Biofunctional Properties of Multilayer Zirconia in Dentistry: Influence of Sintering and Surface Treatments

Abstract

The objective of this study was to analyze the differences in biofunctional properties between conventionally and speed-sintered multilayer monolithic zirconia with different surface finishing properties. Zirconia discs (Katana STML; SagemaxNexxZr T Multi), shade A2, were cut by the dry milling method, according to the fraction of each layer (Katana n=4, Sagemax n=3). The final dimensions were 10 × 13 × 1mm. Milling was followed by two sintering protocols, conventional (i.e. long), and speed ones. After sintering, the samples underwent a surface treatment according to which they were divided into two subgroups: glazed and polished. A digital spectrophotometer, Datacolor SF300, was used for sample color measurements. The structural analysis was performed for the Katana STML samples that, after sintering, exhibited total color change greater than AT values. The refinement of the collected x-ray diffraction data was done based on the Rietveld method. Biocompatibility of the materials was evaluated by direct MTT assay for 72 h on human gingival fibroblasts (hGFs). The polished Katana specimens exhibited the highest color change, with DE* ranging from 1.514 to 3.013 between layers. DE* values for all layers were above the perceptibility threshold (PT) (DE*>1.2), and the enamel layer (EL) exceeded clinically acceptable limits (DE*>2.7). Additionally, lightness difference (DL*=2.453), contributed the most to the color change of this sample. For glazed samples, DE* values ranged from 1.528 to 2.580. The structural and microstructural analysis revealed the nanoparticulate nature of the CS- and SS-Katana. The increasing of microstrain was observed in the SS-Katana, while the quantitative analysis revealed the appearance of an additional tetragonal phase (S.G. P4₂/nmc) in this sample, which coexists with one cubic (S.G. Fm-3m) and one tetragonal phase. On the contrary, DE values for polished and glazed Sagemax samples, obtained from all layers, were clinically acceptable (DE*<2.23). After 24h of direct contact of the hGFs with the polished and glazed surfaces, similar (P>0.05) mitochondrial activity, LDH realize, and cell adhesion were recorded. Increased lightness of speed sintered Katana STML can be due to the presence of the yttria-lean T1 phase with higher tetragonality, whose presence can be caused by the speed sintering protocol.