Effect of position within a blank and thermo-mechanical aging on fracture force and failure mode of strength-gradient multilayer zirconia crowns.

PURPOSE: This study aimed to investigate the effects of blank position and thermo-mechanical aging on the fracture force of strength-gradient multilayer (SGM) zirconia crowns. MATERIALS AND METHODS: Eighty premolar SGM zirconia crowns (KATANA™ Zirconia YML) were assigned to eight groups (n = 10), based on four blank positions (1, 2, 3, and 4 mm below the top edge) and two conditions (with and without thermo-mechanical aging). Aging simulated five years of clinical service through 6,000 thermocycles (5℃ - 55℃) and cyclic fatigue (50 N, 1.2 Hz, 1,200,000 cycles). Fracture force was measured using a universal testing machine. Failure modes and fractographic features were examined. Data were assessed using two-way ANOVA, Bonferroni post hoc tests, independent t-tests, and Spearman correlation (P < .05). RESULTS: Fracture force was significantly influenced by both blank position and thermo-mechanical aging. Crowns from the 1 - 2 mm regions demonstrated significantly lower fracture force and more severe failure modes (Types I - V), while those from the 3 - 4 mm regions showed significantly higher fracture force and milder modes (Types I - III). Thermo-mechanical aging reduced fracture force in all groups, supported by fractographic analysis. A strong negative correlation existed between position depth and failure severity. CONCLUSION: Both blank position and thermo-mechanical aging significantly affected fracture force and failure behavior. Lower blank positions provide better mechanical performance and are recommended for posterior restorations. Conversely, upper blank positions-offering improved translucency-are suitable for anterior restorations where esthetics are prioritized. These findings offer clinically relevant guidance for optimizing strength and appearance in zirconia crown fabrication.