Water on the knee (knee effusion)

Water on the knee, also called knee effusion, happens when extra fluid builds up in or around the knee joint. The swelling can make your knee feel stiff, tight or achy, and it can make everyday movements like walking or bending harder than usual. With the right diagnosis and treatment, most people find relief and get back to their normal activities.

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Illustration of a human knee joint with labeled parts including quadriceps, femur, patella, and ligaments.

Anatomy of the knee

The knee is one of the largest and most complex joints in the body. It's where the thighbone, shinbone and kneecap meet, cushioned by cartilage and stabilized by ligaments and tendons. A thin lining called the synovium surrounds the joint and produces a small amount of fluid that helps the knee move smoothly.

When the knee is injured, irritated or affected by an underlying condition, the synovium can produce too much fluid. That excess fluid collects inside the joint capsule, leading to the swelling commonly known as water on the knee.

Overview

Symptoms

Knee swelling can range from mild to severe depending on the underlying cause. Common signs include:

Visible puffiness or swelling around the knee
A feeling of tightness or fullness in the joint
Stiffness that makes it hard to fully bend or straighten the knee
Pain or tenderness, especially with movement
Warmth or redness around the joint
Difficulty walking, squatting or climbing stairs

Swelling that comes on suddenly may point to an injury, while swelling that develops gradually over time may be linked to an ongoing condition like arthritis. If swelling doesn't improve or keeps returning, it's worth having it checked out.

Causes

Water on the knee can develop for many reasons, and the cause often shapes the best treatment approach. Common causes include:

Injury to the knee, such as a ligament sprain or meniscus tear
Overuse from repetitive activities or sports
Osteoarthritis or other forms of joint inflammation
Infection within the joint
Gout or other crystal-related joint conditions
Underlying autoimmune conditions

Sometimes the exact cause of knee swelling isn't clear right away, and further testing may be needed to identify it.

Risk factors

Certain factors may make knee effusion more likely, though having one or more of these does not mean you'll develop the condition. Risk factors include:

Participating in sports or activities with repetitive knee stress
Being overweight, which adds extra strain on the knee joint
Having a history of knee injuries
Living with osteoarthritis or rheumatoid arthritis
Aging, which naturally increases wear on the joint

Diagnosis

Diagnosing water on the knee starts with a conversation about your symptoms and a physical exam to check for swelling, warmth and range of motion. Your care team may also recommend imaging tests, such as an X-ray or MRI, to look closely at the bones and soft tissue of the knee. In some cases, a small sample of the joint fluid may be removed and examined to help identify infection, gout or other underlying causes.

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Our board-certified specialists are here to help you heal. They can often diagnose the problem in one visit and get you started with a personalized treatment plan. We offer a wide variety of treatment options.