The effect of porcelain veneer extension on strength of metal ceramic crowns.

One goal in metal ceramic substructure design is strengthening the restoration. This investigation compared the strength of four metal ceramic crown designs advocated in the dental literature. Metal ceramic crowns were fabricated, luted to a master die, and loaded to failure on a mechanical testing machine. Crowns with a facial veneer of porcelain covered occlusally with metal withstood significantly higher failure loads than did the other designs. These were followed by crowns having the porcelain extended to the central groove, crowns with facial porcelain veneers without occlusal metal coverage and, the weakest group, crowns with complete occlusal porcelain coverage. There were no significant differences between the latter three designs.