The effect of minimum load on the fluid uptake and wear of highly crosslinked UHMWPE total hip acetabular components

Thirteen total hip replacement acetabular components of the same design (32 mm diameter, cup liners made of highly crosslinked ultra-high molecular weight polyethylene (UHMWPE)) were tested in an AMTI hip simulator (six in wear stations and seven in load-soak stations). Two additional liners were immersed in undiluted bovine calf serum but not loaded. The test conditions were otherwise similar for all wear and load-soak samples, with the following exceptions: (a) no motion was applied onto load-soak liners; (b) the minimum load during the swing phase was set at 40 N for three of the wear liners and four of the load-soak liners (group A: low minimum load), and 230 N for the remaining three wear liners and three load-soak liners (group B: high minimum load). It was found that: (a) load-soak liners gained about 10 times more weight than no-load soak liners; (b) load-soak liners in the low minimum load group gained twice as much weight as those in the high minimum load group, and (c) the observed wear rates were low for both groups; surprisingly, however, the wear rates of the low minimum load liners were also higher than those of the high minimum load liners. This study showed for the first time that the fluid uptake and wear rates of highly crosslinked UHMWPE are sensitive to small differences in the loading pattern.