There are two main types of leg length inequalities:
One is caused by the legs themselves being measurably different in length, usually due to differences in the length of the femur in the thigh or the tibia and fibula bones in the lower leg. This may be a birth defect or it may occur after a broken leg, serious infection, or local damage to one of the growth plates in a leg.
The other, more common, type is seen when the legs themselves are the same length, but due to neuromuscular injuries in the pelvis or upper leg, one leg or hip is held higher and tighter than the other (hypertonicity in the musculature of the pelvis or leg). These unequally tightened muscles cause the legs to seem to be different lengths, even though careful measurement would show equal lengths of the actual leg. This is called leg length alignment asymmetry (LLAA) and can be seen while lying down.
The most common treatment for discrepancies in leg length is the use of a simple heel lift, which can be placed within the shoe. In cases where the length discrepancy is moderate, an external build up to the shoe is usually more comfortable. In severe cases, surgery can be used to make the longer leg shorter (or impede its growth), and/or make the shorter leg longer.
Although prone "functional leg length" is a widely used chiropractic tool in their Activator technique, it is not a recognized anthropometric technique, since legs are usually of unequal length, and measurements in the prone position are not entirely valid estimates of standing X-ray differences. Measurements in the standing position are far more reliable. Another confounding factor is that simply moving the two legs held together and leaning them imperceptibly to one side or the other produces different results.