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Osteonecrosis, which literally means “bone death,” is a painful condition that develops when a segment of bone loses its blood supply and begins to die. Osteonecrosis of the knee most often occurs in the knobby portion of the thighbone, on the inside of the knee (medial femoral condyle). It may also occur on the outside of the knee (lateral femoral condyle) or on the flat top of the lower leg bone (tibial plateau).

In a normal knee, the kneecap fits nicely in the femoral groove, allowing you to walk, run, sit, stand, and move easily. But if the groove is uneven or too shallow, the kneecap can slide off, resulting in a partial or complete dislocation. A sharp blow to the kneecap, as in a fall, can also pop the kneecap out of place. When this happens, the MPFL is usually torn and this makes it more likely for it to happen again.