Leg length difference

What is a leg length difference, and how common is it?

A leg length difference (leg length discrepancy) means the left and right legs are different lengths¹. Nearly everyone has a small one, usually under 1 cm, and it usually needs no action on its own¹².

A structural difference means the leg bones really are different lengths¹³. A functional difference means the legs look uneven, or feel uneven to the person, but the bones match³⁴. Tight soft tissues, stiff joints or a tilted pelvis can cause it⁵.

Walking may change above 1 cm⁶. A difference of 2 cm or more may cause problems, and over 2 cm is rare, about 1 in 1000⁷. Where it starts to matter isn't agreed⁶. Up to 90% of children have some difference, but one that matters is rare⁸.

What causes a leg length difference, and who gets it?

A structural difference can be present from birth or develop later³. Differences usually arise during childhood growth, and a fracture or a problem with a growth plate (the area at the end of a bone that controls how long it grows) may cause unequal leg lengths¹. If one bone grows slowly or stops growing while its pair keeps going, a difference results⁹. Infection and past injury can also cause one³.

Functional differences can come from tight soft tissues, stiff joints, a tilted pelvis or the way the foot works⁵. Most structural differences have no known cause¹⁰. It isn't due to anything you did.

What does a leg length difference feel like, and how is it diagnosed?

In children, a leg length difference is often not noticed until their walking changes, pain appears, or function is limited⁸. It can appear as a limp or toe-walking, back, hip, knee or ankle pain, or a sloping shoulder line³.

The most common way to measure is with a tape measure from the front hip bone to the inside ankle bone, comparing left and right¹. Using the average of at least 2–3 measurements is advised³. A tape measure and standing blocks are useful screening tools, though it isn't clear how their accuracy compares with imaging such as CT³¹¹. A standing X-ray is the most accurate method, though it is seldom needed¹.

Pain near the area doesn't prove the difference is the cause, so it's worth looking for other reasons too¹²⁴.

What helps first?

A small difference usually needs no treatment. The usual first step is a heel lift, shoe insert or insole (a lift) matched to a difference that has been measured in millimetres¹²¹⁰. For differences up to 2 cm, a lift inside the shoe may be used¹⁰. Larger differences may need the shoe sole built up¹. Wear the lift with supportive shoes. If you're on your feet for work, it's worth talking about correcting the difference¹.

Lifts may ease pain and improve how you move for many adults with painful joint or muscle problems¹³. How well they work is uncertain, though, and the best amount of correction isn't clear yet¹³¹⁴. Treatment is always your choice¹⁵.

A child's hip, thigh or knee pain shouldn't be put down to leg length.

What helps if a lift isn't enough?

If a lift hasn't been enough, or the difference is large, a surgeon can talk through the options with you. Surgery is always an elective choice, and the size of the difference alone doesn't decide it¹⁵. A difference of more than 5–6 cm may need surgery¹.

In a growing child, closing the growth plate of the longer leg slows its growth, and this has been used for predicted differences of 2–5 cm¹⁶. Lengthening the shorter leg is another option¹⁷.

Surgery carries real risks of complications, so ask for clear counselling first⁷¹⁶¹⁷.

What is the outlook, and can it come back?

The outlook depends on the size and cause of the difference, and on age. Small differences mostly stay harmless, while 2 cm or more is more likely to cause problems⁷. In children, minor differences are usually benign, but a larger one can point to an underlying cause⁸.

A difference has been linked with low back pain, scoliosis and hip or knee osteoarthritis¹³. These are associations, so a difference shouldn't be assumed to explain new or worsening symptoms⁶¹³.

If your child is still growing, it helps to watch the difference over time. It isn't clear yet whether lifts or other treatments change the long-term outcome, or stop a difference returning or progressing¹⁴.

What can make it serious?

• Your child walks on tiptoe and also can't stand with heels down, has one side different from the other, has very bulky calves, or seems unusually stiff or unsteady.
• You have lost weight without trying.
• You have pain at night or at rest that doesn't settle.
• Your child has a limp that hasn't improved after a week, limps without pain, or doesn't want to put weight on their foot.
• Your child has a fever with a limp or a painful, red, hot or swollen joint, or seems very unwell with foot or leg pain.
• Your child suddenly has severe pain in one joint and is limping.