Nondestructive inspection of phase transformation in zirconia-containing hip joints by confocal Raman spectroscopy.

Environmental metastability of zirconia (ZrO2) ceramic in the human body [represented by a tetragonal-to-monoclinic (t→m) phase transformation] takes place on the surface of the artificial joint and proceeds with time toward its interior. Its quantitative characterization is mandatory for the safety of joint implants and consists of the assessment of the in-depth monoclinic profile fraction as compared to that of the initially untransformed material. We attempt to fully establish a characterization protocol and present two different nondestructive approaches for resolving highly graded phase-transformation profiles along the hip-joint subsurface by confocal Raman microprobe technique. A series of partially transformed tetragonal zirconia polycrystal and zirconia-toughened alumina ceramics are used as screening samples. Probe biases could be eliminated and the real transformation profiles retrieved through a deconvolution procedure of Raman experimental data collected as a function of pinhole aperture and focal depth, respectively. Confirmation of the confocal assessments was made by a destructive cross-sectional inspection by both laser optical microscope and Raman spectral line scans. This study unveils for the first time the real quantitative amount of surface phase-transformation fractions and the related subsurface profiles in zirconia-based retrieved medical samples.