Understanding polyethylene wear mechanisms by modeling of debris size distributions

The bioreactivity of UHMWPE (PE) wear debris from artificial hip joints can result in considerable bone loss. Even with four decades of study a better understanding is needed for the debris/wear relationships. Therefore, we developed a methodology and mathematical model to represent the size distributions for polyethylene (PE) wear debris from hip simulator models. Since the diameter of a debris particle represents a positive number, only distributions bounded by 0 and ∞ were investigated. Also, the model needed to fit the distributions for both the particle number and particle volume fractions. Single and mixture distribution schemes were examined. Debris data from PE hip simulator wear studies of crosslinking and surface roughness effects were used to evaluate the models. Single, uni-modal distributions did not adequately represent the number or volume fraction data. A mixture distribution model best represented the size distributions for both number and volume fractions. Crosslinking and surface roughness affects were reflected in the model with specific regions of the debris distribution (sub-distributions) being influenced more than others. For the first time it has been demonstrated that wear debris data is more complex than it appears. Debris morphology data represents several processes and a factor such as crosslinking PE or changing the counterface surface roughness may be reflected only in a small proportion of the total data set.