based). Ideally after pressing the ingots under heat and pressure and cooling phase
starts, the sprues with the remaining material of the ingot (button) are removed then discarded.
It has been claimed that in some dental laboratories these buttons are recycled. The purpose of
the present study was to assess the influence of repeating pressing on the shade stability and
translucency of three pressable glass ceramics in shade A3; IPS e.max Press, Celtra press and
GC initial LiSi press.
Seventy-two-disc shape specimens were prepared to test their color difference and
translucency changes after repeated pressing. Three different pressable glass ceramic materials,
IPS e.max Press, GC initial LiSi and Celtra press were used. These ceramic materials were used
to construct disc specimens of 14 mm diameter and 1.5 mm thickness (n=12 per group). Color
parameters of pressed and repressed specimens were measured using digital spectrophotometer
(Vita Easyshade Compact) over a black background. Color difference (ΔE) and translucency
between pressed and repressed groups were calculated by the following formula: ΔE= [(Δ L*)
2+ (Δ a*) 2 + (Δ b*) 2
]
1/2
The results revealed that there was a significant difference in color between the 3 tested
material (p=0.005). After repeated pressing, IPS e.max Press and Celtra press groups yielded
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(ΔE) mean values of (3.78±1.11) and (3.03±1.44) respectively. These (ΔE) mean values are
beyond the selected clinical perceptible threshold (ΔE=2.6). On the other hand, GC initial LiSi
recorded (ΔE) mean values of (1.80±1.22) which is considered clinically not perceivable.
Furthermore, IPS e.max Press showed statistically significant decrease in translucency
after repeated pressing (p=0.002). However, after repeated pressing, Celtra press and GC initial
LiSi showed no statistically significant differences in translucency