SiNtx Technologies Inc (SINT)

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Zirconia phase transformation in retrieved, wear simulated, and artificially aged ceramic femoral heads

The monoclinic content for retrieved heads was 28.5%?±?7.8, greater than twice that in virgin, aged, or wear tested heads and did not have a significant correlation with time, contrary to the predictions of the hydrothermal aging model. The surface roughness for retrieved heads in the unworn area was not significantly different to that in virgin, aged, or unworn areas of wear tested heads. However in worn areas of the retrieved heads, the surface roughness was higher than observed in wear simulator testing. These results indicate that current testing methodologies do not fully capture the operational conditions of the material and the real performance of future new materials may not be adequately predicted by current pre-clinical testing methods.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5763359/


TRANSITION METAL IONS ACCELERATE POLYMORPHIC PHASE TRANSFORMATION IN ZIRCONIA-TOUGHENED ALUMINA ARTIFICIAL HIP JOINTS: AN IN VITRO STUDY

Metal ions have an apparent detrimental role in destabilizing the zirconia phase at the surface of ZTA femoral heads which may impact mechanical or wear performance. Metal ions naturally present in the prosthetic joint space, metal staining of the head from hip instability, or metal ions released from modular taper corrosion may contribute to ZTA instability, even in well-functioning THA prostheses

https://online.boneandjoint.org.uk/doi/abs/10.1302/1358-992X.99BSUPP_3.ISTA2016-025

DISCUSSION: Our data suggest that metal staining of ZTA femoral heads leads to a discrepancy in predicted (in vitro) versus observed (in vivo) phase transformation rates. We hypothesize that enhanced phase instability may reflect a catalytic reaction at the ceramic surface (i.e., preferential disassociation of water, with higher hydroxyl concentrations accelerating the zirconia phase transformation). Naturally-occurring Fe-ions in the prosthetic joint space may also trigger phase instability in ZTA heads because enhanced phase transformation in one short-term stain-free retrieval (Case A) was also observed. Experiments to clarify this point are ongoing.
SIGNIFICANCE: Metal ions have an apparent detrimental role in destabilizing the zirconia phase at the surface of ZTA femoral heads which may impact mechanical or wear performance. Metal ions naturally present in the prosthetic joint space, metal staining of the head from hip instability, or metal ions released from modular taper corrosion could all contribute to ZTA instability, even in well-functioning THA pros-theses. Further studies are needed to validate these observations.

http://www.ors.org/Transactions/62/1100.pdf

McEntire et al., demonstrate that the presence of metal ions both naturally and from implants collect on the surface of ZTA femoral heads leading to both surface roughening and phase instability. This observation is confirmed by the real world observations of Parkes et al in the first article quoted above. This leads to increased wear particulate being released into the body and increased friction which can potentially impact hip motion. This combined with the increased wear rates demonstrated at the taper connection of ZTA femoral heads clearly show increased ZTA particulate as compared to what is demonstrated in in-vitro models. This increased particulate has led to clinically toxic levels of both micro & nano sized ZTA particles being released into the body based on cell cultures of surrounding tissue of patients with Ceramic-on-Ceramic hip implants. It is important to note the presence of nano sized ZTA particles as both Alumina & Zirconia appear to be toxic to various organs at the nano level. The degree of wear at the nano level is not studied sufficiently to know if this is indeed a possible side effect of CoC hip implants.