Study On UHMWPE Wear Debris Morphology In Artificial Hip Joints

Wear and generation of wear particles especially UHMWPE debris has been recognized as one of the major causes of long-term failure in total joint replacements (TJRs).Therefore,the study on wear mechanism of artificial hip joints as well as feature of wear debris has become one of the hot topics in biotribology research.The wear particles are important information carriers about the microscopic wear mechanisms of artificial hip joint couples.Nevertheless,the standardization of parameters used to characterize the particles,classification of the UHMWPE wear particles,the feature of every group and the relation between wear debris and wear mechanism have not been formed yet.Therefore,this thesis set out,from the study of UHMWPE wear particles,to explore a reasonable method for debris classification, find out the typical debris in certain wear conditions and describe the feature of the debris.Digitized particle images were analyzed on a computer in this thesis.The following nineteen numerical descriptors were used to characterize the particles: particle area,length,width,perimeter,boundary fractal dimension and shape parameters such as form factor,roundness,convexity,aspect ratio etc.Principal component analysis algorithm was applied to reduce the amount of the parameters in order to simplify the following calculation.However,C-means clustering algorithm was applied to classify the UHMWPE wear debris.Then Xie-Beni index was introduced to determine the optimal number of cluster and illuminate the clustering validity.Samples of the lubricant were investigated from hip simulator stations which simulated the steady wear condition of artificial hip joints.Digitized particle images from four serum samples were analyzed in this thesis.And it is found that flake-like debris,needle debris,fibrous debris,granules and elongated debris are typical debris in steady wear condition.Furthermore,the wear mechanisms were also illustrated according to the morphological feature of the debris.Besides,tissue samples were taken from four failed Charnley total hip replacements.And particle images from four tissue samples were analyzed.The results indicated that flake-like debris,fibrous debris and granules are typical debris in failure stage.Furthermore,the wear mechanisms were also illustrated.Finally,the comparison of the size distribution of debris isolated from the tissue samples and the simulator lubricants was showed in this thesis.The result illustrated that the mode of the percentage number of particles from lubricants was in the size range 0μm to 0.1μm.Analysis suggesated that because of cytophagy,inevitably in the long term in vivo,many nanoscale wear debris are phagocytosed by macrophages already.However,the percentage number of particles from issue samples which was in the area range＞1.0μm~2 was much higher than from lubricants.It is evidence that the size of debris from failure stage is much larger than the debris from steady wear condition.