Fracture resistance of monolithic gradient zirconia crowns with different finish line designs and cement spaces

Objective: This study was aimed at assessing the effects of various finish line designs and cement gap thicknesses on the fracture resistance of gradient zirconia crowns. Methods: Sixty crowns were fabricated on stainless-steel dies with yttria multi-layered (YML) zirconia and categorized into three primary groups according to finish line type (knife-edge, chamfer, and rounded shoulder). Each group was further classified into two subgroups (n = 10) according to cement space thickness (50 or 80 μm). Optical impressions of the dies were acquired with an indirect laboratory scanner, and cement spaces (50 or 80 μm) were established for each finish line type in Exocad software. Subsequently, the zirconia crowns were milled, sintered, cemented onto their respective dies, thermomechanically aged, and subjected to loading until fracture. The data were statistically analyzed with one-way ANOVA and post hoc tests for pairwise comparisons. Additionally, two-way ANOVA was used to investigate interactions between two study variables. Results: No significant differences between chamfer and knife-edge patterns were observed, whereas the rounded shoulder pattern exhibited significantly higher failure load values. Similarly, no significant difference was observed between 50 μm and 80 μm cement space. Conclusions: Knife-edge margins with YML gradient zirconia crowns provide a reliable alternative to shoulder margins, particularly in minimally invasive preparations. A cement space of 80 μm rather than 50 μm is preferred for various finish line designs.