Statement of problem
The effect of anatomic contour design in all or parts of zirconia crowns is uncertain regarding clinical reliability and survival rates.

The purpose of this in vitro study was to compare the load at fracture of monolithic, anatomic contour zirconia crowns with bilayered crowns with and without a cervical zirconia collar.

Materials and methods
Thirty zirconia crowns were fabricated for a shallow chamfer molar preparation, 10 with a normal core-veneer design, 10 with a core-veneer design with an additional cervical collar of zirconia, and 10 with a monolithic, anatomic contour design. Veneering ceramic was applied to the first 20 specimens to create an anatomic form. All crowns were cemented to epoxy abutments and loaded until complete fracture with a clinically relevant test method. The fracture modes and load at fracture were recorded.

Statistically significant differences were found in the load at fracture and fracture modes among the test groups (P<.001). All fractures except one initiated in the crown margin, mostly in the proximal region. The mean load at fracture was 4091 N for the normal core-veneer design, 4712 N for the collar design, and 6517 N for the monolithic, anatomic contour design.

Monolithic, anatomical contour design gave higher loads at fracture than traditional core-veneer design. Crowns with a cervical zirconia collar had higher load at fracture than the core-veneer design, but lower than the monolithic crowns.

Load at fracture of monolithic and bilayered zirconia crowns with and without a cervical zirconia collar.
Øilo M, Kvam K, Gjerdet NR.