Sometimes patients come to me who have been told by another caregiver that they need to wear a heel lift in one shoe. Careful assessment often shows that the patient does, in fact, have one leg shorter than the other, but the difference is not anatomical, but functional.
Anatomical leg length difference is when one leg is actually physically shorter. This may be due to a previously healed fracture in the leg, or it may be congenital. Wearing a heel lift in this case can be the right thing to do. The lift should be prescribed and designed specifically for the patient by a qualified practitioner.
Functional leg length difference is due to distortions or imbalances in the person’s frame---the entirety of the bones and joints of their body. Once the distortions or imbalances have been corrected, which is exactly what I do, the legs will even. In this case, wearing a heel lift is a bad idea. It is only worsening the problem.