Osteoarthritis of the knee is a condition where the cartilage cushioning the knee joint gradually breaks down, causing pain, stiffness, and difficulty moving. It affects roughly 365 million people worldwide, making the knee the most commonly affected joint in osteoarthritis. The condition progresses over years or decades, and while it can’t be reversed, a combination of exercise, weight management, and targeted treatments can significantly reduce pain and keep you moving.
What Happens Inside the Joint
Your knee joint is capped with a layer of smooth, rubbery cartilage that lets the bones glide against each other without friction. In a healthy knee, the cells that maintain this cartilage (called chondrocytes) constantly balance building new tissue with breaking down old tissue. In osteoarthritis, that balance tips toward destruction. The cartilage loses its structural proteins, absorbs more water, and becomes softer and weaker.
As cartilage fragments break off, the joint lining picks them up and responds with inflammation. That inflammation sends chemical signals back to the cartilage cells, telling them to break down even more tissue while producing less of the strong collagen that holds cartilage together. It becomes a self-reinforcing cycle: damage triggers inflammation, and inflammation accelerates damage.
In early stages, the cartilage surface develops small cracks and begins thinning. Over time, those cracks deepen into fissures, and entire patches of cartilage wear away. In advanced cases, the cartilage can be completely destroyed, leaving bare bone grinding against bone. The bone underneath the cartilage changes too. Early on, it becomes thinner and more porous. Later, it thickens and hardens in a process called sclerosis, and the body grows bony spurs (osteophytes) around the joint edges as it tries to stabilize the deteriorating structure. Fluid-filled cysts can also develop within the bone.
What It Feels Like
Pain and stiffness are the hallmark symptoms. The pain is mechanical, meaning it tends to worsen when you use the joint: climbing stairs, standing up from a chair, walking longer distances, or putting weight on the leg. It often improves with rest, at least in earlier stages.
Morning stiffness is common but typically lasts 30 minutes or less, which distinguishes it from inflammatory types of arthritis like rheumatoid arthritis, where stiffness can last hours. If your knee stiffness persists well beyond 30 minutes, there may be significant inflammation inside the joint. You might also notice a grinding or crunching sensation when bending the knee, swelling after activity, and a gradual loss of range of motion. Over time, the joint can develop a visible inward or outward angulation as cartilage wears unevenly.
Who Gets It and Why
Age is the strongest risk factor. The wear-and-repair process in cartilage slows as you get older, and cumulative stress adds up. But osteoarthritis isn’t simply “wear and tear.” Obesity, prior knee injuries (torn ligaments or meniscus), repetitive occupational stress, genetics, and joint alignment all play significant roles. The worldwide number of people with osteoarthritis grew by 113% between 1990 and 2019, driven by aging populations and rising obesity rates.
Body weight deserves special attention. Every pound of body weight translates to roughly three to four pounds of force on the knee during walking. Research from the Arthritis Foundation found that overweight or obese adults with knee osteoarthritis who lost 10% of their body weight saw a 50% reduction in pain over 18 months. Losing 20% or more delivered another 25% improvement in pain and function on top of that. For someone weighing 200 pounds, a 10% loss means 20 pounds, and a 20% loss means 40 pounds.
How It’s Diagnosed
Diagnosis is usually straightforward and based on your symptoms, a physical exam, and X-rays. Doctors look at the space between the bones on an X-ray (which represents the cartilage thickness), the presence and size of bone spurs, and whether the bone has hardened or changed shape. A widely used grading system classifies knee osteoarthritis into four stages:
- Grade 1: Possible slight narrowing of the joint space and tiny bone spurs. You may have no symptoms at all.
- Grade 2: Definite bone spurs visible on X-ray with possible joint space narrowing. Pain after activity is common.
- Grade 3: Multiple bone spurs, clear narrowing of the joint space, some bone hardening, and possible changes in bone shape. Pain becomes more frequent and limits daily activities.
- Grade 4: Large bone spurs, severely narrowed joint space, significant bone hardening, and obvious deformity. Pain is often constant and disabling.
MRIs are sometimes used for a closer look at soft tissue, but X-rays remain the standard tool. It’s worth noting that the severity on imaging doesn’t always match the severity of symptoms. Some people with Grade 3 changes on X-ray have relatively mild pain, while others with milder imaging findings struggle significantly.
First-Line Treatments
The American College of Rheumatology and Arthritis Foundation jointly recommend exercise, weight loss (if overweight), and self-management education as the foundation of treatment. These aren’t just “nice to have” add-ons. They carry the strongest level of recommendation in clinical guidelines.
For exercise, walking, strength training, neuromuscular training, and water-based exercise all have evidence behind them, and no single type is considered superior. The key is consistency. Regular exercise strengthens the muscles around the knee, reduces stiffness, and can decrease pain over time, even though it might seem counterintuitive to exercise a painful joint. Tai chi also has strong evidence for improving knee osteoarthritis symptoms.
A tibiofemoral knee brace is strongly recommended when knee osteoarthritis significantly affects your ability to walk, causes instability, or produces substantial pain. Braces work by redistributing load across the joint, taking pressure off the most damaged compartment. A cane used in the opposite hand can serve a similar purpose by offloading the joint during walking.
Medications and Injections
Topical anti-inflammatory gels applied directly to the knee are a strongly recommended option. They deliver pain relief to the joint with much less systemic exposure than pills, making them a good starting point. Oral anti-inflammatory medications are also strongly recommended but come with more potential side effects, particularly for the stomach and cardiovascular system, so they’re typically used at the lowest effective dose.
Corticosteroid injections directly into the knee joint can provide meaningful pain relief, especially during flare-ups. The effect tends to be strongest in the first month, then gradually fades. Hyaluronic acid injections (sometimes called viscosupplementation) take longer to kick in but may offer more sustained relief, with moderate effects still present at six months in some studies. The trade-off is that corticosteroid injections work faster for acute pain, while hyaluronic acid may be a better fit when you’re looking for longer-lasting improvement.
When Joint Replacement Becomes an Option
Total knee replacement is considered when pain is severe or refractory, daily function is significantly impaired, X-rays show advanced joint damage (generally Grade 3 or higher), and non-surgical treatments have been adequately tried without enough relief. There’s no single pain score or test result that triggers the decision. Guidelines from organizations across multiple countries consistently point to the same combination: persistent pain, meaningful disability, radiographic evidence of osteoarthritis, and failure of conservative measures.
Progressive deformity or joint instability can also be reasons for surgery, even when pain isn’t the dominant complaint. The surgery replaces the damaged bone surfaces with metal and plastic components. Most people experience substantial pain relief and improved mobility, though recovery involves several months of physical therapy and gradual return to full activity. The artificial joint typically lasts 15 to 20 years or longer, which is why surgeons and patients sometimes weigh the timing carefully, particularly for people under 60.