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Layered-type zirconia disks, which offer color gradation from enamel to cervical shade, have been employed in recent years to replicate the shades of natural teeth. The layered structure is effective at replicating colors and has helped popularize monolithic zirconia restorations. However, the sintering shrinkage of zirconia is very large; thus, controlling the sintering distortion is very important. Thus, the objective of this study was to determine the influence exerted by the layered structure of the zirconia disk and the vertical milling area on the sintering distortion. An experimental fixed partial denture (FPD) was designed based on a 4-unit monolithic zirconia FPD. A single-composition (SC)-type disk with no shade and a single-composition-layered (SCL)-type disk with shade gradation were selected for this study. In particular, three milling areas, the top end of the disk (area I), vertical center (area II), and bottom end of the disk (area III), were investigated. Moreover, the sintering distortions generated by the experimental FPDs were measured. Results showed that sintering distortion in 4-unit monolithic zirconia FPDs occurred in all SC and SCL areas. Additionally, the sintering distortions were affected by the layered structure of the zirconia disks, the degree of which depended on the milling area (area I > area II > area III). Thus, when fabricating dental prosthesis using SCL zirconia disks, the milling area must be selected considering both the color adjustment and sintering distortion., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

This study investigated the optical properties and flexural strength of translucent TZP layered with high-translucent PSZ using resin cement of various shades. Zirconia specimens (translucent TZP; Zpex and high-translucent PSZ; ZpexSmile) were 13 mm in diameter, layered at thickness ratios of 0.3/0.7, 0.5/0.5, and 0.7/0.3 mm (ZpexSmile/Zpex), and then luted using resin cement of 3 shades. Monolithic specimens of both were used as controls. CIE L*a*b* color coordinates and translucency parameter (TP) were evaluated as optical properties. Biaxial flexural strength was also determined as a mechanical evaluation. The a* and b* values of layered specimens varied depending on the shade of cement. TP values were not affected by shade of cement and thickness ratio. The biaxial flexural strength was intermediate value between both monolithic specimens. The layering method of zirconia with various translucencies using resin cement of different shades can improve color expression while maintaining clinically sufficient flexural strength.

The technology of physicochemical surface modification is available for enhancing the bioactivity and osseointegration capability of tetragonal zirconia polycrystal (TZP). Hydrophobicity index and electrical charge play important roles in protein adsorption. We previously studied the mechanism underlying the adsorption of bovine serum albumin (BSA) on the surfaces of dental materials and hydroxyapatite in vitro. The aim of the present study was to clarify the correlation among the adsorption of BSA to TZP and physicochemically modified TZP surfaces and the zeta potential of BSA and TZP. We used TZP that was sintered at 1350°C for 2 h in air because this kind of TZP is widely applied in the field of dentistry. Surface physicochemistry was modified with ultraviolet light (UV) and atmospheric-pressure plasma treatment. The zeta potentials were measured with ELSZ-1000 and ELSZ-2000 analyzers (Otsuka Electronics, Hirakata, Japan). All experiments were conducted in 10 mM NaCl (pH 7.0). The zeta potentials of as-received TZP and BSA were negative, but those of UV- and plasma-treated TZP were positive. The reason the zeta potentials of TZP changed positive by physicochemical modification is due to an increase in the amount of basic hydroxyl groups. The zeta potentials of UV- and plasma-treated TZP after BSA adsorption were negative. These results suggested that electrostatic interactions play an important role in BSA adsorption to TZP and modified TZP surfaces, so that this modified surface may control the adsorption of protein.