Comparison of wear of mobile and fixed bearing knees tested in a knee simulator

Abstract

Mobile bearing knees have been developed to provide theoretical increases in tibiofemoral contact area and conformity to decrease contact stresses when compared to fixed bearing designs. The decrease in contact stress has been conjectured to reduce the microabrasive/adhesive wear rate of ultrahigh molecular weight polyethylene (UHMWPE) tibial inserts. Although mobile bearing knees may provide lower contact stresses, the inferior surface of the insert has the potential to encounter large sliding distances against the tibial base plate. These combinations of factors have raised questions of whether mobile bearing knees exhibit less wear than fixed bearing designs. In this study, when both fixed and mobile bearing knees were tested in a knee wear simulator utilizing load/motion inputs for walking gait, there was no statistically significant difference in wear rates measured between fixed and mobile bearing designs ( p=0.37), although a trend toward lower wear was noted with the mobile bearing design. The wear rates for the two fixed bearing designs (mean±standard errors), the I/B II and the NexGen CR, were similar (15.0±0.8 mg/million cycles (Mc), n=9 and 14.4±1.2 mg/Mc, n=6, respectively) and approximately equivalent to the mobile bearing knee design, the NexGen LPS-Flex Mobile knee, (13.0±1.1 mg/Mc, n=6). A major difference in wear rates was noted when a highly crosslinked polyethylene was tested, where the NexGen CR (using conventional gamma irradiated polyethylene) and NexGen CR Prolong™ (using highly crosslinked polyethylene) designs differed only in the relative amounts of polyethylene crosslinking utilized in the manufacture of the fixed bearing surface. The wear rate of the highly crosslinked NexGen CR Prolong™ was significantly lower than the wear rates of the other three design and material combinations tested (2.71±0.38 mg/Mc, n=12, P<0.0001).