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 is different from wear on the outer (lateral) side. The causes differ. The effect on alignment differs. The treatment options differ too. Finding out 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 loss of joint space 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 in the inner half of the knee joint. The knee has 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 along that inner edge.
This is the most common form of one-sided knee arthritis. As the inner gap closes, the leg can slowly bow outward. Clinicians call this 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. This pulls the leg into valgus (knock-knee) alignment. The mechanical effects are close to a mirror image of medial wear. But the treatment pathway can differ, because the outer compartment carries load in a slightly different way.
How do you know which side of your knee is worn?
Where you feel symptoms, and their pattern, often give a strong clue. But imaging is the only way to confirm which compartment is affected and how much joint space is left.
Aching along the inner or outer joint line in particular, not spread across the whole knee.
A gradual outward bow (varus) with medial wear, or an inward lean (valgus) with lateral wear.
The affected compartment feels tight when you get up after sitting, then loosens as you move.
Uneven wear on your soles can reflect a shift in your mechanical axis caused by joint space loss on one side.
What causes arthritis on one side of the knee?
Arthritis does not always affect the whole knee evenly. Several factors decide which compartment carries 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 the 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 change 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 shows the mechanical axis, the line from the centre of your hip to the centre of your ankle. Your clinician can then 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 load. When you lie down for a standard x-ray or MRI, gravity is taken away. The joint opens slightly. The narrowing caused by cartilage loss can then 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 look less advanced. Alignment shifts caused by muscle tone and weight cannot be seen. A lying-down scan may underestimate how severe one-sided wear is.
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 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 work well together.
How is one-sided knee arthritis usually managed?
Management depends on which compartment is affected, how much joint space is left and how your overall leg alignment has changed. A standing scan gives your treating clinician the measurements needed to choose the right path.
Options 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 spread weight differently 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 doctor the detailed alignment picture they need to recommend the right next step.
Before you start a 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.
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 carries most of the load when you walk, 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. Arthritis in a single compartment 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. For patients travelling from abroad, the price is confirmed during the free video consultation.
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 which compartments are involved. You can share these with your surgeon, physiotherapist or doctor back home, 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.