Variable reduced metal support for collarless metal ceramic crowns: a new model for strength evaluation

A newly designed specimen allowed strength testing of simulated collarless metal ceramic crowns. The design provided precision replication of specimens having ceramic veneers that were completely supported by metal or that were extended 1 and 2 mm beyond the metal support. Twelve specimens of each design were loaded to failure at angles of 90, 45, and 6 degrees. Specimens with unsupported extensions approximated the compressive strength of those with complete framework support (means of 291.6 MPa, 320.2 MPa, and 335.9 MPa, respectively, for 1 mm, 2 mm, and 0 mm of porcelain extension) when tested at 90 degrees. However, for corresponding porcelain lengths, mean resistance to fracture at an angle of 45 degrees was 7.3, 11, and 5.8 times smaller, and at an angle of 6 degrees, 45, 60, and 25 times smaller. The differences were highly significant (P < .001). These results suggest that collarless metal ceramic crowns having up to 2 mm of unsupported porcelain could resist the same axial pressure as those restorations with complete metal support, provided a 90-degree shoulder tooth preparation is used.