Any free-floating object in the knee is known as a loose body (sometimes referred to as a joint mouse because it runs away if you try to grab it). It can be a fragment of bone, cartilage, or meniscus. It can also be a piece of glass, metal or any foreign object. They may occur singly or in groups and typically affect only one joint.
Loose bodies are classified as either stable or unstable. Unstable loose bodies are free to move about the joint and cause problems. Stable loose bodies are not actually loose in the joint. Typically they are stuck to the synovium or capsule of the joint in a fixed position and generally don’t cause symptoms. These are often found when investigating the knee for some other reason.
A loose body isn’t usually noticeable until it jams somewhere in the joint. Your symptoms can change depending upon where in your knee it ends up. A loose body can cause catching of the knee, giving way of the knee (instability), pain or swelling. Occasionally the joint actually becomes locked where you cannot move it at all.
- Injury (such as a patella dislocation)
- Torn piece of cartilage (meniscus)
- Foreign body
- Synovial osteochondromatosis
The diagnosis is easy if you can feel the ‘joint mouse’ moving around. A plain xray is always performed of the knee but must include a notch and skyline view. The loose body is often seen on a regular x-ray if it has calcium in it. If the piece is made entirely of cartilage an MRI will be needed to see the fragment (you can’t see those on regular x-rays). Cartilage is nourished by the fluid within the joint (synovial fluid) so loose bodies often increase in size and become smoother over time. It is important to determine where the loose body came from and to treat the cause of the problem.
Loose bodies almost always continue to cause problems. They are typically removed using arthroscopic techniques but a slightly larger cut is often needed to get the loose body out of the joint through the skin. The entire knee is carefully checked during the arthroscopy to see where the loose piece came from and if further treatment for this area is required