Biaxial Flexural Strength and Fracture Behavior of Five-Unit Monolithic Zirconia Fixed Dental Prostheses: An Experimental and Finite Element Study

Higher-translucency zirconia grades (4Y- and 5Y-PSZ) offer esthetic advantages over conventional 3Y-TZP but exhibit reduced flexural strength, raising concerns for restorations with reduced connector dimensions. It is unclear whether biaxial flexural strength, as determined by standardized specimen testing, translates to the fracture resistance of geometrically complex multi-unit bridges. This study compared the biaxial flexural strength (DIN EN ISO 6872:2015, n = 45) of three zirconia grades (3Y-TZP, 4Y-PSZ, and 5Y-PSZ) with the fracture behavior of monolithic five-unit fixed dental prostheses (FDPs) with deliberately undersized connectors (n = 24, teeth 33–37) made from the same three zirconia grades. Finite element (FE) analysis was used to compare the simulated elastic stress distribution with the experimentally observed fracture location. Biaxial flexural strength differed significantly among all groups (3Y-TZP: 1406.5 ± 133.3 MPa, 4Y-PSZ: 1146.9 ± 215.9 MPa, and 5Y-PSZ: 657.0 ± 137.5 MPa; p < 0.001). In contrast, bridge fracture loads differed less between the zirconia grades (3Y-TZP: 542.1 ± 44.9 N, 4Y-PSZ: 466.6 ± 116.6 N, and 5Y-PSZ: 372.1 ± 32.1 N). Only 5Y-PSZ showed significantly lower fracture loads than 3Y-TZP (p < 0.001), whereas 4Y-PSZ did not differ significantly from either 3Y-TZP (p = 0.253) or 5Y-PSZ (p = 0.129). All bridges initially fractured at the undersized connector, matching the site of the highest predicted tensile stress in den FE analysis. With this deliberately undersized connector, the differences in biaxial flexural strength between 3Y-TZP and 4Y-PSZ did not translate proportionally in a significant difference in fracture load. The FE analysis correctly identified the fracture-critical connector. The fracture load was not predicted, since the simulations were calibrated to the experimental fracture load values.