Endurance of Different Thicknesses of Multilayer Zirconia Under Simulated Bruxism: An In Vitro Experiment

Objective: To evaluate the effects of specimen thickness and simulated bruxism loading on the flexural strength of multilayer zirconia at three clinically relevant thicknesses. Materials and Methods: Sixty bar specimens (N = 60) were milled from multilayer zirconia disks (graded 4Y-PSZ dentin/5Y-PSZ enamel composition) (IPS e.max® ZirCAD MT Multi, Ivoclar Vivadent, Schaan, Liechtenstein). Specimens were tested using a modified ISO 6872-based three-point flexural configuration (16 × 4 mm bar geometry) at three thickness levels, 1.5 mm (S), 2.0 mm (M), and 2.5 mm (L), yielding six groups of ten (S1, S2, M1, M2, L1, L2), where the numerical suffix designates uncycled and cycled subgroups respectively. Bruxism simulation was performed for 250,000 cycles at 490.5 N with a 6 mm metallic ball antagonist. Three-point flexural strength testing was performed on a universal testing machine. Surface morphology of cycled specimens was assessed by SEM. Statistical analysis employed a Scheirer–Ray–Hare test and Dunn’s post hoc test with Holm–Bonferroni correction, with significance set at p < 0.05. Results: Specimen thickness exerted a statistically significant effect on flexural strength (p < 0.001). Mean values rose from 267.50 ± 44.207 MPa (S1) and 280.11 ± 69.714 MPa (S2) to 517.00 ± 105.343 MPa (M1) and 604.20 ± 122.469 MPa (M2) to 659.20 ± 145.126 MPa (L1) and 746.90 ± 176.429 MPa (L2). Post hoc analysis confirmed significant differences in all pairwise comparisons involving S against M or L groups, while no significant difference was detected between M and L subgroups at any pairing. Bruxism (loading) exerted no statistically significant main effect on flexural strength in a two-way rank-based analysis accounting for thickness (Scheirer–Ray–Hare, p = 0.277), with no significant thickness × loading interaction (p = 0.876) and no significant loading effect within any individual thickness group after correction for multiple comparisons. SEM imaging revealed a smoother surface topography within the antagonist contact zone relative to surrounding areas, accompanied by larger debris particles at the impact site. Conclusions: Although thickness significantly influenced flexural strength, no statistically significant difference was detected between the 2.0 mm and 2.5 mm groups under the present experimental conditions and sample size. Evaluated only as descriptive ISO 6872 material-classification reference values, the 1.5 mm group’s mean flexural strength falls below, and the 2.0 mm group’s mean meets, the strength class associated with three-unit fixed dental prostheses including molars; because the present modified bar-specimen model does not satisfy ISO 6872’s slenderness recommendation and was not designed to validate a clinical minimum restoration thickness, these comparisons should not be read as an indication-specific clinical thickness recommendation.