Differentiating aseptic loosening from infection as the cause of prosthetic joint failure is difficult because both entities are similar, clinically and histopathologically. Aseptic loosening frequently results from an immune reaction to the prosthesis. There is inflammation with an influx of histiocytes, giant cells, lymphocytes, and plasma cells. Proinflammatory cytokines and proteolytic enzymes are secreted, causing osteolysis and loosening. These same events occur in infection except that neutrophils, rarely present in aseptic loosening, are invariably present in infection. Clinical signs and symptoms, laboratory tests, x-rays and joint aspiration are insensitive, nonspecific or both. Artifacts produced by the metallic hardware hamper cross-sectional imaging modalities. Radionuclide imaging is not affected by the presence of metallic hardware and is very useful for evaluating the painful prosthesis. Bone scintigraphy, with an accuracy of 50%-70% is a useful screening test, since a normal study effectively excludes a prosthetic complication. Adding gallium-67, a nonspecific inflammation-imaging agent, improves the accuracy of bone scintigraphy to 70%-80%. The accuracy of combined leukocyte/marrow imaging, 90%, is the highest among available radionuclide studies. Its success is due to the fact that leukocyte imaging is most sensitive for detecting neutrophil mediated inflammations. Inflammatory conditions that are neutrophil-poor, even though large numbers of other leukocytes may be present, (such as the aseptically loosened joint prosthesis) go undetected. The success of leukocyte/marrow imaging is tempered by the limitations of in-vitro labeling. In-vivo labeling has been investigated and a murine monoclonal antigranulocyte antibody appears promising. Some investigations have focused on F-18 FDG imaging, although specificity is a concern with this agent.
Radionuclide imaging; painful joint replacement; infection