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 tenosynovial giant cell tumor (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 tenosynovial giant cell tumor 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