The patella (also known as the knee cap) normally rides in the a groove at the bottom of the femur at the level of the knee joint. A luxating patella is the when the patella moves out of its normal location, often because the groove is too shallow. Surgery is done to deepen the groove on the femur to keep the patella in place. The soft tissue along the side of the patella is tightened to provide additional support to the knee.