Fracture Strength of Zirconia and Alumina Ceramic Crowns Supported by Implants
Due to the brittleness and limited tensile strength of the veneering glass-ceramic materials, the methods that combine strong core material (as zirconia or alumina) are still under debate. The present study aims to evaluate the fracture strength and the mechanism of failure through fractographic analysis of single all-ceramic crowns supported by implants. Forty premolar cores were fabricated with CAD/CAM technology using alumina (n = 20) and zirconia (n = 20). The specimens were veneered with glass-ceramic, cemented on titanium abutments, and subjected to loading test until fracture. SEM fractographic analysis was also performed. The fracture load was 1165 (±509) N for alumina and 1638 (±662) N for zirconia with a statistically significant difference between the two groups (P = 0.026). Fractographic analysis of alumina-glass-ceramic crowns, showed the presence of catastrophic cracks through the entire thickness of the alumina core; for the zirconia-glass-ceramic crowns, the cracks involved mainly the thickness of the ceramic veneering layer. The sandblast procedure of the zirconia core influenced crack path deflection. Few samples (n = 3) showed limited microcracks of the zirconia core. Zirconia showed a significantly higher fracture strength value in implant-supported restorations, indicating the role played by the high resistant cores for premolar crowns.
Figure 1. (a) The standard ITI abutment analogs. (a1) alumina and zirconia CAD/CAM core copings. (a2) Alumina and zirconia cores positioned on the ITI abutments. Figure 2. Specimens under compressive loading tests.
SEM images of copings after fracture. (a) and (a1) Alumina-ceramic specimen. (b) and (b1) Zirconia-ceramic specimen. White arrows indicate the core microcracks.
SEM images of the fracture plane. (a) Alumina specimens with secondary electrons. (a1) Alumina specimens with back scattered electrons. (b) Zirconia with secondary electrons. (b1) Zirconia with back-scattered electrons.
SEM images for different surface treatments. (a) Sagittal section of coping treated with sandblasting in the area of rectangle (1) and as received by milling center in the area of rectangle (2). (b) Fractographic evaluation of the rectangle (1); white arrows indicate the absence of a detachment line at the ceramic-zirconia interface. (c) Fractographic evaluation of the rectangle (2); white arrows indicate the presence of a detachment line at the ceramic-zirconia interface. Black arrows indicate wake hackle reaching the zirconia-ceramic interface. An arrest line was just associated with a roughness of the zirconia surface.
(a) Scanning electron microscopy images of alumina. The mean grain size was of 1.77 (±0.54) μm with a polygonal shape. (b) Scanning electron microscopy images of 3 mol %Y-TZP ceramics. (b1) The mean grain size was of 0.45 (±0.15) μm approximated with the radius of the circle, which has the same section area as the area of the grains. Sintered at 1450°C for 2 h.
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