A new optical method for determining deformation and volumetric wear in retrieved polyethylene acetabular cups

Aim: Most methods used for the determination of volumetric wear of polyethylene cups are based on the assumption that the head of the prosthesis penetrates the cup in "cylindrical" fashion. The new accurate optical method is independent of this disputable assumption. Method: The articulating surface of the cup is scanned with light and a data set of 60 000 pixels obtained in this way is stored in a computer. Data obtained from used cups were compared with those obtained from unused cups. The volumetric wear was calculated directly by threefold integration. To assess the changes in surface shape, the data are fitted by an ellipsoid whose long axis defines the mean direction of load. A total of 18 retrieved and 3 unused cups of different types were studied. Results: The unused acetabular cups deviated only slightly from ideal hemispheres. The surfaces showed rotational symmetry, and an undulation having an amplitude of 0.1 mm between dome and equator. For all explanted cups, the assumption of cylindrical penetration of the head into the polyethylene was shown not to represent the true situation. The cup expands in all directions, and the volumetric wear is underestimated by 50 % with the traditional methods. The data suggest that long-term survival may be jeopardized when the main direction of loading is centered on the dome of the cup. Ceramic heads were associated with smaller rates of volumetric wear. Conclusion: The new optical method is characterised by short measuring times, precision and simple application. Analysis of the wear patterns of polyethylene components using this technique may contribute to a further understanding of the complex mechanisms of aseptic loosening.