Chemically driven tetragonal-to-monoclinic polymorphic transformation in retrieved ZTA femoral heads from dual mobility hip implants.

Two short-term (two and nine months) retrieved zirconia-toughened alumina (ZTA) femoral heads and nine pristine femoral heads from the same manufacturer have been investigated with respect to their surface stability by means of confocal Raman spectroscopy. Quantitative estimations of monoclinic volume fraction have been carried out in both non-wear and main wear zones of the retrieved heads, which invariantly showed high volume fractions of monoclinic polymorph. In-depth (sub-surface) profiles, non-destructively collected in the main wear zones with the Raman probe in confocal configuration, indeed confirmed that polymorphic transformation was extended down to 100μm below the bearing surface of the femoral heads. Acceleration of tetragonal-to-monoclinic transformation rate leads to unexpectedly high fractions of monoclinic phase within very short-term in-vivo exposures. Phase transformation in-vivo is much more marked than what one could actually predict according to simply simulating a hydrothermal environment in-vitro and could not be simply ascribed to the mechanical stress fields generated during normal service at the bearing surface. Instead, the chemical consequences of metal contamination on the ZTA femoral head surface are shown to play the most detrimental role in phase destabilization.