Burst strength of BIOLOX®delta femoral heads and its dependence on low-temperature environmental degradation

Zirconia-toughened alumina (ZTA) currently represents the bioceramic gold standard for load-bearing components in artificial hip joints. ZTA is long known for its high flexural strength and fracture toughness, both properties arising from a microscopic crack-tip shielding mechanism due to the stress-induced tetragonal-to-monoclinic (t→m) polymorphic transformation of zirconia. However, there have been concerns over the years regarding the long-term structural performance of ZTA since the t→m transformation also spontaneously occurs at the material's surface under low-temperature environmental conditions with a concomitant degradation of mechanical properties. Spontaneous surface degradation has been extensively studied in vitro, but predictive algorithms have underestimated the extent of in vivo degradation observed in retrievals. The present research focused on burst-strength assessments of Ø28 mm ZTA femoral before and after long-term in vitro hydrothermal ageing according to ISO 7206-10. An average burst strength of 52 kN was measured for pristine femoral heads. This value was ∼36% lower than results obtained under the same standard conditions by other authors. A further loss of burst strength ∼13% in ultimate load) was observed after hydrothermal ageing, with increased surface monoclinic content ranging from ∼6% to >50%. Nevertheless, the repetitively stressed and hydrothermally treated ZTA heads exceeded the minimum burst strength stipulated by the US Food and Drug Administration (FDA) despite severe test conditions. Lastly, Raman spectroscopic assessments of phase transformation and residual stresses on the fracture surface of the femoral heads were used to clarify burst-strength fluctuations and the effect of hydrothermal ageing on the material's overall strength degradation.