A swollen knee happens when excess fluid accumulates in or around the knee joint, and the causes range from a single awkward twist to chronic conditions that build up over years. The most common triggers fall into four broad categories: injury, arthritis, infection, and overuse. Figuring out which one is behind your swelling depends on how quickly it came on, where exactly the knee is puffy, and what other symptoms are tagging along.
Injuries That Cause Sudden Swelling
Trauma is one of the most straightforward reasons a knee swells up. A torn ligament (like the ACL or MCL), a meniscus tear, or a fracture around the joint can cause fluid to flood the area within minutes to hours. The swelling often arrives alongside sharp pain, difficulty bending the knee, and a feeling of instability when you try to stand. If the injury involves bleeding into the joint, the knee can balloon noticeably within the first hour.
Sports collisions, falls, and car accidents are typical culprits. But you don’t need a dramatic event. Pivoting on a planted foot, landing awkwardly from a jump, or even stepping off a curb wrong can tear cartilage or strain a ligament enough to trigger swelling. If your knee swelled rapidly after a specific moment you can pinpoint, an acute injury is the most likely explanation.
Osteoarthritis and Wear-Related Swelling
Osteoarthritis is the most common chronic cause of knee swelling, driven by mechanical wear and tear on the cartilage that cushions the joint. As that cartilage thins over time, the bones underneath generate friction, and the joint responds by producing extra fluid. The swelling tends to be mild to moderate, builds gradually, and often worsens after activity or at the end of the day. You may notice stiffness first thing in the morning that loosens up within 20 to 30 minutes.
Osteoarthritis knees typically ache and feel tender but don’t produce the dramatic, tense swelling you see with inflammatory or infectious causes. The joint may look slightly puffy compared to your other knee, and you might feel a grinding or crunching sensation when bending it. Risk factors include age, previous knee injuries, excess body weight, and jobs or activities that place repetitive stress on the joint.
Inflammatory Arthritis
Rheumatoid arthritis and other autoimmune conditions cause the immune system to attack the lining of the joint itself, producing swelling that is noticeably different from osteoarthritis. The knee becomes painful, visibly swollen, and stiff, often warm to the touch. Morning stiffness lasts longer, sometimes more than an hour, and the swelling doesn’t necessarily correlate with how much you’ve been on your feet. Rheumatoid arthritis also tends to affect joints symmetrically, so if one knee is swollen, the other may follow.
Gout and Crystal Deposits
Two types of crystal buildup can trigger intense, rapid knee swelling. Gout occurs when uric acid crystals accumulate in the joint. Pseudogout, a related but distinct condition, involves calcium crystals forming inside the joint space. Both cause episodes of sudden, severe pain with significant swelling and redness that can peak within hours.
Pseudogout hits the knee more often than gout does, and its risk factors are different from what most people expect. Having too much iron in the blood, too little magnesium, an underactive thyroid, or an overactive parathyroid gland all raise the likelihood. Previous trauma or surgery to the joint also increases risk. Some families carry a genetic predisposition. These crystal attacks can mimic an infection, which is why doctors often draw fluid from the knee to look at under a microscope and distinguish the two.
Joint Infection
Septic arthritis is the most urgent cause of a swollen knee. Bacteria enter the joint, usually through the bloodstream or after a wound, injection, or surgery near the knee. The classic presentation is a rapidly swelling, hot, extremely painful knee that you don’t want to move at all. Fever accompanies the swelling in roughly 40% to 60% of cases, but its absence doesn’t rule out infection.
What makes septic arthritis dangerous is speed. Bacteria can destroy cartilage within days if the infection isn’t treated. A knee that is warm to the touch, red or discolored, and significantly more swollen than the other knee warrants immediate medical attention, especially if you also feel feverish or generally unwell. Doctors confirm the diagnosis by withdrawing fluid from the joint and analyzing it. A very high white blood cell count in that fluid points strongly to a bacterial source.
Bursitis: Swelling on Top of the Kneecap
Not all knee swelling comes from inside the joint. Prepatellar bursitis is inflammation of the small fluid-filled sac that sits directly over the kneecap, and it produces a distinctive pocket of swelling right on the front of the knee rather than general puffiness throughout the joint.
Frequent kneeling is the most common cause. That’s why the condition has earned nicknames like housemaid’s knee, carpenter’s knee, and carpet layer’s knee. A direct blow to the kneecap, such as falling onto a hard surface, can also trigger it. There are two patterns: acute bursitis, which comes on suddenly after impact or a bacterial infection of the bursa, and chronic bursitis, which develops gradually from repeated pressure on the knee over weeks or months. If you spend time kneeling for work, gardening, or prayer, and the swelling is concentrated right over the kneecap, bursitis is a strong possibility.
Baker’s Cyst: Swelling Behind the Knee
A Baker’s cyst is a fluid-filled bulge that forms in the soft tissue behind the knee, in the crease called the popliteal fossa. It isn’t a standalone condition. In adults, it almost always develops as a secondary response to something else going on inside the joint. The most common underlying cause is a degenerative meniscal tear. Osteoarthritis, rheumatoid arthritis, and other inflammatory joint diseases also frequently lead to cyst formation.
Baker’s cysts occur most often in adults between ages 35 and 70. They can be painless, discovered incidentally on an MRI done for another reason, or they can cause a tight, aching pressure behind the knee that worsens when you fully bend or straighten the leg. If a Baker’s cyst ruptures, it can send fluid down the calf, causing sudden pain and swelling that mimics a blood clot, which is why any new calf swelling alongside knee problems deserves prompt evaluation.
Overuse Without a Clear Injury
Repetitive stress on the knee from running, jumping, or cycling can create localized pain and mild swelling without a single traumatic event. Patellar tendonitis, often called jumper’s knee, is a common example. It causes tenderness right at the bottom edge of the kneecap where the patellar tendon attaches. The swelling with tendonitis tends to be subtle compared to a torn ligament or arthritis flare, and the pain is most noticeable during activity or when pressing directly on the tendon.
Overuse injuries can be tricky to pin down because their symptoms overlap with bursitis, meniscal tears, and cartilage problems under the kneecap. The distinguishing feature is usually a clear pattern tied to activity: pain that ramps up with exercise, improves with rest, and returns when you push again.
How Doctors Identify the Cause
When the cause of knee swelling isn’t obvious from your history and a physical exam, the single most informative test is drawing fluid from the joint with a needle, a procedure called aspiration. The appearance and lab analysis of that fluid narrows the possibilities quickly. Clear, straw-colored fluid with a low white blood cell count (under 2,000 cells per cubic millimeter) points to a non-inflammatory cause like osteoarthritis or a mechanical injury. Cloudy fluid with a count between 2,000 and 50,000 suggests an inflammatory condition like rheumatoid arthritis or crystal disease. Counts above 50,000 raise strong concern for a bacterial infection.
Imaging plays a supporting role. X-rays can reveal fractures, bone spurs, or joint space narrowing from arthritis. MRI is better for soft tissue problems like ligament tears, meniscal damage, and Baker’s cysts. But for sorting out why the knee is producing extra fluid in the first place, joint aspiration gives the most direct answer.