Femoral heads made from zirconia-toughened alumina (ZTA) are the most advanced bioceramic available for total hip arthroplasty. ZTA's superior mechanical properties result from the polymorphic transformation of its zirconia (ZrO2) phase in the presence of a propagating crack. In vitro derived activation energies predict that several human lifetimes are needed to reach a state of significant transformation;1 but in vivo confirmation of material stability is still lacking. This investigation determined if transition metal ions might be responsible for triggering the tetragonal to monoclinic (t®m-ZrO2) phase transformation in this bioceramic.
Materials and Methods
BIOLOX®delta femoral heads (CeramTec GmbH, Plochingen, Germany) were acquired and characterized for their surface monoclinic content, Vm, using Raman spectroscopy. Then they were physiologically scratched with different metals (i.e., Ti, CoCr, and Fe, n=3 each) to simulate in vivo staining as a result of acetabular shell impingement due to subluxation or dislocation. They were subsequently hydrothermally aged for up to 100 h in an autoclave at 98∼132°C and 1 bar pressure. Raman maps, each consisting of 120 spectra, were compiled and monoclinic contents, Vm, calculated for zones adjacent to and away from the metal stains.2 Activation energies for the t®m transformation in stained and non-stained zones were derived and compared to retrieved heads having service lives of between ∼45 days and ∼8 years.
The fractions of m-ZrO2 in the as-received and treated heads are presented in Table 1. In all cases, significantly greater amounts of m-ZrO2 were found on the metal stained areas, with the transformation amounts similar to retrieved heads. Activation energies for the t®m transformation in non-stained, CoCr-, Fe-, and Ti-stained zones were found to be 79, 60, 62, and 67 kJ/mol, respectively. Extrapolated t®m time-transformation curves at 37°C are shown in Figure 1 along with average m-ZrO2 fractions from 15 short- and medium-term retrievals. Data are compiled for values obtained on the main-wear-zones (MWZ) and non-wear-zones (NWZ) versus their elapsed time in vivo.
This experiment suggests a discrepancy in predicted (in vitro) versus observed (in vivo) phase transformation rates for metal stained ZTA femoral heads.1 This phenomenon should be carefully considered because of its potential effect after reductions of dislocations and in the taper of these heads. While explaining the gap between in vitro predictions and in-vivo observations for phase transformation rates in ZTA components, an intrinsic incompatibility between ZTA and metal ions was also demonstrated. This phenomenon arises from a catalytic reaction at the ceramic surface by enhanced hydroxyl concentrations near the metal stains.
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.