Fracture Resistance of Pressable Zirconia Crowns with Modified Core Design

OBJECTIVE: To determine the fracture resistance and mode of failure of heat-pressed Procera® All-Zircon crowns with modified core designs. METHODS: Two preparation designs, anatomical and flat, and two core designs were tested on a maxillary premolar resin die. One ceramic core had full-shoulder coverage to the margin (control) and the other was modified with a cutback to 3mm above the facial margin of the preparation. Veneering porcelain was heat-pressed to the cores producing a total thickness of 1.5mm axially and 2mm occlusally. Final crowns were cemented onto dies with resin cement and stored for 24 hours in 100% humidity at room temperature. The crowns were axially loaded at 0.5 mm/min and the maximum breaking force was recorded. Fracture mode was evaluated by direct 360° visual inspection under a florescent light source held at 90° to the vertical axis of the crown. RESULTS: Mean maximum loads in Newtons for anatomical and flat all-ceramic crowns with full coverage zirconia cores were 1300.1±365.0 and 1572.2±140.8 respectively. The fracture loads were significantly less for both anatomical (1013.1±159.3) and flat crowns (1243.0±242.6) with the modified copings (Independent t-test, p<0.05). For both coping designs, the fracture loads were significantly greater with the flat preparation design (Independent t-test, p<0.05). Quantitative analysis of fracture modes revealed similar fracture patterns for both anatomical and flat crowns. All fractures occurred in the veneering porcelain; there were with no open core fractures in any of the crowns. CONCLUSIONS: Modification of core design decreased the fracture resistance of the crowns by 21-22%. Flat preparations had 18% higher loads than the anatomical preparations. Regardless of core design, porcelain fracture occurs mainly within the veneering layer for all groups tested and fracture loads were comparable with average masticatory loads for normal dentition.