Knee Osteoarthritis: Causes, Symptoms and Treatment Options

Knee osteoarthritis is the gradual wearing away of the smooth cartilage that cushions the knee joint, which leaves bone rubbing on bone and causes pain, stiffness, and swelling. It is the most common form of arthritis in the knee and the single most common reason people end up needing a knee replacement¹. It develops slowly over years, and for most people the symptoms can be managed for a long time before surgery is ever discussed.

I lived with knee osteoarthritis for the best part of a decade before my own total knee replacement. The hardest part was not knowing how the disease would progress or which treatments were worth my time, so this guide sets that out plainly, checked by a consultant orthopaedic surgeon. If your knee pain has reached the point where it limits daily life, it may be worth reading the signs you may need a knee replacement.

What is knee osteoarthritis?

Knee osteoarthritis is a “wear and repair” condition in which the cartilage lining the joint breaks down faster than the body can maintain it. In a healthy knee, a smooth layer of cartilage lets the bones glide over each other. As that layer thins and roughens, the bones can begin to rub together, the joint becomes inflamed, and the body may grow bony spurs (osteophytes) around the edges.

The knee has three compartments: the inner (medial) side, the outer (lateral) side, and the area behind the kneecap. Osteoarthritis often starts in just one compartment, most commonly the inner side, which is why some people are suitable for a partial knee replacement that resurfaces only the one damaged compartment. Osteoarthritis affects around 1 in 5 adults over the age of 45 in the knee specifically, making it one of the most common long-term conditions there is².

What causes knee osteoarthritis?

Osteoarthritis is caused by cartilage wearing down faster than it can repair, and several things raise the risk. The main ones are:

• Age: the biggest single factor; it becomes far more common from the mid-forties onwards.
• Extra weight: carrying extra weight multiplies the load through the knee with every step and is the most important modifiable risk factor.
• Previous knee injury: a torn cartilage, ligament injury, or fracture can lead to osteoarthritis years later.
• Repetitive heavy use: jobs or sports with heavy kneeling, squatting, or impact.
• Family history and sex: it runs in families and is more common in women.
• Joint alignment: bow legs or knock knees concentrate the load on one side, as covered in how bow legs and knock knees drive knee arthritis.

What are the symptoms of knee osteoarthritis?

The main symptom is pain that is worse with activity and eases with rest, building gradually over months and years rather than appearing overnight. Alongside the pain, most people notice:

• Stiffness, especially first thing in the morning or after sitting still, usually easing within about 30 minutes of moving.
• Swelling around the joint, which can come and go.
• Grinding, clicking, or crunching (crepitus) when the knee moves.
• A feeling the knee may give way or lock.
• Reduced range of movement, so it becomes harder to fully bend or straighten the knee.

In my case the morning stiffness and the ache after a long walk came first; the swelling and the sense that the knee might buckle came later. If your pain is severe, sudden, or comes with fever or a hot, red joint, that points to something other than osteoarthritis, so read when knee pain means you should see a doctor.

How is knee osteoarthritis diagnosed?

Osteoarthritis is usually diagnosed from your symptoms and a physical examination, without needing scans in straightforward cases. NICE advises that a clinician can diagnose osteoarthritis clinically, without imaging, if you are 45 or over, have activity-related joint pain, and have either no morning stiffness or stiffness lasting no longer than 30 minutes³. An X-ray may be used to gauge the severity or to plan surgery, and it can show joint-space narrowing and bony spurs, but the X-ray findings often do not match the level of pain, so treatment is guided by symptoms rather than the picture alone.

How is knee osteoarthritis treated?

Treatment starts with exercise, weight management, and pain relief, which NICE recommends for everyone before surgery is considered³. These are not just delaying tactics; for many people they control the condition for the rest of their lives. The main options, from first-line to last, are:

• Exercise and physiotherapy: strengthening the muscles around the knee is the core treatment, recommended for everyone whatever their age or severity.
• Weight management: losing extra weight meaningfully reduces the load and the pain. If surgery is on the horizon, see losing weight before knee surgery.
• Pain relief: topical anti-inflammatory gels first, then oral medication, used at the lowest effective dose.
• Injections: a corticosteroid injection can give short-term relief of a painful flare.
• Walking aids, braces, and footwear: simple supports that take load off the joint.

These non-surgical options, and how far each can take you, are covered in full in the alternatives to knee replacement.

When does osteoarthritis lead to a knee replacement?

A knee replacement is considered only when the pain limits everyday activities (walking, sleeping, stairs, getting dressed) and exercise, weight management, and pain relief no longer help enough. It resurfaces the worn joint with metal and plastic components, and it is a successful operation: around 90 to 95% of knee replacements are still in place at 10 years, and they typically last 15 to 20+ years⁴.

It is not a decision the X-ray makes for you. I delayed my own surgery far longer than I needed to, partly out of fear, so my honest advice is to judge it on how much the knee is limiting your life rather than on age alone. The full picture of who is a candidate is in the signs you may need a knee replacement, and the operation itself is covered in our complete guide to knee replacement surgery.