Aims: Yttria-stabilised Zirconia (Y-TZP) ball heads were introduced into the market in 1985. Since then these components have had wide diffusion in hip replacements, due to their good mechanical performance and reliability. Namely, only a few papers were published up to now reporting failures of Y-TZP ball heads. The worldwide recall in August 2001 of some Y-TZP batches changed this situation. The recall was due to an high number of fractures, that were somehow linked to the spontaneous phase transition in Y-TZP. The revision surgery of a zirconia head belonging to the batches in above that fractured 34 months after implant gave us the opportunity to retrieve the fragments for analysis, and to draw some conclusions about the cascade of events that led to the ball head fracture.
Methods: The retrieved specimens were submitted to visual inspection by optical microscopy to analyze the fracture pattern, then selected samples were cut and analysed by FEG-SEM equipped with backscattered CEN-TAURUSA8 detector. Specimen were analysed without any coating. Grain size were measured by SEM-coupled computerized image analysis on thermally etched samples. Phase composition was investigated by XRD. Density was determined by the Archimedes’ method. Periprosthetic tissue membrane collected at revision surgery was observed by light and Scanning Electron Microscopy coupled with EDAX.
Results: XRD showed that maximum of about 60% monoclinic phase was present in the inner cone near the chamfer, while in the outer polished surface only 3% monoclinic phase was present. SEM demonstrated that the zone near to the surface of the taper, in the core of the ball head, consisted of an inhomogeneous structure formed by dense agglomerates of Y-TZP grains, about 10 mm in diameter, within a matrix of lower density. A crack network originated from the above mentioned inhomogeneous zone, and extended through the outer part of the sample which consists of well densified material. Histologic sections showed the presence of many metal particles in a poor in vessels stroma; ceramic appeared as polygonal shape granules of different dimension, ranging from 2 to 10 B5m.
Conclusions: The Authors demonstrated that the likely initiator of the failure is to be sought in subcritical crack growth of the defects due the uncontrolled densification during sintering, enhanced by the wet environment and by cyclic loading. The hydrothermal stability of the material had only a secondary role in the fracture.
