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Knee: Intraosseous Lipoma and Synovial Chondromatosis of a Popliteal Cyst

74M w/ pain.  Outside x-ray noted a tibial “lesion”.  Sent for CT to further evaluate.

Findings:  ~5.3 cm long well-defined, lucent medullary bone lesion of the proximal tibia with a narrow zone of transition to normal bone, areas of sclerotic borders, and lack of endosteal/cortical destruction (all benign features).  Internal density of lesion is mainly fat with ring-like dense calcification centrally, consistent with an intraosseous lipoma.  Benign, but probably at some elevated risk for fracture given its size.

2nd interesting finding is a cluster of calcified/ossified bodies, all relatively small and tracking along the expected location of the gastrocnemius/semimembranosis (popliteal) bursa.  No bodies seen in the joint space proper.  This is synovial chondromatosis, a benign metaplastic process where chondral bodies form and detach from the synovium and can become calcified/ossified.  Within joints these bodies can cause erosions and destruction.  MRI may be needed to see non-calcified bodies.  Given the lack of joint effusion and maturity of the bursal calcifications in this case, it’s doubtful the process affects his knee joint (although the most frequently affected joint).