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

Abstract

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°C – 55°C) 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.