An in vitro study of the compressive load at fracture of Procera AllCeram crowns with varying thickness of veneer porcelain

Statement of problem. When designing and cutting a crown preparation, a balance must be achieved between the amount of tooth structure conserved and the space created for the restoration. All-ceramic crowns, which contain a core, are usually veneered with porcelain to produce the desired aesthetics and contour. The effect of veneer porcelain on the strength of the restoration has been a cause of debate. Purpose. The purpose of this study,,vas to investigate the effect of different thickness of veneer porcelain on the compressive load at fracture of Procera AllCeram crowns. Material and Methods. Sixty brass dies were fabricated with a crown-like preparation and a chamfer margin. Sixty crowns were fabricated with a 0.6-mm-thick core: Procera crowns with either a 0.4-min- or 0.9-mm-thick veneer of AllCeram (Groups I and 2 respectively) and In-Ceram crowns with a 0.9-mm-thick veneer of Vitadur Alpha porcelain (Group 3). Each group consisted of 20 crowns. In-Ceram crowns were used as the control group. All crowns were measured at 4 axial and I occlusal random locations before autoglazing. They were air abraded and adhesively bonded onto the appropriate brass die using Clearfil Newbond Bonding Agent, Clearfil Porcelain Bond Activator, and Panavia 21 TC Dental Adhesive as the luting agent. After storage in distilled water at 37degreesC for 24 hours, the specimens were placed in a compressive test rig within an Instron universal testing machine and loaded in the center of the occlusal surface with a 4-mm diameter stainless steel ball. An axial preload of 20 N was applied before compressive testing at a crosshead speed of 0.1 mm/minute until fracture occurred. An analysis of variance revealed no significant difference in the load at fracture between the 3 groups (P=.3). Using a Bonferroni correction, independent t tests revealed no significant difference in load at fracture for any of the pair-wise comparisons between the 3 test groups (P < .05). Results. The mean load at fracture for Group 1 was 2197.6 N (SD = 776.4); Group 2, 2401.4 N (SD = 699.1),and Group 3, 2581.0 N (SD = 715.6). Conclusion. Within the parameters of this study, the axial thickness of veneer porcelain did not have a significant effect on the compressive load at fracture of Procera AllCeram crowns.