[Nonelastic deformations of polyethylene inlays in hemiprostheses]

In most models of hip prosthesis, the inlay is formed by a layer of polyethylene. The attrition of polyethylene results from two components, wear and nonelastic deformation. Former studies analysed total attrition. The aim of this trial was to measure solely the non elastic deformation. Nonelastic deformation was investigated using two different types of bipolar hemiprostheses to exclude influences of different shaped metal caps. The bipolar hemiprostheses were tested with a hydraulic pulser. 1 x 10(6) cycles of a halfsinewave with a stress of 4600 N at a frequency of 8 Hz were used. The whole procedure was carried out in isotonic saline solution. Stress was applied along a single axis to prevent any wear. After the test, the inner surface of the polyethylene inlay was analysed with a touchpeg. As the inlays had different inner diameters, the analysis of the surface was defined along radials of 5 degrees, 25 degrees and 45 degrees. The nonelastic deformation in the direction of the main stress at 45 degrees was higher than at the lateral areas. The nonelastic deformation measured was between 0.2 and 0.6 mm/a, i.e. 5 to 30% of total attrition. There was a significant difference between the two types of inlays. The model with several pegs and edges for a complicated snap mechanism had a significant higher nonelastic deformation (alpha = 0.05) than the smooth shaped inlay.