It commonly causes pain on the inside of the knee, stiffness, swelling, pain at night, and can often limit your daily activities. It is often characterised by exacerbations and remissions and usually gets worse with time. It can worsen with activities that load the knee, such as walking, stairs, and squatting.
A standing X-ray is normally all that is required to make the diagnosis, and often more advanced imaging modalities like MRI scans can cause more confusion than help with the diagnosis.
The severity of arthritis on the X-ray does not determine what the management should be. This is based on the severity of your symptoms. The old saying in orthopaedics is "treat the patient, don't treat the X-ray."
The knee joint explained
Your knee is a hinge joint. The thigh bone (femur) meets the shin bone (tibia) below it, and the kneecap (patella) glides across the front. Where the bones meet, they are capped with articular cartilage, a smooth, slippery surface a few millimetres thick that lets the joint bend and straighten almost without friction.
Two C-shaped shock absorbers, the menisci, sit between the femur and tibia and spread the load with each step. The knee is usually described as having three compartments: the medial (inside), the lateral (outside), and the patellofemoral (under the kneecap). Arthritis can affect one, two, or all three, and which compartments are involved influences both your symptoms and your surgical options.
In osteoarthritis, that smooth cartilage gradually thins and wears away. As it does, the bone beneath takes load it was not built for, thickening and forming bony spurs called osteophytes, while the joint lining becomes inflamed and tight. This is what makes an arthritic knee feel stiff as well as sore, and what produces the swelling you may notice. When the wear is mainly on the inside of the knee, the leg tends to bow inward; when it is on the outside, the knee tends to go knock-kneed.
Osteoarthritis versus inflammatory arthritis
Osteoarthritis is a degenerative condition and is far more common than inflammatory arthritis. It affects approximately 10% of the population. It becomes much more common as we age and is more common in females. Osteoarthritis causes thinning of the cartilage overlying the bone. As this process progresses, it can often lead to a point where there is virtually no cartilage overlying the bone. This is sometimes described as bone-on-bone arthritis.
Inflammatory arthritis is a condition that causes inflammation in the lining of the joint. There are a vast number of inflammatory arthritic conditions, with the most commonly known being rheumatoid arthritis. It often presents in multiple joints and is classically associated with significant swelling. If left untreated, this can result in secondary osteoarthritis. These days, rheumatologists can manage inflammatory arthritis extremely well with various medications, so early diagnosis is important. There is often a strong family history.
What causes it
Most knee osteoarthritis has no single identifiable cause. It develops gradually, usually after fifty, and family history makes it more likely.
Some develops because of an underlying problem with the shape or health of the joint, malalignment (being bow-legged or knock-kneed puts uneven load through one side), a previous meniscectomy, a fracture involving the joint surface, a previous ligament injury such as an ACL tear, or a childhood condition affecting the growth plate. These are covered in the questions below.
Body weight matters more for the knee than for most joints, because the knee carries several times body weight with each step, and even more on stairs. Ordinary exercise does not cause arthritis, in fact, regular activity helps maintain joint health.
How it is diagnosed
From the history, the examination and a plain X-ray.
The standard investigation is a weight-bearing X-ray, taken standing rather than lying down. Cartilage does not show on an X-ray, so it is assessed indirectly by the space between the bones. That space narrows as cartilage is lost, and the narrowing shows most clearly when the joint is loaded. This is why the film is taken standing, and why a lying-down X-ray can underestimate the changes.
We require four X-ray views to properly assess arthritis of the knee. These include:
- an AP (front-to-back) view
- a lateral (side-on) view
- a notch view, which is taken from the front but with the knee flexed
- a skyline view, which assesses the patellofemoral joint
Four features are looked for: narrowing of the joint space, thickening of the bone beneath the surface, osteophytes at the margins, and small cysts within the bone.
An MRI is not usually required. It may be arranged where the diagnosis is unclear, where symptoms do not match the X-ray, or where a meniscal tear, avascular necrosis or a stress fracture is suspected.
Why the X-ray and your symptoms often disagree
This causes a great deal of confusion, so it is worth being explicit.
Some people have marked changes on X-ray and modest symptoms. Others have significant pain with relatively mild change. The relationship between what the film shows and how the knee feels is genuinely loose.
Decisions are therefore based on symptoms and function, not on the X-ray alone. A film that looks worse than last year does not by itself mean anything needs to change. How far you can walk, whether you are sleeping, and whether you can manage stairs matter more.
The old principle in orthopaedics is to treat the patient, not the X-ray.
How it progresses
Knee arthritis is a disease characterized by exacerbations and remissions. Sometimes it's really bad, sometimes it is not so bad. It is a degenerative condition, however, and usually gets worse with time.
Some people remain stable for years with modest symptoms. Others deteriorate more quickly. Symptoms also fluctuate, painful periods followed by quieter phases are normal, and a bad month does not necessarily mean the condition has changed. Decisions are best made on the trend across a year rather than on a single flare.
As arthritis progresses in one compartment, the knee can drift out of alignment, bowing inward if the medial compartment is affected, or outward if the lateral compartment is affected. This in turn concentrates even more load through the worn side, which can accelerate the process.
Most people with knee arthritis do not have surgery. The first question is not usually whether to have an operation, but how to manage the condition well.
What happens next
Treatment is staged. Exercise therapy, weight management where relevant, activity modification and appropriate pain relief come first, and are effective for many people. Injections have a limited but real role. Surgery is considered when symptoms persist despite these and the effect on daily life justifies an operation.
Each of these is covered in the Treatment Options articles, where the decision between non-surgical and surgical management, and between partial and total knee replacement, is explored in detail.
Common questions
What is the difference between osteoarthritis and rheumatoid arthritis?
Osteoarthritis is a degenerative condition in which the cartilage lining a joint gradually wears away, usually affecting one or a few joints. Rheumatoid arthritis is an autoimmune disease in which the immune system attacks the joint lining, typically affecting many joints on both sides of the body, and is managed medically by a rheumatologist in the first instance.
Why is the X-ray taken standing up?
Cartilage does not appear on an X-ray, so its loss is assessed indirectly by measuring the space between the bones. That space narrows most visibly when the joint is bearing weight. A film taken lying down can therefore underestimate how much cartilage has been lost, which is why a standing X-ray is the standard investigation.
Do I need an MRI?
Straightforward knee osteoarthritis is diagnosed on a weight-bearing X-ray and does not usually require an MRI. An MRI may be requested where the diagnosis is uncertain, where symptoms do not match the X-ray findings, or where a condition such as a meniscal tear, avascular necrosis or a stress fracture is suspected.
My X-ray looks bad but my pain is manageable. What does that mean?
X-ray appearance and symptoms often do not match. Some people have marked changes on imaging with modest pain, and others have significant pain with relatively mild changes. Treatment decisions are based on how much the knee is affecting your daily life rather than on the severity of the X-ray alone.
Does knee arthritis always get worse?
Knee osteoarthritis generally progresses over time, but the rate varies considerably and is difficult to predict. Some people remain stable for years with modest symptoms; others deteriorate more quickly. Symptoms also fluctuate, with painful periods followed by quieter phases, so decisions are based on the overall trend rather than a single bad month.
What is patellofemoral arthritis?
Patellofemoral arthritis affects the compartment where the kneecap (patella) meets the front of the femur. It typically causes pain at the front of the knee, particularly when climbing or descending stairs, squatting, or sitting for long periods with the knee bent. It can exist on its own or alongside arthritis in the other compartments of the knee.
What is a meniscal tear, and does it lead to arthritis?
The meniscus is a C-shaped pad of cartilage that acts as a shock absorber between the femur and the tibia. Tears can occur from an injury or from degeneration over time. A degenerative meniscal tear is often part of the osteoarthritis process rather than a separate problem. Large traumatic tears, particularly those treated by removing meniscal tissue (meniscectomy), increase the risk of arthritis developing in that compartment later in life.
What is post-traumatic arthritis?
Post-traumatic arthritis develops after a fracture involving the knee joint surface, or after a significant ligament injury that has altered the way the knee moves. The cartilage over and around the injured area can degrade over years, producing the same symptoms and X-ray changes as primary osteoarthritis. It tends to develop at a younger age than typical osteoarthritis.
Common questions.
What is the difference between osteoarthritis and a meniscal tear?
Osteoarthritis is loss of the joint surface across a compartment. A meniscal tear is damage to one of the two C-shaped cartilage cushions. The complication is that degenerate meniscal tears are part of the arthritic process, so most arthritic knees will show one on MRI. Where arthritis is established, treating the tear surgically does not usually help the pain.
Does knee arthritis always end in a knee replacement?
No. Many people are managed for years or indefinitely with exercise, weight management, activity modification and occasional injections, and never come to surgery. Progression is variable and is not a straight line.
Should I have keyhole surgery for my arthritic knee?
No. Arthroscopy for degenerative knee disease, including where MRI shows a degenerate meniscal tear, has been shown in multiple randomised trials to give no meaningful benefit over placebo surgery or structured exercise. It carries surgical risk without corresponding gain.
Will exercise damage my knee further?
No. Appropriate exercise does not accelerate cartilage loss, and it is one of the few interventions with consistent evidence of benefit. Some discomfort during and shortly after exercise is expected. Pain that persists well beyond the session means the programme needs adjusting, not stopping.
Why is my knee bow-legged?
Because the medial compartment on the inner side of the knee wears first in most people. As that side loses height, the leg angles inward, producing a varus or bow-legged shape. Wear confined to the outer side produces the opposite, a valgus or knock-kneed shape. Alignment is planned and corrected at the time of a knee replacement.
Do I need an MRI for knee arthritis?
Usually not. A weight-bearing X-ray is the appropriate first investigation and is often the only one needed. MRI is reserved for cases where the diagnosis is uncertain, where a true mechanical block is suspected, or where something in the history does not fit.
Am I too young for a knee replacement?
Age is a consideration rather than a barrier. A younger patient will place more demand on the implant over a longer life, so the chance of eventually needing a revision is higher. That is a reason to exhaust non-operative options and to consider whether a partial replacement is appropriate, not a reason to endure symptoms that are genuinely limiting your life.
How do I know which compartment is affected?
The pattern of pain gives a strong clue, and a weight-bearing X-ray with a skyline view usually confirms it. Inner-side pain with a bow-legged alignment suggests the medial compartment. Front-of-knee pain, worse on stairs and standing from a low chair, suggests the patellofemoral compartment.
References.
- Royal Australian College of General Practitioners. Guideline for the management of knee and hip osteoarthritis. 2nd edn. East Melbourne: RACGP.
- Australian Orthopaedic Association National Joint Replacement Registry. Annual Report. Adelaide: AOA.
- Thorlund JB, Juhl CB, Roos EM, Lohmander LS. Arthroscopic surgery for degenerative knee: systematic review and meta-analysis of benefits and harms. BMJ. 2015;350:h2747.
- Skou ST, Roos EM. Good Life with osteoArthritis in Denmark (GLA:D): evidence-based education and supervised neuromuscular exercise delivered to patients with knee or hip osteoarthritis. BMC Musculoskelet Disord. 2017;18(1):72.
This page provides general information about a medical condition and a surgical procedure. It is not individual medical advice and does not replace consultation with a qualified practitioner. Outcomes vary between individuals and all surgery carries risk. A referral is needed for a consultation with Professor Daevyd Rodda; without one, you can start with the practice nurse practitioner, who can provide it.