Burst Strength of BIOLOX®delta Femoral Heads and Its Dependence on Low-Temperature Environmental Degradation

Abstract

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.

Fracture of a 40-mm BIOLOX Delta Femoral Head

“…Based on the “weakest link theory,” these cracks may influence the variability in strength of ceramics [ 21 ] as these fractures and defects undergo chemical reactions within the crack, cleaving oxide bonds [ 14 ] and accumulating inelastic strain within the crack until a threshold is met. This results in fracture propagation away from the largest flaw that is favorably oriented to the tensile strength to which the material is subjected [ 22 ] until the burst strength of BIOLOX delta ceramic femoral heads of 52kN is surpassed [ 23 ]. Catastrophic failure and fracture are nearly always preceded by subcritical crack growth.…”

Analysis of Chemisorbed Tribo-Film for Ceramic-on-Ceramic Hip Joint Prostheses by Raman Spectroscopy

“…The stress-induced tetragonal-to-monoclinic polymorphic transformation of zirconia results in high flexural strength and fracture toughness. These features occur because of the microscopic crack-tip shielding mechanism of ZTA [ 11 ]. Thus, Raman experiments demonstrate compositional enhancement of BIOLOX ® delta [ 12 ].…”

“…Thus, Raman experiments demonstrate compositional enhancement of BIOLOX ® delta [ 12 ]. To investigate the changes in material properties of BIOLOX ® delta femoral heads, Raman spectroscopy was used as an effective technique [ 8 , 10 , 11 , 13 ].…”

“…Consequently, for proper understanding of the joint replacement condition and further improvement of its durability, chemical changes of individual SF components and three model human SF fluids have been considered in our study. As Raman spectroscopy- surpasses conventional techniques in terms of facility, selectivity and stability, it is presumed most suitable to explain the function of the fluids and performance of the ceramic implant material [ 8 , 10 , 11 , 13 ]. Alumina ceramic implants were analyzed previously by this spectroscopic technique several times and practical bio diagnostics under presence of protein interactions has also been reported [ 33 ].…”

Annealing-Induced Off-Stoichiometric and Structural Alterations in Ca2+- and Y3+-Stabilized Zirconia Ceramics

“…However, defect alteration and phase instability of zirconia in specific service environments (e.g., high temperature and humidity) may cause a degradation of these properties, even if the zirconia ceramics are fully stabilized [ 8 ]. For example, 8–9 mol% Y-stabilized zirconia (YSZ) for SOFC applications appears to be situated in the two-phase field (cubic + tetragonal) of the phase diagram at working temperatures, but coarsening of the tetragonal regions and decomposition on the cationic sublattice into Y-enriched and depleted regions on the nanoscale may induce electrical degradation (i.e., a lowered ionic conductivity) during operation [ 9 ].…”