Low temperature degradation of single layers of multilayered zirconia in comparison to conventional unshaded zirconia: Phase transformation and flexural strength

Objectives: The aim of this study was to evaluate the transformation of yttrium stabilized tetragonal zirconia (YTZP) to monoclinic phase and its change in flexural strength of the various layers of multilayered zirconia (enamel layer, transition layer 1, transition layer 2 and body layer) in comparison to two conventional zirconia ceramics. Additionally, the ball-on-three-balls test was compared with a conventional biaxial flexural strength test. Methods: The crystallographic structure of the four layers of Katana Zirconia ML (Kuraray), e.max ZirCAD (Ivoclar Vivadent) and Lava Plus (3M ESPE) was investigated by x-ray-diffraction before and after hydrothermal aging in an autoclave for 5, 10, 15 and 20 h. The biaxial flexural strength was examined with a piston-on-three ball test and a ball-on-three-balls test. Results: There was a significant difference of the ratio of transformation with respect to the tested materials, also between the four different layers of multilayered zirconia. After 20 h of hydrothermal aging the mean ratio of monoclinic phase ranged from 9.9 vol% (LAVA Plus) to 44.2 vol% (Katana Zirconia ML, enamel layer). However, hydrothermal aging had no significant influence on the flexural strength of any material. Furthermore, the flexural strength measured by the ball-on-three-balls test was higher than that measured by the piston-on-three ball test. Conclusion: A ratio of 40% monoclinic phase at the surface did not significantly influence the biaxial flexural strength. Both test methods provide reliable results. However, as the ball-on-three balls test is less dependent on surface roughness, it might be preferred for investigations adapted to clinical reality.