Good question. I recently visited a chiropractor for lower back pain and had x-rays done. The chiro observed in passing that my right leg is 7 mm shorter than the left. Again not observable lying flat and caused by hip rotation I guess. He was somewhat surprised no one had picked this up before though it does explain why my right shoe from childhood has always worn out on the outside of the heel. He suggested getting an orthotic insert. He said a heel insert might work but a full orthotic is preferable. Since I'm having major issues with my right knee I'm going to try this....nothing else seems to have fixed it.