Volume 3, Issue 9 (October 2006)
The Effects of Load Soak Control on the Wear of UHMWPE at Various Hydration Levels in a Joint Simulation Study
The study was to investigate the effects of various hydration levels on the wear of ultra-high molecular weight polyethylene (UHMWPE) in a hip simulation testing, and to assess the effect of load soak control on the implant wear rates. Three types of polyethylene liners (28 mm ID) made from GUR 1050 UHMWPE bars were evaluated: noncrosslinked (XL0), gamma irradiated (thermally-treated) at nominal doses of 5 and 10 Mrads (XL5 and XL10, respectively). Three hydration processes were applied: (1) presoaked in water for four weeks, i.e., standard protocol; (2) exposed in air for four weeks; and (3) presoaked in water at 900 psi (approximately 60 atm) in a pressurized chamber for three weeks. Six groups of interest were selected and tested on a hip simulator with load soak control stations. The group that was subjected to 900 psi pressure gained 11.9 ± 0.4 mg of water after three weeks, which was about eight times the groups that presoaked in water. After the wear simulation started, the highly hydrated group lost almost 10 mg during the first 0.5-million cycle interval, which was due to the pumping motion of the load soak stations. All soak control liners achieved similar hydration levels after one million cycles of testing. The overall wear rating was inversely related to the amount of crosslinking. Without load soak correction, the initial wear rate of XL5 with pressurized soaking was twice the corrected wear; however, the wear rate remained similar after one million cycles with or without corrections.