The effect of finish line preparation and layer thickness on the failure load and fractography of ZrO2 copings

Statement of problem. To prevent tooth weakening or pulp irritation, there is a need for a minimally invasive method of preparing single anterior crowns. Restoration dimensions for reduced coping thicknesses or less invasive finish line preparations are required. Purpose. The purposed of this in vitro was to study investigate the fracture performance of high-strength zirconia copings, compare knife-edge margins with chamfer finish lines, and examine the effect of reducing the layer thickness from 0.5 mm to 0.3 mm. Material and methods. Y-TZP zirconia copings were manufactured on brass dies of a maxillary central incisor. Forty copings, with 2 layer thicknesses (0.5 and 0.3 mm), and 2 finish line preparations (knife edge and chamfer; n=10) were cemented using a conventional glass ionomer cement and stored in distilled water at 37 degrees C for 24 hours. The copings were vertically loaded until fracture using a universal testing machine. Data were analyzed by 2-way ANOVA (alpha=.05). Fractographic examination was performed using scanning electron microscopy and confocal laser scanning microscopy. Results. A significantly higher mean failure load was measured for knife-edge (0.5 mm, 1110 +/- 175 N; 0.3 mm, 730 +/- 160 N) versus chamfer (0.5 mm, 697 +/- 126 N; 0.3 mm, 455 +/- 79 N) preparations (P<.001), and for 0.5-mm versus 0.3-mm thickness layers (P<.001). Conclusions. Knife-edge preparations present a promising alternative to chamfer finish lines; the fracture load required for knife-edge preparations was 38% greater than that required for chamfer preparations, regardless of coping thickness. Reducing the thickness of a single crown coping from 0.5 to 0.3 mm resulted in a 35% reduction in fracture load required for either preparation type.