Intracapsular chondroma of the knee
Anterior knee pain with intermittent locking
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There is a well-circumscribed anterior paraarticular oval mass within the infrapatellar (Hoffa's) fat pad. It appears isointense to the muscles on T1WI with a central area of high signal (isointense to the articular cartilage), heterogeneous high signal on PD fat sat sequences with a central area also isointense to the articular cartilage and peripheral hypointense rim (calcification). No enhancement is seen following IV contrast administration. Note mild edema of high signal within of the infrapatellar fat pad with no lesion of the adjacent bone. An intraarticular effusion is noted. Grade 3 vertical oblique tear of the posterior horn of the medial meniscus.
Intracapsular (osteo-)chondroma of the knee is a rare benign tumor of cartilaginous origin due to an extrasynovial metaplasia, usually located within the infrapatellar fat pad.
The main differential diagnoses include localized pigmented villonodular synovitis (also known as intraarticular giant cell tumor of the tendon sheath), calcified synovial sarcomas, calcifying bursitis, primary synovial chondromatosis, periosteal chondromas, and soft tissue chondrosarcoma.
Localized pigmented villonodular synovitis typically appears as a soft tissue mass of midline location within the infrapatellar fat pad in contact with the patellar tendon. Its signal intensity is variable; however, the presence of low signal intensity on all sequences is a typical feature, indicating hemosiderin deposition.
Additional contributor: R Bouguelaa, MD