Volume 8, Issue 3 (March 2011)
Wear Rate in a Series of Retrieved RP Knee Bearings
Rotating platform mobile bearing knees are an appealing approach to the problems of tibial loosening and rotational mal-alignment that are of concern with fixed bearing knees. A potential disadvantage of rotating platform (RP) bearings is ultra-high molecular weight polyethylene (UHMWPE) wear debris from the large additional backside articular surface that accommodates tibio-femoral rotation. The investigation of a series of 76 rotating platform knees (Sigma® RP, DePuy/J&J, Warsaw, IN) indicates that UHMWPE bearing wear, as measured by change in through-thickness dimension, increases monotonically with time in vivo. Total wear penetration rate is 0.023 mm/year and shows a decreasing trend, though this trend is not statistically significant. The current study results are consistent with the decreasing wear rate previously reported in a series of LCS® RP (DePuy/J&J, Warsaw, IN) knee. This decreasing wear rate stands in contrast to an increasing backside-only wear rate reported in fixed bearing knees. An important contribution of the current study is that it provides a conservative measurement of total wear penetration and penetration rate in one mobile bearing design over time out to >8 years.