Knee arthritis is the breakdown of cartilage inside the knee joint, leading to pain, stiffness, and swelling that typically worsens over time. Around 375 million people worldwide have knee osteoarthritis, making it one of the most common joint conditions on the planet. It most often develops after age 50 and affects women more than men.
Types of Knee Arthritis
Three types account for the vast majority of cases. Osteoarthritis is by far the most common. It’s a degenerative condition where the cartilage cushioning the ends of your bones gradually wears down, exposing bone and triggering inflammation. Most people who say they have “knee arthritis” have this type.
Rheumatoid arthritis is fundamentally different. It’s an autoimmune disease where your immune system mistakenly attacks the lining of the joint capsule, causing chronic inflammation that slowly destroys cartilage. It tends to affect both knees symmetrically and often involves other joints too.
Post-traumatic arthritis develops after a knee injury. Damage to ligaments, cartilage, or meniscus makes the joint less stable, and that instability accelerates cartilage breakdown over the years. A torn ACL, for example, leads to arthritis within 10 to 15 years in about 13% of cases. When that ACL tear is combined with damage to other structures like the meniscus or collateral ligaments, the risk climbs as high as 40%.
A fourth, less discussed type is patellofemoral arthritis, which involves cartilage thinning specifically under the kneecap. It causes pain at the front of the knee, especially when climbing stairs or sitting for long periods.
What Happens Inside the Joint
Healthy knee cartilage is smooth, slippery, and remarkably resilient. It absorbs shock and lets the bones in your knee glide against each other with almost no friction. In osteoarthritis, the cells that maintain this cartilage become overactive and start producing inflammatory signals and enzymes that break down the cartilage matrix faster than it can repair itself. The smooth surface develops cracks and fissures, becomes rough, and gradually thins.
As cartilage erodes, the joint lining becomes inflamed, producing excess fluid that causes swelling. The underlying bone responds by hardening (a process called sclerosis) and growing bony spurs along the joint edges. These spurs, combined with the loss of cushioning, alter the mechanics of the knee and contribute to stiffness and restricted movement. In advanced stages, cartilage can wear away completely, leaving bone grinding directly against bone.
Common Symptoms
Knee arthritis doesn’t usually arrive all at once. Early on, you might notice stiffness when you first get up in the morning or after sitting for a while, and it loosens up within a few minutes. Pain tends to flare during or after activity, particularly walking, climbing stairs, or squatting.
As the condition progresses, symptoms become harder to ignore:
- Creaking, clicking, or grinding sounds (crepitus) when bending or straightening the knee
- Swelling and warmth around the joint, sometimes with visible redness
- Pain that varies with the weather, often worsening in cold or damp conditions
- Stiffness and locking, where the knee feels stuck in one position
- Weakness and buckling, a sensation that the knee might give out
- Difficulty walking, especially on uneven surfaces or for longer distances
These symptoms tend to develop gradually over months or years. Flare-ups, where pain and swelling spike for days or weeks, are common and can be triggered by overuse, weather changes, or minor injuries.
Who Is Most at Risk
Age is the strongest predictor. The cartilage repair mechanisms in your joints slow down as you get older, and decades of use take a cumulative toll. Women are affected at higher rates than men, particularly after menopause, likely due to hormonal changes that influence cartilage health and joint inflammation.
Excess body weight is one of the most modifiable risk factors. The knee is a hinge joint that relies heavily on surrounding ligaments and soft tissue for stability, unlike the hip, which is a ball-and-socket design with more inherent structural support. Extra weight doesn’t just increase the load on the knee; it changes how you walk. People carrying significant extra weight tend to develop a wider stance and altered knee alignment that concentrates force on the inner part of the joint, accelerating cartilage damage in that specific area.
Previous injuries, genetics, and occupations involving repetitive kneeling, squatting, or heavy lifting all increase risk as well. Having one risk factor doesn’t guarantee arthritis, but stacking several of them together raises the likelihood substantially.
How It’s Diagnosed and Graded
Diagnosis typically starts with a physical exam and X-rays. Your doctor will look for joint tenderness, swelling, range of motion, and the presence of crepitus. X-rays reveal the hallmarks of arthritis: narrowing of the space between bones (indicating cartilage loss), bone spurs, and bone hardening.
Doctors grade severity using a scale from 0 to 4. Grade 0 is a normal joint with no visible changes. Grade 1 shows possible early bone spur formation. Grade 2 has definite bone spurs with possible narrowing of the joint space. Grade 3 shows moderate bone spurs, clear joint space narrowing, and some bone hardening. Grade 4, the most severe, features large bone spurs, significant joint space loss, severe bone hardening, and visible deformity of the bone ends.
MRI is sometimes used for a more detailed look at cartilage, meniscus, and ligament damage, particularly when X-ray findings don’t match the severity of symptoms. Blood tests can help distinguish osteoarthritis from rheumatoid arthritis when that’s in question.
Exercise and Physical Therapy
Regular exercise is one of the most effective treatments for knee arthritis, and it’s often underused. A large systematic review comparing five types of exercise found that all of them reduced pain compared to no exercise. Stationary cycling ranked highest for pain relief, followed by resistance training and water-based exercise. Yoga performed best for reducing stiffness and improving daily function and quality of life.
The key is consistency rather than intensity. Strengthening the muscles around the knee, particularly the quadriceps and hamstrings, absorbs force that would otherwise go straight into the joint. Low-impact aerobic exercise like cycling, swimming, or walking on flat ground improves joint mobility and reduces inflammation without pounding the cartilage. Flexibility work helps maintain range of motion.
Many people avoid exercise because they worry about making things worse, but controlled movement actually nourishes cartilage by circulating joint fluid across its surface. Inactivity does the opposite: it leads to muscle weakening, joint stiffness, and faster decline.
Medications and Injections
Anti-inflammatory medications taken by mouth are considered the first-line option for managing arthritis pain, backed by strong evidence. They reduce both pain and inflammation. For people who can’t tolerate oral anti-inflammatories due to stomach or kidney concerns, topical versions applied directly to the knee are an alternative.
Corticosteroid injections directly into the joint can provide quick relief, typically within days, but the effect tends to fade within a few weeks. They’re useful for managing flare-ups but aren’t a long-term solution. Hyaluronic acid injections, which aim to supplement the knee’s natural lubricating fluid, provide more modest initial relief but may last longer, with some benefit still present at six months in people with mild to moderate arthritis. However, major orthopedic guidelines have moved away from recommending hyaluronic acid injections, citing limited overall evidence of meaningful benefit.
Opioid pain medications are generally discouraged for knee arthritis due to the risks of dependence and side effects, especially given that this is a chronic condition requiring long-term management.
When Surgery Becomes an Option
Knee replacement surgery enters the conversation when pain persists despite months of consistent non-surgical treatment, daily activities become significantly limited, and X-rays confirm substantial joint damage. The combination of all three matters. Severe arthritis on an X-ray alone isn’t enough if your symptoms are manageable, and significant pain with minor X-ray findings typically prompts further investigation before considering surgery.
Most guidelines describe the surgical candidate as someone with moderate to severe pain (often including pain at night that disrupts sleep), meaningful difficulty with daily activities like walking, climbing stairs, or getting in and out of chairs, and radiographic evidence of advanced cartilage loss. Failed conservative therapy is a prerequisite in nearly every guideline, meaning you’ve genuinely tried exercise, weight management, and medication before moving to surgery.
Total knee replacement is highly effective for the right candidate, with the majority of people experiencing dramatic pain relief and improved mobility. Modern implants typically last 15 to 20 years or more. Partial knee replacement is an option when damage is limited to just one compartment of the knee, preserving more natural bone and allowing faster recovery.
Weight Management and Joint Load
Losing weight, if you carry extra, is one of the single most impactful things you can do for an arthritic knee. Every pound of body weight translates to roughly three to four pounds of force across the knee joint during walking. Losing even 10 to 15 pounds can meaningfully reduce pain and slow the progression of cartilage loss. The benefit is both mechanical (less load on the joint) and biological, since excess body fat produces inflammatory compounds that circulate through the bloodstream and contribute to cartilage breakdown independent of the weight itself.
Combining weight loss with exercise produces better outcomes than either one alone. The exercise builds muscle that supports the joint while the weight loss reduces the force that joint has to handle with every step.