Particles of wear debris have been implicated in osteolysis around and aseptic loosening of total joint prostheses, but the number and size distribution of particles present in periprosthetic tissues are unknown. A method of particle assay was developed, consisting of nitric-acid digestion of tissue followed by collection of particles, electronic quantitation, and parallel morphological and chemical characterization. Nitric acid had minimum deleterious effects on control samples of titanium, cobalt-chromium alloy, and polyethylene particles, as determined by atomic absorption spectroscopy, scanning electron microscopy, and electronic measurements of the sizes of the particles. Acid digestion of twelve control samples of tissue, including tissue rich in hemosiderin, resulted in particle counts that were no higher than that in the digestion solution background. Other digestion preparations, including hydrochloric acid and sodium hypophosphate, were not as effective as nitric acid, With the low size limit of detection of approximately 0.58 micrometer, particle analysis of tissue adjacent to twenty retrieved total joint implants indicated a range of concentration of 0.85 to 141.85 x 10(9) particles per gram of tissue (dry weight). Although a few particles of more than 100 micrometers were detected, the mode of particle diameter from each sample ranged from the lower limit of detection (approximately 0.58 micrometer) to 0.79 micrometer. The findings of morphological studies and x-ray spectroscopy of isolated particles corresponded with those of light microscopy of the fibrous membranes. These data indicate that most of the particles in implant membranes are smaller than the resolution of the light microscope and that tissue digestion is necessary for quantitation and characterization. CLINICAL RELEVANCE: Particulate wear debris, especially polyethylene, has been implicated in osteolytic lesions as well as in more diffuse loosening of total joint prostheses. The biological consequences of debris probably depend on the composition, size, and amount and, perhaps, on the shape of the debris, as well as on the host response to the particles. It is reasonable to suspect that implants that differ in design or materials, or both, release wear debris that differs in size and amount. Before we could begin to compare design, materials, and patient variables, however, it was necessary to develop an assay to determine the number and size of debris particles accurately. Although the methods described in this study have well recognized limitations, we have shown that tissues adjacent to a failed total joint implant contain billions of particles per gram (dry weight) and that most of these particles have a diameter of less than two micrometers. Slightly modi fled, this method can also be used to determine prospectively the number of particles in joint fluids. It is anticipated that this and other, similar assay procedures can be used to compare the physical characteristics of wear debris produced by implants of different designs or materials, or both, and thereby can contribute to the understanding of the safety and efficacy of total joint prostheses.
