A loose body is a loose piece of cartilage floating around in the knee joint.
Loose bodies are often referred to as 'knee mice' or 'joint mice'. The bits may start off quite small, but may grow, nourished by the joint fluid (synovial fluid). They may be washed into the gutters at either side of the joint (parapatellar gutters) or behind the joint in the poplital space, but sometimes one sweeps out and gets caught between the bones, causing sudden pain and sometimes locking or giving way.
A rare condition - synovial chondromatosis - may result in the formation of multiple loose bodies from abnormal cells in the joint lining.
Removal of loose bodies is surgical.