Optical, Structural and Biological Characteristics of Rapid-Sintered Multichomatic Zirconia

Background: To overcome the aesthetic limitations of dental monolithic zirconia resto-rations, multichromatic systems were developed to combine improved structural integrity with a natural shade gradient that mimics the optical properties of natural teeth. In re-sponse to the clinical demand for time-efficient, i.e. chairside fabrication of zirconia res-torations, rapid sintering protocols became necessary to adjust clinical efficiency with material performance. This study addresses the challenges of rapid sintering protocol related to optical performance and phase transformation of the final restoration, and the zirconia-cell interaction. Methods: The influence of rapid sintering protocol on the color stability of the final dental restoration was evaluated by CIE L*a*b* color space. Phase transformation was assessed through X-ray diffraction analysis. Cellular behavior was evaluated by measuring wettability, materials surface energy and cells mitochondrial activity assay on human gingival fibroblasts. Results: Optical measurements demonstrated that the total color change of enamel layer of the polished samples were being higher than clinically ac-ceptable values. Results of X-ray diffraction analysis, made for the fixed occupancy at Z0.935Y0.065O0.984 revealed that rapid sintering caused a decrease in the cubic (C) phase and an increase in the total amount of tetragonal (T) phases. Additionally, a small amount of the monoclinic (M) phase is noticed. Although glazing as a surface finishing procedure resulted in increased hydrophility both polished and glazed surface-treated specimens showed statistically comparable cell adhesion and proliferation (p>0.05). Conclusion: Five times higher heating and cooling rates caused difficulty in reaching equilibrium, leading to changes in lattice parameters and the formation of the metastable T' phase which could compromise aesthetic appearance of the final restoration.