Fluid in the Knee: Symptoms, Causes, Treatment & Care

Fluid in the knee can cause swelling, pain, stiffness, and reduced movement. This guide covers fluid in the knee: its symptoms, causes, treatment, and when to seek care.

Many injuries and health problems can increase fluid within or around the knee. Common causes in primary care include osteoarthritis, trauma, and gout.

Finding the cause helps guide the right treatment. It also helps clinicians find urgent problems, such as a joint infection ^5,6.

A fever with a swollen knee can signal septic arthritis ^6,8. Septic arthritis is an infection within a joint ⁶.

A knee effusion is an abnormal buildup of fluid within the knee joint. Synovial fluid is the liquid that helps lubricate and cushion the knee.

An effusion is swelling caused by extra fluid in tissues around a joint. Bursitis can cause fluid near a joint rather than inside it.

A Baker cyst is a fluid-filled sac behind the knee. It can form when fluid builds within the knee.

People often use “swollen knee” to describe fluid inside or around the joint. A clinician can examine the knee to find where the swelling began.

Extra fluid can raise pressure within the knee and make walking painful. A large effusion may prevent full bending or straightening.

Fluid can also affect how well the thigh muscles work. This effect may lead to thigh muscle weakness.

Blood within the joint can reduce knee motion. It can also limit normal firing of the thigh muscles.

The amount of swelling does not show the cause by itself. Clinicians also consider its timing, related symptoms, and any recent injury ^5,10.

Knee effusion often causes pain, swelling, stiffness, and limited motion. The affected knee may look larger than the other knee.

Common knee effusion symptoms and signs include:

Pain and swelling can differ based on the cause. Some effusions appear after an injury, while others begin without trauma.

The timing of swelling after an injury can offer useful clues ¹⁰. It does not confirm a diagnosis by itself.

Swelling within minutes or several hours can suggest a ligament rupture ¹⁰. It may also occur with an internal fracture or kneecap dislocation ¹⁰.

Swelling that develops over hours or days may occur with a meniscus tear ¹⁰. The meniscus is firm cartilage that cushions the knee.

Atraumatic swelling with warmth or redness may occur with gout or pseudogout ¹⁰. It can also occur with an arthritis flare or infection ¹⁰.

A painful, swollen knee with fever raises concern for septic arthritis ¹⁰. Symptoms of septic arthritis often begin quickly ⁶.

Damage to knee structures can cause excess joint fluid. An acute injury with an effusion can point to damage inside the joint ¹⁰.

Injury and overuse causes include:

The injury pattern can help guide the examination ¹⁰. It can also help clinicians decide whether imaging is needed ¹⁰.

Osteoarthritis, trauma, and gout are common causes in primary care. Age, activity, and general health can affect the likely cause.

Osteoarthritis develops as knee cartilage breaks down over time. It can cause pain, stiffness, swelling, and difficulty moving.

Rheumatoid arthritis can also cause fluid buildup in the knee. Pain in several joints can raise concern for an inflammatory condition ¹⁰.

Gout and pseudogout are forms of crystal-related joint inflammation. Both conditions can cause knee effusion.

A joint fluid test can look for crystals ^5,6. Finding crystals does not fully rule out septic arthritis.

Infection can occur with gout or pseudogout at the same time. Clinicians may still order a Gram stain and culture when crystals are present.

Septic arthritis develops when bacteria or fungi infect a joint ⁶. The knee is one of the joints commonly affected ⁶.

Symptoms can include intense pain, swelling, redness, and fever ⁶. Movement may make the pain worse ⁶.

Factors linked with septic arthritis include diabetes and rheumatoid arthritis ⁶. Other factors include immune-suppressing medicines and recent joint surgery ⁶.

A recent joint injury or an artificial joint can also raise the risk ⁶. Bacteria may reach the joint through the blood, an injury, or surgery ⁶.

Knee effusion can also result from blood-related or inflammatory conditions. Blood vessel disorders and tumors are less common possible causes ^5,8.

A tumor may be noncancerous or cancerous ⁸. Atraumatic knee swelling can point to infection or a wider health condition.

Clinicians begin by asking when the swelling started. They also ask how quickly it appeared and how long it has lasted.

Important questions may cover the following subjects:

During the examination, the clinician compares both knees. The assessment often includes motion, strength, nerve, and blood flow checks ¹⁰.

Possible examination findings include:

These findings can narrow the list of possible causes. Further tests may be needed when the cause remains unclear ^5,10.

Clinicians select imaging based on the suspected cause ¹⁰. Routine X-rays are not advised for every person with possible osteoarthritis.

X-rays may be considered after an acute knee injury ¹⁰. Ultrasound can show an effusion, Baker cyst, or surface structure ¹⁰.

MRI is rarely used during an emergency knee assessment ¹⁰. Clinicians may consider it when surgery is possible ¹⁰.

MRI may also help when pain continues despite conservative treatment ¹⁰. Conservative treatment means care that does not involve surgery.

Laboratory tests may help confirm a suspected diagnosis when the history and examination are inconclusive ¹⁰. Blood cultures and other blood work may help assess septic arthritis ⁶.

Knee aspiration is also called arthrocentesis. A trained clinician places a needle into the joint and removes fluid.

The procedure can reduce pressure and help find the cause. Clinicians may use it for a large or painful effusion.

Unexplained knee effusion may need aspiration and fluid testing. Testing is important when infection or crystal arthritis is possible ^5,6.

The laboratory may perform these tests:

Crystals do not completely exclude an infection within the joint. Septic arthritis can occur alongside gout or pseudogout.

Aspiration has risks, including harm to nearby nerves, tendons, or blood vessels. A trained and qualified clinician should perform the procedure.

Treatment depends on the cause of the swollen knee ⁸. Mild symptoms may respond to rest, ice, and elevation.

Short-term home care can include these steps:

Seek medical care instead of continuing home care when warning signs appear. The urgent warning signs are listed later in this guide.

Nonsteroidal anti-inflammatory drugs include ibuprofen and naproxen ⁸. These drugs may help swelling linked with arthritis or an injury ⁸.

Ask a clinician which pain medicine fits your health needs. Tell that clinician about all medicines, vitamins, and supplements you take.

Treating the cause may help reduce the swelling ^7,8. The care plan can differ for arthritis, injury, gout, and infection.

First-line osteoarthritis care includes exercise, education, and self-management. Weight loss is also part of first-line care when appropriate.

Regular activity can reduce osteoarthritis pain and improve movement. Walking, swimming, and water exercise are possible activity choices.

Physical therapy uses guided exercises to improve movement and flexibility. It can also help reduce knee pain.

Gout treatment may include colchicine ⁸. This medicine helps with gout pain, inflammation, and joint effusion ⁸.

Rheumatoid arthritis and other inflammatory conditions need diagnosis-based care. Joint fluid tests can help clinicians identify inflammatory or infectious causes.

Exercise therapy is conservative care for many meniscus tears. Severe displaced tears from trauma may need surgery.

Routine arthroscopic surgery does not reduce pain from knee osteoarthritis. It is also not advised for degenerative meniscus tears.

This advice also applies when degenerative tears cause locking or catching. Treatment decisions can differ for severe traumatic tears.

Knee aspiration can relieve pain from a large, painful effusion. Removing fluid may also reduce pressure and improve joint movement.

A clinician may inject medicine after removing the fluid. Steroids can block inflammatory chemicals and may be injected for arthritis ⁸.

Septic arthritis is a bacterial or fungal joint infection ⁶. A needle may drain infected fluid when it builds quickly ⁶.

Severe septic arthritis may require surgery to drain and wash the joint ⁶. Delayed treatment can lead to lasting joint damage ⁶.

Surgery may also help selected severe injuries, such as displaced traumatic meniscus tears. Joint replacement may be considered for end-stage osteoarthritis.

Replacement is considered after suitable conservative treatments have been tried. Routine arthroscopy is not recommended for knee osteoarthritis pain.

Some knee symptoms need emergency assessment rather than home care. Seek emergency care now in these situations:

If one knee changes color and feels warm, seek immediate medical attention. These changes can be a sign of infection within the joint.

Acute trauma with severe pain, swelling, or instability needs urgent assessment ¹⁰. These signs may point to a fracture, dislocation, or serious tissue injury ¹⁰.

Unexplained swelling without trauma also needs medical assessment. Infection, gout, or a wider health condition may be responsible ^5,10.

Arrange a medical visit if ice and rest do not improve symptoms. A clinician can assess the swelling and look for its cause.

Locking, catching, or buckling can suggest internal damage or instability ¹⁰. A knee stuck in a bent position may be linked with a meniscus tear ¹⁰.

Returning swelling also deserves discussion with a clinician. An effusion may signal an injury or another condition, and home or prescribed treatments are available ^5,8.

Strong muscles around the knee can reduce pressure on the joint. Low-impact exercise choices include swimming and water aerobics.

Maintaining a healthy weight can reduce stress on the knees. It may also reduce joint overload linked with knee degeneration.

After knee aspiration, keep the needle site clean and dry as directed. Leave the dressing in place for the period advised by the clinician.

Contact the treating clinician after aspiration if these problems occur:

Fluid in the knee is a sign of an injury or health condition. It is not a diagnosis by itself, so treatment depends on the cause ^5,8.

Mild swelling may respond to short-term rest, ice, and elevation. Ongoing or unexplained swelling needs medical assessment.

If swelling occurs with fever, seek emergency care now ⁸. Do the same if the swollen knee cannot move or bear weight and you have a broken bone or torn ligament ⁸.

Loss of feeling or a suspected broken bone also requires emergency care now ⁸. Early assessment helps clinicians identify the cause and start suitable treatment.