Loose bodies are small pieces of bone or cartilage (or a combination of both) which float inside the cavity of the knee joint. They are most commonly the remnants of injuries to the knee such as cartilage tears, osteochondral fractures or of other conditions such as ostochondritis dissecans.
The main problems caused by loose bodies are instability and locking, that is an inability to move or straighten the knee joint. The locking is often painful and will require the sufferer to manipulate his or her knee to start the joint moving again. When this occurs, a click is often felt or heard.
If loose bodies are problematic the can be removed arthroscopically.