الفهرس 
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Abstract
This study was designed to evaluate the effect of different milling techniques and different degree of tapers on the retention of Y-TZP zirconia crowns.
Three stainless steel dies were designed to simulate all ceramic full coverage crown preparation for a molar tooth. Three different TOC designs were prepared 8º, 14º, 20º. An occlusal notch was prepared for exact positioning of the copings during measurements.
Impressions of the master dies were made with a vinyl polysiloxane impression material using perforated plastic tubes. For CAD/ CAM technique, the impressions of the master dies were poured with the laser visible stone. For MAD/MAM technique, impressions of the master dies were poured using type IV dental stone. The 30 impressions were poured with non shrink epoxy resin, the resin dies were used for the measurements.
Fifteen MAD/MAM copings were fabricated using Zirkon Zahn system. Three non anatomical composite mock ups were fabricated on the three stone dies of different tapers. Composite bar was added to the occlusal surface of the copings, to provide a mean for removal of the coping during retention testing. The thickness of occlusal surface was adjusted to be 1.5mm, the two ends of the bar were extended about 3mm. Each mock up was traced and milled from green zirconia block using Zirkonzahn machine. Another fifteen CAD/CAM copings were fabricated using CEREC InLab system .The laser visible stone dies were laser scanned. Non anatomical copings were designed using the in Lab3D software version 3.65. The occlusal thickness was increased to be 1.5mm by digital addition of a bar to provide a mean of removal of the coping during retention testing. The two ends of the bar were extended about 3mm. VITA In-Ceram YZ presintered Cubes with appropriate size were selected and milled by the MC XL unit.
The copings were milled with an oversize of approximately 25% to compensate for the later sintering shrinkage. The copings were sintered in high temperature furnace to have maximum density.
Each coping was checked on its relevant working die, master steel die and the epoxy die for seating and marginal fit. Internal surfaces of the copings were air particle abraded using 120 µm alumina at 2.5 bar pressure in abrasion unit. Glass ionomer cement was used for cementing the copings to the epoxy resin dies. The coping were seated, during cementation, utilizing static finger pressure then axially loaded with a 3kg load using a specially designed device. The copings were left under the static load for 5 minutes to ensure cement setting. After 24 hr, measurements were performed.
A coping attachment device was machined, had a channel with dimensions to accommodate the occlusal bar. The assembled epoxy resin dies and zirconia copings were placed into this device. The coping attachment device was connected to the upper member of the universal testing and the epoxy dies were connected to the lower member of the testing machine. The retention of the zirconia coping was measured by applying tensile force sufficient to dislodge the coping from the abutment, using a tensile or pull load, with a crosshead speed of 0.5 mm/min. The force in Newtons required to remove the copings was recorded and used for statistical analysis.
The results of this study revealed generally that CAD/CAM technique with total occlusal convergence 20º (subgroup II c)gave the lowest retentive force (15.33 N) while MAD/MAM technique with 14º (subgroup I b) gave the highest retentive force (227.6 N).
Within the limitations of this study, the following conclusions can be drawn:
1-There is significant effect of taper on the retention, Preparations with 14º TOC produced the most retentive zirconia crowns.
2- Increasing the taper to 20º decreased the retention significantly.
3- The two tested milling techniques have significant effect on the retention. Zirkonzahn copings recorded higher retention than CEREC inLab copings.