Graded structures for damage resistant and aesthetic all-ceramic restorations

Objectives: Clinical studies revealed several performance deficiencies with alumina- and zirconia-based all-ceramic restorations: fracture; poor aesthetic properties of ceramic cores (particularly zirconia cores); and difficulty in achieving a strong ceramic-resin-based cement bond. We aim to address these issues by developing a functionally graded glass/zirconia/glass (G/Z/G) structure with improved damage resistance, aesthetics, and cementation properties. Methods: Using a glass powder composition developed in our laboratory and a commercial fine zirconia powder, we have successfully fabricated functionally graded G/Z/G structures. The microstructures of G/Z/G were examined utilizing a scanning electron microscopy (SEM). The crystalline phases present in G/Z/G were identified by X-ray diffraction (XRD). Young's modulus and hardness of G/Z/G were derived from nanoindentations. Critical loads for cementation radial fracture in G/Z/G plates (20 mm × 20 mm, 1.5 or 0.4 mm thick) bonded to polycarbonate substrates were determined by loading with a spherical indenter. Parallel studies were conducted on homogeneous yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) controls. Results: The G/Z/G structure consists of an outer surface aesthetic glass layer, a graded glass-Y-TZP layer, and a dense Y-TZP interior. The Young's modulus and hardness increase from surface to interior following power-law relations. For G/Z/G plates of 1.5 and 0.4 mm thick, critical loads for cementation radial fracture were 1990 ± 107 N (mean ± S.D., n = 6) and 227 ± 20 N (mean ± S.D., n = 6), respectively, which were ∼30 and 50% higher than those for their monolithic Y-TZP counterparts (1388 ± 90 N for 1.5 mm and 113 ± 10 N for 0.4 mm thick; mean ± S.D., n = 6 for each thickness). A 1-sample t-test revealed significant difference (p < 0.001) in critical loads for radial fracture of G/Z/G and homogeneous Y-TZP for both specimen thicknesses. Significance: Our results indicate that functionally graded G/Z/G structures exhibit improved damage resistance, aesthetics, and potentially cementation properties compared to homogeneous Y-TZP.