Effect of changing connector area of three-unit fixed zirconia prosthesis in various silhouette and elliptical shapes on their compressive and flexural strength

Abstract

Introduction: Despite the many advances made in the field of all-ceramic restorations, there is still not enough information about the design and thickness of the connector, and the study in this field continues. Therefore, the purpose of this study was to compare the fracture strength and flexural strength in fixed three-unit zirconia prostheses with silhouette and elliptical connectors made by CAD/CAM method for the posterior region. Material and methods: First molar and first premolar teeth of the lower jaw were selected as abutment for FPD prosthesis, and teeth preparation was done on the plastic model of the jaw. It was molded from a plastic model with silicone and its cast was made with dyestone. Scanned cast and FPD samples were designed and manufactured with CAD/CAM system in two groups with silhouette and elliptical connector shapes, each group having 3 subgroups with connector area of 8 mm2, 10 mm2 and 12 mm2. Then the zirconia samples were cleaned and sintered. The samples were cemented and kept in saliva solution at 37 degrees for 48 hours before the test. The flexural strength and fracture strength of the samples were measured with a three-point bending machine and a universal testing machine. Result: There is no significant difference in fracture strength between silhouette and elliptical (P value > 0.05). The fracture strength in the area of 12 mm2 is significantly higher than the area of 8 and 10 mm2 (P value < 0.05). There is no significant difference in flexural strength between silhouette and elliptical (P value > 0.05). There is no significant difference in flexural strength in different combinations of connector shape and connector area (P value > 0.05). Conclusion: In zirconia three-unit fixed prostheses with elliptical or silhouette connectors with areas of 8, 10 and 12 mm2, the flexural strength is not affected by the shape or the area and the fracture strength is not affected by the shape of the connector. However, the fracture toughness increases with the increase of the connector area.