Arthritis in the knee is a condition where the joint gradually loses its protective cartilage, leading to pain, stiffness, and reduced mobility. It affects roughly 365 million people worldwide, with about 5% of all adults living with some form of knee arthritis. The condition becomes increasingly common after age 30 and peaks in the 80 to 84 age group, where it affects nearly one in four women and one in five men.
What Happens Inside the Joint
Your knee joint is lined with a smooth layer of cartilage that cushions the ends of your thighbone and shinbone, letting them glide against each other with minimal friction. In a healthy knee, your body continuously breaks down and rebuilds this cartilage in a balanced cycle. When arthritis develops, that balance tips. Enzymes that break down cartilage become overactive, and the molecules that normally keep cartilage firm and springy (collagen and proteoglycans) are stripped away faster than they can be replaced.
What makes this worse is that the process feeds on itself. Inflammatory signals in the joint not only accelerate cartilage breakdown but also suppress new cartilage production. These same signals trigger cell death within the cartilage itself and stimulate the production of compounds that cause more inflammation. Over time, the cartilage thins and roughens. In advanced cases, it wears away entirely, leaving bone grinding against bone. The body tries to compensate by growing small bone spurs around the joint, but these often make the stiffness and discomfort worse.
Types of Knee Arthritis
Osteoarthritis is by far the most common type. It’s the “wear and tear” form that develops gradually over years, driven by the cartilage breakdown process described above. It typically affects people over 50, though it can appear earlier after injury.
Rheumatoid arthritis is an autoimmune condition where the immune system attacks the joint lining. Unlike osteoarthritis, it usually affects both knees simultaneously and can develop at any age. Post-traumatic arthritis follows a specific knee injury, such as a torn ligament or meniscus, or a fracture that damages the joint surface. The biology is similar to osteoarthritis, but the timeline is compressed because the injury accelerates cartilage loss.
Who Is Most at Risk
Body weight is the single most modifiable risk factor. Obese women have nearly four times the risk of developing knee osteoarthritis compared to women at a healthy weight. For obese men, the risk is roughly five times greater. People in the heaviest 20% of the population face up to ten times the risk of those in the lightest 20%. Every extra pound of body weight adds roughly four pounds of force to the knee with each step, so even modest weight gain accumulates enormous mechanical stress over decades.
Previous knee injuries significantly raise your risk as well. A torn ACL or meniscus changes the way forces distribute across the joint, speeding up cartilage wear even after surgical repair. Age, female sex, genetics, and occupations that involve repetitive kneeling or heavy lifting also contribute. Women are affected at nearly twice the rate of men globally: about 6% of women versus 3.8% of men.
What Knee Arthritis Feels Like
The earliest symptom is usually pain that worsens with activity, particularly going up or down stairs, rising from a chair, or walking longer distances. You may notice stiffness after sitting for a while or first thing in the morning. Swelling around the knee can come and go, sometimes triggered by a particularly active day.
A hallmark sign is crepitus: a creaking, clicking, or grinding sensation when you bend or straighten the knee. This happens because the roughened cartilage surfaces no longer glide smoothly. As the condition progresses, the knee may feel unstable, occasionally catch or lock, and lose range of motion. Some people develop a visible bowing of the leg inward or outward as cartilage wears unevenly from one side of the joint.
Pain tends to be activity-related in early stages, meaning it improves with rest. In more advanced arthritis, pain can become constant, disrupting sleep and making even short walks difficult.
How Severity Is Measured
Doctors use weight-bearing X-rays to assess how much damage has occurred. The standard grading system runs from 0 to 4. Grade 0 is a normal joint with no narrowing of the space between bones. Grade 1 shows possible tiny bone spurs with questionable narrowing. Grade 2 has definite bone spurs and mild narrowing. Grade 3 shows moderate bone spurs, clear narrowing of the joint space, and early bone changes. Grade 4, the most severe, reveals large bone spurs, significant loss of joint space, and visible deformity of the bone ends.
It’s worth knowing that X-ray severity doesn’t always match how much pain you feel. Some people with Grade 2 changes on imaging have significant discomfort, while others with Grade 3 or 4 changes manage reasonably well. Treatment decisions are based on your pain and functional limitations, not the X-ray alone.
Pain Relief and Medication Options
Topical anti-inflammatory creams or gels applied directly to the knee are a strong first option. A large network analysis comparing treatments found that topical anti-inflammatories work just as well as oral versions for improving knee function, while causing far fewer stomach and digestive side effects. They were also safer than acetaminophen (Tylenol) and actually more effective for function improvement. For people who want to avoid the gastrointestinal risks of daily pills, topical options are a practical choice.
Oral anti-inflammatories remain widely used for flare-ups or more persistent pain, but they carry higher risks for the stomach, kidneys, and cardiovascular system with long-term use. Acetaminophen is gentler but less effective for arthritis-specific symptoms.
Knee Injections
When creams and pills aren’t enough, injections directly into the joint can help. Corticosteroid injections provide fast relief, often within the first week, but the effect fades over a few months. Hyaluronic acid injections (sometimes called “gel shots”) work more slowly but tend to last longer. In one clinical trial, 54% of people receiving hyaluronic acid still had meaningful pain relief at six months, compared to 44% with corticosteroids. However, by the one-year mark, both types of injection tend to lose their effect, and the knee returns to its pre-injection baseline.
Neither injection type reverses the underlying cartilage damage. They buy time and comfort, which can be valuable for staying active and participating in physical therapy.
Exercise and Physical Therapy
Strengthening the muscles around your knee, particularly the quadriceps on the front of your thigh, reduces the load on the joint itself. This is one of the most consistently effective treatments across all stages of knee arthritis. Low-impact activities like swimming, cycling, and walking on flat surfaces keep the joint moving without excessive stress. Physical therapy programs typically focus on building strength, improving flexibility, and teaching movement patterns that protect the knee during daily tasks.
Weight loss, if you’re carrying extra pounds, produces outsized benefits. Because of the mechanical multiplier effect at the knee, losing even 10 pounds removes roughly 40 pounds of force per step. Studies consistently show that combining weight loss with exercise produces better pain relief than either one alone.
When Surgery Becomes the Right Option
Total knee replacement is considered when non-surgical treatments have stopped providing adequate relief. The American Academy of Orthopaedic Surgeons identifies several situations that point toward surgery: chronic inflammation and swelling that doesn’t improve with rest or medications, visible deformity of the knee, and failure to get meaningful improvement from physical therapy, injections, or anti-inflammatory medications.
There are no absolute age or weight cutoffs for the procedure. The decision is based on how much pain and disability you’re experiencing. Most people who undergo knee replacement report significant improvement in pain and mobility, with the artificial joint lasting 15 to 20 years or more in the majority of cases. Recovery involves several weeks of limited mobility followed by months of physical therapy to rebuild strength and range of motion.