Fatigue Performance and Failure Loads of Composition-Gradient Multilayer Zirconia: Partial-Coverage Restorations...

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.

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).

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).

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.

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.