Crystalline Phase Content and Load-bearing Capacity of Gradient Zirconia Crowns

Objectives: The purpose of this study is to evaluate the crystalline phase content and load-bearing capacity of zirconia crowns with gradient yttria and colorant composition before and after autoclave aging.
Methods: Full contour zirconia crowns were designed and fabricated with gradient yttria and colorant composition (Group L4-6). Crowns with identical geometry but unitary composition (Group L4 and Group L6) were also fabricated for comparison. All the crowns were adhesively luted onto carbon fiber reinforced polyether-ether-ketone(CF/PEEK) abutments and subjected to load-to-fracture test before and after autoclave aging. Disc specimens of three individual compositions were fabricated for crystalline phase and mechanical property characterization by X-ray diffraction analysis and nanoidentation.
Results: Lowest mean fracture load was detected in Group L4, while fracture loads of Group L4-6 and Group L6 were not statistically different. As yttria composition increased from 4.1 mol% to 5.3 mol%, the percentage of cubic phase zirconia increased from around 40% to around 65% while that of tetragonal phase decreased correspondingly. None of tested group presented monoclinic phase zirconia. Specimens in the same group showed a similar crystalline phase content and no statistical difference in fracture load before and after autoclave aging.
Conclusions: Gradient yttria and colorant composition does not attenuate load-bearing capacity of zirconia crowns. Autoclave aging poses no effect on crystalline phase content and load bearing capacity of zirconia stabilized with 4.1-5.3 mol% yttria.