Wear and degradation on retrieved zirconia femoral heads.

Zirconia femoral heads retrieved from patients after different implantation periods (up to 13 years) were analysed using vertical scanning interferometry, atomic force microscopy and Raman microspectroscopy. A range of topographical and compositional changes on the surface of the retrievals are reported in this work. The study revealed that changes in roughness are the result of a combination of factors, i.e. scratching, surface upheaval due to transformation to the monoclinic phase and grain pull-out. Clusters of transformed monoclinic grains were observed on heads implanted for more than 3 years. The phase composition of these clusters was confirmed by Raman microspectroscopy. Increased abrasive wear and a higher monoclinic phase content concentrated on the pole of the femoral heads, confirming that the tetragonal to monoclinic phase transformation was not only induced by the tetragonal phase metastability and environmental conditions but mechanical and tribological factors, also affected the transformation kinetics. Additionally, the head implanted for 13 years showed evidence of a self-polishing mechanism leading to a considerable smoothening of the surface. These observations provide an insight into the interrelated mechanisms underlying the wear and transformation process on zirconia ceramics during implantation.