الفهرس 
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Abstract
The purpose of this study was to comparatively evaluate the metallceramic crowns constructed by the conventional lost wax casting technique and those constructed by the electro forming technology as regards to: their initial marginal fit and the effect of firing cycles of porcelain, porcelain application, and cementation on their marginal fit.
Measurement of the vertical marginal discrepancies
Thirty standard machine-cut brass dies were fabricated to simulate the average dimensions of prepared upper premolar. Thirty samples were constructed. Grouping of the samples was first designed in accordance with the metallic substructure. Three main groups (each of 10 samples) were designed as follows: Ni-Cr alloy, high noble gold alloy and electroformed copings. Each group was divided into two subgroups (n=5) according to porcelain application (with or without).
The CG group was subdivided into two subgroups CGI and CG II. The Ni-Cr group was subdivided into subgroups Ni-Cr I and Ni-Cr II. The EG group was subdivided into subgroups EG I and EG II.
Subgroups (CG I, Ni-Cr I, and EG I) were subjected to firing cycles with porcelain application, while the subgroups (CG II, Ni-Cr II, and EG II) were subjected to the same firing cycles without porcelain application.
Then all the samples were cemented using zinc phosphate cements under a static load of 8 kg using a specially designed loading device.
The vertical marginal discrepancies were measured at predetermined points by the use of a universal measuring microscope. The results were confirmed by the use of the stereo microscope.
The measurements were taken at several stages during the study. 1- Initially (metal copings).
2- After glazing cycles.
3- After cementation.
Measurement of the value of distortion
Three different types of discs were fabricated for the measurement of deformation. All the discs had the same diameter but different thicknesses according to the thickness of the copings used in the daily dental practice. Half of the discs of each metal group were subjected to firing cycles with porcelain application, while the other half were subjected to firing cycles without porcelain application.
The resulted distortion was calculated from the equation:
. . Change in legth of the line
DistortIOn - 0·· 11 h f h 1· X 100
ngma engt 0 t e me
Data were collected, tabulated, and statistically analyzed with the following conclusions:
1- The electro formed copings recorded the least vertical marginal discrepancies followed by cast gold copings. The highest values were recorded with cast Ni-Cr copings. There was a statistical significaht difference between the three metal copings before porcelain firing cycles.