Functional or Structural: Which Type of Leg Length Difference Do You Have?
A leg length discrepancy is either structural, meaning one bone is genuinely shorter, or functional, meaning the bones are the same length but something in the pelvis, hip or muscle balance makes one leg act shorter. Telling the two apart matters because each type needs a different approach, and only a standing, weight-bearing image shows the real picture.
Key takeaways
- 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, muscle tightness or joint problems create an imbalance.
- Lying-down scans can miss a functional discrepancy because gravity flattens the pelvis and relaxes the muscles.
- Standing whole-body imaging captures both types under real weight-bearing conditions.
- Knowing which type you have guides whether you need a heel raise, manual therapy, or further investigation.
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 its partner. A functional (sometimes called "apparent") leg length discrepancy means the bones are equal, but the body behaves as though one leg is shorter because of an imbalance elsewhere.
The bone itself is shorter. This can happen because of a previous fracture that healed with some shortening, a hip or knee replacement, a growth-plate injury earlier in life, or a condition that affected bone development. The difference is fixed. It does not change depending on your posture or muscle tone.
Structural differences are sometimes called "true" leg length discrepancies because they show up consistently on imaging regardless of how 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.
Because the source is soft tissue or alignment rather than bone length, a functional discrepancy can change with position. It may appear when you stand but vanish when you lie down, which is exactly why lying-down imaging often misses it.
Many people have a combination of both. A small structural difference can push the pelvis off balance, which then adds a functional component on top. That is why measuring the full chain, from head to feet, while you are standing matters so much.
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 compensation has been building 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 shifts between the hip and the lower back, often worse after standing or walking.
You lean to one side while walking, or people remark that you have an uneven stride.
The "longer" leg absorbs 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 causing the body to compensate.
What causes a leg length discrepancy?
Causes divide along the same structural/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 with slight shortening or lengthening. (Structural)
Hip or knee replacement surgery can alter the effective length of one leg. (Structural)
A tilted or rotated pelvis shifts the hip sockets up or down, making one leg act shorter. (Functional)
A spinal curve tips the pelvis, creating 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, until a new stress, an injury, a change in activity, or age-related stiffness pushes the body past its ability to compensate. That is often when symptoms appear.
Why does a standing scan matter for measuring leg length?
A lying-down scan relaxes the muscles and lets gravity flatten the pelvis. That can mask 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 joint loading all playing their part. At ScanAlign, full-body standing imaging covers both legs, the pelvis and the full spine in a single exposure with about 90% less radiation than a standard X-ray, around ten times less. This means the report shows both bone lengths and any pelvic or spinal tilt that may be creating a functional component.
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 appropriate scan. The two are complementary, not competing.
What can be done once you know which type you have?
The right approach depends entirely on the type of discrepancy and its size. Identifying the type is the essential first step.
- A heel lift or insole may be recommended to level the pelvis
- Footwear modification for larger differences
- Post-surgical review if a replacement caused the change
- Ongoing monitoring of compensation patterns
- Addressing the pelvic tilt, rotation or scoliosis that is causing the apparent difference
- Targeted stretching or strengthening to rebalance muscle tension
- Manual therapy directed at the source, not the symptom
- A heel raise would be unhelpful here and could make things worse
Without knowing the type, any correction is guesswork. A heel lift given for a functional discrepancy, for example, does not fix the pelvic tilt that caused it and may create new problems further up the chain. Clear imaging is what turns guesswork into a plan.
Frequently asked questions
Yes, and this is quite common. A small structural difference in bone length can cause the pelvis to tilt, which then adds a functional component on top. A standing whole-body scan helps separate the two.
They are the same thing described with 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 in standing 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, starting 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 and the spine compensates by curving 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 originates 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.
Pricing details are available on our frequently asked questions page. The first step is a free video consultation to confirm the scan is right for you.
Yes. A spinal curve can tilt the pelvis, which shifts 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 both the curve and the resulting pelvic tilt together.
Find out which type of leg length difference you have
A free video consultation is the first step. We will help you understand whether a standing EOS scan is the right next move 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.