ABSTRACT Objective This study investigated the influence of restoration type (partial‐coverage versus crown) and ceramic‐layer thickness on the fatigue behavior and failure load of monolithic composition‐gradient multilayer zirconia (4Y‐PSZ/5Y‐PSZ) molar restorations. Material and Methods Seventy‐two CAD/CAM‐fabricated monolithic zirconia restorations (IPS e.max ZirCAD‐Prime‐Esthetic, Ivoclar‐Vivadent) were assigned to six groups ( n = 12), based on restoration type—partial‐coverage restoration (PCR) or crown (C)—and ceramic thickness (occlusal/buccal: 0.5/0.4, 1.0/0.6, or 1.5/0.8 mm). All restorations were adhesively bonded to standardized dentin‐analogue dies. Specimens underwent thermomechanical fatigue (1.2 million cycles, 49 N, 1.6 Hz, 5°C–55°C), followed by single load to failure testing. Data were analyzed using ANOVA, Tukey's post hoc test, and independent t‐tests (α = 0.05). Results All specimens survived fatigue. Evident cracks were observed post‐fatigue; one each in PCR‐0.5 and C‐0.5 groups, resulting in an overall success rate of 97.22%. Mean failure loads (N) were: PCR‐0.5: 2047, PCR‐1.0: 2018, PCR‐1.5: 2777, C‐0.5: 695, C‐1.0: 1957, C‐1.5: 3503. Ultrathin (0.5 mm) PCRs demonstrated significantly higher failure loads than ultrathin crowns ( p < 0.001), whereas crowns at 1.5 mm thickness outperformed PCRs ( p = 0.005). No significant difference was observed at 1.0 mm thickness ( p = 0.634). Conclusion At ultrathin thicknesses (0.5 mm), partial‐coverage designs showed superior failure loads over crowns. Both restoration types are mechanically viable at 1.0 mm and 1.5 mm thicknesses. Clinical Significance Composition‐gradient multilayer‐zirconia demonstrates high fatigue resistance and is well‐suited for minimally invasive, non‐retentive PCR molar restorations. However, a minimum thickness of 1.0 mm should be maintained for single crowns.
Spitznagel et al. (Wed,) studied this question.