The kneecap or patella sits over the knee joint and moves up and down along a track of cartilage. Connective tissue on each side called the retinaculum help hold it in place and guide it up and down in the track.
Patellar instability occurs when one side of the retinaculum is tighter or looser than the other. The kneecap can move out of the track and sublux or even dislocate. When this happens over and over the patella doesn't always go back to the middle. One edge hangs over the side (as seen on X-ray).
Conservative care with physical therapy, exercises, and bracing or taping is the usual treatment. If these measures don't help after three months then surgery to rebalance the retinaculum may be considered.