This is a condition where one leg is shorter than the other. This can cause lower back pain, pelvic tilt resulting in scoliosis, hip pain due to the longer leg jamming into the hip joint and knee pain, this can even cause mid back and neck pain. By measurement of CT scan, the accurate difference in leg length is determined and the shorter leg is posted to compensate the difference in leg length and realign the pelvis.
The pic shows a physical examination for leg length difference. Notice the 2 land-mark lines don't line up, with the left leg shorter than the right leg.