Simulated oral environment affects zirconia aging and mechanical strength

Zirconia is an emerging alternative to titanium for dental implant systems. However, zirconia surfaces can deteriorate under aqueous conditions, thereby compromising mechanical performance. The goal of this study was to assess the impact of biological exposure or aging simulating the oral cavity on the surface integrity and mechanical strength of surface-treated zirconia.

Partially yttria- or magnesia-stabilized zirconia specimens were polished, acid-etched, or sandblasted. Zirconia disks were exposed to mammalian cells or bacteria. Surface degradation via tetragonal-to-monoclinic phase transformation was assessed by Raman microscopy. Zirconia bars were subjected to accelerated aging by exposure to autoclave steam. Surface morphology was examined by scanning electron microscopy, and mechanical strength was measured using four-point bend test loading until failure.

Early-colonizing oral bacteria significantly increased monoclinic content ∼5-fold on acid-etched yttria-stabilized zirconia versus control (p < 0.05). Surface monoclinic content on acid-etched zirconia increased rapidly (54.6 ± 9.3 %) after 4 h of aging before plateauing (77.0 ± 5.7 %) after 20 h. In contrast, polished or sandblasted zirconia exhibited minimal monoclinic content (∼4 %) after 4 h that gradually increased to ∼40 % by 20 h. Acid-etching treatment significantly reduced the flexural strength of zirconia (585 ± 34 MPa) while sandblasting variants were statistically similar (922 ± 52 MPa) to machined control (921 ± 74 MPa). Aging reduced the flexural strength of machined or sandblasted zirconia, which was significant (p < 0.05) for machined zirconia (782 ± 178 MPa).

Acid-etching and/or exposure to acidic environment increase zirconia susceptibility to surface degradation, impacting its surface and mechanical properties.