الفهرس 
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Abstract
This in vitro study investigated the marginal accuracy of different impression materials using two ceramic materials by machinable Computer Aided Design / Computer Aided Manufacturing (CAD/CAM).
In the present investigation, thirty ceramic copings were constructed according to the manufacturer’s instructions. Samples were divided in to three groups according to the impression materials it’s made of: * Group(I) Laminated hydrocolloid technique (n=10) * group (II) Addition Silicone (n=10) * group (III) Polyether (n=10)
Each group was further subdivided into two subgroups according to the ceramic material uesd:
* Subgroup A: zirconia samples(n=15)
* Subgroup B: e-max samples(n=15)
One impression was made for each material on the same metal die using perforated trays. All fabrication procedures were accomplished according to the manufacturers instructions for each tested material used.
For each material. crowns were constructed by CAD-CAM according to the manufacturer’s instructions in the shape of non anatomical cylindrical crowns of 6mm diameter, 7mm height, 2mm occlusal thickness and margin thickness 1mm.
All tested crowns were individually seated on the stainless steel die and were examined for vertical marginal fit with respect to the die. A specially designed holding device was used to apply a uniform static load on the tested crowns to ensure the accuracy of their seating on the stainless steel die and to hold them in place during the examination with a stereomicroscope .
Data analysis was performed in several steps. Initially, descriptive statistics for each group results. Two factorial analysis of variance ANOVA test of significance comparing variables affecting mean values (ceramic and impression materials). One way ANOVA followed by pair-wise Tukey’s post-hoc tests were performed to detect significance between subgroups. Student t-test was done between ceramics with each impression. Statistical analysis was performed using Aasistat 7.6 statistics software for Windows (Campina Grande, Paraiba state, Brazil). P values ≤0.05 are considered to be statistically significant in all tests.
The results of this study showed:
1. Laminated hydrocolloid agar-alginate impression techniques have significant lowest marginal gap than polyvinyl and polyether.
2. For zirconia copings group it was found that non-significant difference in marginal gap between polyether and polyvinyl impression materials.
3. For e.max cad copings group it was found that there is significant difference marginal gap between polyether and polyvinyl impression material