Osteoarthritis of the knee is a condition where the cartilage that cushions the ends of your thighbone and shinbone gradually breaks down, causing pain, stiffness, and reduced mobility. It’s the most common form of arthritis, affecting millions of people, and it tends to worsen over time as the joint loses its natural shock-absorbing surface.
What Happens Inside the Knee Joint
A healthy knee has a smooth layer of cartilage covering the ends of each bone, allowing them to glide against each other with very little friction. In osteoarthritis, that cartilage deteriorates in stages. It first begins to swell and soften, then develops tiny cracks on the surface (a process called fibrillation), and eventually wears down to the point where raw bone is exposed.
The damage isn’t random. Cartilage cells become overactive and start producing enzymes that break down the very tissue they’re supposed to maintain. At the same time, these cells multiply abnormally, forming clusters that disrupt the smooth structure of the cartilage. It’s essentially the joint’s own repair system working against itself.
The bone underneath the cartilage changes too. It becomes denser and develops small fluid-filled cysts. New blood vessels and inflammatory cells push into areas that are normally sealed off, accelerating the breakdown. The tissue lining the inside of the joint (the synovium) becomes inflamed, and excess fluid accumulates in the joint, which is why many people with knee osteoarthritis notice swelling. All of these changes together create a cycle: inflammation drives cartilage loss, cartilage loss exposes bone, and exposed bone triggers more inflammation.
Common Symptoms
The hallmark symptom is pain that worsens with activity and improves with rest, at least in the earlier stages. You might notice it most when climbing stairs, getting up from a chair, or walking longer distances. As the condition progresses, pain can persist even at rest or wake you up at night.
Stiffness is another early sign, particularly in the morning or after sitting for a while. This usually loosens up within 20 to 30 minutes of moving around, which distinguishes it from inflammatory types of arthritis where stiffness lasts much longer. Over time, you may hear or feel a grinding, clicking, or crunching sensation when you bend the knee. Some people develop a noticeable decrease in range of motion, making it harder to fully straighten or bend the leg. In advanced cases, the joint can become visibly swollen, and the knee may start to bow inward or outward as cartilage wears unevenly.
Who Is Most at Risk
Age is the strongest predictor. The condition is uncommon before 40 and becomes increasingly prevalent with each decade after that. Women are more likely to develop knee osteoarthritis than men, especially after menopause. Carrying extra body weight is one of the most significant modifiable risk factors because every pound of body weight translates to roughly three to four pounds of force across the knee joint during walking.
Previous knee injuries substantially raise your risk, even decades later. Within 10 years of an ACL tear, about 1 in 3 people show signs of knee osteoarthritis, regardless of whether they had surgical reconstruction. When a meniscus tear accompanies the ACL injury, that number climbs to 52% at 12 years. Jobs or activities that involve repetitive kneeling, squatting, or heavy lifting also increase wear on the joint over time. Genetics play a role as well; if osteoarthritis runs in your family, your cartilage may be more vulnerable to breakdown.
How It’s Diagnosed
Doctors typically diagnose knee osteoarthritis through a combination of your symptom history, a physical exam, and imaging. During the exam, they’ll check for tenderness, swelling, range of motion, and any grinding sensation when the knee moves. X-rays can reveal joint space narrowing (a sign that cartilage has thinned), bone spurs, and changes in bone density. MRI scans are sometimes used to get a more detailed look at the cartilage and surrounding soft tissues, though they’re not always necessary for diagnosis.
One important point: the severity of changes on an X-ray doesn’t always match the severity of your pain. Some people with significant cartilage loss on imaging report mild symptoms, while others with modest changes on X-ray experience considerable discomfort.
Non-Surgical Treatment Options
Weight loss is one of the most effective interventions for knee osteoarthritis if you’re carrying extra weight. Research from Wake Forest University found that losing 10% of body weight meaningfully improves symptoms. Losing 20% or more produces even better results: significantly less inflammation, less pain, and the ability to walk farther. Importantly, these outcomes were achieved through diet and exercise alone, without surgery or medication.
Exercise might seem counterintuitive when your knee hurts, but strengthening the muscles around the joint, particularly the quadriceps, helps stabilize the knee and absorb some of the load that would otherwise fall on damaged cartilage. Low-impact activities like swimming, cycling, and walking are generally well tolerated. Physical therapy can help you build a routine tailored to your specific limitations.
Over-the-counter anti-inflammatory medications are often the first line of pain relief. Some people benefit from corticosteroid injections directly into the joint, which can reduce inflammation and provide temporary relief lasting weeks to months. Knee braces or shoe inserts can also help by shifting weight away from the most damaged part of the joint.
When Joint Replacement Becomes an Option
Knee replacement surgery is generally considered when conservative treatments have been tried for six months or more without adequate relief. Specific situations that point toward surgery include pain that prevents you from sleeping through the night, inability to walk more than a few blocks, and loss of function severe enough to interfere with work or daily activities.
The procedure involves removing the damaged surfaces of the knee bones and capping them with metal and plastic components that recreate a smooth joint surface. Recovery typically takes several months of physical therapy, with most people returning to normal daily activities within 6 to 12 weeks. Modern joint replacements last 15 to 20 years or longer for most patients, which is why surgeons often try to delay the procedure in younger patients when possible.
What Progression Looks Like
Knee osteoarthritis doesn’t follow a single timeline. Some people have mild symptoms that stay relatively stable for years, while others experience a more rapid decline. Factors that accelerate progression include continued excess weight, joint malalignment, previous injury, and high-impact repetitive use.
In early stages, you might only notice pain during or after specific activities. In moderate stages, pain becomes more frequent, stiffness increases, and you may start avoiding certain movements. In advanced osteoarthritis, cartilage is severely worn or gone entirely in parts of the joint, bone rubs against bone, and the knee may become visibly deformed. At this point, even short walks or standing for a few minutes can be painful. Recognizing where you are in this spectrum helps guide which treatments make the most sense and how aggressively to pursue lifestyle changes that can slow things down.