Phase Transformation and Roughening in Artificially Aged and Retrieved Zirconia-Toughened Alumina Femoral Heads.

BACKGROUND: Zirconia-toughened alumina (ZTA) used in hip arthroplasty contains yttria-stabilized zirconia (Y-TZP) as a toughening agent. However, Y-TZP is well known to degrade in vivo from tetragonal to monoclinic phase transformation. The stability of never-implanted ZTA femoral heads was evaluated in a severe artificial aging test, with retrieved ZTA heads also evaluated for clinical relevance. We hypothesized that ZTA would degrade due to tetragonal-to-monoclinic phase transformation, with changes in surface topography and progressive roughening. Y-TZP specimens served as a positive comparison group, while magnesia-stabilized zirconia (Mg-PSZ), which does not undergo phase transformation, served as a stable comparison group. METHODS: Monoclinic phase concentration, surface topography, and roughness of never-implanted ZTA, Y-TZP, and Mg-PSZ heads were measured by X-ray diffraction and optical profilometry, before and after 2 rounds of 24 hours of hydrothermal aging. Explanted ZTA heads were characterized by the same methods. RESULTS: After 48 hours in an autoclave, the surface of ZTA heads exhibited irregularly spaced protrusions about 20-30 nm high by 100-150 µm in diameter, with significant increases in monoclinic phase concentration (from 12.2% to 21.3%) and surface roughness. Similar features were observed on the surface of explanted ZTA heads, with 33% monoclinic phase after 2.7 years in vivo. CONCLUSION: Based on data collected from ZTA retrievals, this artificial aging test underestimated the amount of phase transformation in vivo. Phase transformation and surface roughening of ZTA heads steadily increased without reaching a plateau, which may lead to stress concentrations and weakening of the ceramic material, and could result in late fracture and wear.

Reduced UHMWPE wear using magnesia-stabilized zirconia instead of CoCr femoral components in a knee simulator.

Magnesia-stabilized zirconia (Mg-PSZ) is stable and maintains a scratch-resistant surface in hip replacement, but is untested in knees. We assessed whether using Mg-PSZ instead of cobalt-chromium (CoCr) femoral components resulted in less tibial insert wear, and evaluated changes in topography and roughness of the femoral components. Inserts bearing against CoCr or Mg-PSZ were tested using standard (9 Mc) and aggressive (6 Mc) waveforms. Femoral component surface topography and roughness were evaluated before and after testing by optical profilometry. When bearing against Mg-PSZ, UHMWPE wear rate decreased by 73% (standard) and by 59% (aggressive conditions). After 15 Mc, CoCr components featured deep scratches, and roughness increased five-fold, while Mg-PSZ components were unchanged. Mg-PSZ femoral components may be indicated for high-demand patients and those with metal sensitivity.