Long-term stability of an injection-molded zirconia bone-level implant: A testing protocol considering aging kinetics...

Separately addressing the fatigue resistance (ISO 14801, evaluation of final product) and aging behavior (ISO 13356, standardized sample) of oral implants made from yttria-stabilized zirconia proved to be insufficient in verifying their long-term stability, since (1) implant processing is known to significantly influence transformation kinetics and (2) aging, up from a certain level, is liable to decrease fatigue resistance. Therefore, the aim of this investigation was to apply a new testing protocol considering environmental conditions adequately inducing aging during dynamic fatigue.

Zirconia implants were dynamically loaded (10⁷ cycles), hydrothermally aged (85°, 60 days) or subjected to both treatments simultaneously. Subsequent, monoclinic intensity ratios (X_m) were obtained by locally resolved X-ray microdiffraction (μ-XRD²). Transformation propagation was monitored at cross-sections by μ-Raman spectroscopy and scanning electron microscopy (SEM). Finally, implants were statically loaded to fracture. Linear regression models (fracture load) and mixed models (X_m) were used for statistical analyses.

All treatments resulted in increased fracture load (p≤0.005), indicating the formation of transformation induced compressive stresses around surface defects during all treatment modalities. However, only hydrothermal and combinational treatment were found to increase X_m (p<0.001). No change in X_m was observed for solely dynamically loaded samples (p≥0.524). Depending on the variable observed, a monoclinic layer thickness of 1-2μm (SEM) or 6-8μm (Raman spectroscopy) was measured at surfaces exposed to water during treatments.

Hydrothermal aging was successfully induced during dynamic fatigue. Therefore, the presented setup might serve as reference protocol for ensuring pre-clinically long-term reliability of zirconia oral implants.