Topic Ceramic

Executive Summary

Ageing, Shocks and Wear Mechanisms in ZTA and the Long-Term Performance of Hip Joint Materials

Authors:
A. Perrichon, B. Haochih Liu, J. Chevalier, L. Gremillard, B. Reynard, F. Farizon , J.-D. Liao and J. Geringer
Journal:
Materials 2017, 10, 569; doi:10.3390/ma10060569
Level of Evidence:
Not applicable

Summary

Perrichon et al. studied the failure risk of zirconia-toughened alumina (ZTA) hip implant components. They examined the effects of wear and ageing on ZTA (BIOLOX®delta) hip implant components in the presence of body fluids. They evaluated three degradation conditions (one specimen tested per condition):

  1. The friction condition consisted of a 6-million cycle hip simulation test acc. to ISO 14242-1.
  2. For hydrothermal ageing, a femoral head went through a treatment in an autoclave that simulates hundreds of years in vivo.
  3. Severe shocks of 6 and 9kN were imposed by vertical micro-separation of 1mm on a specific shock device for 1 million cycles.

Sophisticated physical methods were applied to reveal any damage on and below the bearing surfaces. Mechanical properties were determined by nano-indentation. The authors were able to simulate and reproduce degradation mechanisms observed in retrieved ZTA heads, comparing the wear patterns. The friction test produced non-homogenous roughening as wear damage on the surface, which induced only a slight transformation of the zirconia phase. After hydrothermal ageing, the mechanical properties of the ZTA surface showed a negligible optical degradation and no micro-cracks despite an increase in monoclinic phase. Only the mechanical shock test, under severe microseparation, produced the in-vivo-like wear scars seen on retrievals, which revealed also the highest zirconia phase transformation (i.e. stress-induced).

Perrichon et al. concluded that even if performed under simple conditions that limit the validity of extrapolation to the conditions in vivo, hydrothermal ageing is not the primary origin of degradation. According to the authors, shocks are the origin of main wear damage represented by the formation of wear stripes in which phase transformation occurs. The high contact stresses during shocks are the origin of the formation of micro-cracks within the wear stripes, which locally trigger a zirconia phase transformation that reduces their propagation in response. The remaining (limited) crack paths cause a decrease in the mechanical properties in the damaged areas. The occurrence of the phase transformation by shocks is therefore not detrimental for ZTA, but rather a positive effect that limits the extension of the wear stripes.

Study limitations

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