Fracture strength of all-ceramic (Y-TZP) three- and four-unit fixed dental prostheses with different connector design and production history.

Aims: To investigate how different default settings for connector design of two different CAD/CAM systems, i.e. to compare how different radii of curvature in the embrasure area of the connector affect the fracture strength and the fracture mode of 3-unit and 4-unit all-ceramic FDPs made from Y-TZP and to investigate how the number of pontics affect the fracture strength of Y-TZP.

Material and methods: 32 all-ceramic three (3Z) and four (4Z)-unit, Y-TZP, FDP cores were made and divided in 4 groups, with 8 FDP in each group. The groups 3Z:1 and 4Z:1 were generated with a mechanical scanner, Procera Forte and the FDPs in group 3Z:2 and 4Z:2 were generated with an optical scanner, NobelProcera Scanner. The connector dimensions were set to 3 mm x 3 mm and core was set to 0.7 mm. The design of radius of the gingival and occlusal embrasures in the connector areas was set according to default settings and the manufacturer's recommendations. All the FDP cores were subjected to heat treatment, thermocycled for 5,000 cycles, preloaded for 10,000 cycles to simulate ageing and finally loaded to fracture.

Results: Regarding connector design a significant difference was found between group 3Z:1 and 3Z:2 (P<0.05), and group 4Z:1 (50% of the FDPs fractured during preloading 30-300N) and 4Z:2 (P<0.05). An extra pontic decreased the fracture strength up to 45%.

Conclusions: The default settings of the two different CAD/CAM systems had a great impact on the fracture strength. It is important that a CAD/CAM system is equipped with possibilities to design a connector that fulfils the clinical demands of mechanical function and longevity. The most crucial factor for the load-bearing capacity isthe design of the radius of the gingival embrasures. Increasing the number of pontics from three to four decreases the load-bearing capacity nearly twice.