Pairs of sheep knee joints were used in the experiments. One of the joints was worn in a simulator over different periods of time under carefully simulated physiological forces and kinematic cycles while the other, 'control joint', was kept intact for comparison. Wear particles were extracted from a synovial fluid from both the worn and unworn sheep joints by a ferrography technique and then examined in a scanning electron microscope (SEM). The shape of the particles was characterized by boundary fractal dimension, shape factor and convexity. The shape parameters obtained for particle populations were statistically compared using a Wilcoxon rank test. It has been found that subtle changes in the shape of the particle boundary occur during the wear process. Studying the changes in wear particle boundaries provides important information about the cause and progression of wear in synovial joints. (C) 1997 Elsevier Science Ltd.