In-depth profiling of elastic residual stress and the in vivo wear mechanism of self-mating alumina hip joints

Residual (elastic) stress profiles developed in self-mating alumina heads upon long-term exposure in the human body have been quantitatively determined by means of three different piezo-spectroscopic (PS) approaches available to the alumina lattice. The PS measurements, collected at exactly the same area of the sample, included fluorescence spectroscopy (fs), Raman spectroscopy (rs) (in-depth resolutions both in the order of the tens of μm), and cathodoluminescence spectroscopy (cs) (in-depth resolution down to the tens of nm). The lateral spatial resolution was in the order of the tens of μm, of the single μm, and of the hundredths of nm for fs and rs and cs, respectively. Owing to the precise knowledge of the response functions in space of the three probes, deconvoluted stress profiles could be obtained in zone of different wear severity. The profiles revealed high stress gradients along the material subsurface, “cleaned” from averaging effects related to the finite probe volume. The subsurface stress field in heavily worn zones entailed a surface layer under strong compression, an abrupt compressive-to-tensile transition up to a similar stress magnitude, and a deeper-lying region of tension which ultimately relaxed in the unstressed bulk. The opposite trend was observed in mildly worn zones of the femoral head, with moderate tensile stresses in the immediate surface and weak counterbalancing stresses of compressive nature along the subsurface. A wear mechanism for self-mating alumina hip surfaces was then drawn consistently with the newly available residual stress profiles.