Material characteristics of a novel shrinkage-free ZrSiO4 ceramic for the fabrication of posterior crowns

Objective To confirm the clinical applicability of a novel ZrSiO4 (zircon) based shrinkage-free ceramic material, the flexural strength, fracture toughness and chemical solubility were tested. In addition, the fracture load of full crowns made from this material was tested after cyclic thermomechanical loading. Methods Flexural strength of 12 specimens was measured using a biaxial bending test. Fracture toughness was measured using 10 slotted box shaped specimens. The specimens were fractured using a universal testing machine; fracture loads were recorded. A chemical solubility test was performed in accordance with ISO norm 6872. Additionally, 32 ZrSiO4 all-ceramic crowns were fabricated on extracted caries-free human molars. Sixteen Empress 2 and 16 PFM crowns served as a reference control. After artificial aging of 1.2 million cycles in the chewing simulator, the survival rate of the crowns was determined. The fracture load of all surviving specimens was obtained by loading the crowns until fracture in a universal testing machine. Results A flexural strength of 328 MPa was found. The fracture toughness of the ZrSiO4 ceramic was 5.16 MPa m0.5. The chemical solubility amounted to 7.2 μg/cm2. All specimens survived the chewing simulation (survival rate: 100%); no crowns had to be re-cemented. A mean fracture strength of 1790 N was found for Everest HPC for Empress 2 crowns, 1715 N for Empress 2 crowns and 2416 N for PFM crowns. Fracture loads of PFM crowns were significantly higher than for Empress 2 crowns (P = 0.032) as well as ZrSiO4-crowns (P = 0.007). There was no significant difference between ZrSiO4-crowns and Empress 2 crowns (P = 0.743). Significance At the present stage, Everest HPC can be recommended for the fabrication of single crowns as an alternative to conventional PFM and other all-ceramic crowns, because its fracture strength exceeds average masticatory forces in the posterior region.