INFLUENCE OF THICKNESS ON INTERFACIAL FAILURE OF ALL-CERAMIC CROWNS
Previous finite element modeling and physical testing at the Naval Dental School/NIST indicated that failure loads would not increase with ceramic thickness for all-ceramic crowns and inlays loaded in the occlusal fossa. In fact, this research suggested that ceramic restorations could support higher loads at 0.5 mm thick than at 2.0 mm thick, if they were well bonded and uniformly supported. The present investigation extended previous physical testing of simple ceramic bars to full ceramic crowns. Forty molar analogs were machined from epoxy-glass rods and forty matched crown patterns, ten each of 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm occlusal thicknesses were machined from plexiglass rods, allowing an 80-100 micrometer precementation space. Crown patterns were sprued, invested, burned-out and pressed in a leucite reinforced feldspathic ceramic (OPC, Jeneric/Pentron). Pressing was accomplished in an Ivoclar Empress machine programmed for Jeneric Pentron OPC. Both crown analogs and patterns were prepared by acid etching, silage treatment, application of a low viscosity bonding agent, and cemented under a constant load of 100N. Specimens were loaded to failure at 0.5 mm per minute cross head speed using a blunt piston cushioned with a polyethylene sheet in a central fossa location. An acoustic transducer and transillumination were utilized for crack detection during loading. Significant differences among mean failure loads per thickness group were analyzed using ANOVA and a 95% multiple range test.