Leg length discrepancy (LLD) is a condition where one leg is measurably shorter or longer than the other. Nearly everyone has some degree of asymmetry between their legs, but differences beyond about 2 cm typically start causing noticeable changes in posture, gait, and comfort. The condition falls into two main categories, has a wide range of causes, and can be managed with anything from a simple shoe insert to surgery, depending on the size of the difference and how it affects your body.
Two Types of Leg Length Discrepancy
Leg length inequality is divided into two groups: anatomic (structural) and functional. The distinction matters because each type has different causes and different treatment paths.
An anatomic discrepancy means there is a true difference in bone length. One femur (thighbone) or tibia (shinbone) is physically shorter or longer than the one on the other side. This can be measured on imaging and doesn’t change based on how you stand or move.
A functional discrepancy, sometimes called a leg-length alignment asymmetry, means the bones themselves may be equal in length, but something else makes one leg behave as though it’s shorter. Muscle tightness, joint contractures, pelvic rotation, or foot mechanics can all create a functional difference. For example, a stiff hip that won’t fully extend or a flat arch on one foot can shift your pelvis and mimic a true short leg. Correcting the underlying joint or muscle issue often resolves a functional discrepancy without any need to address bone length.
Common Causes
In children, congenital conditions are a major source. Some are mild, involving slightly different growth rates between the two legs. Others are severe, including rare malformations where entire sections of a limb are underdeveloped or absent. Conditions involving abnormal blood vessel growth, like Klippel-Trénaunay-Weber syndrome, can cause one leg to grow longer than the other through overstimulation of tissue growth.
Acquired causes are more varied. A fracture that heals with some shortening, damage to a growth plate from injury or infection during childhood, or bone loss after surgery can all leave one leg shorter. In growing children, even a minor injury to the growth plate at the end of a bone can slow that bone’s development over years, gradually widening the gap. In adults, hip or knee replacement surgery and degenerative joint disease are common contributors.
How Your Body Compensates
The most immediate effect of an anatomic discrepancy is pelvic tilt. When one leg is shorter, the pelvis drops on that side, creating an oblique angle that ripples upward through the spine. Research shows that pelvic obliquity becomes significant once a discrepancy reaches about 2.5 cm, affecting both standing posture and walking. The spine may develop a compensatory curve (measured by what’s called a Cobb angle) to keep the head centered over the body, and larger discrepancies of 3 cm or more have been associated with increased spinal curvature.
Your gait changes in specific, measurable ways. The shorter leg tends to land with more ankle plantar flexion (pointing the toes down slightly to reach the ground), while the longer leg develops increased plantar flexion force at push-off. The pelvis and trunk rotate more toward the shorter side during stance, and hip motion on the longer side becomes restricted. These compensations are subtle at small discrepancies. Studies consistently find that differences under 1 cm produce no significant changes in step length, step duration, or single-leg support time, but differences of 2 to 3 cm do.
Over time, these asymmetric forces can contribute to low back pain, hip discomfort, and uneven wear on joints, though the relationship between small discrepancies and pain is more complicated and varies from person to person.
How It’s Measured
In a clinical setting, the simplest approach is the block test. You stand upright while blocks of known height are placed under your shorter foot until your pelvis feels level. This gives a quick, functional measurement of how much correction your body needs, and it accounts for factors like differences in foot height that a bone measurement alone would miss.
For a precise measurement of bone length, imaging is the standard. Plain radiography remains the most practical method, with several techniques available. A scanogram uses multiple X-ray exposures of the hip, knee, and ankle, stitched together digitally to measure each bone segment. The newer EOS imaging system uses a low-dose biplanar X-ray that produces true-to-size images with minimal magnification error (around negative 0.5% to 0.8%), compared to conventional radiographs that magnify images by roughly 6 to 9%. EOS imaging is particularly useful when precise measurements matter for surgical planning, since even a few millimeters of error can change the treatment approach.
Predicting the Final Discrepancy in Children
In growing children, the current discrepancy isn’t necessarily the final one. A difference of 1 cm at age 5 will likely be larger by the time the skeleton matures. Orthopedic specialists use the multiplier method, developed by Dror Paley, to predict discrepancy at skeletal maturity. For congenital conditions, where the discrepancy grows proportionally with the child, the calculation is straightforward: the current discrepancy multiplied by a factor based on the child’s age and sex. For developmental conditions where growth inhibition is ongoing, the formula accounts for both the current difference and how much growth the affected leg still has ahead of it.
The beauty of this method is its simplicity. It requires as few as one or two measurements, works the same for the thighbone or shinbone, and doesn’t depend on the child being at a specific height percentile. These predictions guide the timing of treatment, particularly for surgical options that depend on remaining growth.
Non-Surgical Treatment
For discrepancies that cause symptoms but don’t warrant surgery, shoe lifts are the primary tool. A lift can be placed inside the shoe (an internal lift) or added to the outside sole (an external lift). Internal lifts work well for smaller corrections, but most shoes can only accommodate an insert up to about 10 mm before there isn’t enough room for both the lift and your foot. Beyond that threshold, an external modification to the sole is the standard approach.
Lifts don’t always correct 100% of the measured difference. The goal is to find the height that makes you feel balanced and reduces symptoms, which may be somewhat less than the full discrepancy. Physical therapy can complement lifts by addressing muscle imbalances and joint restrictions that contribute to a functional component of the discrepancy.
Surgical Options
Surgery enters the picture for larger discrepancies, typically 2 cm or more, and the approach depends heavily on the patient’s age and the size of the gap.
Slowing Growth on the Longer Side
In children with enough growth remaining, a procedure called epiphysiodesis can equalize leg length by temporarily or permanently slowing the growth plate on the longer leg. This is generally used for predicted discrepancies between 2 and 5 cm and is performed between roughly ages 7 and 16. Timing is critical: do it too early and you overcorrect, too late and the growth plate has already closed. The procedure itself is relatively minor compared to lengthening surgery, with a shorter recovery and fewer complications.
Lengthening the Shorter Side
For discrepancies of 5 cm or more, or when slowing the longer leg isn’t an option, limb lengthening through distraction osteogenesis is the primary surgical treatment. The bone is surgically cut, and then gradually pulled apart at a controlled rate so new bone fills the gap. Traditionally this was done with external metal frames attached to the bone through the skin. Modern internal devices, like the Precice magnetic lengthening nail, are placed entirely inside the bone. The nail contains a magnetic mechanism activated by an external remote control held against the skin, allowing the patient to lengthen the bone at home, typically about 1 mm per day (split into three sessions of 0.33 mm). Unlike external frames, the nail also allows controlled shortening if a correction overshoots.
Limb lengthening is a months-long process. The bone lengthens over weeks, then requires additional months to fully harden and bear weight normally. The total timeline from surgery to unrestricted activity varies but often spans six months to a year or more, depending on how much length is being gained.