Fractal dimension and size distribution characteristics of ultrahigh- molecular-weight polyethylene wear particles in condition of artificial knee joint simulator

Abstract

Background: As a typical friction pair, the artificial joints also generate a large number of wear particles due to the wear movement, which has become the direct cause of the debris disease called in clinical. The wear debris with different morphology and size contains a large amount of information relating to the wear status, and has the typical fractal characteristics. These fractal characteristics with a certain degree of regularity are favorable to study the wear mechanism under the different wear conditions. The aim of this study is to evaluate the fractal dimension and size distribution characteristics of UHMWPE wear particles in condition of artificial knee joint simulator by the automatic extraction and fractal identification system of wear debris based on the improved radar graph method. Methods: The forged CoCrMo alloy (ISO5832-12) and ultra-high-molecular-weight polyethylene (UHMWPE, ISO5834-2) were selected as the artificial joint prosthesis materials. The knee joint simulator produced by University of Leeds (Simulation Solutions Ltd.) was used to realize the knee joint wear motion. The wear debris extraction was based on the standard of ISO17853. Scanning electron microscope (Hitachi, S3000N) was used to observe the morphology characteristics of wear debris. The automatic extraction and fractal identification system of wear debris based on the improved radar graph method was used to investigate the fractal characteristics and size distribution of UHMWPE wear particles. Results: The fractal characteristics of the single UHMWPE wear debris are very obvious. When the wear particles changes from the larger sized strip to the smaller sized near-sphere, the radar fractal dimension decreases continuously. The fractal dimension D of ideal spherical shaped wear debris is close to zero. The fractal characteristics of UHMWPE wear debris group directly reflects the transformation trend of wear mechanism. In the initial stage of wear, the strip and flaky wear debris with the larger fractal dimension occupy the largest proportion in total wear debris. The adhesive and ploughing wear are the main wear mechanisms, corresponding to the running-in stage of the wear movement. With the extension of wear running-cycles, the proportion of strip wear debris reduces and the percentages of flaky and blocky wear debris increase obviously. The wear mechanisms gradually change to the abrasive and fatigue spalling wear and the wear movement enters into the composite wear stage. When the friction pair enters the stable wear stage, the proportion of all kinds of wear debris has little change. But the number of small-sized wear debris increases due to the action of cyclic stress, which causes the fractal dimension of wear debris group to decrease to some extent. Conclusions: The fractal dimension and size distribution of wear debris with different profiles can be obtained by the automatic extraction and fractal identification system of wear debris based on the improved radar graph method. It has enough numerical gradients to distinguish the wear debris morphology and avoid the effects of errors. It is not only used for the shape extraction, calculation of fractal dimension and statistics of wear particle parameters for the single and group of wear debris, but also provides a new digital analysis tool for the identification and diagnosis of the artificial joint wear debris.