The patella is the knee cap. This is a bone that looks just like a small cap and slides up and down within a grove on the lower end of the femur, the bone that goes from the hip to the knee. The knee cap is held in its place by the patellar ligament, a tough band of tendon that is attached to the tibial crest.
The tibial crest is a protuberance on the tibia, the bone that goes from the knee to the foot. Some dogs, particularly toy breeds, may have an abnormality where the groove in the femur is too shallow and/or the tibial crest is out of line. As a result, the knee cap may come off the groove when the dog bends the leg. This problem is corrected by moving the tibial crest slightly to the side (by means of transplant) in such a way that the knee cap can move up and down and not slip off the groove.