Wear of bearing couples is one of the most important factors determining the longevity of a total hip implant. Ceramics have been used as bearing materials in hip joints with the expectation of wear reduction due to their smooth surfaces, low friction, and good wetting properties. All these properties should contribute to extend the in vivo lifetime of the hip joint. However, in vivo surface degradation of some zirconia ceramics has been reported, with the presence of asperities at the articular surfaces of the hip joint becoming a factor in precipitating degenerative changes. Recent topographic analyses of residual stress fields, conducted by confocal Raman and fluorescence spectroscopy on retrieved ceramic femoral heads, revealed clear changes in stress distribution with exposure time in vivo, as well as a possible migration of the polar position of maximum stress. Metastability of zirconia, and the potential effect of surface roughening arising from transformation of tetragonal-to-mono- clinic zirconia, represents a typical environmental effect strongly affecting the structural performance of a hip joint. From a materials science viewpoint, the environmental stability of zirconia has been found to strongly depend on grain size and on the amount and type of stabilizing element Some aspects of zirconia metastability are useful to improve the bulk fracture toughness of the joint material and, thus, have been welcomed by joint designers; however, the drawback is a potential embrittlement and a roughening of the bearing surface with aging in vivo. On the one hand, it is somewhat surprising that manu- facturers are still distributing monolithic zirconia ceramic femoral heads (e.g., in Japan), but fundamental information is lacking on the characteristics of degradation with no direct comparison made among zirconia ceramic heads manufactured by different makers. On the other hand, a new generation of alumina/zirconia composite material has recently become avail- able to the orthopedic community.
In this paper, they tested such an advanced composite femoral head with respect to the topologic and phase-stability response of its bearing surface to hydrothermal environment, in comparison with commercially available monolithic zirconia femoral heads. They found and important improvement in terms of stability in the aluminia/zirconia, specially in their response in OH – on the surface. This adds stability, because it inhibits the addition of oxygen to the vacancies of the crystal cell and hydrogen being interstiatilly located in the lattice.
To read more about it, search: Pezzotti, Giuseppe; Saito, Takuma; Takahashi, Yasuhito. “Surface topology of advanced aluminia/zirconia composite femoral head compared with commercial femroral heads of monolithic zirconia”. ACS 94:(3) 2011. 945-950.