A swollen knee happens when excess fluid accumulates in or around the knee joint. The causes range from a sports injury that floods the joint within minutes to a slow buildup from arthritis that worsens over weeks. Understanding where the fluid is coming from and how quickly it appeared are the two most important clues to figuring out what’s going on.
Two Types of Knee Swelling
Not all knee swelling is the same, and the difference matters. Fluid can collect inside the joint capsule itself, a condition called a joint effusion, or it can pool in one of the small fluid-filled sacs (bursae) that sit between skin, tendons, and bone around the knee. Bursae act as cushions to reduce friction between moving structures, and when one gets irritated, it swells in a very localized spot.
The most common example of bursal swelling is “housemaid’s knee,” where the sac directly in front of the kneecap puffs up from repeated kneeling. Another is a Baker’s cyst, a pocket of fluid behind the knee that involves the bursa between two tendons at the back of the joint. A Baker’s cyst sometimes communicates with the knee joint itself, meaning fluid can flow between them, while other bursae around the knee stay completely separate. A joint effusion, by contrast, makes the entire knee look puffy and stiff, often filling in the natural hollows on either side of the kneecap.
Acute Injuries That Cause Rapid Swelling
When a knee swells within seconds to minutes of an injury, blood is usually filling the joint. This is called a hemarthrosis, and it points to damage to a structure with its own blood supply. The classic example is a torn ACL (anterior cruciate ligament). Swelling after an ACL tear is rapid and immediate, and the knee typically feels unstable, as though it might buckle.
A fracture involving the knee joint also causes fast, significant swelling. Any high-energy impact, like a fall from height or a car accident, that results in instant ballooning of the knee should be evaluated urgently.
Meniscus tears follow a different pattern. Because the rubbery cartilage pads inside the knee have limited blood supply, the swelling develops gradually, usually over several hours rather than minutes. You might finish a game or a hike feeling only mild discomfort, then wake up the next morning with a noticeably swollen, stiff knee. This slower timeline is one of the easiest ways to distinguish a meniscus tear from a ligament tear before any imaging is done.
Osteoarthritis
Osteoarthritis is the most common chronic cause of knee swelling, especially after age 50. Years of wear gradually break down the cartilage that cushions the ends of the bones, and the joint lining responds by producing extra fluid. The swelling tends to come and go, flaring after activity and improving with rest. Morning stiffness is typical, but it usually loosens up in less than 30 minutes.
The knee may feel warm and achy but rarely turns hot or deeply red. Over time, the joint can develop bony enlargements that make the swelling look more permanent, even on days when fluid levels are relatively low.
Rheumatoid Arthritis and Other Inflammatory Types
Rheumatoid arthritis (RA) produces swelling through a different mechanism. The immune system attacks the joint lining, triggering persistent inflammation that sends fluid pouring into the joint space. The knee often feels warm or hot, and stiffness tends to be worst in the morning, lasting longer than 30 minutes, sometimes for hours. That duration of morning stiffness is one of the clearest ways to tell inflammatory arthritis apart from osteoarthritis.
RA also tends to affect joints symmetrically. If your right knee is swollen, there’s a good chance your left one is too, or that smaller joints in your hands and feet are also involved. Other inflammatory conditions like psoriatic arthritis or lupus can produce similar knee swelling, often alongside symptoms in other parts of the body like skin changes or fatigue.
Gout and Pseudogout
Both gout and pseudogout cause sudden, intense knee swelling that can appear overnight. The knee becomes hot, red, and exquisitely tender, sometimes too painful to even rest a bedsheet on.
Gout happens when uric acid crystals accumulate in the joint. While gout famously strikes the big toe, the knee is another common target, especially in people who’ve had gout for years. Pseudogout involves a different type of crystal, calcium pyrophosphate, and actually affects the knee more often than gout does. It’s more common in older adults and can mimic a joint infection so closely that doctors frequently need to draw fluid from the knee to tell them apart.
The distinction between the two is made by examining the fluid under a special microscope. Uric acid crystals are needle-shaped, while calcium pyrophosphate crystals are shorter and more diamond-shaped. This analysis is important because the treatments differ.
Infection in the Joint
Septic arthritis, a bacterial infection inside the knee joint, is the cause of knee swelling that doctors worry about most. The knee becomes hot, red, and extremely painful, often with fever and a general feeling of being unwell. Moving the knee even slightly can be agonizing. The swelling typically comes on fast, over a day or two.
Bacteria usually reach the joint through the bloodstream, but a puncture wound, recent surgery, or a nearby skin infection can introduce them directly. People with weakened immune systems, diabetes, or existing joint disease are at higher risk. Septic arthritis is a medical emergency because bacteria can destroy cartilage within days if left untreated. If your knee is hot, swollen, and extremely painful, especially with a fever, get it evaluated the same day.
Overuse and Bursitis
Repetitive stress on the knee causes swelling without a single dramatic injury. Runners, cyclists, and people whose jobs involve kneeling, squatting, or climbing stairs frequently develop knee swelling from overuse. The swelling builds gradually over days to weeks.
Prepatellar bursitis (swelling of the sac in front of the kneecap) is especially common in people who kneel for extended periods: carpet layers, plumbers, gardeners. The swelling sits right on top of the kneecap and feels like a soft, fluid-filled lump, distinct from the generalized puffiness of a joint effusion. Pes anserine bursitis causes swelling and tenderness on the inner side of the knee, just below the joint line, and is common in runners and people with osteoarthritis.
How Doctors Identify the Cause
A physical exam reveals more than you might expect. Two simple bedside tests help determine how much fluid is in the joint. In the bulge sign test, the examiner strokes fluid upward from the inner side of the knee, then pushes it across from the outer side, watching for a visible ripple of fluid returning to the inner hollow. This picks up small effusions. For larger amounts of fluid, the patellar tap test works: the examiner pushes fluid from above the kneecap downward, then presses the kneecap itself. If the kneecap bounces against the bone underneath, there’s a significant effusion.
When the cause isn’t obvious from the history and exam, drawing fluid from the knee with a needle (aspiration) provides direct answers. The fluid’s appearance alone offers clues: clear and straw-colored suggests osteoarthritis, cloudy points toward inflammation or infection, and bloody fluid suggests a fracture or ligament tear. Lab analysis of the fluid measures white blood cell levels. Normal joint fluid has very few white cells (under 200 per cubic millimeter). Inflammatory conditions like RA or gout push that count into the thousands or tens of thousands, while infected fluid often exceeds 50,000. Crystal analysis under a polarized microscope confirms or rules out gout and pseudogout.
Managing Swollen Knees at Home
Many causes of knee swelling respond well to basic self-care. Rest, ice, compression with an elastic bandage, and elevation (keeping the knee above heart level) reduce fluid accumulation from minor injuries and overuse. Ice works best in 15 to 20 minute intervals, several times a day, particularly in the first 48 to 72 hours.
Over-the-counter anti-inflammatory medications help reduce both swelling and pain for conditions like osteoarthritis flares, bursitis, and minor injuries. Maintaining a healthy weight matters more than most people realize. Every extra pound of body weight translates to roughly three to four pounds of additional force on the knee joint, so even modest weight loss can meaningfully reduce chronic swelling.
For large, painful effusions that don’t resolve on their own, aspiration provides immediate relief by physically removing the fluid. This is sometimes followed by an injection of medication into the joint to reduce inflammation and slow reaccumulation. Swelling that returns repeatedly despite conservative treatment, or swelling accompanied by locking, catching, or giving way, typically warrants imaging with an MRI to look for structural damage that may need surgical repair.