Carol Eustice is a writer covering arthritis and chronic illness, who herself has been diagnosed with both rheumatoid arthritis and osteoarthritis.

Riteesha G. Reddy, MD, is a board-certified rheumatologist and internist at a private practice in Dallas, Texas.

Joint effusion is a condition in which excess fluid accumulates in or around a joint, causing a swollen joint. Joint effusion commonly affects the knee, where it is often referred to as "water on the knee" or "fluid on the knee."

Joint effusion can be caused by infection, arthritis, or injury (including repetitive use injuries). In addition to the swelling, the build-up of fluid within the joint space can cause pain and stiffness. When the accumulation of fluid is accompanied by inflammation of the connective tissues lining a joint, it is called joint effusion with synovitis .

Joint effusion tends to affect larger joints such as the knee, shoulder, elbow, or ankle. It is not the same as edema , which is the generalized build-up of fluid in tissues.

Read on to learn more about joint effusion, its causes and symptoms, and what treatments are available.

Regardless of what is causing the accumulation of fluid in a joint, the symptoms are more or less the same. They can range in severity from mild to debilitating.

Classic symptoms of joint effusion include:

Depending on what is causing joint effusion, other symptoms may develop, including:

This video has been medically reviewed by Oluseun Olufade, MD.

Joint effusion is a sign of joint inflammation and can be broadly classified as either infectious (septic) or noninfectious (aseptic).

Joint effusion caused by infection is referred to as septic arthritis. Aseptic joint effusion can be the result of an injury or arthritis.

Septic arthritis is most commonly caused by an infection in the joint. The infection can arise from an open wound, like a deep laceration or invasive medical procedure (including a joint replacement).

An infection in the bloodstream—called a systemic infection—can sometimes take hold in a joint and cause localized swelling and effusion.

When the joint effusion is caused by an infection, the symptoms are typically intense and fast-developing. The condition will usually be extremely painful, particularly with movement. Redness, warmth, and fever are also common.

Certain conditions can increase the risk of septic arthritis, including:

Injury is a common cause of joint effusion, especially of the knee. Injuries—such as those from a car accident, severe fall, or blunt force impact—can also lead to an effusion.

The injury may also involve bone, connective tissues (such as tendons and ligaments), or joint cartilage (like the meniscus).

Repeated stress on a joint can also cause effusion. This type of overuse injury happens after repeating a movement over and over and is usually related to an occupation or sports.

If the joint effusion is caused by a repetitive stress injury, bursitis (the inflammation of the fluid-filled sac that cushions a joint) and tenosynovitis (inflammation of the tendon sheath where a muscle attaches to a bone) are also common.

Pain, swelling, and stiffness, as well as difficulty extending or rotating the joint, are common symptoms of injury-related effusion.

Joint effusion is common in people with arthritis. The condition can be chronic (slowly developing and progressive) or acute (severe and rapidly developing).

There are two broad categories of arthritis: osteoarthritis (a non-inflammatory form of arthritis also known as "wear-and-tear arthritis") and autoimmune arthritis (a group of inflammatory autoimmune disorders in which the immune system attacks its own joints).

With osteoarthritis, joint effusion primarily affects the knee and most commonly occurs when there is extensive joint damage. The knee is one of the most commonly affected joints.

With autoimmune arthritis, joint effusion may be chronic but most often occurs in acute episodes (known as exacerbations). Among some of the autoimmune conditions that can cause joint effusion are:

Diagnosing joint effusion may involve a physical exam, imaging tests, and a lab evaluation of the fluid in your joint. In addition, the healthcare provider will want to review your medical history, current health, and other symptoms.

Your healthcare provider will examine your joint thoroughly. They will touch (palpate) and work (manipulate) the joint to determine the severity of the condition and the possible causes.

Among some of the possible findings:

After examining your knee, the healthcare provider may order imaging tests to determine the exact cause of the effusion. Each test has its benefits and limitations.

The imaging tests may include:

In some cases, your healthcare provider may want to drain (aspirate) fluid from a swollen joint. This can help reduce pressure and relieve pain but can also provide valuable information about the cause of the effusion.

The fluid, known as synovial fluid, is removed with a needle and syringe during a procedure known as arthrocentesis . Synovial fluid is usually clear with the consistency of an egg white. Any changes in the appearance, texture, color, or composition of the fluid can provide clues as to the underlying cause of the effusion.

A lab can analyze the fluid to determine if there are excess white blood cells (a sign of infection), uric acid crystals (a sign of gout), or other abnormalities that may narrow the possible causes.

The standard first-line treatments for joint effusion include rest, ice application, immobilization, and a nonsteroidal anti-inflammatory drug (NSAID) like Advil (ibuprofen) or Aleve (naproxen).

If the swelling is especially severe, your healthcare provider may want to aspirate the joint to reduce pressure inside the joint. They may give you a cortisone injection after the procedure to further reduce inflammation and pain.

Infections can usually be treated with a 14-day course of a broad-spectrum oral antibiotic like ciprofloxacin. Other more serious infections, like systemic gonorrhea or methicillin-resistant Staphylococcus aureus (MRSA), may require a two- and four-week course of intravenous antibiotics.

If you have rheumatoid arthritis or another form of autoimmune arthritis, medications may be prescribed to suppress the inappropriate immune response and relieve symptoms. This may involve immunosuppressive drugs like methotrexate or Humira (adalimumab).

Arthroplasty (joint surgery) is reserved for serious joint injuries or to repair joints immobilized by arthritis. Severe cases may require a total joint replacement.

Arguably the most effective way to ease joint swelling and pain is by applying an ice pack for 15 to 20 minutes several times daily. The does not cure the underlying cause but can help relieve the acute symptoms. A compression bandage can also help limit or reduce swelling, but avoid wrapping the joint too tightly as this can cut off blood circulation.

While joint effusion can’t always be avoided, there are things you can do to significantly lower your risk of it occurring:

Excess fluid around a joint—called a joint effusion—tends to affect larger joints, such as the knee or ankle. A joint effusion can occur as a result of injury, infection, or different types of arthritis.

In many cases, the excess fluid can be drained while steps can be taken to diagnose and treat the underlying cause (such as using antibiotics to treat a knee infection or immunosuppressants to treat rheumatoid arthritis).

No matter the cause of the effusion, there are steps you can take to better protect the joint and avoid future episodes.

Joint pain can be frustrating, especially when it limits your normal activity. If you experience the build-up of fluid in a joint, even mildly, see your healthcare provider right away to determine the cause. Postponing treatment can result in long-term (and possibly irreversible) damage to your joint and surrounding tissues.

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