Flexural Strength and Clinical Classification of Different Layers in 4/5Y-PSZ Zirconia Materials

Multilayer 4Y/5Y-PSZ zirconia materials have been developed to combine strength and translucency in monolithic "all-in-one" dental restorations. This study evaluated the flexural strength of different layers (incisal, transition, and dentin) in four commercially available multilayer zirconia systems using three-point bending tests in accordance with ISO 6872. A total of 360 CAD/CAM-fabricated bar-shaped specimens were prepared from the materials CE (Cercon yo ML, DentsplySirona), KA (Katana YML, Kuraray Noritake), PZ (3D ProZir, Aidite), PE (IPS e.max ZirCAD Prime esthetic), and assigned to layer-specific groups based on their position within the discs. After sintering and standardized surface finishing, specimens were tested under three-point bending conditions. Fracture strength was calculated and statistically analysed. Microstructural and fractographic analyses were performed to assess grain structure and to identify fracture origins. The results demonstrated significant differences in flexural strength both among materials and between layers. In general, dentin layers exhibited the highest strength, reaching mean values up to 1143 MPa, while incisal layers showed significantly lower values, with minima around 572 MPa. Only one material (CE) maintained flexural strength above the ISO threshold of 800 MPa across all layers, qualifying for unrestricted (class 5) clinical use. Other materials showed limitations, particularly in the more translucent incisal regions (KA, PE). One material fell below the ISO threshold (PZ). Weibull moduli revealed differences in reliability, with moduli ranging from 4.7 to 16.5. Fractographic evaluation identified typical fracture patterns such as surface grinding defects and internal porosity, but no abnormal fracture origins. The strength gradient corresponds to microstructural differences, particularly grain size and phase composition, influenced by yttria content. Increased translucency in incisal layers is associated with reduced mechanical performance. These findings emphasize that, despite aesthetic advantages, layer-dependent strength variations must be considered when selecting multilayer zirconia for clinical applications, especially in long-span restorations.