Zirconia-Based Functionally Graded Structures for Dental Restorative Materials

Objectives: Several clinical performance deficiencies with zirconia-based all ceramic restorations persist: chipping of the porcelain veneers; poor aesthetic properties of zirconia cores; and difficulty in achieving a strong zirconia-tooth structure bond. This research aims to address these issues by developing a zirconia-based functionally graded structure with improved damage resistance, aesthetics, and cementation properties for dental crowns and bridges.

Methods: Using glass-ceramic powders of various compositions developed in our laboratory and a commercial fine zirconia powder, we have successfully fabricated functionally graded glass/zirconia/glass (G/Z/G) structures with improved properties compared to monolithic zirconia (Y-TZP). G/Z/G plates (20x20 mm of 1.5 and 0.4 mm thick) were epoxy bonded to polycarbonate substrates (simulating tooth dentin structure) for load-to-fracture tests using a tungsten carbide spherical indenter (r = 3.18 mm) on an universal testing machine (Instron 5566). As a control, monolithic Y-TZP plates were epoxy bonded to polycarbonate substrates. The chemical composition and microstructure of G/Z/G were characterized using x-ray diffraction, optical microscope, and scanning electron microscope.

Results: The surfaces of G/Z/G consisted of aesthetic glass-ceramic, sandwiching a dense Y-TZP interior. For 1.5 mm thick G/Z/G on polycarbonate, critical loads to fracture were 1990±107 (mean±SD, n = 6), approximately 30% higher than those for monolithic Y-TZP (1388±90, n = 6). For 0.4 mm thick specimens, critical loads for G/Z/G (227±20, n = 6) were almost twice as high as those for monolith Y-TZP (113±10, n = 6), suggesting that the impact of graded structure on the flexural damage resistance could be more significant for thin (d < 0.5 mm) ceramic prostheses. 1-sample t-test showed significant differences in test results (p < 0.001) for both thicknesses.

Conclusion: Zirconia-based functionally graded structures exhibit better damage resistance, aesthetics, and cementation properties than monolithic Y-TZP. Supported by NIH/NIDCR 1R01 DE017925 (PI. Zhang).