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. Symptoms may include swelling and pain when bending the knee.
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.
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. An MRI is often done to assess for cartilage injury.
In some cases arthroscopy can be used to 'smooth' the cartilage. Although new cartilage cannot grow to take its place, scar tissue appears.
It is also possible to transplant some cartilage from an uninjured part of the knee. Another option is to remove some normal cartilage cells, reproduce them in a lab and then later reimplant them into the damaged area so that new cartilage will grow.