Signs of in-vivo aging of zirconia from explanted dental implants with porous coating after several years in function

The present study investigated the in vivo aging of yttria-stabilized zirconia (YSZ) oral implants (ZiUnite®) removed after 37 to 181 months. These implants featured a porous zirconia surface to enhance osseointegration. They were placed in prospective clinical investigations and had to be explanted due to peri‑implant bone breakdown. Since no single clinical/non-clinical parameter or combination of parameters were found to influence the bone loss, we have assessed the extent of the in vivo aging, known as low temperature degradation (LTD), on five explants as a possible critical influencing parameter. This research represents, to the best of our knowledge, the first report on low-temperature degradation-induced tetragonal-to-monoclinic (t-m) transformation in clinically applied YSZ oral implants after up to 15 years. Using scanning electron microscopy (SEM) and focused ion beam (FIB)-SEM tomography, the analysis revealed significant surface t-m transformation, pronounced micro-cracking of the porous coating, and degradation. This extensive transformation and resultant volume expansion may have negatively impacted the bone-to-implant interface, contributing to clinical failure. Additionally, differently produced YSZ implants can exhibit varied behaviors, even with identical raw materials. The presented type of assessment of susceptibility to in vivo aging of oral implants could inspire the study protocol for other medical device systems. STATEMENT OF SIGNIFICANCE: This study is the first to document substantial t-m transformation of clinically used YSZ implants after up to 15 years in the oral environment. Extensive t-m transformation in the porous coatings at body temperature occurred rapidly, leading to microcracking and potential loss of cohesion with the implant bulk. The porous, cracked zirconia coatings were linked to brittle fractures in vivo and it is hypothesized that these changes possibly contributed to bone loss, loss of osseointegration and subsequent implant failures. The findings of such transformation underscore the potential clinical risks of these zirconia coatings leading to the mentioned changes. The present assessment method could serve as a valuable protocol model for investigating the in vivo aging susceptibility of other medical devices.