Abstract
The mechanical failure of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups in vivo is due mainly to a combination of excessive plastic flow and fretting. Localised overheating of the bearing surface, due to insufficient lubrication, causes this. The purpose of this study was to determine the amount of creep in UHMWPE under various conditions.
Test pieces were cut from a piece of raw material and tested according to ASTM D2990. In the first test, to determine the anisotropic behaviour of the material, test pieces of raw material were cut at various orientations. The material was then tested in the virgin state and the virgin state at different temperatures. It was also gamma sterilised under different conditions, namely 24 kGy in air, 25 kGy in a nitrogen atmosphere and 25 kGy in air, and heat treated at 80°C to get an annealing effect. Further tests were conducted to determine the effect of cross-linking on creep behaviour. These tests were administered at room temperature, at 50°C and at 60°C.
The material showed extreme anisotropic behaviour. It was more sensitive to creep in the centre of the bar than on the outside (32%). Maximum creep, however, occurred at a 45°-angle. This is significant if we assume that maximum loading of an acetabular cup occurs at an angle of 70.7°. The difference in creep for the virgin material, measured at room temperature and at 60°C, was 87.3% or 0.716 mm. The variance in creep for the different methods of sterilisation was a maximum of 0.3 mm. Creep for the cross-linked material, however, was markedly less than for the virgin material. There was a decrease of 36% (0.58 mm) in creep at room temperature and almost 83% (0.84 mm) at 60°C.
The test results show that the cross-linked material is much more stable. This may explain the good in-vivo service of these products.
The abstracts were prepared by Professor M.B.E. Sweet. Correspondence should be addressed to him at PO Box 47363, Parklands, Johannesburg 2121, South Africa.
