الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
 |  | from | 135 |  |  |
| | | from | 135 | | |
Abstract
This study was designed to evaluate the effect of different milling techniques and different cement spaces on the marginal accuracy of YPS zirconia copings.
A Stainless steel die was prepared to simulate all ceramic full coverage crown preparation for a molar tooth which was called the main master die. An occlusal notch was prepared for exact positioning of the copings during the measurements. Four equidistant marks were engraved on the die 0.5 mm below the margin using size 1 round bur to orient the stereomicroscope for marginal gap measurements. For MAD/MAM technique, standardization of the cement space of each subgroup was questionable. Therefore, another 2 Stainless Steel dies were machined with increased diameters and heights to compensate for the application of die spacer.
Impressions of the master dies were made with a vinyl polysiloxane impression material using perforated plastic tubes. For CAD/ CAM technique, an impression of the main master die was made and poured with the laser visible stone. For MAD/MAM technique, three impressions of the three steel dies were poured using type IV dental stone. Another 30 impressions were poured with non shrink epoxy resin which were used for the measurements.
15 CAD/CAM copings were fabricated using CEREC InLab system.Using the inLab3D software version 3.6, three different coping designs were fabricated by selecting three different cement spaces (zero, 30 & 60 um). Another 15 MAD/MAM copings were fabricated using Zirkonzahn system .Three non anatomical composite mock ups were fabricated on the three stone dies which compensated for the application of the die spacer.While the the stylus of the Zirkograph 025 ECO with 5th axismachinewas tracing the mock up, the carbide cutter was milling the green zirconia block.All copings were milled with an oversize of approximately 25% to compensate for the later sintering shrinkage.
Using stereomicroscope with a digital camera, the vertical marginal gapswere measured before and after cementation of the copings on epoxy dies.Before cementation, a specially designed metal jig was used to seat the copings on the die during the measurements. Internal surfaces of the copings were air particle abraded using alumina and glass ionomer cement was used for cementing the copings to the epoxy resin dies. The copings were seated,during cementation,utilizing static finger pressure then axially loaded with a 3kg load using a specially designed device.The mean vertical gap (in microns) for each specimen was then calculated and saved in an Excel sheet for statistical analysis.
Concerning the effect of the milling technique regardless of the cement space:before cementation,MAD/MAMgroup recorded a higher total marginal gap mean value than CAD/CAM group with no statistically significant difference.After cementation,CAD/CAM group recorded a higher total marginal gap mean value than MAD/MAM groupwith no statistically significant difference. CAD/CAM group recorded a higher total seating discrepancy mean value than MAD/MAM groupwith no statistically significant difference.
Concerning the effect of the milling technique regarding the cement space 60µm:After cementation, MAD/MAMgroup with cement space 60µmrecorded a higher marginal gap mean value than CAD/CAM groupwith cement space 60µm with no statistically significant difference.
Concerning the effect of the cement space, the results of the copings fabricated by both milling techniques showed that subgroups with cement space 0 recorded the highest marginal gap mean value followed by cement space 30µm meanwhile the lowest marginal gap mean value was recorded with cement space 60µmbefore and after cementation and there was no significant difference between subgroups.MAD/MAMsubgroup with cement space 60µm recorded the highest seating discrepancy mean value followed by cement space 30µm meanwhile the lowest seating discrepancy mean value was recorded with cement space 0 with statistically significant difference. On the other hand, CAD/CAMsubgroup with cement space 30 recorded the highest seating discrepancy mean value followed by cement space 0 meanwhile the lowest seating discrepancy mean value was recorded with cement space 60 with no statistically significant difference.