Radiological opacities of the knee after intra-articular injections of osmic acid.

Unusual radiological opacities have been observed in 29 cases after intraarticular injection of osmic acid in 113 knees of 72 patients under treatment for a chronic exudative synovitis in different types of rheumatic diseases. They are statistically more frequent after the injection of 200 mg than after 100 mg (33.9 and 15.7% respectively). Most of the time, they are located in the suprapatellar pouch. The opacities appear soon after the injection and can last for several years. They do not seem to have an unfavorable influence on the clinical course. They appear to be caused by a deposition of osmium in synovial fat tissue. From here the osmium might stimulate fibrosis. The radiological image has to be differentiated from intra-articular calcifications, synovial ossifications and various periosteal osteogenic processes.