The effect of surface topography of retrieved femoral heads on the wear of UHMWPE sockets.

The study was undertaken to investigate the association, if any, between the surface roughness of 35 explanted femoral heads and the clinical wear factors of the corresponding polyethylene sockets. The wear of the socket was measured using a shadowgraph technique. The surface topography was investigated using a Rodenstock RM 600 non-contacting profilometer, and six parameters were used to characterise the roughness. Further, qualitative inspections of the femoral surface were undertaken using both a Joel JSM-IC-848 scanning electron microscope and a Zeiss Axiotech microscope with a differential interference contrast facility. Two parameters were found to correlate with the clinical wear factor, namely the skewness of the amplitude distribution function, Sk, and the arithmetic mean roughness, Ra. Simple parameters describing the peak heights of the asperities were found not to have a significant association with the clinical wear factor. The exponent in the power relationship between the arithmetic mean roughness and kclinical was found to be equal to 0.5 (SE: 0.2). This value is significantly smaller than that found in laboratory experiments and may be due to the non-uniform nature of the roughening of the femoral head, three-body wear and the effect of other clinical factors on the wear process. Further, extremely strong correlations were detected between the differing roughness parameters, which would suggest that any attempt to deduce which one is the most important in affecting the wear of the polymeric counterface is fraught with difficulty. However, further investigation of those parameters, such as the reduced peak height or the material filled profile peak area, which may better describe the effect of the counterface on the wear of the UHMWPE surface would appear to be prudent.