Radionuclide arthrography of the knee is described using Tc-99m sulfur colloid, which is injected into the synovial cavity following partial distension of the joint space with saline. The preliminary findings in Baker's cyst are described. There appear to be four patterns: 1) a herniation or cyst that can be posterior, inferior, medial or lateral to the knee synovial space; 2) a limited leak inferiorly towards the calf or superiorly toward the thigh; 3) extensive leak: giant cyst found in patients with rheumatoid arthritis; 4) multiple 'cysts' or communications with the knee synovial space. The series is small, and more work is needed before these preliminary findings can be used as firm categories. Radionuclide arthrography should be considered in patients with unexplained calf pain or pain behind the knee, especially with a history of previous internal knee derangement, or rheumatoid arthritis in whom ruptured cysts can closely mimic acute thrombophlebitis. Radionuclide arthrography has certain advantages over contrast arthrography and also arthroscopy in the diagnosis of Baker's cyst.
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