Predicting UHMWPE wear: decreasing wear rate following a change in sliding direction

The patient demographic for total joint replacement is becoming younger and more active leading to increased performance expectations [1]. To develop new products that meet these demands, computer simulations that predict wear are emerging as potential design tools [2]. The algorithms for these simulations typically assume a linear relationship between wear and sliding distance [3-5] and therefore do not account for the possibility of a variable wear rate along a path (Fig 1). In a previous study we demonstrated that the wear of crosslinked polyethylene did not depend on the sliding distance, rather on the number of crossing events (ie changes in direction) [6], implying that large portions of sliding did not contribute to the total wear. We were unable, however, to identify the transition from high wearing multidirectional sliding to the low wearing unidirectional sliding (Fig 1). The purpose of this study was to further investigate this transition by examining a broader range of sliding distances. We hypothesized that small increases in sliding distance after a direction change would produce additional cumulative wear while no additional wear would be produced at longer sliding distances sufficient to reestablish ‘unidirectional’ sliding.