A stiff, swollen knee almost always means excess fluid has built up inside the joint. Your knee normally contains a small amount of lubricating fluid, but when something goes wrong, whether from an injury, arthritis, or infection, the tissue lining your joint overproduces that fluid. The extra fluid increases pressure inside the joint capsule, which physically limits how far you can bend or straighten your leg. With a large amount of swelling, your knee may naturally rest at a slight bend, around 15 degrees, because that position relieves some of the internal pressure.
How Swelling Restricts Your Knee
The membrane lining your knee joint acts like a filter. When it’s healthy, it keeps fluid production balanced. When it’s inflamed, large gaps open in that membrane, allowing molecules of almost any size to pass through and flood the joint space. This is why swelling can develop so quickly after certain injuries.
That excess fluid does more than just make your knee look puffy. It triggers a reflex that inhibits your quadriceps from fully firing, which is why a swollen knee often feels weak or unstable in addition to stiff. Weight-bearing activities like walking become painful because each step compresses the extra fluid, increasing pressure on the surrounding tissues.
Injuries That Cause Sudden Swelling
If your knee became stiff and swollen after a specific incident, like twisting during sports, landing awkwardly, or taking a direct hit, an injury is the most likely explanation. The most common culprits are ACL tears, MCL tears, meniscus tears, hyperextended knees, sprains, and bone fractures. Less frequently, a kneecap dislocation or patellofemoral pain syndrome is responsible.
The speed at which swelling appears can help narrow down the cause. An ACL tear typically produces a popping sensation at the moment of injury, and the knee swells rapidly within a few hours. You may also feel the knee “give out” when you try to stand on it. A meniscus tear, by contrast, tends to swell more gradually over the next day or two. If your knee ballooned up within hours of an injury, that rapid onset points toward ligament damage or bleeding inside the joint. Slower swelling suggests cartilage or soft tissue inflammation.
Arthritis: The Most Common Chronic Cause
If your knee stiffness and swelling developed without a clear injury and has been coming and going for weeks or months, arthritis is the leading suspect. Knee osteoarthritis affects roughly 15% of adults aged 45 and older worldwide, and that number has been rising. It happens when the cartilage cushioning your knee joint gradually wears down, leading to inflammation, fluid buildup, and pain that worsens with activity.
A simple way to distinguish osteoarthritis from rheumatoid arthritis is the “30-minute rule” for morning stiffness. If your knee loosens up in under 30 minutes after you start moving, osteoarthritis is more likely. If stiffness lingers beyond 30 minutes, it suggests an inflammatory type of arthritis like rheumatoid arthritis. Osteoarthritis also tends to affect one knee more than the other and often starts in a single joint. Rheumatoid arthritis is typically symmetrical, affecting the same joint on both sides of the body, and it more commonly begins in the hands and feet before involving the knees.
Baker’s Cyst: Swelling Behind the Knee
If the swelling is concentrated behind your knee rather than around the kneecap, a Baker’s cyst may be the cause. This is a fluid-filled pouch that forms in the back of the knee, creating a visible bulge and a feeling of tightness. The pain tends to worsen with activity and when you try to fully straighten or fully bend the knee. Baker’s cysts often develop as a secondary problem when another condition, like arthritis or a meniscus tear, causes your knee to overproduce fluid.
In rare cases, a Baker’s cyst can rupture. When that happens, the fluid leaks down into your calf, causing sharp knee pain, calf swelling, and sometimes redness or a sensation of water running down the back of your lower leg. A ruptured cyst can mimic the symptoms of a blood clot, so sudden calf swelling alongside knee pain warrants prompt medical evaluation.
Signs That Point to Infection
A knee joint infection, called septic arthritis, is uncommon but serious. The hallmark presentation is a single knee that becomes red, hot to the touch, swollen, and extremely painful to move. Even gentle pressure on the joint causes significant discomfort. Up to 90% of people with a septic joint have at least a low-grade fever, and higher fevers occur in more than half of cases. However, the absence of fever alone doesn’t rule it out.
If your knee is red and warm, especially if you also feel feverish or generally unwell, this combination needs urgent attention. Joint infections can cause permanent damage within days if untreated. People with diabetes, rheumatoid arthritis, recent joint surgery, or a weakened immune system are at higher risk.
What the Fluid Itself Reveals
When the cause of knee swelling isn’t obvious, a doctor may drain some fluid from the joint with a needle. The appearance of that fluid provides immediate clues. Clear, straw-colored fluid is typical of osteoarthritis or minor irritation. Bloody or red-tinged fluid suggests trauma, a ligament tear, or bleeding inside the joint. Cloudy or opaque fluid, sometimes yellowish-green, raises concern for infection or an inflammatory condition like gout. The fluid can also be tested for bacteria, crystals (which indicate gout or pseudogout), and inflammatory cell counts to pinpoint the diagnosis.
Managing Swelling at Home
For a knee that’s recently become swollen from a minor injury or a flare of known arthritis, the classic approach is protecting the joint, resting from aggravating activities, applying ice, using compression (like an elastic bandage), and elevating your leg. This combination helps control swelling in the first 48 to 72 hours. Some physiotherapy guidelines have moved away from strict rest, recommending that you begin gentle, pain-free movement as soon as you can tolerate it rather than immobilizing the joint entirely. Prolonged inactivity can weaken the muscles around your knee and delay recovery.
Ice works best in 15 to 20 minute intervals with a barrier between the ice and your skin. Compression should feel snug but not tight enough to cause numbness or tingling below the wrap. Keeping your leg elevated above heart level when you’re sitting or lying down helps fluid drain away from the joint.
If your swelling doesn’t improve within a few days, keeps returning, or is accompanied by inability to bear weight, a locked knee that won’t fully bend or straighten, visible deformity, or redness and warmth, those patterns suggest something beyond a minor strain that needs professional evaluation.