Functional or Structural: Which Type of Leg Length Difference Do You Have?
A leg length discrepancy comes in two types. It is structural when one bone is really shorter. It is functional when the bones are the same length, but something in the pelvis, the hip or the balance of the muscles makes one leg act shorter. It matters which type you have, because each needs a different approach. Only a standing, weight-bearing image shows the real picture.
Key points
- A structural leg length difference means one femur or tibia is physically shorter than the other.
- A functional (apparent) difference means the bones match, but pelvic tilt, tight muscles or joint problems create an imbalance.
- Scans done lying down can miss a functional difference, because gravity flattens the pelvis and the muscles relax.
- Standing whole-body imaging captures both types while your body carries its real weight.
- Knowing your type guides whether you need a heel raise, manual therapy or more tests.
What is the difference between a structural and a functional leg length discrepancy?
A structural leg length difference means one leg bone is measurably shorter than the bone on the other side. A functional (sometimes called "apparent") leg length discrepancy means the bones are equal. The body behaves as though one leg is shorter because of an imbalance somewhere else.
The bone itself is shorter. This can happen after an old fracture that healed with some shortening, after a hip or knee replacement, after a growth-plate injury earlier in life, or because of a condition that affected how the bone developed. The difference is fixed. It does not change with your posture or muscle tone.
Structural differences are sometimes called "true" leg length discrepancies. They show up the same way on imaging, however you are positioned.
The bones match, but something else creates the mismatch. Common causes include a lateral pelvic tilt, a rotated pelvis, tight muscles on one side (such as the hip flexors or adductors), scoliosis, or a joint contracture in the hip or knee.
The source is soft tissue or alignment, not bone length. So a functional difference can change with position. It may appear when you stand and vanish when you lie down. This is exactly why imaging done lying down often misses it.
Many people have a mix of both. A small structural difference can push the pelvis off balance, which then adds a functional part on top. That is why it matters so much to measure the whole chain, from head to feet, while you are standing.
How do you know if your legs are different lengths?
Most small differences cause no symptoms at all. When the gap is large enough, or when the body has been compensating for years, you may notice one or more of these signs.
Clothes hang unevenly. Trousers drag on one side. A tailor notices one leg is "longer."
Persistent aching that moves between the hip and the lower back, often worse after standing or walking.
You lean to one side as you walk, or people tell you that your stride is uneven.
The "longer" leg takes more impact. Over time, the knee, ankle or foot on that side may become sore.
If you also have uneven hips, this is often the first visible clue that something further down the chain is making the body compensate.
What causes a leg length discrepancy?
The causes split along the same structural and functional line. A structural difference starts in the bone itself. A functional difference starts in the soft tissue or alignment above or below.
A break in the femur or tibia that healed slightly shorter or longer. (Structural)
Hip or knee replacement surgery can change the effective length of one leg. (Structural)
A tilted or rotated pelvis moves the hip sockets up or down, so one leg acts shorter. (Functional)
A curve in the spine tips the pelvis. This creates an apparent leg length difference even though both legs are equal. (Functional)
Tight hip flexors, adductors or hamstrings on one side can pull the pelvis out of balance. (Functional)
Some people live with a small structural difference for decades without trouble. Then a new stress, an injury, a change in activity or stiffness with age pushes the body past what it can compensate for. That is often when symptoms appear.
Why does a standing scan matter for measuring leg length?
A scan done lying down relaxes the muscles and lets gravity flatten the pelvis. That can hide a functional leg length difference completely, because the body's compensations switch off the moment you lie on a table.
A standing, weight-bearing image captures your skeleton exactly as it behaves in daily life, with gravity, muscle tension and the load on your joints all playing their part. At ScanAlign, full-body standing imaging covers both legs, the pelvis and the whole spine in a single exposure with about 90% less radiation than a standard X-ray, around ten times less. So the report shows both bone lengths and any tilt in the pelvis or spine that may be adding a functional part.
Wondering whether your leg length difference is structural, functional or both? A standing EOS scan can show you clearly.
Book a free video consultationWhen MRI is the better tool: EOS shows bones, joints and alignment. If your clinician suspects a soft-tissue problem, such as a disc issue, nerve compression, or a muscle or tendon tear, MRI is usually the more suitable scan. The two work together. They do not compete.
What can be done once you know which type you have?
The right approach depends entirely on the type of difference and its size. Finding out the type is the essential first step.
- A heel lift or insole may be recommended to level the pelvis
- Changes to footwear for larger differences
- A review after surgery if a replacement caused the change
- Ongoing monitoring of how the body compensates
- Addressing the pelvic tilt, rotation or scoliosis that is causing the apparent difference
- Targeted stretching or strengthening to rebalance muscle tension
- Manual therapy aimed at the source, not the symptom
- A heel raise would not help here and could make things worse
Without knowing the type, any correction is guesswork. For example, a heel lift given for a functional difference does not fix the pelvic tilt that caused it. It may also create new problems further up the chain. Clear imaging is what turns guesswork into a plan.
So before you start treatment, or change the one you are on, it is worth getting a detailed diagnosis. Knowing whether the cause sits in the pelvis, the legs or the spine changes what will actually help.
Frequently asked questions
Yes, and this is quite common. A small structural difference in bone length can make the pelvis tilt, which then adds a functional part on top. A standing whole-body scan helps separate the two.
They are the same thing in different words. "Apparent" and "functional" both mean the bones are equal in length, but something else, such as pelvic tilt or muscle imbalance, makes one leg act shorter.
When you lie flat, gravity flattens the pelvis and your muscles relax. The tilt or rotation that creates the apparent difference when you stand may disappear completely. So a supine X-ray or CT can report equal leg lengths, even when the body behaves very differently upright.
No. ScanAlign accepts self-referrals. You can book a leg length measurement scan directly. It starts with a free video consultation to make sure an EOS scan is suitable for you.
The EOS system uses about 90% less radiation than a standard X-ray, around ten times less. It captures the full body in a single standing sweep, so there is no need for multiple exposures.
It can. When one leg is shorter, or acts shorter, the pelvis tilts. The spine then curves to keep your head level. Over time, that compensation can overload the lower back muscles and joints on one side.
Pelvic obliquity means the pelvis is tilted to one side. It can be caused by a structural leg length difference pulling one side of the pelvis down. Or it can be the cause of a functional difference, if the tilt starts in the spine or hip muscles.
A heel lift can help level the pelvis when the difference is structural. If the difference is functional, a lift does not address the underlying cause, such as muscle imbalance or pelvic rotation, and could make things worse. Knowing the type first is essential.
The price for patients travelling from abroad is confirmed during the free video consultation. That consultation is also the first step to confirm the scan is right for you.
Yes. A curve in the spine can tilt the pelvis, which moves one hip socket higher than the other. This creates an apparent leg length difference even when the femur and tibia on each side are the same length. Standing full-body imaging can show the curve and the pelvic tilt it causes together.
Find out which type of leg length difference you have
A free video consultation is the first step, before you plan any trip to London. We will help you understand whether a standing EOS scan at 19 Harley Street, London is the right next step for you.
Book a free video consultationWritten by Abbas Dhami (Specialist Diagnostic Radiographer)
Sources
- Brady RJ, Dean JB, Skinner TM, Gross MT. Limb length inequality: clinical implications for assessment and intervention. J Orthop Sports Phys Ther. 2003;33(5):221-234.
- Gurney B. Leg length discrepancy. Gait & Posture. 2002;15(2):195-206.
- NHS. Back pain: causes. nhs.uk/conditions/back-pain.
This page is general information and not a diagnosis.
ScanAlign is the trading name of CoreMed Solutions Ltd and operates under The Harley Street Hospital's CQC licence.