Identification of scratch mechanisms on a retrieved metallic femoral head

The degradation of the femoral head roughness caused by scratching is known to increase the wear rate of the polyethylene counterface of the Charnley total hip prostheses and therefore to have an impact on osteolysis. The knowledge of the scratching mechanism is hence of paramount importance to improve the long-term clinical performance of these prostheses. The aim of this study is to identify the mechanism of scratch generation of a retrieved titanium (TA6V) femoral head through an accurate characterisation of its surface topography. The surface topography features have been assessed both qualitatively and quantitatively by respectively using an optical microscope and a three dimensional contacting profilometer. Based on the scanning of about 80 areas spread over the entire surface of the retrieved femoral head, hypotheses on the mechanism of scratch generation have been formulated after analysing the observed topographical patterns, the arithmetic mean roughness and the amplitude density function (ADF). The formulated hypotheses have finally been verified comparing the ADF and the bearing area curves of profiles related to the most severely scratched areas with those of simulated worn profiles. This study highlights the major influence of foreign third bodies on the scratching of metallic femoral heads of the Charnley type prostheses.