Injury to the knee can cause damage to the articular lining cartilage in the knee joint, or sometimes to both the cartilage and the bone.
If the injury is restricted to the cartilage, it will not show up in a plain X-ray; it may be noted on an MRI. An arthroscopy (using a special instrument to look inside the joint) can thoroughly identify it.
Detached Cartilage of Bone in the Knee Joint
If a piece of cartilage or bone has become detached in the knee and the injury is not treated immediately, the loose part can 'swim around' in the joint. This means that it may occasionally get stuck, causing pain and a feeling that the knee is locked. The knee may also click and swell up. Such a condition is called a loose body in the knee.
As cartilage does not show up on an X-ray, the loose body will only be visible if it consists of bone.
Symptoms may include swelling and pain when bending the knee.