Effect of Thickness, Substrate, Cement, and Storage Time on the Fracture Strength of Zirconia Crowns

Objectives: To investigate the influence of crown thickness, abutment material properties, and dental cements used on the fracture resistance and failure mode of monolithic computer-aided design/manufacturing (CAD/CAM) zirconia crowns for short and long-term water storage conditions. Methods: Forty-eight minimal-thickness (0.5-mm thick) and conventional-thickness (1.0-mm thick) CAD/CAM monolithic zirconia (IPS e.max ZirCAD MT) crowns were cemented on lithium disilicate (IPS e.max) abutments to simulate enamel structure, and on composite (Paradigm MZ100) abutments to simulate dentin structure (elastic modulus). Cements used were resin cement and resin-modified glass ionomer cement. The specimens were divided into 4 groups (n=12): MER (minimal-thickness crown cemented on e.max abutments with resin cement), MDR (minimal-thickness crown cemented on composite abutments with resin cement), CDR (conventional-thickness crown cemented on composite abutments with resin cement), and CDG (conventional-thickness crown cemented on composite abutment with glass ionomer cement). Specimens were stored in water for 7 days (short-term) or 3 years (long-term) (n=6). All specimens were tested for fracture resistance by a load-to-failure test using a universal testing machine. Crown geometry and thicknesses were precisely controlled using digital design and fabrication processes. Fracture load and type of fracture were analyzed. A scanning electron microscope (SEM) was used to perform fractographic analysis. A Welch’s analysis of variance (ANOVA), Games-Howell post hoc tests were performed to analyze the results (p≤0.05). Results: The mean fracture load for group CDR was the highest for both short-term and long-term results (2009 ±263 N and 2557 ±235 N) (p≤0.05). Group MDR showed the lowest mean fracture load for both short- and long-term data (1301 ±161 N and 1441 ±993 N) (p≤0.05). The mean fracture load of groups MER and CDG were comparable (1933.00 ±202.81 N and 1919.67 ±367.28 N) at 7 days (p>0.05). Conclusions: Minimal-thickness and conventional-thickness zirconia crowns had high fracture resistance, exceeding average maximum occlusal forces. Minimal-thickness zirconia crowns cemented to an enamel like substrate had comparable fracture strengths when compared to conventional-thickness crowns cemented to a dentin like substrate for short-term water storage conditions. However, conventional-thickness crowns had a more predictable fracture resistance and fracture pattern than minimal-thickness crowns after long-term water storage.