Aesthetic Outcomes of Zirconia Surface Using Different Sterilization Methods

Objectives: The aim of the study was to investigate the aesthetic outcomes of zirconia surface using different sterilization methods.
Methods: Forty eight zirconia disks (Y-TZP, Cercon, Dentsply) were cut (diameter 25mm, thickness 1mm), prepared and polished to the same surface roughness (R_a~0.2mm) by profilometer. The specimen were then exposed to steam autoclave sterilization (Group S), UV irradiation (Group UV) and gamma ray irradiation (Group γ). After the treatments, zirconia surface were characterized and compared with the control group (i.e. without any sterilization treatment, Group C) by colour using naked eye under 6500K light illumination, surface roughness, surface free energies (SFE), scanning electron microscopy (SEM), and X-Ray Diffraction (XRD).
Results: The colour of the white zirconia sample did not have obvious changes in Group S compared to Group C. However, the colour of the zirconia changed to light yellow in Group UV and dark brown in Group γ. Surface roughness result showed the highest mean R_a value (0.61±0.04mm) in Group γ, which was significantly higher than all the other four groups (p<0.05). There was no significant difference between the other three groups of group C (0.24±0.08mm), group S (0.22±0.08mm), and group UV (0.18±0.03mm) (p>0.05). The SFE increased after different sterilization treatments compared to control group, with the rank of 30.41mJ/m² (Group C) < 34.52mJ/m² (Group S) < 37.38mJ/m² (Group UV) < 43.28mJ/m² (Group γ). SEM revealed the surface morphological of zirconia appeared to be similar in all groups except in group UV, which appears to be more flat with less pits and shallow grooves than the other three groups. XRD showed that only tetragonal (T) phase structure could be detected on all the four groups.
Conclusions: It seems to be steam autoclave sterilization is the best sterilization method to preserve the best aesthetic outcome for zirconia.