Knee Problem: Loose Body
This is a small piece of calcified cartilage that becomes caught in between the bones of the knee causing sudden, sharp pain and occasional giving away. There is usually moderate to severe swelling of the knee and sometimes the swelling reduces within 48 hours of the sharp pain. Occasionally, the knee actually becomes locked and you are unable to straighten the knee.
The loose body is often seen on a regular x-ray. Sometimes, an MRI is ordered to see a loose piece of cartilage that does not have any bone in it (you can't see those on regular x-rays).
If you think you have this problem, and especially if your knee locks, please seek medical treatment immediately.