Fluid on the Knee (Joint Effusion)

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What is fluid on the knee?

Fluid on the knee is a condition in which too much fluid builds up in or around the knee. The knee is one of the largest joints in the body. It is a hinge joint formed by three bones—the thighbone (femur), shinbone (tibia), and kneecap (patella). The joint also consists of several ligaments, tendons, cartilage structures, and bursae, which are fluid-filled sacs. Joint fluid—or synovial fluid—bathes the joint structures to cushion them and reduce friction between them.

Fluid on the knee can occur when there is too much synovial fluid or when blood or lymph fluid leaks into the joint. Excess fluid accumulates in the synovial cavity, which is the closed space enveloping the joint that holds synovial fluid. In cases of trauma or severe inflammation additional blood and fluid can accumulate within the surrounding soft tissues. Knee joint effusion is the medical name for the condition.

When there is fluid on/in the knee, it can cause the knee to swell. Pain and joint stiffness can also occur. Trauma, including acute injuries and overuse injuries, and diseases, such as gout and arthritis, can cause fluid on the knee. Depending on the cause, the symptoms of swelling, pain and stiffness can develop slowly or immediately after an injury.

Knee injuries are one of the most common types of joint injuries. Diseases affecting the knee, such as arthritis, are also very common. As a result, almost one-third of people will experience fluid on the knee at some point in their lives. You are more likely to have a problem with fluid on the knee as you age or if you play sports or are obese.

Treating fluid in the knee depends on the underlying cause. When injuries are to blame, this may involve rest, along with ice and elevation. When the cause isn’t clear, doctors may order radiographic imaging studies (X-ray, MRI) and perform joint fluid aspiration to take a sample of fluid for testing. Removing the fluid with joint arthrocentesis (needle) can also help relieve symptoms. In some cases, surgery may be necessary.

Left untreated, fluid on the knee can limit joint movement and cause supporting muscles to weaken and atrophy. See your doctor if you notice knee swelling that isn’t improving within a day or two, despite home treatment. Seek immediate medical care if you have an injury involving the knee.

What are the symptoms of fluid on the knee?

See your doctor if you have knee swelling and other symptoms that persist or worsen despite home treatment. You should seek prompt medical care if the swollen knee is also warm or red compared to the other knee or if you also have a fever. Seek immediate medical care any time you have significant trauma involving the knee.

The symptoms that occur with fluid on the knee can vary with the underlying cause. When only one knee is affected, symptoms are noticeable compared to the other knee.

Common symptoms of fluid on the knee

Common symptoms of fluid on the knee include:

  • Inability to bear weight on the affected leg
  • Joint stiffness and limited range of motion
  • Pain
  • Swelling

What causes fluid on the knee?

Fluid on the knee occurs when there is excess synovial fluid, blood, inflammatory exudation, or lymph leak into or around the joint. There are a variety of acute and chronic conditions that can cause this. They fall under two broad categories—traumatic and non-traumatic. Traumatic causes include both acute injuries, such as meniscus tears, and chronic overuse injuries, such as tendonitis. Non-traumatic causes of fluid on the knee include joint conditions, such as arthritis, gout, bursitis, infection, cysts and tumors. Tumors in the knee can be cancerous or noncancerous.

What are the risk factors for knee joint effusion?

You are at risk of developing fluid on the knee if you have a knee condition that results in disease or damage to the joint. This includes:

  • Bursitis, which is a main cause of fluid on the kneecap
  • Cartilage tears or injuries, especially meniscal tears
  • Gout
  • Ligament tears, including ACL (anterior cruciate ligament) tears

Your risk increases with age and if you play sports, especially sports involving pivoting, twisting, or quick changes in direction. This includes basketball, soccer, tennis, and downhill skiing. Obesity also increases your risk, as extra weight puts extra stress and strain on the joint.

Reducing your risk of fluid on the knee

Preventing diseases relies on changing risk factors that are under your control. You may be able to lower your risk of fluid on the knee by:

  • Avoiding high impact activities that stress the knees
  • Getting regular low impact physical exercise to strengthen the muscles supporting the knee
  • Maintaining a healthy weight
  • Treating known knee conditions
  • Wearing properly fitting shoes and protective equipment, such as knee pads, while playing sports

If you are concerned about your risk of injury or knee problems, talk with your doctor. Find out what changes you need to make to protect your knees.

How is knee joint effusion treated?

Treatment of a knee joint effusion depends on the underlying cause. The goal is to improve symptoms and heal the underlying injury or disease, if possible. Common treatment approaches to fluid on the knee include:

  • Anti-inflammatory medicines, including NSAIDs (nonsteroidal anti-inflammatory drugs), such as ibuprofen (Advil, Motrin), and corticosteroid injections into the knee
  • Drainage or aspiration of the knee to remove the fluid
  • Physical therapy to strengthen the muscles supporting the knee and improve range of motion
  • Rest, ice and elevation

In some cases, doctors recommend surgery to treat the underlying injury or disease process. This may include repair of damaged knee structures or joint replacement if the damage is severe.

What are the potential complications of fluid on the knee?

Usually, joint effusions will go away once you address the underlying problem. Left untreated, injuries and diseases causing fluid on the knee can lead to disability. When fluid on the knee limits knee movement, the supporting muscles can become weak and atrophy. Effusions related to an infection can result in the spread of infection to the blood and other body parts. Without prompt treatment, joint infections can also lead to permanent joint damage. In general, the sooner you seek treatment for fluid on the knee, the better.

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2021 Jan 4
THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
  1. A to Z: Joint Effusion. Nemours Foundation. https://kidshealth.org/AetnaBetterHealthKentucky/en/parents/az-joint-effusion.html
  2. Anatomy of the Knee. Arthritis Foundation. https://www.arthritis.org/health-wellness/about-arthritis/where-it-hurts/anatomy-of-the-knee
  3. Anterior Cruciate Ligament (ACL) Injuries. American Academy of Orthopedic Surgeons. https://orthoinfo.aaos.org/en/diseases--conditions/anterior-cruciate-ligament-acl-injuries
  4. Gerena LA, DeCastro A. Knee Effusion. [Updated 2020 Oct 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK532279/
  5. Johnson MW. Acute knee effusions: a systematic approach to diagnosis. Am Fam Physician. 2000 Apr 15;61(8):2391-400.
  6. Joint Aspiration. Cleveland Clinic. https://my.clevelandclinic.org/health/treatments/14512-joint-aspiration
  7. Prepatellar (Kneecap) Bursitis. American Academy of Orthopedic Surgeons. https://orthoinfo.aaos.org/en/diseases--conditions/prepatellar-kneecap-bursitis
  8. Swollen Knee. Cleveland Clinic. https://my.clevelandclinic.org/health/symptoms/17678-swollen-knee
  9. Swollen Knee. Mayo Foundation for Medical Education and Research. https://www.mayoclinic.org/diseases-conditions/swollen-knee/symptoms-causes/syc-20378129

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