الفهرس 
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Abstract
Purpose: The aim of this in-vitro study was to evaluate the passivity of milled bar constructed by different impression materials and transfer techniques to be screwed into four implants installed according to All-On-Four implant placement concept. Materials and methods: U-shaped mandibular edentulous model was constructed from heat cured acrylic resin and covered with 2mm simulated mucosa. Two anterior implants (3.7mm x 13mm) were placed vertically in the canine region bilaterally, while the posterior implants (4.2mm x 16mm) were placed in 2nd premolar regions with 30 degree distal to the anterior implants. According to type of impression material and transfer technique, twenty milled bars were constructed and equally grouped (n=5) as follows: group 1:- where milled bars were constructed on casts from open-tray impression technique (LOT) with polyvinyl siloxane material (PVS). group 2:- where milled bars were constructed on casts from open-tray impression technique (LOT) with polyether impression material (PE). group 3:- where milled bars were constructed on casts from snap-on impression technique (SCT) with polyvinyl siloxane material (PVS). group 3:- where milled bars were constructed on casts from snap-on impression technique (SCT) with polyether impression material (PE). Milled bars were screwed onto the acrylic model and the vertical gap distance was measured between the bar and implant prefabricated multiunit abutment by using an optical microscope at x 10 magnification and 1 micron of measurement accuracy. Results: Regardless the impression material, there was a high statistically significant difference between (LOT) and (SCT)techniques at the anterior implants(p<0.001)only.Regardless the transfer technique, there was no statistically significant difference between (PE) and (PVS) impression materials at the anterior implants(p<0.054) or at posterior implants(p<0.057). Regardless the implant position and angulation, there was a high statistically significant difference between (LOT) and (SCT) techniques with polyether (p<0.001). Conclusion: Within the limitations of this in vitro study, it could be concluded that; absolute fit of triple bar; measured in terms of vertical gap fit (µm); constructed to be screwed into four implants installed according to all in four concept was not observed in this study regardless the type of impression material or transfer techniques. Polyether impressionfor implant transferred with snap-on closed tray technique may be the method of choice to produce accepted passive fit of the triple bar superstructure used for all in four implant concept.Keywords: Implant supported overdenture, Milled bar, All-on-4, Implant transfer, Gap distance