History: 50 year old male with knee pain.
This is a case of synovial osteochondromatosis, which can be either primary or secondary. Primary synovial osteochondromatosis (or chondromatosis alone, if the bodies are not all ossified) is a benign idiopathic neoplasm of the synovium characterized by multiple loose intra-articular cartilaginous bodies, some of which may be ossified. Secondary synovial osteochondromatosis is seen with coexistent osteoarthosis, as seen in the image above. A general distinction in the secondary form is multiple bodies of differing sizes with concentric rings of growth.
The differential diagnosis includes primary versus secondary osteochondromatosis, pigmented villonodular synovitis, intra-articular chondroma, synovial chondrosarcoma, and periosteal chondroma.
Credit to Quinn Meisinger, M.D. for this classic example of secondary synovial osteochondromatosis.