Objectives: The purpose of this study was to evaluate the effect of coping thickness and cement space on the fracture resistance (FR) of alumina and zirconia ceramic copings fabricated with CAD/CAM technology. Methods: A melamine tooth # 14 was prepared for a full coverage restoration with a 1mm marginal width, 4mm abutment height, and 12 degrees convergence angle. A cast metal master die of the preparation was fabricated. Cerec inLab (Sirona) was used to mill thirty In-Ceram (Vivadent) alumina and In-Ceram (Vivadent) zirconia copings from the master die. The copings were designed with thicknesses of either 0.6 or 0.8mm, and with cement spaces of 20, 30, or 40 microns (n=5). Sixty identical composite resin (Surefil, Dentsply-Caulk) dies simulating the prepared tooth were fabricated from a polyvinyl siloxane impression of the master die. Copings were glass infiltrated and luted to the composite resin dies using Panavia 21 TC (Kuraray) bonding agent.  Specimens were stored in distilled water at 37C for 24h. The occlusal surface of each coping was covered with a small square of soft rubber and loaded axially with a flat steel plate on an Instron Universal Testing Machine (crosshead speed of 0.5 mm/minute) until fracture of the ceramic copings occurred. The data were analyzed with multiple ANOVA (alpha=0.05).Result: ANOVA revealed no significant difference in FR (P > 0.05) based on material, coping thickness, or cement space. Mean FR in newtons and one standard deviation (SD) are shown below. Conclusion: For the coping thicknesses and cement spaces tested, zirconia and alumina copings fabricated with Cerec inLab CAD/CAM technology demonstrated similar values of fracture resistance.

Coping Thickness  (mm)	Cement Space (microns)	FR Alumina (SD) (newtons)	FR Zirconia (SD) (newtons)
0.6	20	1504(27)	1525(80)
	30	1555(70)	1482(101)
	40	1448(69)	1604(100)
0.8	20	1544(79)	1672(121)
	30	1569(111)	1636(85)
	40	1659(140)	1554(128)