It is not uncommon for people to have one leg that is longer than the other. For most people this is of no concern and symptoms do not arise. Muscles in the legs, back and hips will work to balance out minor differences. In some cases the difference is too large for the body to accommodate or the muscles are weakened and a lift needs to be added to the shortened side. This will level out the pelvis and can help with painful nerve impingement. Please note that functional leg length discrepancies caused by muscle imbalance are generally resolved with strengthening and stretching the affected muscle groups. Anatomical/skeletal differences are situations in which a physical lift can be considered.
There are multiple ways to measure leg length discrepancies, each with their own positives and negatives. It is generally worth measuring the patient multiple times on each side, this allows for a better picture of what is really going on and helps eliminate some inconsistencies. Once a patient has been measured and the difference calculated it is generally wise to start with a smaller lift and gradually increase the height until the symptoms are relieved. A good starting place with a lift is to add one half of the difference in leg lengths to the shortened side. If the patient has been walking without a lift for 50 years the body may be slow in adapting and you have to treat each individual on a per case basis. Most people do not need the full amount compensated for and if a very aggressive initial approach is taken compliance can become a significant factor.
If the required lift is small and less than 3mm (1/8th inch) it is generally not suggested to intervene unless there are symptoms. For larger lifts of 8mm (3/8th inch) the shoe must be considered. Shallow casual style shoes with no removable liners cannot comfortably fit lifts or are restricted to very small ones. Most athletic shoes with liners that come out will only have room for less than 1 cm of lift. If the lift required is larger than this, the shoe will have to have the sole modified. Building the lift right into the shoe itself alleviates potential fit issues but does limit the patient to only having that shoe with the proper support. Not all shoes can be modified and they external appearance of the footwear will be altered.
Children may show signs of leg length discrepancy as they grow and develop. They may have a short left leg on their initial visit and after a few months of growth the right leg may now be short. For this reason it is generally wise to observe and take note rather than to intervene. There are circumstances where an injury has damaged a growth plate, the difference is large or even suggested by a physician/specialist. These cases would be taken as exceptions and lifts can easily be done for a smaller foot.