This 54-year-old male presents with right knee pain that has continued for a year with no known injury. Have a look at images 1 through 3, and see if anything stands out.
Q1 – What is the most likely diagnosis?
Q2 – What is the differential diagnosis for the intraarticular bodies?
A1 – The most likely diagnosis is primary synovial chondromatosis. Synovial chondromatosis is a rare monoarticular process characterized by cartilaginous bodies. Primary synovial chondromatosis has a predilection for the large joints, including the knee, hip, elbow, and shoulder, although it can be identified in any joint or tendon. It affects males more frequently than females, and it is most common in the third to fifth decades. The MRI signal characteristics of the intraarticular bodies varies depending on the amount of calcification and / or ossification (images 5 and 6, arrows). Rare, but primary synovial chondromatosis can undergo malignant degeneration.
A2 – Secondary synovial chondromatosis is more common than primary synovial chondromatosis. It usually presents in the setting of longstanding osteoarthritis. In this patient, note that the medial and lateral compartments of the joint space are preserved. Only a small marginal erosion of the medial tibial plateau is noted (image 4, green arrow). Secondary synovial chondromatosis does not undergo malignant degeneration.
Rice bodies would be an unlikely consideration in this individual who is male, middle-aged, and presents with a monoarticular arthritis as rice bodies are commonly identified in inflammatory arthropathies such as rheumatoid arthritis and tuberculous infections. They are smaller than synovial chondromatosis. Other intraarticular masses to potentially consider are pigmented villonodular synovitis (PVNS) which has diffuse and focal forms, and would not present as nodular densities, but as a mass. Blooming may occur on gradient images due to a lack of refocusing pulse on this sequence, and due to siderotic components. Lipoma arborescens is most commonly found in the suprapatellar bursa and follows fat signal. Synovial hemangiomas are typically high in signal on T2-weighted images.
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