الفهرس 
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Abstract
Aging of zirconia with low temperature degradation presented an aggravating factor for properties of the yttrium-stabilized zirconia. This study compared the effect of low temperature degradation on fracture resistance, shade and shear bond strength to adhesive resin cement of two types of CAD/CAM monolithic zirconia along the vertical marginal gap and internal fit.A maxillary premolar was prepared to receive all ceramic crown. A silicon mold was made out of the prepared tooth to produce a master metal die that was used to fabricate 20 fully contoured crowns of the two types of the translucent zirconia. The crowns were fabricated through milling of pre-sintered bar coded zirconia blocks using Cerec and ZirkonZahn Cad/CAM milling systems. The milled crowns were then sintered. Five crowns of each type of the fabricated zirconia crowns were aged under standard autoclave condition of at 134 °C at 2 bar pressure for 10 hours. The fabricated crowns were adhesively cemented on the master metal die one after another to evaluate the facture resistance before and after aging, thus giving an idea about the effect of aging on the facture resistance of zirconia crowns. Results showed Incoris TZI crowns had higher fracture resistance (before 3898 ± 124.8 N and 3399 ± 159.5 N after aging) than Prettau Crowns (before 3504 ± 336.2 N and 3077 ± 170.8 N after aging).The prepared tooth was again duplicated using silicon mold to produce 20 epoxy dies. After which 10 fully contoured crowns of each type of zirconia were fabricated on the epoxy dies. Again 5 crowns of each type of zirconia were artificially aged under standard autoclave conditions for 10 hours. Measurements of the vertical marginal gap before cementation were under taken on the non-aged crowns using a metallurgic microscope. Results showed that Prettau (96.91 ± 2.78 µm) crowns had a statistically significant higher marginal gap than that of Incoris TZI crowns (63.17 ± 6.4 µm).
The crowns were then adhesively cemented on theircorresponding epoxy dies. The color change ΔE of all crown groups was measured before and after aging using intra oral spectrophotometer (Vita Easyshade). Results showed that Incoris TZI crowns had lower mean value of ΔE (before 3.56 ± 0.28 and 4.36 ± 0.34 after aging) than that of Prettau crowns (before 4.26 ± 0.51 and 5.6 ± 0.47 after aging), while only Prettau crowns had Easyshade evaluation “Adjust” after aging.The cemented crown samples were sectioned using low speed saw and internal fit measurements for both types of crowns were undertaken using a metallurgic microscope. Results showed that the highest mean value was reported for the Prettau crowns (117 ± 3.67µm) while the lowest value was reported with Incoris TZI crowns (103.2 ± 3.61 µm).
For evaluating the zirconia/ resin cement shear bond strength, pre-sintered Incoris TZI and Prettau blocks were milled into rectangular shaped samples (4 x 19 x 15 mm) using low speed saw and ZirkonZahn Cad/CAM milling machine respectively. The samples were sintered and disc shaped composite samples were then adhesively cemented to zirconia samples. Each type of the Zirconia/ Panavia/ composite sample and of zirconia samples were then further subdivided into two subgroups according to the thermal cycling. Computerized universal testing machine was used to evaluate the shear bond strength before and after thermal cycling. Results showed that the highest mean shear bond strength value were reported for the Incoris TZI (before 12.75± 1.32 Mpa, and 3.002 ± 0.52 Mpa after aging), while the lowest value was reported for the Prettau zirconia (before 8.58 ± 0.92 Mpa and 1.584 ± 0.48 after aging).