A functional leg length difference suggests that it isn't the bones that are the problem. When one leg bone is longer than the same one on the other side, it's said to cause a structural leg length difference. When they measure out to be the same from top to bottom then it's called a functional leg length difference. Something in the soft tissue or contractile tissue is the problem. Often this is scar tissue or muscle tightness.
For some people a curve in the spine can cause one leg to seem longer than the other. Patients who have had a hip or knee replacement can develop a functional or structural leg length difference from the surgery. With time and a little attention, a functional leg length difference often resolves.
Careful measurement is needed to clearly identify a structural versus a functional leg length difference. The examiner must use the belly button as the starting point and measure down to the ankle bone on the inside. The same measure is taken on both sides.
Some people use the pelvic bone as the starting point but any pelvic tilt or muscle tightness will give a false reading. X-rays can be used to confirm clinical suspicions. Again careful measurements are needed when using X-rays to guide the process.