Quick answer: Leg length discrepancy (LLD) means one leg measures shorter than the other. Up to 5 mm is normal and usually harmless. Between 5–25 mm, a custom shoe lift is the fix most orthopedists recommend before surgery. Here’s how to tell what you have, what’s causing it, and what to do about it.
What Is Leg Length Discrepancy (LLD)?
Leg length discrepancy is exactly what it sounds like — one leg measures shorter than the other. Doctors also call it anisomelia, or list it on insurance paperwork as ICD-10 code M21.7. It affects more people than you’d think. Studies from the American Academy of Orthopaedic Surgeons suggest that 40–70% of adults have some measurable difference between their legs. Most never notice. The problem starts when the difference crosses a certain threshold and your body can’t quietly compensate anymore.
. It affects more people than you’d think. Studies from the American Academy of Orthopaedic Surgeons suggest that 40–70% of adults have some measurable difference between their legs. Most never notice. The problem starts when the difference crosses a certain threshold and your body can’t quietly compensate anymore.There are two kinds, and the distinction matters because the treatment is different.
- Structural LLD — there’s an actual difference in bone length. The femur, the tibia, or both. Once your growth plates close in your late teens, structural differences are permanent without surgery.
- Functional LLD — the bones are equal, but something above the legs (a tilted pelvis, scoliosis, hip contracture, or muscle imbalance) makes one leg behave shorter. This kind sometimes corrects with physical therapy or chiropractic work.
In my fifteen years modifying shoes for customers across the country, the average difference I see is around 12 mm. The biggest I’ve ever built for was 47 mm — a customer in Texas who broke his femur as a kid and grew up with one leg shorter than the other. The smallest I’ve worked on was 4 mm, for a runner who’d noticed she was finishing every long run with knee pain on the same side.
What Causes Leg Length Discrepancy?
Different people, different stories. Here are the causes I hear most often from customers when I ask why they’re calling:
- Congenital — born with it. About 0.1% of newborns have a measurable structural LLD. Usually small, sometimes part of a larger condition like fibular hemimelia.
- Childhood fractures — break a leg before your growth plates close (roughly age 16 for girls, 18 for boys) and if the bone heals shorter, you grow up with a permanent difference.
- Hip replacement surgery — one of the most common reasons I get calls. Surgeons aim for equal leg length, but a 5–15 mm difference post-surgery isn’t unusual. Some surgeons actually leave a slight difference on purpose to take pressure off the sciatic nerve.
- Knee replacement — same story as hip, smaller magnitude.
- ACL or femur fractures in adulthood — when there’s hardware involved, the repaired leg can end up a few millimeters off.
- Scoliosis — a curved spine pulls the pelvis up on one side. The leg looks shorter even though the bones are equal. Classic functional LLD.
- Pregnancy and postpartum changes — the pelvis shifts during pregnancy and sometimes doesn’t fully reset. New mothers calling about hip and low-back pain are a real category in our business.
- Aging and disc compression — uneven disc degeneration tilts the pelvis. Common after age 50.
When to see a specialist
If you’re noticing any of these, get measured properly by an orthopedic doctor or a physical therapist before you start buying inserts off Amazon:
- Persistent low back pain that’s worse on one side
- A knee that hurts on the shorter leg side (the body compensates in weird ways)
- Uneven shoe wear — one outsole worn down faster than the other
- A visible limp you didn’t have a year ago
- Hip pain on either side that started after a surgery or injury
A standing X-ray called a scanogram takes about five minutes and tells you to the millimeter. Don’t guess on this. The cost of treatment if you’re wrong about the size of your discrepancy is real — both in money and in months of wearing the wrong lift.
How to Measure Leg Length Discrepancy at Home
Before you book an X-ray, you can get a rough number. The method orthopedists actually use in their clinics is called the block test, and you can do a simplified version of it on your kitchen floor.
What you’ll need: a few hardcover books or thin pieces of plywood, a flat hard floor (no carpet), and either a full-length mirror or a helper.
- Stand barefoot, feet about hip-width apart, weight even on both legs.
- Have your helper look at the level of your hip bones (the bony points on either side of your waist — those are your iliac crests) from behind.
- If one hip is higher, slide thin books under the foot of the lower side until both hips look level.
- Measure the total height of the books. That’s roughly your discrepancy.
This isn’t surgical-grade accurate. It can be off by 3–5 mm because of muscle tightness, posture habits, and the fact that hip bones aren’t always perfectly symmetric. But it’s enough to tell you whether you’re sitting at 2 mm (don’t worry about it) or 15 mm (call a specialist). For real precision, ask your doctor for a CT scanogram or EOS imaging. Both measure to the millimeter.
Treatment Options: From Insoles to Shoe Lifts
What works depends on how much difference you have. Here’s the short version, then I’ll walk through each one.
| Option | Best for | Cost | Drawback |
|---|---|---|---|
| OTC heel pad / insert | Under 5 mm | $10–30 | Slips around, wears out fast, only lifts the heel |
| Custom orthotic insole | 5–8 mm + arch issues | $80–400 | Limited lift height, takes up shoe volume |
| Custom shoe modification | 5–50 mm, any shoe | $95–225 | You mail your shoes in (7–10 business days) |
| Limb-lengthening surgery | Over 25 mm structural | $50,000+ | 6–12 months recovery, surgical risk |
For most adults with 5–25 mm of LLD, the practical, non-surgical answer is a custom shoe modification — meaning a lift is built into the sole of your shoe, so your shoe walks like normal, just slightly thicker on one side.
Insertable heel pads (the $20 kind on Amazon) work for very small differences and on low-activity days. The two problems with them: they slide around inside the shoe no matter how much double-sided tape you use, and they only lift the heel — not the ball of the foot. That means your stride still rolls wrong, which is what causes the knee and back pain in the first place.
Custom orthotics go a step further. A podiatrist makes them from a mold of your foot and they fix some of the slippage. But you’re still limited to the height the shoe was designed for. Above about 8–10 mm, you run out of room — your heel pops out of the shoe with every step.
A custom shoe modification — where the lift is built into the outsole itself — has no real height limit (I’ve built 50 mm lifts), doesn’t slide, doesn’t take up shoe volume, and works on any shoe you mail in. If you want the full breakdown of how we decide on the right height for you, I wrote a separate piece on choosing the correct height for a shoe lift.
How CNET Orthopedic Builds Yours
I started CNET Orthopedic because I couldn’t find a custom shoe-lift shop that talked to customers like people. Here’s how it works: you mail us your shoes (we send a prepaid USPS label), we measure your discrepancy needs from your doctor’s note or our home-measurement guide, and we build the lift into the outsole using a split-sole technique. When it comes back to you, it looks almost stock — no platform-shoe vibe. Most customers can’t tell which shoe is modified from a normal walking distance.
Turnaround is 7–10 business days from when your shoes arrive at our shop. If you’ve recently had hip surgery, take a look at the guide I put together on shoe lifts after hip replacement surgery — it covers what to expect from your surgeon’s measurement and how to time the order.
Frequently Asked Questions
How much leg length difference is normal?
Up to 5 mm (about 1/5 of an inch) is considered normal and most people in this range have no symptoms. Between 5 and 20 mm, a shoe lift is typically recommended. Above 20 mm, your doctor may discuss limb-lengthening surgery as an option, especially if you’re under 25.
Can a shoe lift fix leg length discrepancy permanently?
A shoe lift doesn’t change the bone — it compensates for the difference so your stride evens out. Worn consistently, it relieves the back, hip, and knee strain that uneven legs cause. For structural LLD, a shoe lift is a permanent management tool. For functional LLD, your doctor may try physical therapy first to see if the underlying cause can be corrected.
Do I need a prescription to order a shoe lift?
No. We work with both medically-referred customers and self-referred customers. If you have a doctor’s measurement, send it. If you don’t, our home-measurement guide and a phone call with our shop gets you close, and we can fine-tune after your first wear.
How long does a custom shoe lift last?
The lift itself lasts as long as the shoe. We use a polyurethane material that’s more durable than most factory outsole rubber. When the shoe wears out, you send us a new pair and we build the same lift on those.
Can I get shoe lifts for any kind of shoe?
Almost any. We modify sneakers, work boots, dress shoes, cowboy boots, hiking boots, golf shoes, and most sandals. We can’t modify shoes with extremely thin or flexible soles (ballet flats, for example) because there isn’t enough structure to attach the lift to.
Medical disclaimer: This article is for informational purposes and does not replace medical advice. Consult a licensed orthopedist, podiatrist, or physical therapist before changing footwear for any medical condition. Last reviewed: 2026-05-18.