New Insights in the Veneering Chipping of Zirconium-Oxide Fixed-Dental-Prostheses (FDPs)

Objectives: Previous mouth-motion fatigue study has shown that veneer chipping at the buccal cusp pontic area of zirconium-oxide FDPs occurs at significantly lower loads in over-pressed relative to hand-veneered specimens. Here, we provide new insights into veneer chipping behavior in view of effective fracture toughness and micro-hardness of the over-pressed and hand-veneered porcelain. Methods: Three over-pressed and three-hand veneered FDPs were obtained from Nobel Biocare (Gothenburg, Sweden). Each sample included two zirconium-oxide abutments fabricated on NobelReplace implant-replicas and a zirconium-oxide bridge framework CAD/CAM manufactured. Abutments were screw-retained with 32 N/cm torque to titanium implants. FDPs were embedded in epoxy-resin (Epofix, Struers, Copenhagen, Denmark) to allow for longitudinally sectioning (Isomet 1000, Buehler, Lake Bluff, IL). In each cross-section, twelve indentations were performed in the porcelain occlusal pontic area with a peak load of 9.8 N at a dwell time of 5 s using a Vickers micro-hardness tester (Leco, St. Joseph, MI, USA). To avoid interactions, indentations were placed at a distance of at least twice the crack length from artifacts and porcelain borders. Effective fracture toughness (MPa•m^(1/2)) and micro-hardness (GPa) were computed using validated formulas. A t-test data analysis (α = 0.05; SigmaPlot 11.0, Ashburn, VA, US) was performed to determine whether fracture toughness and micro-hardness were significantly different between the two porcelains. Results: The effective fracture toughness was significantly higher for hand-veneered (0.95±0.13 MPa•m^(1/2) (mean±SD)) than over-pressed porcelain (0.53±0.12 MPa•m^(1/2) (mean±SD)) (p < 0.001). Micro-hardness was significantly lower in hand-veneered (6.26±0.9 GPa (mean±SD)) than over-pressed porcelain (8.54±0.6 GPa (mean±SD)) (p < 0.001). Conclusion: Higher fracture toughness and lower hardness are more effective to suppress crack initiation and propagation, leading to improved resistance to fatigue chipping in the hand-veneered porcelain relative to over-pressed veneer. Supported in part by NIH/NIDCR-R01DE017925 and Nobel Biocare grant 2007-561.