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Knee: Focal Nodular Synovitis

47M w/ 5 years of pain.
Findings:  Focal mass along the anterior joint capsule, protruding into Hoffa’s fat.  Mixed, predominately low-intermediate signal intensity on fat-sat and non fat-sat images.  No other masses. 

This entity is thought to be a focal form of the more widespread synovial joint disease of pigmented villonodular synovitis (PVNS).  This is the classic location, usually less than 4cm.  PVNS involves the entire joint capsule surface with areas of bone erosion.  Blood products are broken down and hemosiderin is deposited within histiocytes, giving the low singal intensity areas that are enhanced using gradient echo MRI sequences.  The focal nodular form tends to have less hemosiderin deposited.  PVNS is treated by complete synovectomy, and because this is extrememly difficult to accomplish, the disease tends to recur.