Medial or Lateral Knee Arthritis: Why Does It Matter Which Side Wears Out?
It matters because wear on the inner (medial) side of your knee has different causes, different alignment consequences and different treatment options to wear on the outer (lateral) side. Identifying which compartment is affected, and how much your leg alignment has changed, helps your clinician choose the right plan for you.
Key takeaways
- Your knee has three compartments. Arthritis rarely affects all three equally.
- Medial compartment osteoarthritis is the most common pattern and is linked to bow-legged alignment.
- Lateral compartment osteoarthritis is less common and is linked to knock knees.
- Standing, weight-bearing imaging shows the true joint space loss that lying-down scans can hide.
- Knowing the compartment can open the door to less invasive options such as partial replacement.
What is medial compartment osteoarthritis?
Medial compartment osteoarthritis is arthritis affecting the inner half of the knee joint. The knee is divided into three compartments: the medial (inner) side, the lateral (outer) side and the patellofemoral compartment behind the kneecap. When cartilage wears away on the medial side, the gap between the thigh bone (femur) and shin bone (tibia) narrows on that inner edge.
This is the most common form of one-sided knee arthritis. As the inner gap closes, the leg can gradually bow outward into what clinicians call varus alignment. You may notice your trousers wearing unevenly, or that your weight seems to fall more on the inside of your foot.
What about lateral compartment osteoarthritis?
Lateral compartment osteoarthritis is less common. Here the cartilage wears on the outer side of the joint, pulling the leg into valgus (knock-knee) alignment. The mechanical consequences are effectively the mirror image of medial wear, but the treatment pathway can differ because the outer compartment bears load in a slightly different way.
How do you know which side of your knee is worn?
The location and pattern of your symptoms often give a strong clue, but imaging is the only way to confirm which compartment is affected and how much joint space remains.
Aching specifically along the inner or outer joint line, not spread across the whole knee.
A gradual bow outward (varus) with medial wear, or inward lean (valgus) with lateral wear.
The affected compartment feels tight when you stand after sitting, then loosens as you move.
Uneven sole wear can reflect a shift in your mechanical axis caused by one-sided joint space loss.
What causes arthritis on one side of the knee?
Arthritis does not always affect the whole knee evenly. Several factors determine which compartment bears the most load and wears first.
If your mechanical axis runs through the inner half of the knee, the medial compartment takes more load. A valgus axis overloads the lateral side.
A meniscus tear, ligament damage or fracture on one side can speed up wear in that compartment years later.
Higher body weight increases force through the knee. If alignment is already off-centre, the overloaded compartment deteriorates faster.
Your body works as a connected chain from your feet through your hips to your spine. A problem at any level can shift the way your knee is loaded. That is why a full-leg standing x-ray that captures the hip, knee and ankle together is so valuable. It reveals the mechanical axis, the line from the centre of your hip to the centre of your ankle, so your clinician can see exactly where the load falls.
Why does a standing scan show knee arthritis more accurately?
A standing, weight-bearing image shows the true gap between your bones under real-life loading. When you lie down for a standard x-ray or MRI, gravity is removed, the joint opens slightly, and the narrowing caused by cartilage loss can look less severe than it really is.
What it shows: True joint space narrowing under your body weight. The mechanical axis from hip to ankle. How your alignment changes under load. This is the information surgeons need to plan an osteotomy, an unloader brace or a total or partial knee replacement.
What it misses: Without gravity loading the joint, the gap between the bones relaxes. Cartilage loss can appear less advanced. Alignment shifts caused by muscle tone and weight are invisible. A lying-down scan may underestimate the severity of one-sided wear.
At ScanAlign, weight-bearing standing imaging uses EOS technology to capture a full-length view of both legs while you stand naturally. The scan uses about 90% less radiation than a standard x-ray, around ten times less. Because the whole leg is in one image, the mechanical axis is measured directly rather than estimated from a smaller field of view.
When MRI is the better choice: EOS shows bones, joints and alignment clearly. If your clinician needs to look at soft tissue, cartilage surface detail, ligaments or meniscal tears, MRI is usually the better tool. The two often complement each other.
How is one-sided knee arthritis typically managed?
Management depends on which compartment is affected, how much joint space remains and how your overall leg alignment has changed. A standing scan gives your treating clinician the measurements needed to choose the right path.
Options that your clinician may discuss include:
- Activity and weight management to reduce load through the affected compartment.
- Physiotherapy to strengthen the muscles that stabilise your knee and hip.
- An unloader brace designed to shift pressure away from the worn side.
- Osteotomy, a procedure to realign the bone and redistribute weight across the joint.
- Partial knee replacement, which resurfaces only the worn compartment and leaves healthy bone untouched.
- Total knee replacement, when more than one compartment is significantly affected.
ScanAlign is a diagnostic imaging clinic, not a treatment provider. The scan and report give your surgeon, physiotherapist or GP the detailed alignment picture they need to recommend the right next step.
Wondering which compartment is causing your knee pain? A standing scan can show the joint space and alignment your clinician needs to see.
Book a free video consultationFrequently asked questions
The medial (inner) compartment is the most commonly affected. It bears the majority of load during walking, so it tends to show wear earlier, especially if your leg alignment pushes weight inward.
Medial arthritis affects the inner half of the joint and is linked to bow-legged alignment. Lateral arthritis affects the outer half and is linked to knock-knee alignment. Each pattern responds to different treatments and bracing strategies.
Yes. Single-compartment arthritis is common, particularly in the medial compartment. When only one side is significantly worn, options like a partial replacement or osteotomy may be considered.
Without your body weight pressing through the joint, the bones move apart slightly. This makes the remaining gap look larger than it really is. A standing, weight-bearing image shows the true space under real-life load.
It is an imaginary line from the centre of your hip to the centre of your ankle. Where that line crosses the knee tells clinicians which compartment is overloaded. A full-leg standing image is the standard way to measure it.
Yes. EOS uses about 90% less radiation than a standard x-ray, around ten times less. You stand naturally during the scan. It takes seconds and there is no enclosed tunnel.
No. ScanAlign accepts self-referrals. You can book a standing knee scan directly through a free video consultation, where a specialist radiographer reviews your situation first.
ScanAlign is a private, self-pay clinic. Current pricing is listed on the frequently asked questions page.
Yes. It shows exactly which compartments are affected and how much the alignment has shifted. Surgeons rely on this information when deciding whether a partial replacement is suitable or whether a total replacement is needed.
You receive a detailed report and images showing joint space, alignment and compartment involvement. You can share these with your surgeon, physiotherapist or GP so they have accurate, weight-bearing data to plan your next step.
Find out which compartment is affected
A standing scan shows the joint space and alignment your clinician needs to choose the right plan. Start with a free video consultation.
Book a free video consultationWritten by Abbas Dhami (Specialist Diagnostic Radiographer)
Sources
- Sharma L, Song J, Felson DT, et al. The role of knee alignment in disease progression and functional decline in knee osteoarthritis. JAMA. 2001;286(2):188-195.
- National Institute for Health and Care Excellence (NICE). Osteoarthritis: care and management. Clinical guideline CG177. 2014 (updated 2022).
- Illés T, Somoskeöy S. The EOS imaging system and its uses in daily orthopaedic practice. International Orthopaedics. 2012;36(7):1405-1411.
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.