A lift or a sole extension is a permanent, external modification of the midsole and outsole. It adds a prescribed amount of height to one or both shoes to help a patient who has a leg length discrepancy feel and walk level. Walking and feeling level can reduce unwanted compensations of other soft tissue and joint structures that would be required to support a shorter limb.
When constructing a lift modification, there is a potential of utilizing an internal lift instead of external modification. If the lift required is below 2cm, and there is sufficient space within the footwear, the inclusion on an internal modification is possible.