Possible triggers for phase transformation in zirconia hip balls

The clinical history of yttria-stabilized, zirconia (Zr) ceramic has been controversial. In the patient, combinations of hydrothermal and mechanical shocks may trigger detrimental changes in Zr balls that have inferior metastability. Transformations from tetragonal to monoclinic phase may be influenced by impingement, dislocation, and disassociation in certain patients. Hydrothermal stability was measured in Zr balls from four vendors by autoclave and mechanical models that included "cup-impingement," "abrasive" wear, and "3rd-body" wear. Standard simulator tests for polyethylene (PE) wear studies combined pristine and previously transformed Zr and were also used to test lubricant effects (Zr/Zr-serum, Zr/Zr-water, Zr/PE-water). For in-vivo comparisons we studied retrieved Zr balls at 1-15 years follow-up by laser interferometry, SEM, EDS, XRD, and Raman spectroscopy. We found that severe mechanical shock triggered local surface destruction but little transformation. In contrast, hydrothermal processes revealed 5-13% monoclinic by 7 h, increasing at a rate of 0.56%/h for 22-mm balls and 0.81%/h for 26-mm balls. The all-ceramic Zr/Zr bearings were very sensitive to lubrication mode, showing early catastrophic failure when run in water but surviving 20 million cycles when run with serum lubrication. Wear with Zr/PE combination did not trigger phase changes in water or serum but decreased the monoclinic content measured on previously transformed surfaces. Most retrieved Zr balls showed high transformation (30-85% monoclinic) but some showed no transformation. The ball areas with major monoclinic changes corresponded to PE contact, suggesting that tribological conditions under the cup were the trigger. This indicated that we understand little of the hydrothermal conditions operating under Zr/PE hip joints in-vivo.